PreAllocSplitting.cpp

//===-- PreAllocSplitting.cpp - Pre-allocation Interval Spltting 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 machine instruction level pre-register allocation
// live interval splitting pass. It finds live interval barriers, i.e.
// instructions which will kill all physical registers in certain register
// classes, and split all live intervals which cross the barrier.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "pre-alloc-split"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/LiveStackAnalysis.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/RegisterCoalescer.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

static cl::opt<int> PreSplitLimit("pre-split-limit", cl::init(-1), cl::Hidden);
static cl::opt<int> DeadSplitLimit("dead-split-limit", cl::init(-1), cl::Hidden);

STATISTIC(NumSplits, "Number of intervals split");
STATISTIC(NumRemats, "Number of intervals split by rematerialization");
STATISTIC(NumFolds, "Number of intervals split with spill folding");
STATISTIC(NumRenumbers, "Number of intervals renumbered into new registers");
STATISTIC(NumDeadSpills, "Number of dead spills removed");

namespace {
  class VISIBILITY_HIDDEN PreAllocSplitting : public MachineFunctionPass {
    MachineFunction       *CurrMF;
    const TargetMachine   *TM;
    const TargetInstrInfo *TII;
    const TargetRegisterInfo* TRI;
    MachineFrameInfo      *MFI;
    MachineRegisterInfo   *MRI;
    LiveIntervals         *LIs;
    LiveStacks            *LSs;

    // Barrier - Current barrier being processed.
    MachineInstr          *Barrier;

    // BarrierMBB - Basic block where the barrier resides in.
    MachineBasicBlock     *BarrierMBB;

    // Barrier - Current barrier index.
    unsigned              BarrierIdx;

    // CurrLI - Current live interval being split.
    LiveInterval          *CurrLI;

    // CurrSLI - Current stack slot live interval.
    LiveInterval          *CurrSLI;

    // CurrSValNo - Current val# for the stack slot live interval.
    VNInfo                *CurrSValNo;

    // IntervalSSMap - A map from live interval to spill slots.
    DenseMap<unsigned, int> IntervalSSMap;

    // Def2SpillMap - A map from a def instruction index to spill index.
    DenseMap<unsigned, unsigned> Def2SpillMap;

  public:
    static char ID;
    PreAllocSplitting() : MachineFunctionPass(&ID) {}

    virtual bool runOnMachineFunction(MachineFunction &MF);

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<LiveIntervals>();
      AU.addPreserved<LiveIntervals>();
      AU.addRequired<LiveStacks>();
      AU.addPreserved<LiveStacks>();
      AU.addPreserved<RegisterCoalescer>();
      if (StrongPHIElim)
        AU.addPreservedID(StrongPHIEliminationID);
      else
        AU.addPreservedID(PHIEliminationID);
      AU.addRequired<MachineDominatorTree>();
      AU.addRequired<MachineLoopInfo>();
      AU.addPreserved<MachineDominatorTree>();
      AU.addPreserved<MachineLoopInfo>();
      MachineFunctionPass::getAnalysisUsage(AU);
    }
    
    virtual void releaseMemory() {
      IntervalSSMap.clear();
      Def2SpillMap.clear();
    }

    virtual const char *getPassName() const {
      return "Pre-Register Allocaton Live Interval Splitting";
    }

    /// print - Implement the dump method.
    virtual void print(std::ostream &O, const Module* M = 0) const {
      LIs->print(O, M);
    }

    void print(std::ostream *O, const Module* M = 0) const {
      if (O) print(*O, M);
    }

  private:
    MachineBasicBlock::iterator
      findNextEmptySlot(MachineBasicBlock*, MachineInstr*,
                        unsigned&);

    MachineBasicBlock::iterator
      findSpillPoint(MachineBasicBlock*, MachineInstr*, MachineInstr*,
                     SmallPtrSet<MachineInstr*, 4>&, unsigned&);

    MachineBasicBlock::iterator
      findRestorePoint(MachineBasicBlock*, MachineInstr*, unsigned,
                     SmallPtrSet<MachineInstr*, 4>&, unsigned&);

    int CreateSpillStackSlot(unsigned, const TargetRegisterClass *);

    bool IsAvailableInStack(MachineBasicBlock*, unsigned, unsigned, unsigned,
                            unsigned&, int&) const;

    void UpdateSpillSlotInterval(VNInfo*, unsigned, unsigned);

    bool SplitRegLiveInterval(LiveInterval*);

    bool SplitRegLiveIntervals(const TargetRegisterClass **,
                               SmallPtrSet<LiveInterval*, 8>&);
    
    bool createsNewJoin(LiveRange* LR, MachineBasicBlock* DefMBB,
                        MachineBasicBlock* BarrierMBB);
    bool Rematerialize(unsigned vreg, VNInfo* ValNo,
                       MachineInstr* DefMI,
                       MachineBasicBlock::iterator RestorePt,
                       unsigned RestoreIdx,
                       SmallPtrSet<MachineInstr*, 4>& RefsInMBB);
    MachineInstr* FoldSpill(unsigned vreg, const TargetRegisterClass* RC,
                            MachineInstr* DefMI,
                            MachineInstr* Barrier,
                            MachineBasicBlock* MBB,
                            int& SS,
                            SmallPtrSet<MachineInstr*, 4>& RefsInMBB);
    void RenumberValno(VNInfo* VN);
    void ReconstructLiveInterval(LiveInterval* LI);
    bool removeDeadSpills(SmallPtrSet<LiveInterval*, 8>& split);
    unsigned getNumberOfNonSpills(SmallPtrSet<MachineInstr*, 4>& MIs,
                               unsigned Reg, int FrameIndex, bool& TwoAddr);
    VNInfo* PerformPHIConstruction(MachineBasicBlock::iterator use,
                                   MachineBasicBlock* MBB,
                                   LiveInterval* LI,
                                   SmallPtrSet<MachineInstr*, 4>& Visited,
            DenseMap<MachineBasicBlock*, SmallPtrSet<MachineInstr*, 2> >& Defs,
            DenseMap<MachineBasicBlock*, SmallPtrSet<MachineInstr*, 2> >& Uses,
                                      DenseMap<MachineInstr*, VNInfo*>& NewVNs,
                                DenseMap<MachineBasicBlock*, VNInfo*>& LiveOut,
                                DenseMap<MachineBasicBlock*, VNInfo*>& Phis,
                                        bool toplevel, bool intrablock);
};
} // end anonymous namespace

char PreAllocSplitting::ID = 0;

static RegisterPass<PreAllocSplitting>
X("pre-alloc-splitting", "Pre-Register Allocation Live Interval Splitting");

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


/// findNextEmptySlot - Find a gap after the given machine instruction in the
/// instruction index map. If there isn't one, return end().
MachineBasicBlock::iterator
PreAllocSplitting::findNextEmptySlot(MachineBasicBlock *MBB, MachineInstr *MI,
                                     unsigned &SpotIndex) {
  MachineBasicBlock::iterator MII = MI;
  if (++MII != MBB->end()) {
    unsigned Index = LIs->findGapBeforeInstr(LIs->getInstructionIndex(MII));
    if (Index) {
      SpotIndex = Index;
      return MII;
    }
  }
  return MBB->end();
}

/// findSpillPoint - Find a gap as far away from the given MI that's suitable
/// for spilling the current live interval. The index must be before any
/// defs and uses of the live interval register in the mbb. Return begin() if
/// none is found.
MachineBasicBlock::iterator
PreAllocSplitting::findSpillPoint(MachineBasicBlock *MBB, MachineInstr *MI,
                                  MachineInstr *DefMI,
                                  SmallPtrSet<MachineInstr*, 4> &RefsInMBB,
                                  unsigned &SpillIndex) {
  MachineBasicBlock::iterator Pt = MBB->begin();

  // Go top down if RefsInMBB is empty.
  if (RefsInMBB.empty() && !DefMI) {
    MachineBasicBlock::iterator MII = MBB->begin();
    MachineBasicBlock::iterator EndPt = MI;
    do {
      ++MII;
      unsigned Index = LIs->getInstructionIndex(MII);
      unsigned Gap = LIs->findGapBeforeInstr(Index);
      if (Gap) {
        Pt = MII;
        SpillIndex = Gap;
        break;
      
      // We can't insert the spill between the barrier (a call), and its
      // corresponding call frame setup.
      } else if (prior(MII)->getOpcode() == TRI->getCallFrameSetupOpcode() &&
                 MII == MachineBasicBlock::iterator(MI))
        break;
    } while (MII != EndPt);
  } else {
    MachineBasicBlock::iterator MII = MI;
    MachineBasicBlock::iterator EndPt = DefMI
      ? MachineBasicBlock::iterator(DefMI) : MBB->begin();
    
    // We can't insert the spill between the barrier (a call), and its
    // corresponding call frame setup.
    if (prior(MII)->getOpcode() == TRI->getCallFrameSetupOpcode()) --MII;
    while (MII != EndPt && !RefsInMBB.count(MII)) {
      unsigned Index = LIs->getInstructionIndex(MII);
      if (LIs->hasGapBeforeInstr(Index)) {
        Pt = MII;
        SpillIndex = LIs->findGapBeforeInstr(Index, true);
      }
      --MII;
    }
  }

  return Pt;
}

/// findRestorePoint - Find a gap in the instruction index map that's suitable
/// for restoring the current live interval value. The index must be before any
/// uses of the live interval register in the mbb. Return end() if none is
/// found.
MachineBasicBlock::iterator
PreAllocSplitting::findRestorePoint(MachineBasicBlock *MBB, MachineInstr *MI,
                                    unsigned LastIdx,
                                    SmallPtrSet<MachineInstr*, 4> &RefsInMBB,
                                    unsigned &RestoreIndex) {
  // FIXME: Allow spill to be inserted to the beginning of the mbb. Update mbb
  // begin index accordingly.
  MachineBasicBlock::iterator Pt = MBB->end();
  unsigned EndIdx = LIs->getMBBEndIdx(MBB);

  // Go bottom up if RefsInMBB is empty and the end of the mbb isn't beyond
  // the last index in the live range.
  if (RefsInMBB.empty() && LastIdx >= EndIdx) {
    MachineBasicBlock::iterator MII = MBB->getFirstTerminator();
    MachineBasicBlock::iterator EndPt = MI;
    --MII;
    do {
      unsigned Index = LIs->getInstructionIndex(MII);
      unsigned Gap = LIs->findGapBeforeInstr(Index);
      if (Gap) {
        Pt = MII;
        RestoreIndex = Gap;
        break;
      
      // We can't insert a restore between the barrier (a call) and its 
      // corresponding call frame teardown.
      } else if (MII->getOpcode() == TRI->getCallFrameDestroyOpcode() &&
                 prior(MII) == MachineBasicBlock::iterator(MI))
        break;
      --MII;
    } while (MII != EndPt);
  } else {
    MachineBasicBlock::iterator MII = MI;
    MII = ++MII;
    // We can't insert a restore between the barrier (a call) and its 
    // corresponding call frame teardown.
    if (MII->getOpcode() == TRI->getCallFrameDestroyOpcode())
      ++MII;
    
    // FIXME: Limit the number of instructions to examine to reduce
    // compile time?
    while (MII != MBB->getFirstTerminator()) {
      unsigned Index = LIs->getInstructionIndex(MII);
      if (Index > LastIdx)
        break;
      unsigned Gap = LIs->findGapBeforeInstr(Index);
      if (Gap) {
        Pt = MII;
        RestoreIndex = Gap;
      }
      if (RefsInMBB.count(MII))
        break;
      ++MII;
    }
  }

  return Pt;
}

/// CreateSpillStackSlot - Create a stack slot for the live interval being
/// split. If the live interval was previously split, just reuse the same
/// slot.
int PreAllocSplitting::CreateSpillStackSlot(unsigned Reg,
                                            const TargetRegisterClass *RC) {
  int SS;
  DenseMap<unsigned, int>::iterator I = IntervalSSMap.find(Reg);
  if (I != IntervalSSMap.end()) {
    SS = I->second;
  } else {
    SS = MFI->CreateStackObject(RC->getSize(), RC->getAlignment());
    IntervalSSMap[Reg] = SS;
  }

  // Create live interval for stack slot.
  CurrSLI = &LSs->getOrCreateInterval(SS);
  if (CurrSLI->hasAtLeastOneValue())
    CurrSValNo = CurrSLI->getValNumInfo(0);
  else
    CurrSValNo = CurrSLI->getNextValue(~0U, 0, LSs->getVNInfoAllocator());
  return SS;
}

/// IsAvailableInStack - Return true if register is available in a split stack
/// slot at the specified index.
bool
PreAllocSplitting::IsAvailableInStack(MachineBasicBlock *DefMBB,
                                    unsigned Reg, unsigned DefIndex,
                                    unsigned RestoreIndex, unsigned &SpillIndex,
                                    int& SS) const {
  if (!DefMBB)
    return false;

  DenseMap<unsigned, int>::iterator I = IntervalSSMap.find(Reg);
  if (I == IntervalSSMap.end())
    return false;
  DenseMap<unsigned, unsigned>::iterator II = Def2SpillMap.find(DefIndex);
  if (II == Def2SpillMap.end())
    return false;

  // If last spill of def is in the same mbb as barrier mbb (where restore will
  // be), make sure it's not below the intended restore index.
  // FIXME: Undo the previous spill?
  assert(LIs->getMBBFromIndex(II->second) == DefMBB);
  if (DefMBB == BarrierMBB && II->second >= RestoreIndex)
    return false;

  SS = I->second;
  SpillIndex = II->second;
  return true;
}

/// UpdateSpillSlotInterval - Given the specified val# of the register live
/// interval being split, and the spill and restore indicies, update the live
/// interval of the spill stack slot.
void
PreAllocSplitting::UpdateSpillSlotInterval(VNInfo *ValNo, unsigned SpillIndex,
                                           unsigned RestoreIndex) {
  assert(LIs->getMBBFromIndex(RestoreIndex) == BarrierMBB &&
         "Expect restore in the barrier mbb");

  MachineBasicBlock *MBB = LIs->getMBBFromIndex(SpillIndex);
  if (MBB == BarrierMBB) {
    // Intra-block spill + restore. We are done.
    LiveRange SLR(SpillIndex, RestoreIndex, CurrSValNo);
    CurrSLI->addRange(SLR);
    return;
  }

  SmallPtrSet<MachineBasicBlock*, 4> Processed;
  unsigned EndIdx = LIs->getMBBEndIdx(MBB);
  LiveRange SLR(SpillIndex, EndIdx+1, CurrSValNo);
  CurrSLI->addRange(SLR);
  Processed.insert(MBB);

  // Start from the spill mbb, figure out the extend of the spill slot's
  // live interval.
  SmallVector<MachineBasicBlock*, 4> WorkList;
  const LiveRange *LR = CurrLI->getLiveRangeContaining(SpillIndex);
  if (LR->end > EndIdx)
    // If live range extend beyond end of mbb, add successors to work list.
    for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
           SE = MBB->succ_end(); SI != SE; ++SI)
      WorkList.push_back(*SI);

  while (!WorkList.empty()) {
    MachineBasicBlock *MBB = WorkList.back();
    WorkList.pop_back();
    if (Processed.count(MBB))
      continue;
    unsigned Idx = LIs->getMBBStartIdx(MBB);
    LR = CurrLI->getLiveRangeContaining(Idx);
    if (LR && LR->valno == ValNo) {
      EndIdx = LIs->getMBBEndIdx(MBB);
      if (Idx <= RestoreIndex && RestoreIndex < EndIdx) {
        // Spill slot live interval stops at the restore.
        LiveRange SLR(Idx, RestoreIndex, CurrSValNo);
        CurrSLI->addRange(SLR);
      } else if (LR->end > EndIdx) {
        // Live range extends beyond end of mbb, process successors.
        LiveRange SLR(Idx, EndIdx+1, CurrSValNo);
        CurrSLI->addRange(SLR);
        for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
               SE = MBB->succ_end(); SI != SE; ++SI)
          WorkList.push_back(*SI);
      } else {
        LiveRange SLR(Idx, LR->end, CurrSValNo);
        CurrSLI->addRange(SLR);
      }
      Processed.insert(MBB);
    }
  }
}

/// PerformPHIConstruction - From properly set up use and def lists, use a PHI
/// construction algorithm to compute the ranges and valnos for an interval.
VNInfo* PreAllocSplitting::PerformPHIConstruction(
                                                MachineBasicBlock::iterator use,
                                                         MachineBasicBlock* MBB,
                                                               LiveInterval* LI,
                                       SmallPtrSet<MachineInstr*, 4>& Visited,
             DenseMap<MachineBasicBlock*, SmallPtrSet<MachineInstr*, 2> >& Defs,
             DenseMap<MachineBasicBlock*, SmallPtrSet<MachineInstr*, 2> >& Uses,
                                       DenseMap<MachineInstr*, VNInfo*>& NewVNs,
                                 DenseMap<MachineBasicBlock*, VNInfo*>& LiveOut,
                                 DenseMap<MachineBasicBlock*, VNInfo*>& Phis,
                                              bool toplevel, bool intrablock) {
  // Return memoized result if it's available.
  if (toplevel && Visited.count(use) && NewVNs.count(use))
    return NewVNs[use];
  else if (!toplevel && intrablock && NewVNs.count(use))
    return NewVNs[use];
  else if (!intrablock && LiveOut.count(MBB))
    return LiveOut[MBB];
  
  typedef DenseMap<MachineBasicBlock*, SmallPtrSet<MachineInstr*, 2> > RegMap;
  
  // Check if our block contains any uses or defs.
  bool ContainsDefs = Defs.count(MBB);
  bool ContainsUses = Uses.count(MBB);
  
  VNInfo* ret = 0;
  
  // Enumerate the cases of use/def contaning blocks.
  if (!ContainsDefs && !ContainsUses) {
  Fallback:
    // NOTE: Because this is the fallback case from other cases, we do NOT
    // assume that we are not intrablock here.
    if (Phis.count(MBB)) return Phis[MBB];
    
    unsigned StartIndex = LIs->getMBBStartIdx(MBB);
    
    if (MBB->pred_size() == 1) {
      Phis[MBB] = ret = PerformPHIConstruction((*MBB->pred_begin())->end(),
                                          *(MBB->pred_begin()), LI, Visited,
                                          Defs, Uses, NewVNs, LiveOut, Phis,
                                          false, false);
      unsigned EndIndex = 0;
      if (intrablock) {
        EndIndex = LIs->getInstructionIndex(use);
        EndIndex = LiveIntervals::getUseIndex(EndIndex);
      } else
        EndIndex = LIs->getMBBEndIdx(MBB);
      
      LI->addRange(LiveRange(StartIndex, EndIndex+1, ret));
      if (intrablock)
        LI->addKill(ret, EndIndex);
    } else {
      Phis[MBB] = ret = LI->getNextValue(~0U, /*FIXME*/ 0,
                                          LIs->getVNInfoAllocator());
      if (!intrablock) LiveOut[MBB] = ret;
    
      // If there are no uses or defs between our starting point and the
      // beginning of the block, then recursive perform phi construction
      // on our predecessors.
      DenseMap<MachineBasicBlock*, VNInfo*> IncomingVNs;
      for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
           PE = MBB->pred_end(); PI != PE; ++PI) {
        VNInfo* Incoming = PerformPHIConstruction((*PI)->end(), *PI, LI, 
                                            Visited, Defs, Uses, NewVNs,
                                            LiveOut, Phis, false, false);
        if (Incoming != 0)
          IncomingVNs[*PI] = Incoming;
      }
    
      // Otherwise, merge the incoming VNInfos with a phi join.  Create a new
      // VNInfo to represent the joined value.
      for (DenseMap<MachineBasicBlock*, VNInfo*>::iterator I =
           IncomingVNs.begin(), E = IncomingVNs.end(); I != E; ++I) {
        I->second->hasPHIKill = true;
        unsigned KillIndex = LIs->getMBBEndIdx(I->first);
        if (!LiveInterval::isKill(I->second, KillIndex))
          LI->addKill(I->second, KillIndex);
      }
      
      unsigned EndIndex = 0;
      if (intrablock) {
        EndIndex = LIs->getInstructionIndex(use);
        EndIndex = LiveIntervals::getUseIndex(EndIndex);
      } else
        EndIndex = LIs->getMBBEndIdx(MBB);
      LI->addRange(LiveRange(StartIndex, EndIndex+1, ret));
      if (intrablock)
        LI->addKill(ret, EndIndex);
    }
  } else if (ContainsDefs && !ContainsUses) {
    SmallPtrSet<MachineInstr*, 2>& BlockDefs = Defs[MBB];

    // Search for the def in this block.  If we don't find it before the
    // instruction we care about, go to the fallback case.  Note that that
    // should never happen: this cannot be intrablock, so use should
    // always be an end() iterator.
    assert(use == MBB->end() && "No use marked in intrablock");
    
    MachineBasicBlock::iterator walker = use;
    --walker;
    while (walker != MBB->begin())
      if (BlockDefs.count(walker)) {
        break;
      } else
        --walker;
    
    // Once we've found it, extend its VNInfo to our instruction.
    unsigned DefIndex = LIs->getInstructionIndex(walker);
    DefIndex = LiveIntervals::getDefIndex(DefIndex);
    unsigned EndIndex = LIs->getMBBEndIdx(MBB);
    
    ret = NewVNs[walker];
    LI->addRange(LiveRange(DefIndex, EndIndex+1, ret));
  } else if (!ContainsDefs && ContainsUses) {
    SmallPtrSet<MachineInstr*, 2>& BlockUses = Uses[MBB];
    
    // Search for the use in this block that precedes the instruction we care 
    // about, going to the fallback case if we don't find it.
    
    if (use == MBB->begin())
      goto Fallback;
    
    MachineBasicBlock::iterator walker = use;
    --walker;
    bool found = false;
    while (walker != MBB->begin())
      if (BlockUses.count(walker)) {
        found = true;
        break;
      } else
        --walker;
        
    // Must check begin() too.
    if (!found) {
      if (BlockUses.count(walker))
        found = true;
      else
        goto Fallback;
    }

    unsigned UseIndex = LIs->getInstructionIndex(walker);
    UseIndex = LiveIntervals::getUseIndex(UseIndex);
    unsigned EndIndex = 0;
    if (intrablock) {
      EndIndex = LIs->getInstructionIndex(use);
      EndIndex = LiveIntervals::getUseIndex(EndIndex);
    } else
      EndIndex = LIs->getMBBEndIdx(MBB);

    // Now, recursively phi construct the VNInfo for the use we found,
    // and then extend it to include the instruction we care about
    ret = PerformPHIConstruction(walker, MBB, LI, Visited, Defs, Uses,
                                 NewVNs, LiveOut, Phis, false, true);
    
    LI->addRange(LiveRange(UseIndex, EndIndex+1, ret));
    
    // FIXME: Need to set kills properly for inter-block stuff.
    if (LI->isKill(ret, UseIndex)) LI->removeKill(ret, UseIndex);
    if (intrablock)
      LI->addKill(ret, EndIndex);
  } else if (ContainsDefs && ContainsUses){
    SmallPtrSet<MachineInstr*, 2>& BlockDefs = Defs[MBB];
    SmallPtrSet<MachineInstr*, 2>& BlockUses = Uses[MBB];
    
    // This case is basically a merging of the two preceding case, with the
    // special note that checking for defs must take precedence over checking
    // for uses, because of two-address instructions.
    
    if (use == MBB->begin())
      goto Fallback;
    
    MachineBasicBlock::iterator walker = use;
    --walker;
    bool foundDef = false;
    bool foundUse = false;
    while (walker != MBB->begin())
      if (BlockDefs.count(walker)) {
        foundDef = true;
        break;
      } else if (BlockUses.count(walker)) {
        foundUse = true;
        break;
      } else
        --walker;
        
    // Must check begin() too.
    if (!foundDef && !foundUse) {
      if (BlockDefs.count(walker))
        foundDef = true;
      else if (BlockUses.count(walker))
        foundUse = true;
      else
        goto Fallback;
    }

    unsigned StartIndex = LIs->getInstructionIndex(walker);
    StartIndex = foundDef ? LiveIntervals::getDefIndex(StartIndex) :
                            LiveIntervals::getUseIndex(StartIndex);
    unsigned EndIndex = 0;
    if (intrablock) {
      EndIndex = LIs->getInstructionIndex(use);
      EndIndex = LiveIntervals::getUseIndex(EndIndex);
    } else
      EndIndex = LIs->getMBBEndIdx(MBB);

    if (foundDef)
      ret = NewVNs[walker];
    else
      ret = PerformPHIConstruction(walker, MBB, LI, Visited, Defs, Uses,
                                   NewVNs, LiveOut, Phis, false, true);

    LI->addRange(LiveRange(StartIndex, EndIndex+1, ret));
    
    if (foundUse && LI->isKill(ret, StartIndex))
      LI->removeKill(ret, StartIndex);
    if (intrablock) {
      LI->addKill(ret, EndIndex);
    }
  }
  
  // Memoize results so we don't have to recompute them.
  if (!intrablock) LiveOut[MBB] = ret;
  else {
    if (!NewVNs.count(use))
      NewVNs[use] = ret;
    Visited.insert(use);
  }

  return ret;
}

/// ReconstructLiveInterval - Recompute a live interval from scratch.
void PreAllocSplitting::ReconstructLiveInterval(LiveInterval* LI) {
  BumpPtrAllocator& Alloc = LIs->getVNInfoAllocator();
  
  // Clear the old ranges and valnos;
  LI->clear();
  
  // Cache the uses and defs of the register
  typedef DenseMap<MachineBasicBlock*, SmallPtrSet<MachineInstr*, 2> > RegMap;
  RegMap Defs, Uses;
  
  // Keep track of the new VNs we're creating.
  DenseMap<MachineInstr*, VNInfo*> NewVNs;
  SmallPtrSet<VNInfo*, 2> PhiVNs;
  
  // Cache defs, and create a new VNInfo for each def.
  for (MachineRegisterInfo::def_iterator DI = MRI->def_begin(LI->reg),
       DE = MRI->def_end(); DI != DE; ++DI) {
    Defs[(*DI).getParent()].insert(&*DI);
    
    unsigned DefIdx = LIs->getInstructionIndex(&*DI);
    DefIdx = LiveIntervals::getDefIndex(DefIdx);
    
    VNInfo* NewVN = LI->getNextValue(DefIdx, 0, Alloc);
    
    // If the def is a move, set the copy field.
    unsigned SrcReg, DstReg, SrcSubIdx, DstSubIdx;
    if (TII->isMoveInstr(*DI, SrcReg, DstReg, SrcSubIdx, DstSubIdx))
      if (DstReg == LI->reg)
        NewVN->copy = &*DI;
    
    NewVNs[&*DI] = NewVN;
  }
  
  // Cache uses as a separate pass from actually processing them.
  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(LI->reg),
       UE = MRI->use_end(); UI != UE; ++UI)
    Uses[(*UI).getParent()].insert(&*UI);
    
  // Now, actually process every use and use a phi construction algorithm
  // to walk from it to its reaching definitions, building VNInfos along
  // the way.
  DenseMap<MachineBasicBlock*, VNInfo*> LiveOut;
  DenseMap<MachineBasicBlock*, VNInfo*> Phis;
  SmallPtrSet<MachineInstr*, 4> Visited;
  for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(LI->reg),
       UE = MRI->use_end(); UI != UE; ++UI) {
    PerformPHIConstruction(&*UI, UI->getParent(), LI, Visited, Defs,
                           Uses, NewVNs, LiveOut, Phis, true, true); 
  }
  
  // Add ranges for dead defs
  for (MachineRegisterInfo::def_iterator DI = MRI->def_begin(LI->reg),
       DE = MRI->def_end(); DI != DE; ++DI) {
    unsigned DefIdx = LIs->getInstructionIndex(&*DI);
    DefIdx = LiveIntervals::getDefIndex(DefIdx);
    
    if (LI->liveAt(DefIdx)) continue;
    
    VNInfo* DeadVN = NewVNs[&*DI];
    LI->addRange(LiveRange(DefIdx, DefIdx+1, DeadVN));
    LI->addKill(DeadVN, DefIdx);
  }
}

/// RenumberValno - Split the given valno out into a new vreg, allowing it to
/// be allocated to a different register.  This function creates a new vreg,
/// copies the valno and its live ranges over to the new vreg's interval,
/// removes them from the old interval, and rewrites all uses and defs of
/// the original reg to the new vreg within those ranges.
void PreAllocSplitting::RenumberValno(VNInfo* VN) {
  SmallVector<VNInfo*, 4> Stack;
  SmallVector<VNInfo*, 4> VNsToCopy;
  Stack.push_back(VN);

  // Walk through and copy the valno we care about, and any other valnos
  // that are two-address redefinitions of the one we care about.  These
  // will need to be rewritten as well.  We also check for safety of the 
  // renumbering here, by making sure that none of the valno involved has
  // phi kills.
  while (!Stack.empty()) {
    VNInfo* OldVN = Stack.back();
    Stack.pop_back();
    
    // Bail out if we ever encounter a valno that has a PHI kill.  We can't
    // renumber these.
    if (OldVN->hasPHIKill) return;
    
    VNsToCopy.push_back(OldVN);
    
    // Locate two-address redefinitions
    for (SmallVector<unsigned, 4>::iterator KI = OldVN->kills.begin(),
         KE = OldVN->kills.end(); KI != KE; ++KI) {
      MachineInstr* MI = LIs->getInstructionFromIndex(*KI);
      unsigned DefIdx = MI->findRegisterDefOperandIdx(CurrLI->reg);
      if (DefIdx == ~0U) continue;
      if (MI->isRegReDefinedByTwoAddr(DefIdx)) {
        VNInfo* NextVN =
                     CurrLI->findDefinedVNInfo(LiveIntervals::getDefIndex(*KI));
        if (NextVN == OldVN) continue;
        Stack.push_back(NextVN);
      }
    }
  }
  
  // Create the new vreg
  unsigned NewVReg = MRI->createVirtualRegister(MRI->getRegClass(CurrLI->reg));
  
  // Create the new live interval
  LiveInterval& NewLI = LIs->getOrCreateInterval(NewVReg);
  
  for (SmallVector<VNInfo*, 4>::iterator OI = VNsToCopy.begin(), OE = 
       VNsToCopy.end(); OI != OE; ++OI) {
    VNInfo* OldVN = *OI;
    
    // Copy the valno over
    VNInfo* NewVN = NewLI.getNextValue(OldVN->def, OldVN->copy, 
                                       LIs->getVNInfoAllocator());
    NewLI.copyValNumInfo(NewVN, OldVN);
    NewLI.MergeValueInAsValue(*CurrLI, OldVN, NewVN);

    // Remove the valno from the old interval
    CurrLI->removeValNo(OldVN);
  }
  
  // Rewrite defs and uses.  This is done in two stages to avoid invalidating
  // the reg_iterator.
  SmallVector<std::pair<MachineInstr*, unsigned>, 8> OpsToChange;
  
  for (MachineRegisterInfo::reg_iterator I = MRI->reg_begin(CurrLI->reg),
         E = MRI->reg_end(); I != E; ++I) {
    MachineOperand& MO = I.getOperand();
    unsigned InstrIdx = LIs->getInstructionIndex(&*I);
    
    if ((MO.isUse() && NewLI.liveAt(LiveIntervals::getUseIndex(InstrIdx))) ||
        (MO.isDef() && NewLI.liveAt(LiveIntervals::getDefIndex(InstrIdx))))
      OpsToChange.push_back(std::make_pair(&*I, I.getOperandNo()));
  }
  
  for (SmallVector<std::pair<MachineInstr*, unsigned>, 8>::iterator I =
       OpsToChange.begin(), E = OpsToChange.end(); I != E; ++I) {
    MachineInstr* Inst = I->first;
    unsigned OpIdx = I->second;
    MachineOperand& MO = Inst->getOperand(OpIdx);
    MO.setReg(NewVReg);
  }
  
  // The renumbered vreg shares a stack slot with the old register.
  if (IntervalSSMap.count(CurrLI->reg))
    IntervalSSMap[NewVReg] = IntervalSSMap[CurrLI->reg];
  
  NumRenumbers++;
}

bool PreAllocSplitting::Rematerialize(unsigned vreg, VNInfo* ValNo,
                                      MachineInstr* DefMI,
                                      MachineBasicBlock::iterator RestorePt,
                                      unsigned RestoreIdx,
                                    SmallPtrSet<MachineInstr*, 4>& RefsInMBB) {
  MachineBasicBlock& MBB = *RestorePt->getParent();
  
  MachineBasicBlock::iterator KillPt = BarrierMBB->end();
  unsigned KillIdx = 0;
  if (ValNo->def == ~0U || DefMI->getParent() == BarrierMBB)
    KillPt = findSpillPoint(BarrierMBB, Barrier, NULL, RefsInMBB, KillIdx);
  else
    KillPt = findNextEmptySlot(DefMI->getParent(), DefMI, KillIdx);
  
  if (KillPt == DefMI->getParent()->end())
    return false;
  
  TII->reMaterialize(MBB, RestorePt, vreg, DefMI);
  LIs->InsertMachineInstrInMaps(prior(RestorePt), RestoreIdx);
  
  ReconstructLiveInterval(CurrLI);
  unsigned RematIdx = LIs->getInstructionIndex(prior(RestorePt));
  RematIdx = LiveIntervals::getDefIndex(RematIdx);
  RenumberValno(CurrLI->findDefinedVNInfo(RematIdx));
  
  ++NumSplits;
  ++NumRemats;
  return true;  
}

MachineInstr* PreAllocSplitting::FoldSpill(unsigned vreg, 
                                           const TargetRegisterClass* RC,
                                           MachineInstr* DefMI,
                                           MachineInstr* Barrier,
                                           MachineBasicBlock* MBB,
                                           int& SS,
                                    SmallPtrSet<MachineInstr*, 4>& RefsInMBB) {
  MachineBasicBlock::iterator Pt = MBB->begin();

  // Go top down if RefsInMBB is empty.
  if (RefsInMBB.empty())
    return 0;
  
  MachineBasicBlock::iterator FoldPt = Barrier;
  while (&*FoldPt != DefMI && FoldPt != MBB->begin() &&
         !RefsInMBB.count(FoldPt))
    --FoldPt;
  
  int OpIdx = FoldPt->findRegisterDefOperandIdx(vreg, false);
  if (OpIdx == -1)
    return 0;
  
  SmallVector<unsigned, 1> Ops;
  Ops.push_back(OpIdx);
  
  if (!TII->canFoldMemoryOperand(FoldPt, Ops))
    return 0;
  
  DenseMap<unsigned, int>::iterator I = IntervalSSMap.find(vreg);
  if (I != IntervalSSMap.end()) {
    SS = I->second;
  } else {
    SS = MFI->CreateStackObject(RC->getSize(), RC->getAlignment());
    
  }
  
  MachineInstr* FMI = TII->foldMemoryOperand(*MBB->getParent(),
                                             FoldPt, Ops, SS);
  
  if (FMI) {
    LIs->ReplaceMachineInstrInMaps(FoldPt, FMI);
    FMI = MBB->insert(MBB->erase(FoldPt), FMI);
    ++NumFolds;
    
    IntervalSSMap[vreg] = SS;
    CurrSLI = &LSs->getOrCreateInterval(SS);
    if (CurrSLI->hasAtLeastOneValue())
      CurrSValNo = CurrSLI->getValNumInfo(0);
    else
      CurrSValNo = CurrSLI->getNextValue(~0U, 0, LSs->getVNInfoAllocator());
  }
  
  return FMI;
}

/// SplitRegLiveInterval - Split (spill and restore) the given live interval
/// so it would not cross the barrier that's being processed. Shrink wrap
/// (minimize) the live interval to the last uses.
bool PreAllocSplitting::SplitRegLiveInterval(LiveInterval *LI) {
  CurrLI = LI;

  // Find live range where current interval cross the barrier.
  LiveInterval::iterator LR =
    CurrLI->FindLiveRangeContaining(LIs->getUseIndex(BarrierIdx));
  VNInfo *ValNo = LR->valno;

  if (ValNo->def == ~1U) {
    // Defined by a dead def? How can this be?
    assert(0 && "Val# is defined by a dead def?");
    abort();
  }

  MachineInstr *DefMI = (ValNo->def != ~0U)
    ? LIs->getInstructionFromIndex(ValNo->def) : NULL;

  // If this would create a new join point, do not split.
  if (DefMI && createsNewJoin(LR, DefMI->getParent(), Barrier->getParent()))
    return false;

  // Find all references in the barrier mbb.
  SmallPtrSet<MachineInstr*, 4> RefsInMBB;
  for (MachineRegisterInfo::reg_iterator I = MRI->reg_begin(CurrLI->reg),
         E = MRI->reg_end(); I != E; ++I) {
    MachineInstr *RefMI = &*I;
    if (RefMI->getParent() == BarrierMBB)
      RefsInMBB.insert(RefMI);
  }

  // Find a point to restore the value after the barrier.
  unsigned RestoreIndex = 0;
  MachineBasicBlock::iterator RestorePt =
    findRestorePoint(BarrierMBB, Barrier, LR->end, RefsInMBB, RestoreIndex);
  if (RestorePt == BarrierMBB->end())
    return false;

  if (DefMI && LIs->isReMaterializable(*LI, ValNo, DefMI))
    if (Rematerialize(LI->reg, ValNo, DefMI, RestorePt,
                      RestoreIndex, RefsInMBB))
    return true;

  // Add a spill either before the barrier or after the definition.
  MachineBasicBlock *DefMBB = DefMI ? DefMI->getParent() : NULL;
  const TargetRegisterClass *RC = MRI->getRegClass(CurrLI->reg);
  unsigned SpillIndex = 0;
  MachineInstr *SpillMI = NULL;
  int SS = -1;
  if (ValNo->def == ~0U) {
    // If it's defined by a phi, we must split just before the barrier.
    if ((SpillMI = FoldSpill(LI->reg, RC, 0, Barrier,
                            BarrierMBB, SS, RefsInMBB))) {
      SpillIndex = LIs->getInstructionIndex(SpillMI);
    } else {
      MachineBasicBlock::iterator SpillPt = 
        findSpillPoint(BarrierMBB, Barrier, NULL, RefsInMBB, SpillIndex);
      if (SpillPt == BarrierMBB->begin())
        return false; // No gap to insert spill.
      // Add spill.
    
      SS = CreateSpillStackSlot(CurrLI->reg, RC);
      TII->storeRegToStackSlot(*BarrierMBB, SpillPt, CurrLI->reg, true, SS, RC);
      SpillMI = prior(SpillPt);
      LIs->InsertMachineInstrInMaps(SpillMI, SpillIndex);
    }
  } else if (!IsAvailableInStack(DefMBB, CurrLI->reg, ValNo->def,
                                 RestoreIndex, SpillIndex, SS)) {
    // If it's already split, just restore the value. There is no need to spill
    // the def again.
    if (!DefMI)
      return false; // Def is dead. Do nothing.
    
    if ((SpillMI = FoldSpill(LI->reg, RC, DefMI, Barrier,
                            BarrierMBB, SS, RefsInMBB))) {
      SpillIndex = LIs->getInstructionIndex(SpillMI);
    } else {
      // Check if it's possible to insert a spill after the def MI.
      MachineBasicBlock::iterator SpillPt;
      if (DefMBB == BarrierMBB) {
        // Add spill after the def and the last use before the barrier.
        SpillPt = findSpillPoint(BarrierMBB, Barrier, DefMI,
                                 RefsInMBB, SpillIndex);
        if (SpillPt == DefMBB->begin())
          return false; // No gap to insert spill.
      } else {
        SpillPt = findNextEmptySlot(DefMBB, DefMI, SpillIndex);
        if (SpillPt == DefMBB->end())
          return false; // No gap to insert spill.
      }
      // Add spill. The store instruction kills the register if def is before
      // the barrier in the barrier block.
      SS = CreateSpillStackSlot(CurrLI->reg, RC);
      TII->storeRegToStackSlot(*DefMBB, SpillPt, CurrLI->reg,
                               DefMBB == BarrierMBB, SS, RC);