lib/CodeGen/TwoAddressInstructionPass.cpp

//===-- TwoAddressInstructionPass.cpp - Two-Address instruction pass ------===//
//
//                     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 TwoAddress instruction pass which is used
// by most register allocators. Two-Address instructions are rewritten
// from:
//
//     A = B op C
//
// to:
//
//     A = B
//     A op= C
//
// Note that if a register allocator chooses to use this pass, that it
// has to be capable of handling the non-SSA nature of these rewritten
// virtual registers.
//
// It is also worth noting that the duplicate operand of the two
// address instruction is removed.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "twoaddrinstr"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Function.h"
#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
using namespace llvm;

STATISTIC(NumTwoAddressInstrs, "Number of two-address instructions");
STATISTIC(NumCommuted        , "Number of instructions commuted to coalesce");
STATISTIC(NumAggrCommuted    , "Number of instructions aggressively commuted");
STATISTIC(NumConvertedTo3Addr, "Number of instructions promoted to 3-address");
STATISTIC(Num3AddrSunk,        "Number of 3-address instructions sunk");
STATISTIC(NumReMats,           "Number of instructions re-materialized");
STATISTIC(NumDeletes,          "Number of dead instructions deleted");

namespace {
  class TwoAddressInstructionPass : public MachineFunctionPass {
    const TargetInstrInfo *TII;
    const TargetRegisterInfo *TRI;
    MachineRegisterInfo *MRI;
    LiveVariables *LV;
    AliasAnalysis *AA;

    // DistanceMap - Keep track the distance of a MI from the start of the
    // current basic block.
    DenseMap<MachineInstr*, unsigned> DistanceMap;

    // SrcRegMap - A map from virtual registers to physical registers which
    // are likely targets to be coalesced to due to copies from physical
    // registers to virtual registers. e.g. v1024 = move r0.
    DenseMap<unsigned, unsigned> SrcRegMap;

    // DstRegMap - A map from virtual registers to physical registers which
    // are likely targets to be coalesced to due to copies to physical
    // registers from virtual registers. e.g. r1 = move v1024.
    DenseMap<unsigned, unsigned> DstRegMap;

    /// RegSequences - Keep track the list of REG_SEQUENCE instructions seen
    /// during the initial walk of the machine function.
    SmallVector<MachineInstr*, 16> RegSequences;

    bool Sink3AddrInstruction(MachineBasicBlock *MBB, MachineInstr *MI,
                              unsigned Reg,
                              MachineBasicBlock::iterator OldPos);

    bool isProfitableToReMat(unsigned Reg, const TargetRegisterClass *RC,
                             MachineInstr *MI, MachineInstr *DefMI,
                             MachineBasicBlock *MBB, unsigned Loc);

    bool NoUseAfterLastDef(unsigned Reg, MachineBasicBlock *MBB, unsigned Dist,
                           unsigned &LastDef);

    MachineInstr *FindLastUseInMBB(unsigned Reg, MachineBasicBlock *MBB,
                                   unsigned Dist);

    bool isProfitableToCommute(unsigned regB, unsigned regC,
                               MachineInstr *MI, MachineBasicBlock *MBB,
                               unsigned Dist);

    bool CommuteInstruction(MachineBasicBlock::iterator &mi,
                            MachineFunction::iterator &mbbi,
                            unsigned RegB, unsigned RegC, unsigned Dist);

    bool isProfitableToConv3Addr(unsigned RegA);

    bool ConvertInstTo3Addr(MachineBasicBlock::iterator &mi,
                            MachineBasicBlock::iterator &nmi,
                            MachineFunction::iterator &mbbi,
                            unsigned RegB, unsigned Dist);

    typedef std::pair<std::pair<unsigned, bool>, MachineInstr*> NewKill;
    bool canUpdateDeletedKills(SmallVector<unsigned, 4> &Kills,
                               SmallVector<NewKill, 4> &NewKills,
                               MachineBasicBlock *MBB, unsigned Dist);
    bool DeleteUnusedInstr(MachineBasicBlock::iterator &mi,
                           MachineBasicBlock::iterator &nmi,
                           MachineFunction::iterator &mbbi, unsigned Dist);

    bool TryInstructionTransform(MachineBasicBlock::iterator &mi,
                                 MachineBasicBlock::iterator &nmi,
                                 MachineFunction::iterator &mbbi,
                                 unsigned SrcIdx, unsigned DstIdx,
                                 unsigned Dist);

    void ProcessCopy(MachineInstr *MI, MachineBasicBlock *MBB,
                     SmallPtrSet<MachineInstr*, 8> &Processed);

    void CoalesceExtSubRegs(SmallVector<unsigned,4> &Srcs, unsigned DstReg);

    /// EliminateRegSequences - Eliminate REG_SEQUENCE instructions as part
    /// of the de-ssa process. This replaces sources of REG_SEQUENCE as
    /// sub-register references of the register defined by REG_SEQUENCE.
    bool EliminateRegSequences();

  public:
    static char ID; // Pass identification, replacement for typeid
    TwoAddressInstructionPass() : MachineFunctionPass(&ID) {}

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
      AU.addRequired<AliasAnalysis>();
      AU.addPreserved<LiveVariables>();
      AU.addPreservedID(MachineLoopInfoID);
      AU.addPreservedID(MachineDominatorsID);
      if (StrongPHIElim)
        AU.addPreservedID(StrongPHIEliminationID);
      else
        AU.addPreservedID(PHIEliminationID);
      MachineFunctionPass::getAnalysisUsage(AU);
    }

    /// runOnMachineFunction - Pass entry point.
    bool runOnMachineFunction(MachineFunction&);
  };
}

char TwoAddressInstructionPass::ID = 0;
static RegisterPass<TwoAddressInstructionPass>
X("twoaddressinstruction", "Two-Address instruction pass");

const PassInfo *const llvm::TwoAddressInstructionPassID = &X;

/// Sink3AddrInstruction - A two-address instruction has been converted to a
/// three-address instruction to avoid clobbering a register. Try to sink it
/// past the instruction that would kill the above mentioned register to reduce
/// register pressure.
bool TwoAddressInstructionPass::Sink3AddrInstruction(MachineBasicBlock *MBB,
                                           MachineInstr *MI, unsigned SavedReg,
                                           MachineBasicBlock::iterator OldPos) {
  // Check if it's safe to move this instruction.
  bool SeenStore = true; // Be conservative.
  if (!MI->isSafeToMove(TII, AA, SeenStore))
    return false;

  unsigned DefReg = 0;
  SmallSet<unsigned, 4> UseRegs;

  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    const MachineOperand &MO = MI->getOperand(i);
    if (!MO.isReg())
      continue;
    unsigned MOReg = MO.getReg();
    if (!MOReg)
      continue;
    if (MO.isUse() && MOReg != SavedReg)
      UseRegs.insert(MO.getReg());
    if (!MO.isDef())
      continue;
    if (MO.isImplicit())
      // Don't try to move it if it implicitly defines a register.
      return false;
    if (DefReg)
      // For now, don't move any instructions that define multiple registers.
      return false;
    DefReg = MO.getReg();
  }

  // Find the instruction that kills SavedReg.
  MachineInstr *KillMI = NULL;
  for (MachineRegisterInfo::use_nodbg_iterator
         UI = MRI->use_nodbg_begin(SavedReg),
         UE = MRI->use_nodbg_end(); UI != UE; ++UI) {
    MachineOperand &UseMO = UI.getOperand();
    if (!UseMO.isKill())
      continue;
    KillMI = UseMO.getParent();
    break;
  }

  if (!KillMI || KillMI->getParent() != MBB || KillMI == MI)
    return false;

  // If any of the definitions are used by another instruction between the
  // position and the kill use, then it's not safe to sink it.
  // 
  // FIXME: This can be sped up if there is an easy way to query whether an
  // instruction is before or after another instruction. Then we can use
  // MachineRegisterInfo def / use instead.
  MachineOperand *KillMO = NULL;
  MachineBasicBlock::iterator KillPos = KillMI;
  ++KillPos;

  unsigned NumVisited = 0;
  for (MachineBasicBlock::iterator I = llvm::next(OldPos); I != KillPos; ++I) {
    MachineInstr *OtherMI = I;
    // DBG_VALUE cannot be counted against the limit.
    if (OtherMI->isDebugValue())
      continue;
    if (NumVisited > 30)  // FIXME: Arbitrary limit to reduce compile time cost.
      return false;
    ++NumVisited;
    for (unsigned i = 0, e = OtherMI->getNumOperands(); i != e; ++i) {
      MachineOperand &MO = OtherMI->getOperand(i);
      if (!MO.isReg())
        continue;
      unsigned MOReg = MO.getReg();
      if (!MOReg)
        continue;
      if (DefReg == MOReg)
        return false;

      if (MO.isKill()) {
        if (OtherMI == KillMI && MOReg == SavedReg)
          // Save the operand that kills the register. We want to unset the kill
          // marker if we can sink MI past it.
          KillMO = &MO;
        else if (UseRegs.count(MOReg))
          // One of the uses is killed before the destination.
          return false;
      }
    }
  }

  // Update kill and LV information.
  KillMO->setIsKill(false);
  KillMO = MI->findRegisterUseOperand(SavedReg, false, TRI);
  KillMO->setIsKill(true);
  
  if (LV)
    LV->replaceKillInstruction(SavedReg, KillMI, MI);

  // Move instruction to its destination.
  MBB->remove(MI);
  MBB->insert(KillPos, MI);

  ++Num3AddrSunk;
  return true;
}

/// isTwoAddrUse - Return true if the specified MI is using the specified
/// register as a two-address operand.
static bool isTwoAddrUse(MachineInstr *UseMI, unsigned Reg) {
  const TargetInstrDesc &TID = UseMI->getDesc();
  for (unsigned i = 0, e = TID.getNumOperands(); i != e; ++i) {
    MachineOperand &MO = UseMI->getOperand(i);
    if (MO.isReg() && MO.getReg() == Reg &&
        (MO.isDef() || UseMI->isRegTiedToDefOperand(i)))
      // Earlier use is a two-address one.
      return true;
  }
  return false;
}

/// isProfitableToReMat - Return true if the heuristics determines it is likely
/// to be profitable to re-materialize the definition of Reg rather than copy
/// the register.
bool
TwoAddressInstructionPass::isProfitableToReMat(unsigned Reg,
                                         const TargetRegisterClass *RC,
                                         MachineInstr *MI, MachineInstr *DefMI,
                                         MachineBasicBlock *MBB, unsigned Loc) {
  bool OtherUse = false;
  for (MachineRegisterInfo::use_nodbg_iterator UI = MRI->use_nodbg_begin(Reg),
         UE = MRI->use_nodbg_end(); UI != UE; ++UI) {
    MachineOperand &UseMO = UI.getOperand();
    MachineInstr *UseMI = UseMO.getParent();
    MachineBasicBlock *UseMBB = UseMI->getParent();
    if (UseMBB == MBB) {
      DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(UseMI);
      if (DI != DistanceMap.end() && DI->second == Loc)
        continue;  // Current use.
      OtherUse = true;
      // There is at least one other use in the MBB that will clobber the
      // register. 
      if (isTwoAddrUse(UseMI, Reg))
        return true;
    }
  }

  // If other uses in MBB are not two-address uses, then don't remat.
  if (OtherUse)
    return false;

  // No other uses in the same block, remat if it's defined in the same
  // block so it does not unnecessarily extend the live range.
  return MBB == DefMI->getParent();
}

/// NoUseAfterLastDef - Return true if there are no intervening uses between the
/// last instruction in the MBB that defines the specified register and the
/// two-address instruction which is being processed. It also returns the last
/// def location by reference
bool TwoAddressInstructionPass::NoUseAfterLastDef(unsigned Reg,
                                           MachineBasicBlock *MBB, unsigned Dist,
                                           unsigned &LastDef) {
  LastDef = 0;
  unsigned LastUse = Dist;
  for (MachineRegisterInfo::reg_iterator I = MRI->reg_begin(Reg),
         E = MRI->reg_end(); I != E; ++I) {
    MachineOperand &MO = I.getOperand();
    MachineInstr *MI = MO.getParent();
    if (MI->getParent() != MBB || MI->isDebugValue())
      continue;
    DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(MI);
    if (DI == DistanceMap.end())
      continue;
    if (MO.isUse() && DI->second < LastUse)
      LastUse = DI->second;
    if (MO.isDef() && DI->second > LastDef)
      LastDef = DI->second;
  }

  return !(LastUse > LastDef && LastUse < Dist);
}

MachineInstr *TwoAddressInstructionPass::FindLastUseInMBB(unsigned Reg,
                                                         MachineBasicBlock *MBB,
                                                         unsigned Dist) {
  unsigned LastUseDist = 0;
  MachineInstr *LastUse = 0;
  for (MachineRegisterInfo::reg_iterator I = MRI->reg_begin(Reg),
         E = MRI->reg_end(); I != E; ++I) {
    MachineOperand &MO = I.getOperand();
    MachineInstr *MI = MO.getParent();
    if (MI->getParent() != MBB || MI->isDebugValue())
      continue;
    DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(MI);
    if (DI == DistanceMap.end())
      continue;
    if (DI->second >= Dist)
      continue;

    if (MO.isUse() && DI->second > LastUseDist) {
      LastUse = DI->first;
      LastUseDist = DI->second;
    }
  }
  return LastUse;
}

/// isCopyToReg - Return true if the specified MI is a copy instruction or
/// a extract_subreg instruction. It also returns the source and destination
/// registers and whether they are physical registers by reference.
static bool isCopyToReg(MachineInstr &MI, const TargetInstrInfo *TII,
                        unsigned &SrcReg, unsigned &DstReg,
                        bool &IsSrcPhys, bool &IsDstPhys) {
  SrcReg = 0;
  DstReg = 0;
  unsigned SrcSubIdx, DstSubIdx;
  if (!TII->isMoveInstr(MI, SrcReg, DstReg, SrcSubIdx, DstSubIdx)) {
    if (MI.isCopy()) {
      DstReg = MI.getOperand(0).getReg();
      SrcReg = MI.getOperand(1).getReg();
    } else if (MI.isInsertSubreg()) {
      DstReg = MI.getOperand(0).getReg();
      SrcReg = MI.getOperand(2).getReg();
    } else if (MI.isSubregToReg()) {
      DstReg = MI.getOperand(0).getReg();
      SrcReg = MI.getOperand(2).getReg();
    }
  }

  if (DstReg) {
    IsSrcPhys = TargetRegisterInfo::isPhysicalRegister(SrcReg);
    IsDstPhys = TargetRegisterInfo::isPhysicalRegister(DstReg);
    return true;
  }
  return false;
}

/// isKilled - Test if the given register value, which is used by the given
/// instruction, is killed by the given instruction. This looks through
/// coalescable copies to see if the original value is potentially not killed.
///
/// For example, in this code:
///
///   %reg1034 = copy %reg1024
///   %reg1035 = copy %reg1025<kill>
///   %reg1036 = add %reg1034<kill>, %reg1035<kill>
///
/// %reg1034 is not considered to be killed, since it is copied from a
/// register which is not killed. Treating it as not killed lets the
/// normal heuristics commute the (two-address) add, which lets
/// coalescing eliminate the extra copy.
///
static bool isKilled(MachineInstr &MI, unsigned Reg,
                     const MachineRegisterInfo *MRI,
                     const TargetInstrInfo *TII) {
  MachineInstr *DefMI = &MI;
  for (;;) {
    if (!DefMI->killsRegister(Reg))
      return false;
    if (TargetRegisterInfo::isPhysicalRegister(Reg))
      return true;
    MachineRegisterInfo::def_iterator Begin = MRI->def_begin(Reg);
    // If there are multiple defs, we can't do a simple analysis, so just
    // go with what the kill flag says.
    if (llvm::next(Begin) != MRI->def_end())
      return true;
    DefMI = &*Begin;
    bool IsSrcPhys, IsDstPhys;
    unsigned SrcReg,  DstReg;
    // If the def is something other than a copy, then it isn't going to
    // be coalesced, so follow the kill flag.
    if (!isCopyToReg(*DefMI, TII, SrcReg, DstReg, IsSrcPhys, IsDstPhys))
      return true;
    Reg = SrcReg;
  }
}

/// isTwoAddrUse - Return true if the specified MI uses the specified register
/// as a two-address use. If so, return the destination register by reference.
static bool isTwoAddrUse(MachineInstr &MI, unsigned Reg, unsigned &DstReg) {
  const TargetInstrDesc &TID = MI.getDesc();
  unsigned NumOps = MI.isInlineAsm() ? MI.getNumOperands():TID.getNumOperands();
  for (unsigned i = 0; i != NumOps; ++i) {
    const MachineOperand &MO = MI.getOperand(i);
    if (!MO.isReg() || !MO.isUse() || MO.getReg() != Reg)
      continue;
    unsigned ti;
    if (MI.isRegTiedToDefOperand(i, &ti)) {
      DstReg = MI.getOperand(ti).getReg();
      return true;
    }
  }
  return false;
}

/// findOnlyInterestingUse - Given a register, if has a single in-basic block
/// use, return the use instruction if it's a copy or a two-address use.
static
MachineInstr *findOnlyInterestingUse(unsigned Reg, MachineBasicBlock *MBB,
                                     MachineRegisterInfo *MRI,
                                     const TargetInstrInfo *TII,
                                     bool &IsCopy,
                                     unsigned &DstReg, bool &IsDstPhys) {
  if (!MRI->hasOneNonDBGUse(Reg))
    // None or more than one use.
    return 0;
  MachineInstr &UseMI = *MRI->use_nodbg_begin(Reg);
  if (UseMI.getParent() != MBB)
    return 0;
  unsigned SrcReg;
  bool IsSrcPhys;
  if (isCopyToReg(UseMI, TII, SrcReg, DstReg, IsSrcPhys, IsDstPhys)) {
    IsCopy = true;
    return &UseMI;
  }
  IsDstPhys = false;
  if (isTwoAddrUse(UseMI, Reg, DstReg)) {
    IsDstPhys = TargetRegisterInfo::isPhysicalRegister(DstReg);
    return &UseMI;
  }
  return 0;
}

/// getMappedReg - Return the physical register the specified virtual register
/// might be mapped to.
static unsigned
getMappedReg(unsigned Reg, DenseMap<unsigned, unsigned> &RegMap) {
  while (TargetRegisterInfo::isVirtualRegister(Reg))  {
    DenseMap<unsigned, unsigned>::iterator SI = RegMap.find(Reg);
    if (SI == RegMap.end())
      return 0;
    Reg = SI->second;
  }
  if (TargetRegisterInfo::isPhysicalRegister(Reg))
    return Reg;
  return 0;
}

/// regsAreCompatible - Return true if the two registers are equal or aliased.
///
static bool
regsAreCompatible(unsigned RegA, unsigned RegB, const TargetRegisterInfo *TRI) {
  if (RegA == RegB)
    return true;
  if (!RegA || !RegB)
    return false;
  return TRI->regsOverlap(RegA, RegB);
}


/// isProfitableToReMat - Return true if it's potentially profitable to commute
/// the two-address instruction that's being processed.
bool
TwoAddressInstructionPass::isProfitableToCommute(unsigned regB, unsigned regC,
                                       MachineInstr *MI, MachineBasicBlock *MBB,
                                       unsigned Dist) {
  // Determine if it's profitable to commute this two address instruction. In
  // general, we want no uses between this instruction and the definition of
  // the two-address register.
  // e.g.
  // %reg1028<def> = EXTRACT_SUBREG %reg1027<kill>, 1
  // %reg1029<def> = MOV8rr %reg1028
  // %reg1029<def> = SHR8ri %reg1029, 7, %EFLAGS<imp-def,dead>
  // insert => %reg1030<def> = MOV8rr %reg1028
  // %reg1030<def> = ADD8rr %reg1028<kill>, %reg1029<kill>, %EFLAGS<imp-def,dead>
  // In this case, it might not be possible to coalesce the second MOV8rr
  // instruction if the first one is coalesced. So it would be profitable to
  // commute it:
  // %reg1028<def> = EXTRACT_SUBREG %reg1027<kill>, 1
  // %reg1029<def> = MOV8rr %reg1028
  // %reg1029<def> = SHR8ri %reg1029, 7, %EFLAGS<imp-def,dead>
  // insert => %reg1030<def> = MOV8rr %reg1029
  // %reg1030<def> = ADD8rr %reg1029<kill>, %reg1028<kill>, %EFLAGS<imp-def,dead>  

  if (!MI->killsRegister(regC))
    return false;

  // Ok, we have something like:
  // %reg1030<def> = ADD8rr %reg1028<kill>, %reg1029<kill>, %EFLAGS<imp-def,dead>
  // let's see if it's worth commuting it.

  // Look for situations like this:
  // %reg1024<def> = MOV r1
  // %reg1025<def> = MOV r0
  // %reg1026<def> = ADD %reg1024, %reg1025
  // r0            = MOV %reg1026
  // Commute the ADD to hopefully eliminate an otherwise unavoidable copy.
  unsigned FromRegB = getMappedReg(regB, SrcRegMap);
  unsigned FromRegC = getMappedReg(regC, SrcRegMap);
  unsigned ToRegB = getMappedReg(regB, DstRegMap);
  unsigned ToRegC = getMappedReg(regC, DstRegMap);
  if (!regsAreCompatible(FromRegB, ToRegB, TRI) &&
      (regsAreCompatible(FromRegB, ToRegC, TRI) ||
       regsAreCompatible(FromRegC, ToRegB, TRI)))
    return true;

  // If there is a use of regC between its last def (could be livein) and this
  // instruction, then bail.
  unsigned LastDefC = 0;
  if (!NoUseAfterLastDef(regC, MBB, Dist, LastDefC))
    return false;

  // If there is a use of regB between its last def (could be livein) and this
  // instruction, then go ahead and make this transformation.
  unsigned LastDefB = 0;
  if (!NoUseAfterLastDef(regB, MBB, Dist, LastDefB))
    return true;

  // Since there are no intervening uses for both registers, then commute
  // if the def of regC is closer. Its live interval is shorter.
  return LastDefB && LastDefC && LastDefC > LastDefB;
}

/// CommuteInstruction - Commute a two-address instruction and update the basic
/// block, distance map, and live variables if needed. Return true if it is
/// successful.
bool
TwoAddressInstructionPass::CommuteInstruction(MachineBasicBlock::iterator &mi,
                               MachineFunction::iterator &mbbi,
                               unsigned RegB, unsigned RegC, unsigned Dist) {
  MachineInstr *MI = mi;
  DEBUG(dbgs() << "2addr: COMMUTING  : " << *MI);
  MachineInstr *NewMI = TII->commuteInstruction(MI);

  if (NewMI == 0) {
    DEBUG(dbgs() << "2addr: COMMUTING FAILED!\n");
    return false;
  }

  DEBUG(dbgs() << "2addr: COMMUTED TO: " << *NewMI);
  // If the instruction changed to commute it, update livevar.
  if (NewMI != MI) {
    if (LV)
      // Update live variables
      LV->replaceKillInstruction(RegC, MI, NewMI);

    mbbi->insert(mi, NewMI);           // Insert the new inst
    mbbi->erase(mi);                   // Nuke the old inst.
    mi = NewMI;
    DistanceMap.insert(std::make_pair(NewMI, Dist));
  }

  // Update source register map.
  unsigned FromRegC = getMappedReg(RegC, SrcRegMap);
  if (FromRegC) {
    unsigned RegA = MI->getOperand(0).getReg();
    SrcRegMap[RegA] = FromRegC;
  }

  return true;
}

/// isProfitableToConv3Addr - Return true if it is profitable to convert the
/// given 2-address instruction to a 3-address one.
bool
TwoAddressInstructionPass::isProfitableToConv3Addr(unsigned RegA) {
  // Look for situations like this:
  // %reg1024<def> = MOV r1
  // %reg1025<def> = MOV r0
  // %reg1026<def> = ADD %reg1024, %reg1025
  // r2            = MOV %reg1026
  // Turn ADD into a 3-address instruction to avoid a copy.
  unsigned FromRegA = getMappedReg(RegA, SrcRegMap);
  unsigned ToRegA = getMappedReg(RegA, DstRegMap);
  return (FromRegA && ToRegA && !regsAreCompatible(FromRegA, ToRegA, TRI));
}

/// ConvertInstTo3Addr - Convert the specified two-address instruction into a
/// three address one. Return true if this transformation was successful.
bool
TwoAddressInstructionPass::ConvertInstTo3Addr(MachineBasicBlock::iterator &mi,
                                              MachineBasicBlock::iterator &nmi,
                                              MachineFunction::iterator &mbbi,
                                              unsigned RegB, unsigned Dist) {
  MachineInstr *NewMI = TII->convertToThreeAddress(mbbi, mi, LV);
  if (NewMI) {
    DEBUG(dbgs() << "2addr: CONVERTING 2-ADDR: " << *mi);
    DEBUG(dbgs() << "2addr:         TO 3-ADDR: " << *NewMI);
    bool Sunk = false;

    if (NewMI->findRegisterUseOperand(RegB, false, TRI))
      // FIXME: Temporary workaround. If the new instruction doesn't
      // uses RegB, convertToThreeAddress must have created more
      // then one instruction.
      Sunk = Sink3AddrInstruction(mbbi, NewMI, RegB, mi);

    mbbi->erase(mi); // Nuke the old inst.

    if (!Sunk) {
      DistanceMap.insert(std::make_pair(NewMI, Dist));
      mi = NewMI;
      nmi = llvm::next(mi);
    }
    return true;
  }

  return false;
}

/// ProcessCopy - If the specified instruction is not yet processed, process it
/// if it's a copy. For a copy instruction, we find the physical registers the
/// source and destination registers might be mapped to. These are kept in
/// point-to maps used to determine future optimizations. e.g.
/// v1024 = mov r0
/// v1025 = mov r1
/// v1026 = add v1024, v1025
/// r1    = mov r1026
/// If 'add' is a two-address instruction, v1024, v1026 are both potentially
/// coalesced to r0 (from the input side). v1025 is mapped to r1. v1026 is
/// potentially joined with r1 on the output side. It's worthwhile to commute
/// 'add' to eliminate a copy.
void TwoAddressInstructionPass::ProcessCopy(MachineInstr *MI,
                                     MachineBasicBlock *MBB,
                                     SmallPtrSet<MachineInstr*, 8> &Processed) {
  if (Processed.count(MI))
    return;

  bool IsSrcPhys, IsDstPhys;
  unsigned SrcReg, DstReg;
  if (!isCopyToReg(*MI, TII, SrcReg, DstReg, IsSrcPhys, IsDstPhys))
    return;

  if (IsDstPhys && !IsSrcPhys)
    DstRegMap.insert(std::make_pair(SrcReg, DstReg));
  else if (!IsDstPhys && IsSrcPhys) {
    bool isNew = SrcRegMap.insert(std::make_pair(DstReg, SrcReg)).second;
    if (!isNew)
      assert(SrcRegMap[DstReg] == SrcReg &&
             "Can't map to two src physical registers!");

    SmallVector<unsigned, 4> VirtRegPairs;
    bool IsCopy = false;
    unsigned NewReg = 0;
    while (MachineInstr *UseMI = findOnlyInterestingUse(DstReg, MBB, MRI,TII,
                                                   IsCopy, NewReg, IsDstPhys)) {
      if (IsCopy) {
        if (!Processed.insert(UseMI))
          break;
      }

      DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(UseMI);
      if (DI != DistanceMap.end())
        // Earlier in the same MBB.Reached via a back edge.
        break;

      if (IsDstPhys) {
        VirtRegPairs.push_back(NewReg);
        break;
      }
      bool isNew = SrcRegMap.insert(std::make_pair(NewReg, DstReg)).second;
      if (!isNew)
        assert(SrcRegMap[NewReg] == DstReg &&
               "Can't map to two src physical registers!");
      VirtRegPairs.push_back(NewReg);
      DstReg = NewReg;
    }

    if (!VirtRegPairs.empty()) {
      unsigned ToReg = VirtRegPairs.back();
      VirtRegPairs.pop_back();
      while (!VirtRegPairs.empty()) {
        unsigned FromReg = VirtRegPairs.back();
        VirtRegPairs.pop_back();
        bool isNew = DstRegMap.insert(std::make_pair(FromReg, ToReg)).second;
        if (!isNew)
          assert(DstRegMap[FromReg] == ToReg &&
                 "Can't map to two dst physical registers!");
        ToReg = FromReg;
      }
    }
  }

  Processed.insert(MI);
}

/// isSafeToDelete - If the specified instruction does not produce any side
/// effects and all of its defs are dead, then it's safe to delete.
static bool isSafeToDelete(MachineInstr *MI,
                           const TargetInstrInfo *TII,
                           SmallVector<unsigned, 4> &Kills) {
  const TargetInstrDesc &TID = MI->getDesc();
  if (TID.mayStore() || TID.isCall())
    return false;
  if (TID.isTerminator() || TID.hasUnmodeledSideEffects())
    return false;

  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI->getOperand(i);
    if (!MO.isReg())
      continue;
    if (MO.isDef() && !MO.isDead())
      return false;
    if (MO.isUse() && MO.isKill())
      Kills.push_back(MO.getReg());
  }
  return true;
}

/// canUpdateDeletedKills - Check if all the registers listed in Kills are
/// killed by instructions in MBB preceding the current instruction at
/// position Dist.  If so, return true and record information about the
/// preceding kills in NewKills.
bool TwoAddressInstructionPass::
canUpdateDeletedKills(SmallVector<unsigned, 4> &Kills,
                      SmallVector<NewKill, 4> &NewKills,
                      MachineBasicBlock *MBB, unsigned Dist) {
  while (!Kills.empty()) {
    unsigned Kill = Kills.back();
    Kills.pop_back();
    if (TargetRegisterInfo::isPhysicalRegister(Kill))
      return false;

    MachineInstr *LastKill = FindLastUseInMBB(Kill, MBB, Dist);
    if (!LastKill)
      return false;

    bool isModRef = LastKill->definesRegister(Kill);
    NewKills.push_back(std::make_pair(std::make_pair(Kill, isModRef),
                                      LastKill));
  }
  return true;
}

/// DeleteUnusedInstr - If an instruction with a tied register operand can
/// be safely deleted, just delete it.
bool
TwoAddressInstructionPass::DeleteUnusedInstr(MachineBasicBlock::iterator &mi,
                                             MachineBasicBlock::iterator &nmi,
                                             MachineFunction::iterator &mbbi,
                                             unsigned Dist) {
  // Check if the instruction has no side effects and if all its defs are dead.
  SmallVector<unsigned, 4> Kills;
  if (!isSafeToDelete(mi, TII, Kills))
    return false;

  // If this instruction kills some virtual registers, we need to
  // update the kill information. If it's not possible to do so,
  // then bail out.
  SmallVector<NewKill, 4> NewKills;
  if (!canUpdateDeletedKills(Kills, NewKills, &*mbbi, Dist))
    return false;

  if (LV) {
    while (!NewKills.empty()) {
      MachineInstr *NewKill = NewKills.back().second;
      unsigned Kill = NewKills.back().first.first;
      bool isDead = NewKills.back().first.second;
      NewKills.pop_back();
      if (LV->removeVirtualRegisterKilled(Kill, mi)) {
        if (isDead)
          LV->addVirtualRegisterDead(Kill, NewKill);
        else
          LV->addVirtualRegisterKilled(Kill, NewKill);
      }
    }
  }

  mbbi->erase(mi); // Nuke the old inst.
  mi = nmi;
  return true;
}

/// TryInstructionTransform - For the case where an instruction has a single
/// pair of tied register operands, attempt some transformations that may
/// either eliminate the tied operands or improve the opportunities for
/// coalescing away the register copy.  Returns true if the tied operands
/// are eliminated altogether.
bool TwoAddressInstructionPass::
TryInstructionTransform(MachineBasicBlock::iterator &mi,
                        MachineBasicBlock::iterator &nmi,
                        MachineFunction::iterator &mbbi,
                        unsigned SrcIdx, unsigned DstIdx, unsigned Dist) {
  const TargetInstrDesc &TID = mi->getDesc();
  unsigned regA = mi->getOperand(DstIdx).getReg();
  unsigned regB = mi->getOperand(SrcIdx).getReg();

  assert(TargetRegisterInfo::isVirtualRegister(regB) &&
         "cannot make instruction into two-address form");

  // If regA is dead and the instruction can be deleted, just delete
  // it so it doesn't clobber regB.
  bool regBKilled = isKilled(*mi, regB, MRI, TII);
  if (!regBKilled && mi->getOperand(DstIdx).isDead() &&
      DeleteUnusedInstr(mi, nmi, mbbi, Dist)) {
    ++NumDeletes;
    return true; // Done with this instruction.
  }

  // Check if it is profitable to commute the operands.
  unsigned SrcOp1, SrcOp2;
  unsigned regC = 0;
  unsigned regCIdx = ~0U;
  bool TryCommute = false;
  bool AggressiveCommute = false;
  if (TID.isCommutable() && mi->getNumOperands() >= 3 &&
      TII->findCommutedOpIndices(mi, SrcOp1, SrcOp2)) {
    if (SrcIdx == SrcOp1)
      regCIdx = SrcOp2;
    else if (SrcIdx == SrcOp2)
      regCIdx = SrcOp1;

    if (regCIdx != ~0U) {
      regC = mi->getOperand(regCIdx).getReg();
      if (!regBKilled && isKilled(*mi, regC, MRI, TII))
        // If C dies but B does not, swap the B and C operands.
        // This makes the live ranges of A and C joinable.
        TryCommute = true;
      else if (isProfitableToCommute(regB, regC, mi, mbbi, Dist)) {
        TryCommute = true;
        AggressiveCommute = true;
      }
    }
  }

  // If it's profitable to commute, try to do so.
  if (TryCommute && CommuteInstruction(mi, mbbi, regB, regC, Dist)) {
    ++NumCommuted;
    if (AggressiveCommute)
      ++NumAggrCommuted;
    return false;
  }

  if (TID.isConvertibleTo3Addr()) {
    // This instruction is potentially convertible to a true
    // three-address instruction.  Check if it is profitable.
    if (!regBKilled || isProfitableToConv3Addr(regA)) {
      // Try to convert it.
      if (ConvertInstTo3Addr(mi, nmi, mbbi, regB, Dist)) {
        ++NumConvertedTo3Addr;
        return true; // Done with this instruction.
      }
    }
  }

  // If this is an instruction with a load folded into it, try unfolding
  // the load, e.g. avoid this:
  //   movq %rdx, %rcx
  //   addq (%rax), %rcx
  // in favor of this:
  //   movq (%rax), %rcx
  //   addq %rdx, %rcx
  // because it's preferable to schedule a load than a register copy.
  if (TID.mayLoad() && !regBKilled) {
    // Determine if a load can be unfolded.
    unsigned LoadRegIndex;
    unsigned NewOpc =
      TII->getOpcodeAfterMemoryUnfold(mi->getOpcode(),
                                      /*UnfoldLoad=*/true,
                                      /*UnfoldStore=*/false,
                                      &LoadRegIndex);
    if (NewOpc != 0) {
      const TargetInstrDesc &UnfoldTID = TII->get(NewOpc);
      if (UnfoldTID.getNumDefs() == 1) {
        MachineFunction &MF = *mbbi->getParent();

        // Unfold the load.
        DEBUG(dbgs() << "2addr:   UNFOLDING: " << *mi);
        const TargetRegisterClass *RC =
          UnfoldTID.OpInfo[LoadRegIndex].getRegClass(TRI);
        unsigned Reg = MRI->createVirtualRegister(RC);
        SmallVector<MachineInstr *, 2> NewMIs;
        if (!TII->unfoldMemoryOperand(MF, mi, Reg,
                                      /*UnfoldLoad=*/true,/*UnfoldStore=*/false,
                                      NewMIs)) {
          DEBUG(dbgs() << "2addr: ABANDONING UNFOLD\n");
          return false;
        }
        assert(NewMIs.size() == 2 &&
               "Unfolded a load into multiple instructions!");
        // The load was previously folded, so this is the only use.
        NewMIs[1]->addRegisterKilled(Reg, TRI);

        // Tentatively insert the instructions into the block so that they
        // look "normal" to the transformation logic.
        mbbi->insert(mi, NewMIs[0]);
        mbbi->insert(mi, NewMIs[1]);

        DEBUG(dbgs() << "2addr:    NEW LOAD: " << *NewMIs[0]
                     << "2addr:    NEW INST: " << *NewMIs[1]);

        // Transform the instruction, now that it no longer has a load.
        unsigned NewDstIdx = NewMIs[1]->findRegisterDefOperandIdx(regA);
        unsigned NewSrcIdx = NewMIs[1]->findRegisterUseOperandIdx(regB);
        MachineBasicBlock::iterator NewMI = NewMIs[1];
        bool TransformSuccess =
          TryInstructionTransform(NewMI, mi, mbbi,
                                  NewSrcIdx, NewDstIdx, Dist);
        if (TransformSuccess ||
            NewMIs[1]->getOperand(NewSrcIdx).isKill()) {
          // Success, or at least we made an improvement. Keep the unfolded
          // instructions and discard the original.
          if (LV) {
            for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) {
              MachineOperand &MO = mi->getOperand(i);
              if (MO.isReg() && MO.getReg() != 0 &&
                  TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
                if (MO.isUse()) {
                  if (MO.isKill()) {
                    if (NewMIs[0]->killsRegister(MO.getReg()))
                      LV->replaceKillInstruction(MO.getReg(), mi, NewMIs[0]);
                    else {
                      assert(NewMIs[1]->killsRegister(MO.getReg()) &&
                             "Kill missing after load unfold!");
                      LV->replaceKillInstruction(MO.getReg(), mi, NewMIs[1]);
                    }
                  }
                } else if (LV->removeVirtualRegisterDead(MO.getReg(), mi)) {
                  if (NewMIs[1]->registerDefIsDead(MO.getReg()))
                    LV->addVirtualRegisterDead(MO.getReg(), NewMIs[1]);
                  else {
                    assert(NewMIs[0]->registerDefIsDead(MO.getReg()) &&
                           "Dead flag missing after load unfold!");
                    LV->addVirtualRegisterDead(MO.getReg(), NewMIs[0]);
                  }
                }
              }
            }
            LV->addVirtualRegisterKilled(Reg, NewMIs[1]);
          }
          mi->eraseFromParent();
          mi = NewMIs[1];
          if (TransformSuccess)
            return true;
        } else {
          // Transforming didn't eliminate the tie and didn't lead to an
          // improvement. Clean up the unfolded instructions and keep the
          // original.
          DEBUG(dbgs() << "2addr: ABANDONING UNFOLD\n");
          NewMIs[0]->eraseFromParent();
          NewMIs[1]->eraseFromParent();
        }
      }
    }
  }

  return false;
}

/// runOnMachineFunction - Reduce two-address instructions to two operands.
///
bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
  DEBUG(dbgs() << "Machine Function\n");
  const TargetMachine &TM = MF.getTarget();
  MRI = &MF.getRegInfo();
  TII = TM.getInstrInfo();
  TRI = TM.getRegisterInfo();
  LV = getAnalysisIfAvailable<LiveVariables>();
  AA = &getAnalysis<AliasAnalysis>();

  bool MadeChange = false;

  DEBUG(dbgs() << "********** REWRITING TWO-ADDR INSTRS **********\n");
  DEBUG(dbgs() << "********** Function: " 
        << MF.getFunction()->getName() << '\n');

  // ReMatRegs - Keep track of the registers whose def's are remat'ed.
  BitVector ReMatRegs;
  ReMatRegs.resize(MRI->getLastVirtReg()+1);

  typedef DenseMap<unsigned, SmallVector<std::pair<unsigned, unsigned>, 4> >
    TiedOperandMap;
  TiedOperandMap TiedOperands(4);

  SmallPtrSet<MachineInstr*, 8> Processed;
  for (MachineFunction::iterator mbbi = MF.begin(), mbbe = MF.end();
       mbbi != mbbe; ++mbbi) {
    unsigned Dist = 0;
    DistanceMap.clear();
    SrcRegMap.clear();
    DstRegMap.clear();
    Processed.clear();
    for (MachineBasicBlock::iterator mi = mbbi->begin(), me = mbbi->end();
         mi != me; ) {
      MachineBasicBlock::iterator nmi = llvm::next(mi);
      if (mi->isDebugValue()) {
        mi = nmi;
        continue;
      }

      // Remember REG_SEQUENCE instructions, we'll deal with them later.
      if (mi->isRegSequence())
        RegSequences.push_back(&*mi);

      const TargetInstrDesc &TID = mi->getDesc();
      bool FirstTied = true;

      DistanceMap.insert(std::make_pair(mi, ++Dist));

      ProcessCopy(&*mi, &*mbbi, Processed);

      // First scan through all the tied register uses in this instruction
      // and record a list of pairs of tied operands for each register.
      unsigned NumOps = mi->isInlineAsm()
        ? mi->getNumOperands() : TID.getNumOperands();
      for (unsigned SrcIdx = 0; SrcIdx < NumOps; ++SrcIdx) {
        unsigned DstIdx = 0;
        if (!mi->isRegTiedToDefOperand(SrcIdx, &DstIdx))
          continue;

        if (FirstTied) {
          FirstTied = false;
          ++NumTwoAddressInstrs;
          DEBUG(dbgs() << '\t' << *mi);
        }

        assert(mi->getOperand(SrcIdx).isReg() &&
               mi->getOperand(SrcIdx).getReg() &&
               mi->getOperand(SrcIdx).isUse() &&
               "two address instruction invalid");

        unsigned regB = mi->getOperand(SrcIdx).getReg();
        TiedOperandMap::iterator OI = TiedOperands.find(regB);
        if (OI == TiedOperands.end()) {
          SmallVector<std::pair<unsigned, unsigned>, 4> TiedPair;
          OI = TiedOperands.insert(std::make_pair(regB, TiedPair)).first;
        }
        OI->second.push_back(std::make_pair(SrcIdx, DstIdx));
      }

      // Now iterate over the information collected above.
      for (TiedOperandMap::iterator OI = TiedOperands.begin(),
             OE = TiedOperands.end(); OI != OE; ++OI) {
        SmallVector<std::pair<unsigned, unsigned>, 4> &TiedPairs = OI->second;

        // If the instruction has a single pair of tied operands, try some
        // transformations that may either eliminate the tied operands or
        // improve the opportunities for coalescing away the register copy.
        if (TiedOperands.size() == 1 && TiedPairs.size() == 1) {
          unsigned SrcIdx = TiedPairs[0].first;
          unsigned DstIdx = TiedPairs[0].second;

          // If the registers are already equal, nothing needs to be done.
          if (mi->getOperand(SrcIdx).getReg() ==
              mi->getOperand(DstIdx).getReg())
            break; // Done with this instruction.

          if (TryInstructionTransform(mi, nmi, mbbi, SrcIdx, DstIdx, Dist))
            break; // The tied operands have been eliminated.
        }

        bool RemovedKillFlag = false;
        bool AllUsesCopied = true;
        unsigned LastCopiedReg = 0;
        unsigned regB = OI->first;
        for (unsigned tpi = 0, tpe = TiedPairs.size(); tpi != tpe; ++tpi) {
          unsigned SrcIdx = TiedPairs[tpi].first;
          unsigned DstIdx = TiedPairs[tpi].second;
          unsigned regA = mi->getOperand(DstIdx).getReg();
          // Grab regB from the instruction because it may have changed if the
          // instruction was commuted.
          regB = mi->getOperand(SrcIdx).getReg();

          if (regA == regB) {
            // The register is tied to multiple destinations (or else we would
            // not have continued this far), but this use of the register
            // already matches the tied destination.  Leave it.
            AllUsesCopied = false;
            continue;
          }
          LastCopiedReg = regA;

          assert(TargetRegisterInfo::isVirtualRegister(regB) &&
                 "cannot make instruction into two-address form");

#ifndef NDEBUG
          // First, verify that we don't have a use of "a" in the instruction
          // (a = b + a for example) because our transformation will not
          // work. This should never occur because we are in SSA form.
          for (unsigned i = 0; i != mi->getNumOperands(); ++i)
            assert(i == DstIdx ||
                   !mi->getOperand(i).isReg() ||
                   mi->getOperand(i).getReg() != regA);
#endif

          // Emit a copy or rematerialize the definition.
          const TargetRegisterClass *rc = MRI->getRegClass(regB);
          MachineInstr *DefMI = MRI->getVRegDef(regB);
          // If it's safe and profitable, remat the definition instead of
          // copying it.
          if (DefMI &&
              DefMI->getDesc().isAsCheapAsAMove() &&
              DefMI->isSafeToReMat(TII, AA, regB) &&
              isProfitableToReMat(regB, rc, mi, DefMI, mbbi, Dist)){
            DEBUG(dbgs() << "2addr: REMATTING : " << *DefMI << "\n");
            unsigned regASubIdx = mi->getOperand(DstIdx).getSubReg();
            TII->reMaterialize(*mbbi, mi, regA, regASubIdx, DefMI, *TRI);
            ReMatRegs.set(regB);
            ++NumReMats;
          } else {
            bool Emitted = TII->copyRegToReg(*mbbi, mi, regA, regB, rc, rc,
                                             mi->getDebugLoc());
            (void)Emitted;
            assert(Emitted && "Unable to issue a copy instruction!\n");
          }

          MachineBasicBlock::iterator prevMI = prior(mi);
          // Update DistanceMap.
          DistanceMap.insert(std::make_pair(prevMI, Dist));
          DistanceMap[mi] = ++Dist;

          DEBUG(dbgs() << "\t\tprepend:\t" << *prevMI);

          MachineOperand &MO = mi->getOperand(SrcIdx);
          assert(MO.isReg() && MO.getReg() == regB && MO.isUse() &&
                 "inconsistent operand info for 2-reg pass");
          if (MO.isKill()) {
            MO.setIsKill(false);
            RemovedKillFlag = true;
          }
          MO.setReg(regA);
        }

        if (AllUsesCopied) {
          // Replace other (un-tied) uses of regB with LastCopiedReg.
          for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) {
            MachineOperand &MO = mi->getOperand(i);
            if (MO.isReg() && MO.getReg() == regB && MO.isUse()) {
              if (MO.isKill()) {
                MO.setIsKill(false);
                RemovedKillFlag = true;
              }
              MO.setReg(LastCopiedReg);
            }
          }

          // Update live variables for regB.
          if (RemovedKillFlag && LV && LV->getVarInfo(regB).removeKill(mi))
            LV->addVirtualRegisterKilled(regB, prior(mi));

        } else if (RemovedKillFlag) {
          // Some tied uses of regB matched their destination registers, so
          // regB is still used in this instruction, but a kill flag was
          // removed from a different tied use of regB, so now we need to add
          // a kill flag to one of the remaining uses of regB.
          for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) {
            MachineOperand &MO = mi->getOperand(i);
            if (MO.isReg() && MO.getReg() == regB && MO.isUse()) {
              MO.setIsKill(true);
              break;
            }
          }
        }

        // Schedule the source copy / remat inserted to form two-address
        // instruction. FIXME: Does it matter the distance map may not be
        // accurate after it's scheduled?
        TII->scheduleTwoAddrSource(prior(mi), mi, *TRI);

        MadeChange = true;

        DEBUG(dbgs() << "\t\trewrite to:\t" << *mi);
      }

      // Rewrite INSERT_SUBREG as COPY now that we no longer need SSA form.
      if (mi->isInsertSubreg()) {
        // From %reg = INSERT_SUBREG %reg, %subreg, subidx
        // To   %reg:subidx = COPY %subreg
        unsigned SubIdx = mi->getOperand(3).getImm();
        mi->RemoveOperand(3);
        assert(mi->getOperand(0).getSubReg() == 0 && "Unexpected subreg idx");
        mi->getOperand(0).setSubReg(SubIdx);
        mi->RemoveOperand(1);
        mi->setDesc(TII->get(TargetOpcode::COPY));
        DEBUG(dbgs() << "\t\tconvert to:\t" << *mi);
      }

      // Clear TiedOperands here instead of at the top of the loop
      // since most instructions do not have tied operands.
      TiedOperands.clear();
      mi = nmi;
    }
  }

  // Some remat'ed instructions are dead.
  int VReg = ReMatRegs.find_first();
  while (VReg != -1) {
    if (MRI->use_nodbg_empty(VReg)) {
      MachineInstr *DefMI = MRI->getVRegDef(VReg);
      DefMI->eraseFromParent();
    }
    VReg = ReMatRegs.find_next(VReg);
  }

  // Eliminate REG_SEQUENCE instructions. Their whole purpose was to preseve
  // SSA form. It's now safe to de-SSA.
  MadeChange |= EliminateRegSequences();

  return MadeChange;
}

static void UpdateRegSequenceSrcs(unsigned SrcReg,
                                  unsigned DstReg, unsigned SubIdx,
                                  MachineRegisterInfo *MRI,
                                  const TargetRegisterInfo &TRI) {
  for (MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(SrcReg),
         RE = MRI->reg_end(); RI != RE; ) {
    MachineOperand &MO = RI.getOperand();
    ++RI;
    MO.substVirtReg(DstReg, SubIdx, TRI);
  }
}

/// CoalesceExtSubRegs - If a number of sources of the REG_SEQUENCE are
/// EXTRACT_SUBREG from the same register and to the same virtual register
/// with different sub-register indices, attempt to combine the
/// EXTRACT_SUBREGs and pre-coalesce them. e.g.
/// %reg1026<def> = VLDMQ %reg1025<kill>, 260, pred:14, pred:%reg0
/// %reg1029:6<def> = EXTRACT_SUBREG %reg1026, 6
/// %reg1029:5<def> = EXTRACT_SUBREG %reg1026<kill>, 5
/// Since D subregs 5, 6 can combine to a Q register, we can coalesce
/// reg1026 to reg1029.
void
TwoAddressInstructionPass::CoalesceExtSubRegs(SmallVector<unsigned,4> &Srcs,
                                              unsigned DstReg) {
  SmallSet<unsigned, 4> Seen;
  for (unsigned i = 0, e = Srcs.size(); i != e; ++i) {
    unsigned SrcReg = Srcs[i];
    if (!Seen.insert(SrcReg))
      continue;

    // Check that the instructions are all in the same basic block.
    MachineInstr *SrcDefMI = MRI->getVRegDef(SrcReg);
    MachineInstr *DstDefMI = MRI->getVRegDef(DstReg);
    if (SrcDefMI->getParent() != DstDefMI->getParent())
      continue;

    // If there are no other uses than copies which feed into
    // the reg_sequence, then we might be able to coalesce them.
    bool CanCoalesce = true;
    SmallVector<unsigned, 4> SrcSubIndices, DstSubIndices;
    for (MachineRegisterInfo::use_nodbg_iterator
           UI = MRI->use_nodbg_begin(SrcReg),
           UE = MRI->use_nodbg_end(); UI != UE; ++UI) {
      MachineInstr *UseMI = &*UI;
      if (!UseMI->isCopy() || UseMI->getOperand(0).getReg() != DstReg) {
        CanCoalesce = false;
        break;
      }
      SrcSubIndices.push_back(UseMI->getOperand(1).getSubReg());
      DstSubIndices.push_back(UseMI->getOperand(0).getSubReg());
    }

    if (!CanCoalesce || SrcSubIndices.size() < 2)
      continue;

    // Check that the source subregisters can be combined.
    std::sort(SrcSubIndices.begin(), SrcSubIndices.end());
    unsigned NewSrcSubIdx = 0;
    if (!TRI->canCombineSubRegIndices(MRI->getRegClass(SrcReg), SrcSubIndices,
                                      NewSrcSubIdx))
      continue;

    // Check that the destination subregisters can also be combined.
    std::sort(DstSubIndices.begin(), DstSubIndices.end());
    unsigned NewDstSubIdx = 0;
    if (!TRI->canCombineSubRegIndices(MRI->getRegClass(DstReg), DstSubIndices,
                                      NewDstSubIdx))
      continue;

    // If neither source nor destination can be combined to the full register,
    // just give up.  This could be improved if it ever matters.
    if (NewSrcSubIdx != 0 && NewDstSubIdx != 0)
      continue;

    // Now that we know that all the uses are extract_subregs and that those
    // subregs can somehow be combined, scan all the extract_subregs again to
    // make sure the subregs are in the right order and can be composed.
    MachineInstr *SomeMI = 0;
    CanCoalesce = true;
    for (MachineRegisterInfo::use_nodbg_iterator
           UI = MRI->use_nodbg_begin(SrcReg),
           UE = MRI->use_nodbg_end(); UI != UE; ++UI) {
      MachineInstr *UseMI = &*UI;
      assert(UseMI->isCopy());
      unsigned DstSubIdx = UseMI->getOperand(0).getSubReg();
      unsigned SrcSubIdx = UseMI->getOperand(1).getSubReg();
      assert(DstSubIdx != 0 && "missing subreg from RegSequence elimination");
      if ((NewDstSubIdx == 0 &&
           TRI->composeSubRegIndices(NewSrcSubIdx, DstSubIdx) != SrcSubIdx) ||
          (NewSrcSubIdx == 0 &&
           TRI->composeSubRegIndices(NewDstSubIdx, SrcSubIdx) != DstSubIdx)) {
        CanCoalesce = false;
        break;
      }
      // Keep track of one of the uses.
      SomeMI = UseMI;
    }
    if (!CanCoalesce)
      continue;

    // Insert a copy to replace the original.
    MachineBasicBlock::iterator InsertLoc = SomeMI;
    MachineInstr *CopyMI = BuildMI(*SomeMI->getParent(), SomeMI,
                                   SomeMI->getDebugLoc(),
                                   TII->get(TargetOpcode::COPY))
      .addReg(DstReg, RegState::Define, NewDstSubIdx)
      .addReg(SrcReg, 0, NewSrcSubIdx);

    // Remove all the old extract instructions.
    for (MachineRegisterInfo::use_nodbg_iterator
           UI = MRI->use_nodbg_begin(SrcReg),
           UE = MRI->use_nodbg_end(); UI != UE; ) {
      MachineInstr *UseMI = &*UI;
      ++UI;
      if (UseMI == CopyMI)
        continue;
      assert(UseMI->isCopy());
      // Move any kills to the new copy or extract instruction.
      if (UseMI->getOperand(1).isKill()) {
        CopyMI->getOperand(1).setIsKill();
        if (LV)
          // Update live variables
          LV->replaceKillInstruction(SrcReg, UseMI, &*CopyMI);
      }
      UseMI->eraseFromParent();
    }
  }
}

static bool HasOtherRegSequenceUses(unsigned Reg, MachineInstr *RegSeq,
                                    MachineRegisterInfo *MRI) {
  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg),
         UE = MRI->use_end(); UI != UE; ++UI) {
    MachineInstr *UseMI = &*UI;
    if (UseMI != RegSeq && UseMI->isRegSequence())
      return true;
  }
  return false;
}

/// EliminateRegSequences - Eliminate REG_SEQUENCE instructions as part
/// of the de-ssa process. This replaces sources of REG_SEQUENCE as
/// sub-register references of the register defined by REG_SEQUENCE. e.g.
///
/// %reg1029<def>, %reg1030<def> = VLD1q16 %reg1024<kill>, ...
/// %reg1031<def> = REG_SEQUENCE %reg1029<kill>, 5, %reg1030<kill>, 6
/// =>
/// %reg1031:5<def>, %reg1031:6<def> = VLD1q16 %reg1024<kill>, ...
bool TwoAddressInstructionPass::EliminateRegSequences() {
  if (RegSequences.empty())
    return false;

  for (unsigned i = 0, e = RegSequences.size(); i != e; ++i) {
    MachineInstr *MI = RegSequences[i];
    unsigned DstReg = MI->getOperand(0).getReg();
    if (MI->getOperand(0).getSubReg() ||
        TargetRegisterInfo::isPhysicalRegister(DstReg) ||
        !(MI->getNumOperands() & 1)) {
      DEBUG(dbgs() << "Illegal REG_SEQUENCE instruction:" << *MI);
      llvm_unreachable(0);
    }

    bool IsImpDef = true;
    SmallVector<unsigned, 4> RealSrcs;
    SmallSet<unsigned, 4> Seen;
    for (unsigned i = 1, e = MI->getNumOperands(); i < e; i += 2) {
      unsigned SrcReg = MI->getOperand(i).getReg();
      if (MI->getOperand(i).getSubReg() ||
          TargetRegisterInfo::isPhysicalRegister(SrcReg)) {
        DEBUG(dbgs() << "Illegal REG_SEQUENCE instruction:" << *MI);
        llvm_unreachable(0);
      }

      MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
      if (DefMI->isImplicitDef()) {
        DefMI->eraseFromParent();
        continue;
      }
      IsImpDef = false;

      // Remember COPY sources. These might be candidate for coalescing.
      if (DefMI->isCopy())
        RealSrcs.push_back(DefMI->getOperand(1).getReg());

      if (!Seen.insert(SrcReg) ||
          MI->getParent() != DefMI->getParent() ||
          !MI->getOperand(i).isKill() ||
          HasOtherRegSequenceUses(SrcReg, MI, MRI)) {
        // REG_SEQUENCE cannot have duplicated operands, add a copy.
        // Also add an copy if the source is live-in the block. We don't want
        // to end up with a partial-redef of a livein, e.g.
        // BB0:
        // reg1051:10<def> =
        // ...
        // BB1:
        // ... = reg1051:10
        // BB2:
        // reg1051:9<def> =
        // LiveIntervalAnalysis won't like it.
        //
        // If the REG_SEQUENCE doesn't kill its source, keeping live variables
        // correctly up to date becomes very difficult. Insert a copy.
        //
        const TargetRegisterClass *RC = MRI->getRegClass(SrcReg);
        unsigned NewReg = MRI->createVirtualRegister(RC);
        MachineBasicBlock::iterator InsertLoc = MI;
        bool Emitted =
          TII->copyRegToReg(*MI->getParent(), InsertLoc, NewReg, SrcReg, RC, RC,
                            MI->getDebugLoc());
        (void)Emitted;
        assert(Emitted && "Unable to issue a copy instruction!\n");
        MI->getOperand(i).setReg(NewReg);
        if (MI->getOperand(i).isKill()) {
          MachineBasicBlock::iterator CopyMI = prior(InsertLoc);
          MachineOperand *KillMO = CopyMI->findRegisterUseOperand(SrcReg);
          KillMO->setIsKill();
          if (LV)
            // Update live variables
            LV->replaceKillInstruction(SrcReg, MI, &*CopyMI);
        }
      }
    }

    for (unsigned i = 1, e = MI->getNumOperands(); i < e; i += 2) {
      unsigned SrcReg = MI->getOperand(i).getReg();
      unsigned SubIdx = MI->getOperand(i+1).getImm();
      UpdateRegSequenceSrcs(SrcReg, DstReg, SubIdx, MRI, *TRI);
    }

    if (IsImpDef) {
      DEBUG(dbgs() << "Turned: " << *MI << " into an IMPLICIT_DEF");
      MI->setDesc(TII->get(TargetOpcode::IMPLICIT_DEF));
      for (int j = MI->getNumOperands() - 1, ee = 0; j > ee; --j)
        MI->RemoveOperand(j);      
    } else {
      DEBUG(dbgs() << "Eliminated: " << *MI);
      MI->eraseFromParent();
    }

    // Try coalescing some EXTRACT_SUBREG instructions. This can create
    // INSERT_SUBREG instructions that must have <undef> flags added by
    // LiveIntervalAnalysis, so only run it when LiveVariables is available.
    if (LV)
      CoalesceExtSubRegs(RealSrcs, DstReg);
  }

  RegSequences.clear();
  return true;
}