xref: /llvm-project-15.0.7/llvm/lib/CodeGen/RegisterCoalescer.cpp (revision b13eb8dc)

//===- RegisterCoalescer.cpp - Generic Register Coalescing Interface -------==//
//
//                     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 generic RegisterCoalescer interface which
// is used as the common interface used by all clients and
// implementations of register coalescing.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "regalloc"
#include "RegisterCoalescer.h"
#include "LiveDebugVariables.h"
#include "VirtRegMap.h"

#include "llvm/Pass.h"
#include "llvm/Value.h"
#include "llvm/ADT/OwningPtr.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/LiveRangeEdit.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/RegisterClassInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include <algorithm>
#include <cmath>
using namespace llvm;

STATISTIC(numJoins    , "Number of interval joins performed");
STATISTIC(numCrossRCs , "Number of cross class joins performed");
STATISTIC(numCommutes , "Number of instruction commuting performed");
STATISTIC(numExtends  , "Number of copies extended");
STATISTIC(NumReMats   , "Number of instructions re-materialized");
STATISTIC(NumInflated , "Number of register classes inflated");

static cl::opt<bool>
EnableJoining("join-liveintervals",
              cl::desc("Coalesce copies (default=true)"),
              cl::init(true));

static cl::opt<bool>
VerifyCoalescing("verify-coalescing",
         cl::desc("Verify machine instrs before and after register coalescing"),
         cl::Hidden);

namespace {
  class RegisterCoalescer : public MachineFunctionPass,
                            private LiveRangeEdit::Delegate {
    MachineFunction* MF;
    MachineRegisterInfo* MRI;
    const TargetMachine* TM;
    const TargetRegisterInfo* TRI;
    const TargetInstrInfo* TII;
    LiveIntervals *LIS;
    LiveDebugVariables *LDV;
    const MachineLoopInfo* Loops;
    AliasAnalysis *AA;
    RegisterClassInfo RegClassInfo;

    /// WorkList - Copy instructions yet to be coalesced.
    SmallVector<MachineInstr*, 8> WorkList;

    /// ErasedInstrs - Set of instruction pointers that have been erased, and
    /// that may be present in WorkList.
    SmallPtrSet<MachineInstr*, 8> ErasedInstrs;

    /// Dead instructions that are about to be deleted.
    SmallVector<MachineInstr*, 8> DeadDefs;

    /// Virtual registers to be considered for register class inflation.
    SmallVector<unsigned, 8> InflateRegs;

    /// Recursively eliminate dead defs in DeadDefs.
    void eliminateDeadDefs();

    /// LiveRangeEdit callback.
    void LRE_WillEraseInstruction(MachineInstr *MI);

    /// joinAllIntervals - join compatible live intervals
    void joinAllIntervals();

    /// copyCoalesceInMBB - Coalesce copies in the specified MBB, putting
    /// copies that cannot yet be coalesced into WorkList.
    void copyCoalesceInMBB(MachineBasicBlock *MBB);

    /// copyCoalesceWorkList - Try to coalesce all copies in WorkList after
    /// position From. Return true if any progress was made.
    bool copyCoalesceWorkList(unsigned From = 0);

    /// joinCopy - Attempt to join intervals corresponding to SrcReg/DstReg,
    /// which are the src/dst of the copy instruction CopyMI.  This returns
    /// true if the copy was successfully coalesced away. If it is not
    /// currently possible to coalesce this interval, but it may be possible if
    /// other things get coalesced, then it returns true by reference in
    /// 'Again'.
    bool joinCopy(MachineInstr *TheCopy, bool &Again);

    /// joinIntervals - Attempt to join these two intervals.  On failure, this
    /// returns false.  The output "SrcInt" will not have been modified, so we
    /// can use this information below to update aliases.
    bool joinIntervals(CoalescerPair &CP);

    /// Attempt joining with a reserved physreg.
    bool joinReservedPhysReg(CoalescerPair &CP);

    /// adjustCopiesBackFrom - We found a non-trivially-coalescable copy. If
    /// the source value number is defined by a copy from the destination reg
    /// see if we can merge these two destination reg valno# into a single
    /// value number, eliminating a copy.
    bool adjustCopiesBackFrom(const CoalescerPair &CP, MachineInstr *CopyMI);

    /// hasOtherReachingDefs - Return true if there are definitions of IntB
    /// other than BValNo val# that can reach uses of AValno val# of IntA.
    bool hasOtherReachingDefs(LiveInterval &IntA, LiveInterval &IntB,
                              VNInfo *AValNo, VNInfo *BValNo);

    /// removeCopyByCommutingDef - We found a non-trivially-coalescable copy.
    /// If the source value number is defined by a commutable instruction and
    /// its other operand is coalesced to the copy dest register, see if we
    /// can transform the copy into a noop by commuting the definition.
    bool removeCopyByCommutingDef(const CoalescerPair &CP,MachineInstr *CopyMI);

    /// reMaterializeTrivialDef - If the source of a copy is defined by a
    /// trivial computation, replace the copy by rematerialize the definition.
    bool reMaterializeTrivialDef(LiveInterval &SrcInt, unsigned DstReg,
                                 MachineInstr *CopyMI);

    /// canJoinPhys - Return true if a physreg copy should be joined.
    bool canJoinPhys(CoalescerPair &CP);

    /// updateRegDefsUses - Replace all defs and uses of SrcReg to DstReg and
    /// update the subregister number if it is not zero. If DstReg is a
    /// physical register and the existing subregister number of the def / use
    /// being updated is not zero, make sure to set it to the correct physical
    /// subregister.
    void updateRegDefsUses(unsigned SrcReg, unsigned DstReg, unsigned SubIdx);

    /// eliminateUndefCopy - Handle copies of undef values.
    bool eliminateUndefCopy(MachineInstr *CopyMI, const CoalescerPair &CP);

  public:
    static char ID; // Class identification, replacement for typeinfo
    RegisterCoalescer() : MachineFunctionPass(ID) {
      initializeRegisterCoalescerPass(*PassRegistry::getPassRegistry());
    }

    virtual void getAnalysisUsage(AnalysisUsage &AU) const;

    virtual void releaseMemory();

    /// runOnMachineFunction - pass entry point
    virtual bool runOnMachineFunction(MachineFunction&);

    /// print - Implement the dump method.
    virtual void print(raw_ostream &O, const Module* = 0) const;
  };
} /// end anonymous namespace

char &llvm::RegisterCoalescerID = RegisterCoalescer::ID;

INITIALIZE_PASS_BEGIN(RegisterCoalescer, "simple-register-coalescing",
                      "Simple Register Coalescing", false, false)
INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
INITIALIZE_PASS_DEPENDENCY(LiveDebugVariables)
INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(RegisterCoalescer, "simple-register-coalescing",
                    "Simple Register Coalescing", false, false)

char RegisterCoalescer::ID = 0;

static unsigned compose(const TargetRegisterInfo &tri, unsigned a, unsigned b) {
  if (!a) return b;
  if (!b) return a;
  return tri.composeSubRegIndices(a, b);
}

static bool isMoveInstr(const TargetRegisterInfo &tri, const MachineInstr *MI,
                        unsigned &Src, unsigned &Dst,
                        unsigned &SrcSub, unsigned &DstSub) {
  if (MI->isCopy()) {
    Dst = MI->getOperand(0).getReg();
    DstSub = MI->getOperand(0).getSubReg();
    Src = MI->getOperand(1).getReg();
    SrcSub = MI->getOperand(1).getSubReg();
  } else if (MI->isSubregToReg()) {
    Dst = MI->getOperand(0).getReg();
    DstSub = compose(tri, MI->getOperand(0).getSubReg(),
                     MI->getOperand(3).getImm());
    Src = MI->getOperand(2).getReg();
    SrcSub = MI->getOperand(2).getSubReg();
  } else
    return false;
  return true;
}

bool CoalescerPair::setRegisters(const MachineInstr *MI) {
  SrcReg = DstReg = 0;
  SrcIdx = DstIdx = 0;
  NewRC = 0;
  Flipped = CrossClass = false;

  unsigned Src, Dst, SrcSub, DstSub;
  if (!isMoveInstr(TRI, MI, Src, Dst, SrcSub, DstSub))
    return false;
  Partial = SrcSub || DstSub;

  // If one register is a physreg, it must be Dst.
  if (TargetRegisterInfo::isPhysicalRegister(Src)) {
    if (TargetRegisterInfo::isPhysicalRegister(Dst))
      return false;
    std::swap(Src, Dst);
    std::swap(SrcSub, DstSub);
    Flipped = true;
  }

  const MachineRegisterInfo &MRI = MI->getParent()->getParent()->getRegInfo();

  if (TargetRegisterInfo::isPhysicalRegister(Dst)) {
    // Eliminate DstSub on a physreg.
    if (DstSub) {
      Dst = TRI.getSubReg(Dst, DstSub);
      if (!Dst) return false;
      DstSub = 0;
    }

    // Eliminate SrcSub by picking a corresponding Dst superregister.
    if (SrcSub) {
      Dst = TRI.getMatchingSuperReg(Dst, SrcSub, MRI.getRegClass(Src));
      if (!Dst) return false;
      SrcSub = 0;
    } else if (!MRI.getRegClass(Src)->contains(Dst)) {
      return false;
    }
  } else {
    // Both registers are virtual.
    const TargetRegisterClass *SrcRC = MRI.getRegClass(Src);
    const TargetRegisterClass *DstRC = MRI.getRegClass(Dst);

    // Both registers have subreg indices.
    if (SrcSub && DstSub) {
      // Copies between different sub-registers are never coalescable.
      if (Src == Dst && SrcSub != DstSub)
        return false;

      NewRC = TRI.getCommonSuperRegClass(SrcRC, SrcSub, DstRC, DstSub,
                                         SrcIdx, DstIdx);
      if (!NewRC)
        return false;
    } else if (DstSub) {
      // SrcReg will be merged with a sub-register of DstReg.
      SrcIdx = DstSub;
      NewRC = TRI.getMatchingSuperRegClass(DstRC, SrcRC, DstSub);
    } else if (SrcSub) {
      // DstReg will be merged with a sub-register of SrcReg.
      DstIdx = SrcSub;
      NewRC = TRI.getMatchingSuperRegClass(SrcRC, DstRC, SrcSub);
    } else {
      // This is a straight copy without sub-registers.
      NewRC = TRI.getCommonSubClass(DstRC, SrcRC);
    }

    // The combined constraint may be impossible to satisfy.
    if (!NewRC)
      return false;

    // Prefer SrcReg to be a sub-register of DstReg.
    // FIXME: Coalescer should support subregs symmetrically.
    if (DstIdx && !SrcIdx) {
      std::swap(Src, Dst);
      std::swap(SrcIdx, DstIdx);
      Flipped = !Flipped;
    }

    CrossClass = NewRC != DstRC || NewRC != SrcRC;
  }
  // Check our invariants
  assert(TargetRegisterInfo::isVirtualRegister(Src) && "Src must be virtual");
  assert(!(TargetRegisterInfo::isPhysicalRegister(Dst) && DstSub) &&
         "Cannot have a physical SubIdx");
  SrcReg = Src;
  DstReg = Dst;
  return true;
}

bool CoalescerPair::flip() {
  if (TargetRegisterInfo::isPhysicalRegister(DstReg))
    return false;
  std::swap(SrcReg, DstReg);
  std::swap(SrcIdx, DstIdx);
  Flipped = !Flipped;
  return true;
}

bool CoalescerPair::isCoalescable(const MachineInstr *MI) const {
  if (!MI)
    return false;
  unsigned Src, Dst, SrcSub, DstSub;
  if (!isMoveInstr(TRI, MI, Src, Dst, SrcSub, DstSub))
    return false;

  // Find the virtual register that is SrcReg.
  if (Dst == SrcReg) {
    std::swap(Src, Dst);
    std::swap(SrcSub, DstSub);
  } else if (Src != SrcReg) {
    return false;
  }

  // Now check that Dst matches DstReg.
  if (TargetRegisterInfo::isPhysicalRegister(DstReg)) {
    if (!TargetRegisterInfo::isPhysicalRegister(Dst))
      return false;
    assert(!DstIdx && !SrcIdx && "Inconsistent CoalescerPair state.");
    // DstSub could be set for a physreg from INSERT_SUBREG.
    if (DstSub)
      Dst = TRI.getSubReg(Dst, DstSub);
    // Full copy of Src.
    if (!SrcSub)
      return DstReg == Dst;
    // This is a partial register copy. Check that the parts match.
    return TRI.getSubReg(DstReg, SrcSub) == Dst;
  } else {
    // DstReg is virtual.
    if (DstReg != Dst)
      return false;
    // Registers match, do the subregisters line up?
    return compose(TRI, SrcIdx, SrcSub) == compose(TRI, DstIdx, DstSub);
  }
}

void RegisterCoalescer::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesCFG();
  AU.addRequired<AliasAnalysis>();
  AU.addRequired<LiveIntervals>();
  AU.addPreserved<LiveIntervals>();
  AU.addRequired<LiveDebugVariables>();
  AU.addPreserved<LiveDebugVariables>();
  AU.addPreserved<SlotIndexes>();
  AU.addRequired<MachineLoopInfo>();
  AU.addPreserved<MachineLoopInfo>();
  AU.addPreservedID(MachineDominatorsID);
  MachineFunctionPass::getAnalysisUsage(AU);
}

void RegisterCoalescer::eliminateDeadDefs() {
  SmallVector<LiveInterval*, 8> NewRegs;
  LiveRangeEdit(0, NewRegs, *MF, *LIS, 0, this).eliminateDeadDefs(DeadDefs);
}

// Callback from eliminateDeadDefs().
void RegisterCoalescer::LRE_WillEraseInstruction(MachineInstr *MI) {
  // MI may be in WorkList. Make sure we don't visit it.
  ErasedInstrs.insert(MI);
}

/// adjustCopiesBackFrom - We found a non-trivially-coalescable copy with IntA
/// being the source and IntB being the dest, thus this defines a value number
/// in IntB.  If the source value number (in IntA) is defined by a copy from B,
/// see if we can merge these two pieces of B into a single value number,
/// eliminating a copy.  For example:
///
///  A3 = B0
///    ...
///  B1 = A3      <- this copy
///
/// In this case, B0 can be extended to where the B1 copy lives, allowing the B1
/// value number to be replaced with B0 (which simplifies the B liveinterval).
///
/// This returns true if an interval was modified.
///
bool RegisterCoalescer::adjustCopiesBackFrom(const CoalescerPair &CP,
                                             MachineInstr *CopyMI) {
  assert(!CP.isPartial() && "This doesn't work for partial copies.");
  assert(!CP.isPhys() && "This doesn't work for physreg copies.");

  LiveInterval &IntA =
    LIS->getInterval(CP.isFlipped() ? CP.getDstReg() : CP.getSrcReg());
  LiveInterval &IntB =
    LIS->getInterval(CP.isFlipped() ? CP.getSrcReg() : CP.getDstReg());
  SlotIndex CopyIdx = LIS->getInstructionIndex(CopyMI).getRegSlot();

  // BValNo is a value number in B that is defined by a copy from A.  'B3' in
  // the example above.
  LiveInterval::iterator BLR = IntB.FindLiveRangeContaining(CopyIdx);
  if (BLR == IntB.end()) return false;
  VNInfo *BValNo = BLR->valno;

  // Get the location that B is defined at.  Two options: either this value has
  // an unknown definition point or it is defined at CopyIdx.  If unknown, we
  // can't process it.
  if (BValNo->def != CopyIdx) return false;

  // AValNo is the value number in A that defines the copy, A3 in the example.
  SlotIndex CopyUseIdx = CopyIdx.getRegSlot(true);
  LiveInterval::iterator ALR = IntA.FindLiveRangeContaining(CopyUseIdx);
  // The live range might not exist after fun with physreg coalescing.
  if (ALR == IntA.end()) return false;
  VNInfo *AValNo = ALR->valno;

  // If AValNo is defined as a copy from IntB, we can potentially process this.
  // Get the instruction that defines this value number.
  MachineInstr *ACopyMI = LIS->getInstructionFromIndex(AValNo->def);
  if (!CP.isCoalescable(ACopyMI))
    return false;

  // Get the LiveRange in IntB that this value number starts with.
  LiveInterval::iterator ValLR =
    IntB.FindLiveRangeContaining(AValNo->def.getPrevSlot());
  if (ValLR == IntB.end())
    return false;

  // Make sure that the end of the live range is inside the same block as
  // CopyMI.
  MachineInstr *ValLREndInst =
    LIS->getInstructionFromIndex(ValLR->end.getPrevSlot());
  if (!ValLREndInst || ValLREndInst->getParent() != CopyMI->getParent())
    return false;

  // Okay, we now know that ValLR ends in the same block that the CopyMI
  // live-range starts.  If there are no intervening live ranges between them in
  // IntB, we can merge them.
  if (ValLR+1 != BLR) return false;

  DEBUG(dbgs() << "Extending: " << PrintReg(IntB.reg, TRI));

  SlotIndex FillerStart = ValLR->end, FillerEnd = BLR->start;
  // We are about to delete CopyMI, so need to remove it as the 'instruction
  // that defines this value #'. Update the valnum with the new defining
  // instruction #.
  BValNo->def = FillerStart;

  // Okay, we can merge them.  We need to insert a new liverange:
  // [ValLR.end, BLR.begin) of either value number, then we merge the
  // two value numbers.
  IntB.addRange(LiveRange(FillerStart, FillerEnd, BValNo));

  // Okay, merge "B1" into the same value number as "B0".
  if (BValNo != ValLR->valno) {
    // If B1 is killed by a PHI, then the merged live range must also be killed
    // by the same PHI, as B0 and B1 can not overlap.
    bool HasPHIKill = BValNo->hasPHIKill();
    IntB.MergeValueNumberInto(BValNo, ValLR->valno);
    if (HasPHIKill)
      ValLR->valno->setHasPHIKill(true);
  }
  DEBUG(dbgs() << "   result = " << IntB << '\n');

  // If the source instruction was killing the source register before the
  // merge, unset the isKill marker given the live range has been extended.
  int UIdx = ValLREndInst->findRegisterUseOperandIdx(IntB.reg, true);
  if (UIdx != -1) {
    ValLREndInst->getOperand(UIdx).setIsKill(false);
  }

  // Rewrite the copy. If the copy instruction was killing the destination
  // register before the merge, find the last use and trim the live range. That
  // will also add the isKill marker.
  CopyMI->substituteRegister(IntA.reg, IntB.reg, 0, *TRI);
  if (ALR->end == CopyIdx)
    LIS->shrinkToUses(&IntA);

  ++numExtends;
  return true;
}

/// hasOtherReachingDefs - Return true if there are definitions of IntB
/// other than BValNo val# that can reach uses of AValno val# of IntA.
bool RegisterCoalescer::hasOtherReachingDefs(LiveInterval &IntA,
                                             LiveInterval &IntB,
                                             VNInfo *AValNo,
                                             VNInfo *BValNo) {
  for (LiveInterval::iterator AI = IntA.begin(), AE = IntA.end();
       AI != AE; ++AI) {
    if (AI->valno != AValNo) continue;
    LiveInterval::Ranges::iterator BI =
      std::upper_bound(IntB.ranges.begin(), IntB.ranges.end(), AI->start);
    if (BI != IntB.ranges.begin())
      --BI;
    for (; BI != IntB.ranges.end() && AI->end >= BI->start; ++BI) {
      if (BI->valno == BValNo)
        continue;
      if (BI->start <= AI->start && BI->end > AI->start)
        return true;
      if (BI->start > AI->start && BI->start < AI->end)
        return true;
    }
  }
  return false;
}

/// removeCopyByCommutingDef - We found a non-trivially-coalescable copy with
/// IntA being the source and IntB being the dest, thus this defines a value
/// number in IntB.  If the source value number (in IntA) is defined by a
/// commutable instruction and its other operand is coalesced to the copy dest
/// register, see if we can transform the copy into a noop by commuting the
/// definition. For example,
///
///  A3 = op A2 B0<kill>
///    ...
///  B1 = A3      <- this copy
///    ...
///     = op A3   <- more uses
///
/// ==>
///
///  B2 = op B0 A2<kill>
///    ...
///  B1 = B2      <- now an identify copy
///    ...
///     = op B2   <- more uses
///
/// This returns true if an interval was modified.
///
bool RegisterCoalescer::removeCopyByCommutingDef(const CoalescerPair &CP,
                                                 MachineInstr *CopyMI) {
  assert (!CP.isPhys());

  SlotIndex CopyIdx = LIS->getInstructionIndex(CopyMI).getRegSlot();

  LiveInterval &IntA =
    LIS->getInterval(CP.isFlipped() ? CP.getDstReg() : CP.getSrcReg());
  LiveInterval &IntB =
    LIS->getInterval(CP.isFlipped() ? CP.getSrcReg() : CP.getDstReg());

  // BValNo is a value number in B that is defined by a copy from A. 'B3' in
  // the example above.
  VNInfo *BValNo = IntB.getVNInfoAt(CopyIdx);
  if (!BValNo || BValNo->def != CopyIdx)
    return false;

  assert(BValNo->def == CopyIdx && "Copy doesn't define the value?");

  // AValNo is the value number in A that defines the copy, A3 in the example.
  VNInfo *AValNo = IntA.getVNInfoAt(CopyIdx.getRegSlot(true));
  assert(AValNo && "COPY source not live");

  // If other defs can reach uses of this def, then it's not safe to perform
  // the optimization.
  if (AValNo->isPHIDef() || AValNo->isUnused() || AValNo->hasPHIKill())
    return false;
  MachineInstr *DefMI = LIS->getInstructionFromIndex(AValNo->def);
  if (!DefMI)
    return false;
  if (!DefMI->isCommutable())
    return false;
  // If DefMI is a two-address instruction then commuting it will change the
  // destination register.
  int DefIdx = DefMI->findRegisterDefOperandIdx(IntA.reg);
  assert(DefIdx != -1);
  unsigned UseOpIdx;
  if (!DefMI->isRegTiedToUseOperand(DefIdx, &UseOpIdx))
    return false;
  unsigned Op1, Op2, NewDstIdx;
  if (!TII->findCommutedOpIndices(DefMI, Op1, Op2))
    return false;
  if (Op1 == UseOpIdx)
    NewDstIdx = Op2;
  else if (Op2 == UseOpIdx)
    NewDstIdx = Op1;
  else
    return false;

  MachineOperand &NewDstMO = DefMI->getOperand(NewDstIdx);
  unsigned NewReg = NewDstMO.getReg();
  if (NewReg != IntB.reg || !NewDstMO.isKill())
    return false;

  // Make sure there are no other definitions of IntB that would reach the
  // uses which the new definition can reach.
  if (hasOtherReachingDefs(IntA, IntB, AValNo, BValNo))
    return false;

  // If some of the uses of IntA.reg is already coalesced away, return false.
  // It's not possible to determine whether it's safe to perform the coalescing.
  for (MachineRegisterInfo::use_nodbg_iterator UI =
         MRI->use_nodbg_begin(IntA.reg),
       UE = MRI->use_nodbg_end(); UI != UE; ++UI) {
    MachineInstr *UseMI = &*UI;
    SlotIndex UseIdx = LIS->getInstructionIndex(UseMI);
    LiveInterval::iterator ULR = IntA.FindLiveRangeContaining(UseIdx);
    if (ULR == IntA.end() || ULR->valno != AValNo)
      continue;
    // If this use is tied to a def, we can't rewrite the register.
    if (UseMI->isRegTiedToDefOperand(UI.getOperandNo()))
      return false;
  }

  DEBUG(dbgs() << "\tremoveCopyByCommutingDef: " << AValNo->def << '\t'
               << *DefMI);

  // At this point we have decided that it is legal to do this
  // transformation.  Start by commuting the instruction.
  MachineBasicBlock *MBB = DefMI->getParent();
  MachineInstr *NewMI = TII->commuteInstruction(DefMI);
  if (!NewMI)
    return false;
  if (TargetRegisterInfo::isVirtualRegister(IntA.reg) &&
      TargetRegisterInfo::isVirtualRegister(IntB.reg) &&
      !MRI->constrainRegClass(IntB.reg, MRI->getRegClass(IntA.reg)))
    return false;
  if (NewMI != DefMI) {
    LIS->ReplaceMachineInstrInMaps(DefMI, NewMI);
    MachineBasicBlock::iterator Pos = DefMI;
    MBB->insert(Pos, NewMI);
    MBB->erase(DefMI);
  }
  unsigned OpIdx = NewMI->findRegisterUseOperandIdx(IntA.reg, false);
  NewMI->getOperand(OpIdx).setIsKill();

  // If ALR and BLR overlaps and end of BLR extends beyond end of ALR, e.g.
  // A = or A, B
  // ...
  // B = A
  // ...
  // C = A<kill>
  // ...
  //   = B

  // Update uses of IntA of the specific Val# with IntB.
  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(IntA.reg),
         UE = MRI->use_end(); UI != UE;) {
    MachineOperand &UseMO = UI.getOperand();
    MachineInstr *UseMI = &*UI;
    ++UI;
    if (UseMI->isDebugValue()) {
      // FIXME These don't have an instruction index.  Not clear we have enough
      // info to decide whether to do this replacement or not.  For now do it.
      UseMO.setReg(NewReg);
      continue;
    }
    SlotIndex UseIdx = LIS->getInstructionIndex(UseMI).getRegSlot(true);
    LiveInterval::iterator ULR = IntA.FindLiveRangeContaining(UseIdx);
    if (ULR == IntA.end() || ULR->valno != AValNo)
      continue;
    if (TargetRegisterInfo::isPhysicalRegister(NewReg))
      UseMO.substPhysReg(NewReg, *TRI);
    else
      UseMO.setReg(NewReg);
    if (UseMI == CopyMI)
      continue;
    if (!UseMI->isCopy())
      continue;
    if (UseMI->getOperand(0).getReg() != IntB.reg ||
        UseMI->getOperand(0).getSubReg())
      continue;

    // This copy will become a noop. If it's defining a new val#, merge it into
    // BValNo.
    SlotIndex DefIdx = UseIdx.getRegSlot();
    VNInfo *DVNI = IntB.getVNInfoAt(DefIdx);
    if (!DVNI)
      continue;
    DEBUG(dbgs() << "\t\tnoop: " << DefIdx << '\t' << *UseMI);
    assert(DVNI->def == DefIdx);
    BValNo = IntB.MergeValueNumberInto(BValNo, DVNI);
    ErasedInstrs.insert(UseMI);
    LIS->RemoveMachineInstrFromMaps(UseMI);
    UseMI->eraseFromParent();
  }

  // Extend BValNo by merging in IntA live ranges of AValNo. Val# definition
  // is updated.
  VNInfo *ValNo = BValNo;
  ValNo->def = AValNo->def;
  for (LiveInterval::iterator AI = IntA.begin(), AE = IntA.end();
       AI != AE; ++AI) {
    if (AI->valno != AValNo) continue;
    IntB.addRange(LiveRange(AI->start, AI->end, ValNo));
  }
  DEBUG(dbgs() << "\t\textended: " << IntB << '\n');

  IntA.removeValNo(AValNo);
  DEBUG(dbgs() << "\t\ttrimmed:  " << IntA << '\n');
  ++numCommutes;
  return true;
}

/// reMaterializeTrivialDef - If the source of a copy is defined by a trivial
/// computation, replace the copy by rematerialize the definition.
bool RegisterCoalescer::reMaterializeTrivialDef(LiveInterval &SrcInt,
                                                unsigned DstReg,
                                                MachineInstr *CopyMI) {
  SlotIndex CopyIdx = LIS->getInstructionIndex(CopyMI).getRegSlot(true);
  LiveInterval::iterator SrcLR = SrcInt.FindLiveRangeContaining(CopyIdx);
  assert(SrcLR != SrcInt.end() && "Live range not found!");
  VNInfo *ValNo = SrcLR->valno;
  if (ValNo->isPHIDef() || ValNo->isUnused())
    return false;
  MachineInstr *DefMI = LIS->getInstructionFromIndex(ValNo->def);
  if (!DefMI)
    return false;
  assert(DefMI && "Defining instruction disappeared");
  if (!DefMI->isAsCheapAsAMove())
    return false;
  if (!TII->isTriviallyReMaterializable(DefMI, AA))
    return false;
  bool SawStore = false;
  if (!DefMI->isSafeToMove(TII, AA, SawStore))
    return false;
  const MCInstrDesc &MCID = DefMI->getDesc();
  if (MCID.getNumDefs() != 1)
    return false;
  if (!DefMI->isImplicitDef()) {
    // Make sure the copy destination register class fits the instruction
    // definition register class. The mismatch can happen as a result of earlier
    // extract_subreg, insert_subreg, subreg_to_reg coalescing.
    const TargetRegisterClass *RC = TII->getRegClass(MCID, 0, TRI, *MF);
    if (TargetRegisterInfo::isVirtualRegister(DstReg)) {
      if (MRI->getRegClass(DstReg) != RC)
        return false;
    } else if (!RC->contains(DstReg))
      return false;
  }

  MachineBasicBlock *MBB = CopyMI->getParent();
  MachineBasicBlock::iterator MII =
    llvm::next(MachineBasicBlock::iterator(CopyMI));
  TII->reMaterialize(*MBB, MII, DstReg, 0, DefMI, *TRI);
  MachineInstr *NewMI = prior(MII);

  // NewMI may have dead implicit defs (E.g. EFLAGS for MOV<bits>r0 on X86).
  // We need to remember these so we can add intervals once we insert
  // NewMI into SlotIndexes.
  SmallVector<unsigned, 4> NewMIImplDefs;
  for (unsigned i = NewMI->getDesc().getNumOperands(),
         e = NewMI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = NewMI->getOperand(i);
    if (MO.isReg()) {
      assert(MO.isDef() && MO.isImplicit() && MO.isDead() &&
             TargetRegisterInfo::isPhysicalRegister(MO.getReg()));
      NewMIImplDefs.push_back(MO.getReg());
    }
  }

  // CopyMI may have implicit operands, transfer them over to the newly
  // rematerialized instruction. And update implicit def interval valnos.
  for (unsigned i = CopyMI->getDesc().getNumOperands(),
         e = CopyMI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = CopyMI->getOperand(i);
    if (MO.isReg()) {
      assert(MO.isImplicit() && "No explicit operands after implict operands.");
      // Discard VReg implicit defs.
      if (TargetRegisterInfo::isPhysicalRegister(MO.getReg())) {
        NewMI->addOperand(MO);
      }
    }
  }

  LIS->ReplaceMachineInstrInMaps(CopyMI, NewMI);

  SlotIndex NewMIIdx = LIS->getInstructionIndex(NewMI);
  for (unsigned i = 0, e = NewMIImplDefs.size(); i != e; ++i) {
    unsigned Reg = NewMIImplDefs[i];
    for (MCRegUnitIterator Units(Reg, TRI); Units.isValid(); ++Units)
      if (LiveInterval *LI = LIS->getCachedRegUnit(*Units))
        LI->createDeadDef(NewMIIdx.getRegSlot(), LIS->getVNInfoAllocator());
  }

  CopyMI->eraseFromParent();
  ErasedInstrs.insert(CopyMI);
  DEBUG(dbgs() << "Remat: " << *NewMI);
  ++NumReMats;

  // The source interval can become smaller because we removed a use.
  LIS->shrinkToUses(&SrcInt, &DeadDefs);
  if (!DeadDefs.empty())
    eliminateDeadDefs();

  return true;
}

/// eliminateUndefCopy - ProcessImpicitDefs may leave some copies of <undef>
/// values, it only removes local variables. When we have a copy like:
///
///   %vreg1 = COPY %vreg2<undef>
///
/// We delete the copy and remove the corresponding value number from %vreg1.
/// Any uses of that value number are marked as <undef>.
bool RegisterCoalescer::eliminateUndefCopy(MachineInstr *CopyMI,
                                           const CoalescerPair &CP) {
  SlotIndex Idx = LIS->getInstructionIndex(CopyMI);
  LiveInterval *SrcInt = &LIS->getInterval(CP.getSrcReg());
  if (SrcInt->liveAt(Idx))
    return false;
  LiveInterval *DstInt = &LIS->getInterval(CP.getDstReg());
  if (DstInt->liveAt(Idx))
    return false;

  // No intervals are live-in to CopyMI - it is undef.
  if (CP.isFlipped())
    DstInt = SrcInt;
  SrcInt = 0;

  VNInfo *DeadVNI = DstInt->getVNInfoAt(Idx.getRegSlot());
  assert(DeadVNI && "No value defined in DstInt");
  DstInt->removeValNo(DeadVNI);

  // Find new undef uses.
  for (MachineRegisterInfo::reg_nodbg_iterator
         I = MRI->reg_nodbg_begin(DstInt->reg), E = MRI->reg_nodbg_end();
       I != E; ++I) {
    MachineOperand &MO = I.getOperand();
    if (MO.isDef() || MO.isUndef())
      continue;
    MachineInstr *MI = MO.getParent();
    SlotIndex Idx = LIS->getInstructionIndex(MI);
    if (DstInt->liveAt(Idx))
      continue;
    MO.setIsUndef(true);
    DEBUG(dbgs() << "\tnew undef: " << Idx << '\t' << *MI);
  }
  return true;
}

/// updateRegDefsUses - Replace all defs and uses of SrcReg to DstReg and
/// update the subregister number if it is not zero. If DstReg is a
/// physical register and the existing subregister number of the def / use
/// being updated is not zero, make sure to set it to the correct physical
/// subregister.
void RegisterCoalescer::updateRegDefsUses(unsigned SrcReg,
                                          unsigned DstReg,
                                          unsigned SubIdx) {
  bool DstIsPhys = TargetRegisterInfo::isPhysicalRegister(DstReg);
  LiveInterval *DstInt = DstIsPhys ? 0 : &LIS->getInterval(DstReg);

  // Update LiveDebugVariables.
  LDV->renameRegister(SrcReg, DstReg, SubIdx);

  for (MachineRegisterInfo::reg_iterator I = MRI->reg_begin(SrcReg);
       MachineInstr *UseMI = I.skipInstruction();) {
    SmallVector<unsigned,8> Ops;
    bool Reads, Writes;
    tie(Reads, Writes) = UseMI->readsWritesVirtualRegister(SrcReg, &Ops);

    // If SrcReg wasn't read, it may still be the case that DstReg is live-in
    // because SrcReg is a sub-register.
    if (DstInt && !Reads && SubIdx)
      Reads = DstInt->liveAt(LIS->getInstructionIndex(UseMI));

    // Replace SrcReg with DstReg in all UseMI operands.
    for (unsigned i = 0, e = Ops.size(); i != e; ++i) {
      MachineOperand &MO = UseMI->getOperand(Ops[i]);

      // Adjust <undef> flags in case of sub-register joins. We don't want to
      // turn a full def into a read-modify-write sub-register def and vice
      // versa.
      if (SubIdx && MO.isDef())
        MO.setIsUndef(!Reads);

      if (DstIsPhys)
        MO.substPhysReg(DstReg, *TRI);
      else
        MO.substVirtReg(DstReg, SubIdx, *TRI);
    }

    DEBUG({
        dbgs() << "\t\tupdated: ";
        if (!UseMI->isDebugValue())
          dbgs() << LIS->getInstructionIndex(UseMI) << "\t";
        dbgs() << *UseMI;
      });
  }
}

/// canJoinPhys - Return true if a copy involving a physreg should be joined.
bool RegisterCoalescer::canJoinPhys(CoalescerPair &CP) {
  /// Always join simple intervals that are defined by a single copy from a
  /// reserved register. This doesn't increase register pressure, so it is
  /// always beneficial.
  if (!RegClassInfo.isReserved(CP.getDstReg())) {
    DEBUG(dbgs() << "\tCan only merge into reserved registers.\n");
    return false;
  }

  LiveInterval &JoinVInt = LIS->getInterval(CP.getSrcReg());
  if (CP.isFlipped() && JoinVInt.containsOneValue())
    return true;

  DEBUG(dbgs() << "\tCannot join defs into reserved register.\n");
  return false;
}

/// joinCopy - Attempt to join intervals corresponding to SrcReg/DstReg,
/// which are the src/dst of the copy instruction CopyMI.  This returns true
/// if the copy was successfully coalesced away. If it is not currently
/// possible to coalesce this interval, but it may be possible if other
/// things get coalesced, then it returns true by reference in 'Again'.
bool RegisterCoalescer::joinCopy(MachineInstr *CopyMI, bool &Again) {

  Again = false;
  DEBUG(dbgs() << LIS->getInstructionIndex(CopyMI) << '\t' << *CopyMI);

  CoalescerPair CP(*TRI);
  if (!CP.setRegisters(CopyMI)) {
    DEBUG(dbgs() << "\tNot coalescable.\n");
    return false;
  }

  // Dead code elimination. This really should be handled by MachineDCE, but
  // sometimes dead copies slip through, and we can't generate invalid live
  // ranges.
  if (!CP.isPhys() && CopyMI->allDefsAreDead()) {
    DEBUG(dbgs() << "\tCopy is dead.\n");
    DeadDefs.push_back(CopyMI);
    eliminateDeadDefs();
    return true;
  }

  // Eliminate undefs.
  if (!CP.isPhys() && eliminateUndefCopy(CopyMI, CP)) {
    DEBUG(dbgs() << "\tEliminated copy of <undef> value.\n");
    LIS->RemoveMachineInstrFromMaps(CopyMI);
    CopyMI->eraseFromParent();
    return false;  // Not coalescable.
  }

  // Coalesced copies are normally removed immediately, but transformations
  // like removeCopyByCommutingDef() can inadvertently create identity copies.
  // When that happens, just join the values and remove the copy.
  if (CP.getSrcReg() == CP.getDstReg()) {
    LiveInterval &LI = LIS->getInterval(CP.getSrcReg());
    DEBUG(dbgs() << "\tCopy already coalesced: " << LI << '\n');
    LiveRangeQuery LRQ(LI, LIS->getInstructionIndex(CopyMI));
    if (VNInfo *DefVNI = LRQ.valueDefined()) {
      VNInfo *ReadVNI = LRQ.valueIn();
      assert(ReadVNI && "No value before copy and no <undef> flag.");
      assert(ReadVNI != DefVNI && "Cannot read and define the same value.");
      LI.MergeValueNumberInto(DefVNI, ReadVNI);
      DEBUG(dbgs() << "\tMerged values:          " << LI << '\n');
    }
    LIS->RemoveMachineInstrFromMaps(CopyMI);
    CopyMI->eraseFromParent();
    return true;
  }

  // Enforce policies.
  if (CP.isPhys()) {
    DEBUG(dbgs() << "\tConsidering merging " << PrintReg(CP.getSrcReg(), TRI)
                 << " with " << PrintReg(CP.getDstReg(), TRI, CP.getSrcIdx())
                 << '\n');
    if (!canJoinPhys(CP)) {
      // Before giving up coalescing, if definition of source is defined by
      // trivial computation, try rematerializing it.
      if (!CP.isFlipped() &&
          reMaterializeTrivialDef(LIS->getInterval(CP.getSrcReg()),
                                  CP.getDstReg(), CopyMI))
        return true;
      return false;
    }
  } else {
    DEBUG({
      dbgs() << "\tConsidering merging to " << CP.getNewRC()->getName()
             << " with ";
      if (CP.getDstIdx() && CP.getSrcIdx())
        dbgs() << PrintReg(CP.getDstReg()) << " in "
               << TRI->getSubRegIndexName(CP.getDstIdx()) << " and "
               << PrintReg(CP.getSrcReg()) << " in "
               << TRI->getSubRegIndexName(CP.getSrcIdx()) << '\n';
      else
        dbgs() << PrintReg(CP.getSrcReg(), TRI) << " in "
               << PrintReg(CP.getDstReg(), TRI, CP.getSrcIdx()) << '\n';
    });

    // When possible, let DstReg be the larger interval.
    if (!CP.isPartial() && LIS->getInterval(CP.getSrcReg()).ranges.size() >
                           LIS->getInterval(CP.getDstReg()).ranges.size())
      CP.flip();
  }

  // Okay, attempt to join these two intervals.  On failure, this returns false.
  // Otherwise, if one of the intervals being joined is a physreg, this method
  // always canonicalizes DstInt to be it.  The output "SrcInt" will not have
  // been modified, so we can use this information below to update aliases.
  if (!joinIntervals(CP)) {
    // Coalescing failed.

    // If definition of source is defined by trivial computation, try
    // rematerializing it.
    if (!CP.isFlipped() &&
        reMaterializeTrivialDef(LIS->getInterval(CP.getSrcReg()),
                                CP.getDstReg(), CopyMI))
      return true;

    // If we can eliminate the copy without merging the live ranges, do so now.
    if (!CP.isPartial() && !CP.isPhys()) {
      if (adjustCopiesBackFrom(CP, CopyMI) ||
          removeCopyByCommutingDef(CP, CopyMI)) {
        LIS->RemoveMachineInstrFromMaps(CopyMI);
        CopyMI->eraseFromParent();
        DEBUG(dbgs() << "\tTrivial!\n");
        return true;
      }
    }

    // Otherwise, we are unable to join the intervals.
    DEBUG(dbgs() << "\tInterference!\n");
    Again = true;  // May be possible to coalesce later.
    return false;
  }

  // Coalescing to a virtual register that is of a sub-register class of the
  // other. Make sure the resulting register is set to the right register class.
  if (CP.isCrossClass()) {
    ++numCrossRCs;
    MRI->setRegClass(CP.getDstReg(), CP.getNewRC());
  }

  // Removing sub-register copies can ease the register class constraints.
  // Make sure we attempt to inflate the register class of DstReg.
  if (!CP.isPhys() && RegClassInfo.isProperSubClass(CP.getNewRC()))
    InflateRegs.push_back(CP.getDstReg());

  // CopyMI has been erased by joinIntervals at this point. Remove it from
  // ErasedInstrs since copyCoalesceWorkList() won't add a successful join back
  // to the work list. This keeps ErasedInstrs from growing needlessly.
  ErasedInstrs.erase(CopyMI);

  // Rewrite all SrcReg operands to DstReg.
  // Also update DstReg operands to include DstIdx if it is set.
  if (CP.getDstIdx())
    updateRegDefsUses(CP.getDstReg(), CP.getDstReg(), CP.getDstIdx());
  updateRegDefsUses(CP.getSrcReg(), CP.getDstReg(), CP.getSrcIdx());

  // SrcReg is guaranteed to be the register whose live interval that is
  // being merged.
  LIS->removeInterval(CP.getSrcReg());

  // Update regalloc hint.
  TRI->UpdateRegAllocHint(CP.getSrcReg(), CP.getDstReg(), *MF);

  DEBUG({
    dbgs() << "\tJoined. Result = " << PrintReg(CP.getDstReg(), TRI);
    if (!CP.isPhys())
      dbgs() << LIS->getInterval(CP.getDstReg());
     dbgs() << '\n';
  });

  ++numJoins;
  return true;
}

/// Attempt joining with a reserved physreg.
bool RegisterCoalescer::joinReservedPhysReg(CoalescerPair &CP) {
  assert(CP.isPhys() && "Must be a physreg copy");
  assert(RegClassInfo.isReserved(CP.getDstReg()) && "Not a reserved register");
  LiveInterval &RHS = LIS->getInterval(CP.getSrcReg());
  DEBUG(dbgs() << "\t\tRHS = " << PrintReg(CP.getSrcReg()) << ' ' << RHS
               << '\n');

  assert(CP.isFlipped() && RHS.containsOneValue() &&
         "Invalid join with reserved register");

  // Optimization for reserved registers like ESP. We can only merge with a
  // reserved physreg if RHS has a single value that is a copy of CP.DstReg().
  // The live range of the reserved register will look like a set of dead defs
  // - we don't properly track the live range of reserved registers.

  // Deny any overlapping intervals.  This depends on all the reserved
  // register live ranges to look like dead defs.
  for (MCRegUnitIterator UI(CP.getDstReg(), TRI); UI.isValid(); ++UI)
    if (RHS.overlaps(LIS->getRegUnit(*UI))) {
      DEBUG(dbgs() << "\t\tInterference: " << PrintRegUnit(*UI, TRI) << '\n');
      return false;
    }

  // Skip any value computations, we are not adding new values to the
  // reserved register.  Also skip merging the live ranges, the reserved
  // register live range doesn't need to be accurate as long as all the
  // defs are there.

  // We don't track kills for reserved registers.
  MRI->clearKillFlags(CP.getSrcReg());

  return true;
}

/// ComputeUltimateVN - Assuming we are going to join two live intervals,
/// compute what the resultant value numbers for each value in the input two
/// ranges will be.  This is complicated by copies between the two which can
/// and will commonly cause multiple value numbers to be merged into one.
///
/// VN is the value number that we're trying to resolve.  InstDefiningValue
/// keeps track of the new InstDefiningValue assignment for the result
/// LiveInterval.  ThisFromOther/OtherFromThis are sets that keep track of
/// whether a value in this or other is a copy from the opposite set.
/// ThisValNoAssignments/OtherValNoAssignments keep track of value #'s that have
/// already been assigned.
///
/// ThisFromOther[x] - If x is defined as a copy from the other interval, this
/// contains the value number the copy is from.
///
static unsigned ComputeUltimateVN(VNInfo *VNI,
                                  SmallVector<VNInfo*, 16> &NewVNInfo,
                                  DenseMap<VNInfo*, VNInfo*> &ThisFromOther,
                                  DenseMap<VNInfo*, VNInfo*> &OtherFromThis,
                                  SmallVector<int, 16> &ThisValNoAssignments,
                                  SmallVector<int, 16> &OtherValNoAssignments) {
  unsigned VN = VNI->id;

  // If the VN has already been computed, just return it.
  if (ThisValNoAssignments[VN] >= 0)
    return ThisValNoAssignments[VN];
  assert(ThisValNoAssignments[VN] != -2 && "Cyclic value numbers");

  // If this val is not a copy from the other val, then it must be a new value
  // number in the destination.
  DenseMap<VNInfo*, VNInfo*>::iterator I = ThisFromOther.find(VNI);
  if (I == ThisFromOther.end()) {
    NewVNInfo.push_back(VNI);
    return ThisValNoAssignments[VN] = NewVNInfo.size()-1;
  }
  VNInfo *OtherValNo = I->second;

  // Otherwise, this *is* a copy from the RHS.  If the other side has already
  // been computed, return it.
  if (OtherValNoAssignments[OtherValNo->id] >= 0)
    return ThisValNoAssignments[VN] = OtherValNoAssignments[OtherValNo->id];

  // Mark this value number as currently being computed, then ask what the
  // ultimate value # of the other value is.
  ThisValNoAssignments[VN] = -2;
  unsigned UltimateVN =
    ComputeUltimateVN(OtherValNo, NewVNInfo, OtherFromThis, ThisFromOther,
                      OtherValNoAssignments, ThisValNoAssignments);
  return ThisValNoAssignments[VN] = UltimateVN;
}


// Find out if we have something like
// A = X
// B = X
// if so, we can pretend this is actually
// A = X
// B = A
// which allows us to coalesce A and B.
// VNI is the definition of B. LR is the life range of A that includes
// the slot just before B. If we return true, we add "B = X" to DupCopies.
// This implies that A dominates B.
static bool RegistersDefinedFromSameValue(LiveIntervals &li,
                                          const TargetRegisterInfo &tri,
                                          CoalescerPair &CP,
                                          VNInfo *VNI,
                                          VNInfo *OtherVNI,
                                     SmallVector<MachineInstr*, 8> &DupCopies) {
  // FIXME: This is very conservative. For example, we don't handle
  // physical registers.

  MachineInstr *MI = li.getInstructionFromIndex(VNI->def);

  if (!MI || CP.isPartial() || CP.isPhys())
    return false;

  unsigned A = CP.getDstReg();
  if (!TargetRegisterInfo::isVirtualRegister(A))
    return false;

  unsigned B = CP.getSrcReg();
  if (!TargetRegisterInfo::isVirtualRegister(B))
    return false;

  MachineInstr *OtherMI = li.getInstructionFromIndex(OtherVNI->def);
  if (!OtherMI)
    return false;

  if (MI->isImplicitDef()) {
    DupCopies.push_back(MI);
    return true;
  } else {
    if (!MI->isFullCopy())
      return false;
    unsigned Src = MI->getOperand(1).getReg();
    if (!TargetRegisterInfo::isVirtualRegister(Src))
      return false;
    if (!OtherMI->isFullCopy())
      return false;
    unsigned OtherSrc = OtherMI->getOperand(1).getReg();
    if (!TargetRegisterInfo::isVirtualRegister(OtherSrc))
      return false;

    if (Src != OtherSrc)
      return false;

    // If the copies use two different value numbers of X, we cannot merge
    // A and B.
    LiveInterval &SrcInt = li.getInterval(Src);
    // getVNInfoBefore returns NULL for undef copies. In this case, the
    // optimization is still safe.
    if (SrcInt.getVNInfoBefore(OtherVNI->def) !=
        SrcInt.getVNInfoBefore(VNI->def))
      return false;

    DupCopies.push_back(MI);
    return true;
  }
}

/// joinIntervals - Attempt to join these two intervals.  On failure, this
/// returns false.
bool RegisterCoalescer::joinIntervals(CoalescerPair &CP) {
  // Handle physreg joins separately.
  if (CP.isPhys())
    return joinReservedPhysReg(CP);

  LiveInterval &RHS = LIS->getInterval(CP.getSrcReg());
  DEBUG(dbgs() << "\t\tRHS = " << PrintReg(CP.getSrcReg()) << ' ' << RHS
               << '\n');

  // Compute the final value assignment, assuming that the live ranges can be
  // coalesced.
  SmallVector<int, 16> LHSValNoAssignments;
  SmallVector<int, 16> RHSValNoAssignments;
  DenseMap<VNInfo*, VNInfo*> LHSValsDefinedFromRHS;
  DenseMap<VNInfo*, VNInfo*> RHSValsDefinedFromLHS;
  SmallVector<VNInfo*, 16> NewVNInfo;

  SmallVector<MachineInstr*, 8> DupCopies;
  SmallVector<MachineInstr*, 8> DeadCopies;

  LiveInterval &LHS = LIS->getOrCreateInterval(CP.getDstReg());
  DEBUG(dbgs() << "\t\tLHS = " << PrintReg(CP.getDstReg(), TRI) << ' ' << LHS
               << '\n');

  // Loop over the value numbers of the LHS, seeing if any are defined from
  // the RHS.
  for (LiveInterval::vni_iterator i = LHS.vni_begin(), e = LHS.vni_end();
       i != e; ++i) {
    VNInfo *VNI = *i;
    if (VNI->isUnused() || VNI->isPHIDef())
      continue;
    MachineInstr *MI = LIS->getInstructionFromIndex(VNI->def);
    assert(MI && "Missing def");
    if (!MI->isCopyLike() && !MI->isImplicitDef()) // Src not defined by a copy?
      continue;

    // Figure out the value # from the RHS.
    VNInfo *OtherVNI = RHS.getVNInfoBefore(VNI->def);
    // The copy could be to an aliased physreg.
    if (!OtherVNI)
      continue;

    // DstReg is known to be a register in the LHS interval.  If the src is
    // from the RHS interval, we can use its value #.
    if (CP.isCoalescable(MI))
      DeadCopies.push_back(MI);
    else if (!RegistersDefinedFromSameValue(*LIS, *TRI, CP, VNI, OtherVNI,
                                            DupCopies))
      continue;

    LHSValsDefinedFromRHS[VNI] = OtherVNI;
  }

  // Loop over the value numbers of the RHS, seeing if any are defined from
  // the LHS.
  for (LiveInterval::vni_iterator i = RHS.vni_begin(), e = RHS.vni_end();
       i != e; ++i) {
    VNInfo *VNI = *i;
    if (VNI->isUnused() || VNI->isPHIDef())
      continue;
    MachineInstr *MI = LIS->getInstructionFromIndex(VNI->def);
    assert(MI && "Missing def");
    if (!MI->isCopyLike() && !MI->isImplicitDef()) // Src not defined by a copy?
      continue;

    // Figure out the value # from the LHS.
    VNInfo *OtherVNI = LHS.getVNInfoBefore(VNI->def);
    // The copy could be to an aliased physreg.
    if (!OtherVNI)
      continue;

    // DstReg is known to be a register in the RHS interval.  If the src is
    // from the LHS interval, we can use its value #.
    if (CP.isCoalescable(MI))
      DeadCopies.push_back(MI);
    else if (!RegistersDefinedFromSameValue(*LIS, *TRI, CP, VNI, OtherVNI,
                                            DupCopies))
        continue;

    RHSValsDefinedFromLHS[VNI] = OtherVNI;
  }

  LHSValNoAssignments.resize(LHS.getNumValNums(), -1);
  RHSValNoAssignments.resize(RHS.getNumValNums(), -1);
  NewVNInfo.reserve(LHS.getNumValNums() + RHS.getNumValNums());

  for (LiveInterval::vni_iterator i = LHS.vni_begin(), e = LHS.vni_end();
       i != e; ++i) {
    VNInfo *VNI = *i;
    unsigned VN = VNI->id;
    if (LHSValNoAssignments[VN] >= 0 || VNI->isUnused())
      continue;
    ComputeUltimateVN(VNI, NewVNInfo,
                      LHSValsDefinedFromRHS, RHSValsDefinedFromLHS,
                      LHSValNoAssignments, RHSValNoAssignments);
  }
  for (LiveInterval::vni_iterator i = RHS.vni_begin(), e = RHS.vni_end();
       i != e; ++i) {
    VNInfo *VNI = *i;
    unsigned VN = VNI->id;
    if (RHSValNoAssignments[VN] >= 0 || VNI->isUnused())
      continue;
    // If this value number isn't a copy from the LHS, it's a new number.
    if (RHSValsDefinedFromLHS.find(VNI) == RHSValsDefinedFromLHS.end()) {
      NewVNInfo.push_back(VNI);
      RHSValNoAssignments[VN] = NewVNInfo.size()-1;
      continue;
    }

    ComputeUltimateVN(VNI, NewVNInfo,
                      RHSValsDefinedFromLHS, LHSValsDefinedFromRHS,
                      RHSValNoAssignments, LHSValNoAssignments);
  }

  // Armed with the mappings of LHS/RHS values to ultimate values, walk the
  // interval lists to see if these intervals are coalescable.
  LiveInterval::const_iterator I = LHS.begin();
  LiveInterval::const_iterator IE = LHS.end();
  LiveInterval::const_iterator J = RHS.begin();
  LiveInterval::const_iterator JE = RHS.end();

  // Collect interval end points that will no longer be kills.
  SmallVector<MachineInstr*, 8> LHSOldKills;
  SmallVector<MachineInstr*, 8> RHSOldKills;

  // Skip ahead until the first place of potential sharing.
  if (I != IE && J != JE) {
    if (I->start < J->start) {
      I = std::upper_bound(I, IE, J->start);
      if (I != LHS.begin()) --I;
    } else if (J->start < I->start) {
      J = std::upper_bound(J, JE, I->start);
      if (J != RHS.begin()) --J;
    }
  }

  while (I != IE && J != JE) {
    // Determine if these two live ranges overlap.
    // If so, check value # info to determine if they are really different.
    if (I->end > J->start && J->end > I->start) {
      // If the live range overlap will map to the same value number in the
      // result liverange, we can still coalesce them.  If not, we can't.
      if (LHSValNoAssignments[I->valno->id] !=
          RHSValNoAssignments[J->valno->id])
        return false;

      // Extended live ranges should no longer be killed.
      if (!I->end.isBlock() && I->end < J->end)
        if (MachineInstr *MI = LIS->getInstructionFromIndex(I->end))
          LHSOldKills.push_back(MI);
      if (!J->end.isBlock() && J->end < I->end)
        if (MachineInstr *MI = LIS->getInstructionFromIndex(J->end))
          RHSOldKills.push_back(MI);
    }

    if (I->end < J->end)
      ++I;
    else
      ++J;
  }

  // Update kill info. Some live ranges are extended due to copy coalescing.
  for (DenseMap<VNInfo*, VNInfo*>::iterator I = LHSValsDefinedFromRHS.begin(),
         E = LHSValsDefinedFromRHS.end(); I != E; ++I) {
    VNInfo *VNI = I->first;
    unsigned LHSValID = LHSValNoAssignments[VNI->id];
    if (VNI->hasPHIKill())
      NewVNInfo[LHSValID]->setHasPHIKill(true);
  }

  // Update kill info. Some live ranges are extended due to copy coalescing.
  for (DenseMap<VNInfo*, VNInfo*>::iterator I = RHSValsDefinedFromLHS.begin(),
         E = RHSValsDefinedFromLHS.end(); I != E; ++I) {
    VNInfo *VNI = I->first;
    unsigned RHSValID = RHSValNoAssignments[VNI->id];
    if (VNI->hasPHIKill())
      NewVNInfo[RHSValID]->setHasPHIKill(true);
  }

  // Clear kill flags where live ranges are extended.
  while (!LHSOldKills.empty())
    LHSOldKills.pop_back_val()->clearRegisterKills(LHS.reg, TRI);
  while (!RHSOldKills.empty())
    RHSOldKills.pop_back_val()->clearRegisterKills(RHS.reg, TRI);

  if (LHSValNoAssignments.empty())
    LHSValNoAssignments.push_back(-1);
  if (RHSValNoAssignments.empty())
    RHSValNoAssignments.push_back(-1);

  // Now erase all the redundant copies.
  for (unsigned i = 0, e = DeadCopies.size(); i != e; ++i) {
    MachineInstr *MI = DeadCopies[i];
    if (!ErasedInstrs.insert(MI))
      continue;
    DEBUG(dbgs() << "\t\terased:\t" << LIS->getInstructionIndex(MI)
                 << '\t' << *MI);
    LIS->RemoveMachineInstrFromMaps(MI);
    MI->eraseFromParent();
  }

  SmallVector<unsigned, 8> SourceRegisters;
  for (SmallVector<MachineInstr*, 8>::iterator I = DupCopies.begin(),
         E = DupCopies.end(); I != E; ++I) {
    MachineInstr *MI = *I;
    if (!ErasedInstrs.insert(MI))
      continue;

    // If MI is a copy, then we have pretended that the assignment to B in
    // A = X
    // B = X
    // was actually a copy from A. Now that we decided to coalesce A and B,
    // transform the code into
    // A = X
    // In the case of the implicit_def, we just have to remove it.
    if (!MI->isImplicitDef()) {
      unsigned Src = MI->getOperand(1).getReg();
      SourceRegisters.push_back(Src);
    }
    LIS->RemoveMachineInstrFromMaps(MI);
    MI->eraseFromParent();
  }

  // If B = X was the last use of X in a liverange, we have to shrink it now
  // that B = X is gone.
  for (SmallVector<unsigned, 8>::iterator I = SourceRegisters.begin(),
         E = SourceRegisters.end(); I != E; ++I) {
    LIS->shrinkToUses(&LIS->getInterval(*I));
  }

  // If we get here, we know that we can coalesce the live ranges.  Ask the
  // intervals to coalesce themselves now.
  LHS.join(RHS, &LHSValNoAssignments[0], &RHSValNoAssignments[0], NewVNInfo,
           MRI);
  return true;
}

namespace {
  // DepthMBBCompare - Comparison predicate that sort first based on the loop
  // depth of the basic block (the unsigned), and then on the MBB number.
  struct DepthMBBCompare {
    typedef std::pair<unsigned, MachineBasicBlock*> DepthMBBPair;
    bool operator()(const DepthMBBPair &LHS, const DepthMBBPair &RHS) const {
      // Deeper loops first
      if (LHS.first != RHS.first)
        return LHS.first > RHS.first;

      // Prefer blocks that are more connected in the CFG. This takes care of
      // the most difficult copies first while intervals are short.
      unsigned cl = LHS.second->pred_size() + LHS.second->succ_size();
      unsigned cr = RHS.second->pred_size() + RHS.second->succ_size();
      if (cl != cr)
        return cl > cr;

      // As a last resort, sort by block number.
      return LHS.second->getNumber() < RHS.second->getNumber();
    }
  };
}

// Try joining WorkList copies starting from index From.
// Null out any successful joins.
bool RegisterCoalescer::copyCoalesceWorkList(unsigned From) {
  assert(From <= WorkList.size() && "Out of range");
  bool Progress = false;
  for (unsigned i = From, e = WorkList.size(); i != e; ++i) {
    if (!WorkList[i])
      continue;
    // Skip instruction pointers that have already been erased, for example by
    // dead code elimination.
    if (ErasedInstrs.erase(WorkList[i])) {
      WorkList[i] = 0;
      continue;
    }
    bool Again = false;
    bool Success = joinCopy(WorkList[i], Again);
    Progress |= Success;
    if (Success || !Again)
      WorkList[i] = 0;
  }
  return Progress;
}

void
RegisterCoalescer::copyCoalesceInMBB(MachineBasicBlock *MBB) {
  DEBUG(dbgs() << MBB->getName() << ":\n");

  // Collect all copy-like instructions in MBB. Don't start coalescing anything
  // yet, it might invalidate the iterator.
  const unsigned PrevSize = WorkList.size();
  for (MachineBasicBlock::iterator MII = MBB->begin(), E = MBB->end();
       MII != E; ++MII)
    if (MII->isCopyLike())
      WorkList.push_back(MII);

  // Try coalescing the collected copies immediately, and remove the nulls.
  // This prevents the WorkList from getting too large since most copies are
  // joinable on the first attempt.
  if (copyCoalesceWorkList(PrevSize))
    WorkList.erase(std::remove(WorkList.begin() + PrevSize, WorkList.end(),
                               (MachineInstr*)0), WorkList.end());
}

void RegisterCoalescer::joinAllIntervals() {
  DEBUG(dbgs() << "********** JOINING INTERVALS ***********\n");
  assert(WorkList.empty() && "Old data still around.");

  if (Loops->empty()) {
    // If there are no loops in the function, join intervals in function order.
    for (MachineFunction::iterator I = MF->begin(), E = MF->end();
         I != E; ++I)
      copyCoalesceInMBB(I);
  } else {
    // Otherwise, join intervals in inner loops before other intervals.
    // Unfortunately we can't just iterate over loop hierarchy here because
    // there may be more MBB's than BB's.  Collect MBB's for sorting.

    // Join intervals in the function prolog first. We want to join physical
    // registers with virtual registers before the intervals got too long.
    std::vector<std::pair<unsigned, MachineBasicBlock*> > MBBs;
    for (MachineFunction::iterator I = MF->begin(), E = MF->end();I != E;++I){
      MachineBasicBlock *MBB = I;
      MBBs.push_back(std::make_pair(Loops->getLoopDepth(MBB), I));
    }

    // Sort by loop depth.
    std::sort(MBBs.begin(), MBBs.end(), DepthMBBCompare());

    // Finally, join intervals in loop nest order.
    for (unsigned i = 0, e = MBBs.size(); i != e; ++i)
      copyCoalesceInMBB(MBBs[i].second);
  }

  // Joining intervals can allow other intervals to be joined.  Iteratively join
  // until we make no progress.
  while (copyCoalesceWorkList())
    /* empty */ ;
}

void RegisterCoalescer::releaseMemory() {
  ErasedInstrs.clear();
  WorkList.clear();
  DeadDefs.clear();
  InflateRegs.clear();
}

bool RegisterCoalescer::runOnMachineFunction(MachineFunction &fn) {
  MF = &fn;
  MRI = &fn.getRegInfo();
  TM = &fn.getTarget();
  TRI = TM->getRegisterInfo();
  TII = TM->getInstrInfo();
  LIS = &getAnalysis<LiveIntervals>();
  LDV = &getAnalysis<LiveDebugVariables>();
  AA = &getAnalysis<AliasAnalysis>();
  Loops = &getAnalysis<MachineLoopInfo>();

  DEBUG(dbgs() << "********** SIMPLE REGISTER COALESCING **********\n"
               << "********** Function: "
               << ((Value*)MF->getFunction())->getName() << '\n');

  if (VerifyCoalescing)
    MF->verify(this, "Before register coalescing");

  RegClassInfo.runOnMachineFunction(fn);

  // Join (coalesce) intervals if requested.
  if (EnableJoining)
    joinAllIntervals();

  // After deleting a lot of copies, register classes may be less constrained.
  // Removing sub-register operands may allow GR32_ABCD -> GR32 and DPR_VFP2 ->
  // DPR inflation.
  array_pod_sort(InflateRegs.begin(), InflateRegs.end());
  InflateRegs.erase(std::unique(InflateRegs.begin(), InflateRegs.end()),
                    InflateRegs.end());
  DEBUG(dbgs() << "Trying to inflate " << InflateRegs.size() << " regs.\n");
  for (unsigned i = 0, e = InflateRegs.size(); i != e; ++i) {
    unsigned Reg = InflateRegs[i];
    if (MRI->reg_nodbg_empty(Reg))
      continue;
    if (MRI->recomputeRegClass(Reg, *TM)) {
      DEBUG(dbgs() << PrintReg(Reg) << " inflated to "
                   << MRI->getRegClass(Reg)->getName() << '\n');
      ++NumInflated;
    }
  }

  DEBUG(dump());
  DEBUG(LDV->dump());
  if (VerifyCoalescing)
    MF->verify(this, "After register coalescing");
  return true;
}

/// print - Implement the dump method.
void RegisterCoalescer::print(raw_ostream &O, const Module* m) const {
   LIS->print(O, m);
}