lib/CodeGen/StackSlotColoring.cpp

//===-- StackSlotColoring.cpp - Stack slot coloring 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 stack slot coloring pass.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "stackcoloring"
#include "VirtRegMap.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/LiveStackAnalysis.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/PseudoSourceValue.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include <vector>
using namespace llvm;

static cl::opt<bool>
DisableSharing("no-stack-slot-sharing",
             cl::init(false), cl::Hidden,
             cl::desc("Suppress slot sharing during stack coloring"));

static cl::opt<bool>
ColorWithRegsOpt("color-ss-with-regs",
                 cl::init(false), cl::Hidden,
                 cl::desc("Color stack slots with free registers"));


static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);

STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
STATISTIC(NumRegRepl,    "Number of stack slot refs replaced with reg refs");
STATISTIC(NumLoadElim,   "Number of loads eliminated");
STATISTIC(NumStoreElim,  "Number of stores eliminated");
STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");

namespace {
  class VISIBILITY_HIDDEN StackSlotColoring : public MachineFunctionPass {
    bool ColorWithRegs;
    LiveStacks* LS;
    VirtRegMap* VRM;
    MachineFrameInfo *MFI;
    MachineRegisterInfo *MRI;
    const TargetInstrInfo  *TII;
    const TargetRegisterInfo *TRI;
    const MachineLoopInfo *loopInfo;

    // SSIntervals - Spill slot intervals.
    std::vector<LiveInterval*> SSIntervals;

    // SSRefs - Keep a list of frame index references for each spill slot.
    SmallVector<SmallVector<MachineInstr*, 8>, 16> SSRefs;

    // OrigAlignments - Alignments of stack objects before coloring.
    SmallVector<unsigned, 16> OrigAlignments;

    // OrigSizes - Sizess of stack objects before coloring.
    SmallVector<unsigned, 16> OrigSizes;

    // AllColors - If index is set, it's a spill slot, i.e. color.
    // FIXME: This assumes PEI locate spill slot with smaller indices
    // closest to stack pointer / frame pointer. Therefore, smaller
    // index == better color.
    BitVector AllColors;

    // NextColor - Next "color" that's not yet used.
    int NextColor;

    // UsedColors - "Colors" that have been assigned.
    BitVector UsedColors;

    // Assignments - Color to intervals mapping.
    SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;

  public:
    static char ID; // Pass identification
    StackSlotColoring() :
      MachineFunctionPass(&ID), ColorWithRegs(false), NextColor(-1) {}
    StackSlotColoring(bool RegColor) :
      MachineFunctionPass(&ID), ColorWithRegs(RegColor), NextColor(-1) {}

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<LiveStacks>();
      AU.addRequired<VirtRegMap>();
      AU.addPreserved<VirtRegMap>();
      AU.addRequired<MachineLoopInfo>();
      AU.addPreserved<MachineLoopInfo>();
      AU.addPreservedID(MachineDominatorsID);
      MachineFunctionPass::getAnalysisUsage(AU);
    }

    virtual bool runOnMachineFunction(MachineFunction &MF);
    virtual const char* getPassName() const {
      return "Stack Slot Coloring";
    }

  private:
    void InitializeSlots();
    void ScanForSpillSlotRefs(MachineFunction &MF);
    bool OverlapWithAssignments(LiveInterval *li, int Color) const;
    int ColorSlot(LiveInterval *li);
    bool ColorSlots(MachineFunction &MF);
    bool ColorSlotsWithFreeRegs(SmallVector<int, 16> &SlotMapping,
                                SmallVector<SmallVector<int, 4>, 16> &RevMap,
                                BitVector &SlotIsReg);
    void RewriteInstruction(MachineInstr *MI, int OldFI, int NewFI,
                            MachineFunction &MF);
    bool PropagateBackward(MachineBasicBlock::iterator MII,
                           MachineBasicBlock *MBB,
                           unsigned OldReg, unsigned NewReg);
    bool PropagateForward(MachineBasicBlock::iterator MII,
                          MachineBasicBlock *MBB,
                          unsigned OldReg, unsigned NewReg);
    void UnfoldAndRewriteInstruction(MachineInstr *MI, int OldFI,
                                    unsigned Reg, const TargetRegisterClass *RC,
                                    SmallSet<unsigned, 4> &Defs,
                                    MachineFunction &MF);
    bool AllMemRefsCanBeUnfolded(int SS);
    bool RemoveDeadStores(MachineBasicBlock* MBB);
  };
} // end anonymous namespace

char StackSlotColoring::ID = 0;

static RegisterPass<StackSlotColoring>
X("stack-slot-coloring", "Stack Slot Coloring");

FunctionPass *llvm::createStackSlotColoringPass(bool RegColor) {
  return new StackSlotColoring(RegColor);
}

namespace {
  // IntervalSorter - Comparison predicate that sort live intervals by
  // their weight.
  struct IntervalSorter {
    bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
      return LHS->weight > RHS->weight;
    }
  };
}

/// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
/// references and update spill slot weights.
void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
  SSRefs.resize(MFI->getObjectIndexEnd());

  // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
  for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
       MBBI != E; ++MBBI) {
    MachineBasicBlock *MBB = &*MBBI;
    unsigned loopDepth = loopInfo->getLoopDepth(MBB);
    for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
         MII != EE; ++MII) {
      MachineInstr *MI = &*MII;
      for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
        MachineOperand &MO = MI->getOperand(i);
        if (!MO.isFI())
          continue;
        int FI = MO.getIndex();
        if (FI < 0)
          continue;
        if (!LS->hasInterval(FI))
          continue;
        LiveInterval &li = LS->getInterval(FI);
        li.weight += LiveIntervals::getSpillWeight(false, true, loopDepth);
        SSRefs[FI].push_back(MI);
      }
    }
  }
}

/// InitializeSlots - Process all spill stack slot liveintervals and add them
/// to a sorted (by weight) list.
void StackSlotColoring::InitializeSlots() {
  int LastFI = MFI->getObjectIndexEnd();
  OrigAlignments.resize(LastFI);
  OrigSizes.resize(LastFI);
  AllColors.resize(LastFI);
  UsedColors.resize(LastFI);
  Assignments.resize(LastFI);

  // Gather all spill slots into a list.
  DOUT << "Spill slot intervals:\n";
  for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
    LiveInterval &li = i->second;
    DEBUG(li.dump());
    int FI = li.getStackSlotIndex();
    if (MFI->isDeadObjectIndex(FI))
      continue;
    SSIntervals.push_back(&li);
    OrigAlignments[FI] = MFI->getObjectAlignment(FI);
    OrigSizes[FI]      = MFI->getObjectSize(FI);
    AllColors.set(FI);
  }
  DOUT << '\n';

  // Sort them by weight.
  std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());

  // Get first "color".
  NextColor = AllColors.find_first();
}

/// OverlapWithAssignments - Return true if LiveInterval overlaps with any
/// LiveIntervals that have already been assigned to the specified color.
bool
StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
  const SmallVector<LiveInterval*,4> &OtherLIs = Assignments[Color];
  for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
    LiveInterval *OtherLI = OtherLIs[i];
    if (OtherLI->overlaps(*li))
      return true;
  }
  return false;
}

/// ColorSlotsWithFreeRegs - If there are any free registers available, try
/// replacing spill slots references with registers instead.
bool
StackSlotColoring::ColorSlotsWithFreeRegs(SmallVector<int, 16> &SlotMapping,
                                   SmallVector<SmallVector<int, 4>, 16> &RevMap,
                                   BitVector &SlotIsReg) {
  if (!(ColorWithRegs || ColorWithRegsOpt) || !VRM->HasUnusedRegisters())
    return false;

  bool Changed = false;
  DOUT << "Assigning unused registers to spill slots:\n";
  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
    LiveInterval *li = SSIntervals[i];
    int SS = li->getStackSlotIndex();
    if (!UsedColors[SS] || li->weight < 20)
      // If the weight is < 20, i.e. two references in a loop with depth 1,
      // don't bother with it.
      continue;

    // These slots allow to share the same registers.
    bool AllColored = true;
    SmallVector<unsigned, 4> ColoredRegs;
    for (unsigned j = 0, ee = RevMap[SS].size(); j != ee; ++j) {
      int RSS = RevMap[SS][j];
      const TargetRegisterClass *RC = LS->getIntervalRegClass(RSS);
      // If it's not colored to another stack slot, try coloring it
      // to a "free" register.
      if (!RC) {
        AllColored = false;
        continue;
      }
      unsigned Reg = VRM->getFirstUnusedRegister(RC);
      if (!Reg) {
        AllColored = false;
        continue;
      }
      if (!AllMemRefsCanBeUnfolded(RSS)) {
        AllColored = false;
        continue;
      } else {
        DOUT << "Assigning fi#" << RSS << " to " << TRI->getName(Reg) << '\n';
        ColoredRegs.push_back(Reg);
        SlotMapping[RSS] = Reg;
        SlotIsReg.set(RSS);
        Changed = true;
      }
    }

    // Register and its sub-registers are no longer free.
    while (!ColoredRegs.empty()) {
      unsigned Reg = ColoredRegs.back();
      ColoredRegs.pop_back();
      VRM->setRegisterUsed(Reg);
      // If reg is a callee-saved register, it will have to be spilled in
      // the prologue.
      MRI->setPhysRegUsed(Reg);
      for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
        VRM->setRegisterUsed(*AS);
        MRI->setPhysRegUsed(*AS);
      }
    }
    // This spill slot is dead after the rewrites
    if (AllColored) {
      MFI->RemoveStackObject(SS);
      ++NumEliminated;
    }
  }
  DOUT << '\n';

  return Changed;
}

/// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
///
int StackSlotColoring::ColorSlot(LiveInterval *li) {
  int Color = -1;
  bool Share = false;
  if (!DisableSharing) {
    // Check if it's possible to reuse any of the used colors.
    Color = UsedColors.find_first();
    while (Color != -1) {
      if (!OverlapWithAssignments(li, Color)) {
        Share = true;
        ++NumEliminated;
        break;
      }
      Color = UsedColors.find_next(Color);
    }
  }

  // Assign it to the first available color (assumed to be the best) if it's
  // not possible to share a used color with other objects.
  if (!Share) {
    assert(NextColor != -1 && "No more spill slots?");
    Color = NextColor;
    UsedColors.set(Color);
    NextColor = AllColors.find_next(NextColor);
  }

  // Record the assignment.
  Assignments[Color].push_back(li);
  int FI = li->getStackSlotIndex();
  DOUT << "Assigning fi#" << FI << " to fi#" << Color << "\n";

  // Change size and alignment of the allocated slot. If there are multiple
  // objects sharing the same slot, then make sure the size and alignment
  // are large enough for all.
  unsigned Align = OrigAlignments[FI];
  if (!Share || Align > MFI->getObjectAlignment(Color))
    MFI->setObjectAlignment(Color, Align);
  int64_t Size = OrigSizes[FI];
  if (!Share || Size > MFI->getObjectSize(Color))
    MFI->setObjectSize(Color, Size);
  return Color;
}

/// Colorslots - Color all spill stack slots and rewrite all frameindex machine
/// operands in the function.
bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
  unsigned NumObjs = MFI->getObjectIndexEnd();
  SmallVector<int, 16> SlotMapping(NumObjs, -1);
  SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
  SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
  BitVector SlotIsReg(NumObjs);
  BitVector UsedColors(NumObjs);

  DOUT << "Color spill slot intervals:\n";
  bool Changed = false;
  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
    LiveInterval *li = SSIntervals[i];
    int SS = li->getStackSlotIndex();
    int NewSS = ColorSlot(li);
    assert(NewSS >= 0 && "Stack coloring failed?");
    SlotMapping[SS] = NewSS;
    RevMap[NewSS].push_back(SS);
    SlotWeights[NewSS] += li->weight;
    UsedColors.set(NewSS);
    Changed |= (SS != NewSS);
  }

  DOUT << "\nSpill slots after coloring:\n";
  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
    LiveInterval *li = SSIntervals[i];
    int SS = li->getStackSlotIndex();
    li->weight = SlotWeights[SS];
  }
  // Sort them by new weight.
  std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());

#ifndef NDEBUG
  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
    DEBUG(SSIntervals[i]->dump());
  DOUT << '\n';
#endif

  // Can we "color" a stack slot with a unused register?
  Changed |= ColorSlotsWithFreeRegs(SlotMapping, RevMap, SlotIsReg);

  if (!Changed)
    return false;

  // Rewrite all MO_FrameIndex operands.
  SmallVector<SmallSet<unsigned, 4>, 4> NewDefs(MF.getNumBlockIDs());
  for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
    bool isReg = SlotIsReg[SS];
    int NewFI = SlotMapping[SS];
    if (NewFI == -1 || (NewFI == (int)SS && !isReg))
      continue;

    const TargetRegisterClass *RC = LS->getIntervalRegClass(SS);
    SmallVector<MachineInstr*, 8> &RefMIs = SSRefs[SS];
    for (unsigned i = 0, e = RefMIs.size(); i != e; ++i)
      if (!isReg)
        RewriteInstruction(RefMIs[i], SS, NewFI, MF);
      else {
        // Rewrite to use a register instead.
        unsigned MBBId = RefMIs[i]->getParent()->getNumber();
        SmallSet<unsigned, 4> &Defs = NewDefs[MBBId];
        UnfoldAndRewriteInstruction(RefMIs[i], SS, NewFI, RC, Defs, MF);
      }
  }

  // Delete unused stack slots.
  while (NextColor != -1) {
    DOUT << "Removing unused stack object fi#" << NextColor << "\n";
    MFI->RemoveStackObject(NextColor);
    NextColor = AllColors.find_next(NextColor);
  }

  return true;
}

/// AllMemRefsCanBeUnfolded - Return true if all references of the specified
/// spill slot index can be unfolded.
bool StackSlotColoring::AllMemRefsCanBeUnfolded(int SS) {
  SmallVector<MachineInstr*, 8> &RefMIs = SSRefs[SS];
  for (unsigned i = 0, e = RefMIs.size(); i != e; ++i) {
    MachineInstr *MI = RefMIs[i];
    if (TII->isLoadFromStackSlot(MI, SS) ||
        TII->isStoreToStackSlot(MI, SS))
      // Restore and spill will become copies.
      return true;
    if (!TII->getOpcodeAfterMemoryUnfold(MI->getOpcode(), false, false))
      return false;
    for (unsigned j = 0, ee = MI->getNumOperands(); j != ee; ++j) {
      MachineOperand &MO = MI->getOperand(j);
      if (MO.isFI() && MO.getIndex() != SS)
        // If it uses another frameindex, we can, currently* unfold it.
        return false;
    }
  }
  return true;
}

/// RewriteInstruction - Rewrite specified instruction by replacing references
/// to old frame index with new one.
void StackSlotColoring::RewriteInstruction(MachineInstr *MI, int OldFI,
                                           int NewFI, MachineFunction &MF) {
  for (unsigned i = 0, ee = MI->getNumOperands(); i != ee; ++i) {
    MachineOperand &MO = MI->getOperand(i);
    if (!MO.isFI())
      continue;
    int FI = MO.getIndex();
    if (FI != OldFI)
      continue;
    MO.setIndex(NewFI);
  }

  // Update the MachineMemOperand for the new memory location.
  // FIXME: We need a better method of managing these too.
  SmallVector<MachineMemOperand, 2> MMOs(MI->memoperands_begin(),
                                         MI->memoperands_end());
  MI->clearMemOperands(MF);
  const Value *OldSV = PseudoSourceValue::getFixedStack(OldFI);
  for (unsigned i = 0, ee = MMOs.size(); i != ee; ++i) {
    if (MMOs[i].getValue() != OldSV)
      MI->addMemOperand(MF, MMOs[i]);
    else {
      MachineMemOperand MMO(PseudoSourceValue::getFixedStack(NewFI),
                            MMOs[i].getFlags(), MMOs[i].getOffset(),
                            MMOs[i].getSize(),  MMOs[i].getAlignment());
      MI->addMemOperand(MF, MMO);
    }
  }
}

/// PropagateBackward - Traverse backward and look for the definition of
/// OldReg. If it can successfully update all of the references with NewReg,
/// do so and return true.
bool StackSlotColoring::PropagateBackward(MachineBasicBlock::iterator MII,
                                          MachineBasicBlock *MBB,
                                          unsigned OldReg, unsigned NewReg) {
  if (MII == MBB->begin())
    return false;

  SmallVector<MachineOperand*, 4> Uses;
  SmallVector<MachineOperand*, 4> Refs;
  while (--MII != MBB->begin()) {
    bool FoundDef = false;  // Not counting 2address def.

    Uses.clear();
    const TargetInstrDesc &TID = MII->getDesc();
    for (unsigned i = 0, e = MII->getNumOperands(); i != e; ++i) {
      MachineOperand &MO = MII->getOperand(i);
      if (!MO.isReg())
        continue;
      unsigned Reg = MO.getReg();
      if (Reg == 0)
        continue;
      if (Reg == OldReg) {
        if (MO.isImplicit())
          return false;
        const TargetRegisterClass *RC = getInstrOperandRegClass(TRI, TID, i);
        if (RC && !RC->contains(NewReg))
          return false;

        if (MO.isUse()) {
          Uses.push_back(&MO);
        } else {
          Refs.push_back(&MO);
          if (!MII->isRegTiedToUseOperand(i))
            FoundDef = true;
        }
      } else if (TRI->regsOverlap(Reg, NewReg)) {
        return false;
      } else if (TRI->regsOverlap(Reg, OldReg)) {
        if (!MO.isUse() || !MO.isKill())
          return false;
      }
    }

    if (FoundDef) {
      // Found non-two-address def. Stop here.
      for (unsigned i = 0, e = Refs.size(); i != e; ++i)
        Refs[i]->setReg(NewReg);
      return true;
    }

    // Two-address uses must be updated as well.
    for (unsigned i = 0, e = Uses.size(); i != e; ++i)
      Refs.push_back(Uses[i]);
  }
  return false;
}

/// PropagateForward - Traverse forward and look for the kill of OldReg. If
/// it can successfully update all of the uses with NewReg, do so and
/// return true.
bool StackSlotColoring::PropagateForward(MachineBasicBlock::iterator MII,
                                         MachineBasicBlock *MBB,
                                         unsigned OldReg, unsigned NewReg) {
  if (MII == MBB->end())
    return false;

  SmallVector<MachineOperand*, 4> Uses;
  while (++MII != MBB->end()) {
    bool FoundUse = false;
    bool FoundKill = false;
    const TargetInstrDesc &TID = MII->getDesc();
    for (unsigned i = 0, e = MII->getNumOperands(); i != e; ++i) {
      MachineOperand &MO = MII->getOperand(i);
      if (!MO.isReg())
        continue;
      unsigned Reg = MO.getReg();
      if (Reg == 0)
        continue;
      if (Reg == OldReg) {
        if (MO.isDef() || MO.isImplicit())
          return false;

        const TargetRegisterClass *RC = getInstrOperandRegClass(TRI, TID, i);
        if (RC && !RC->contains(NewReg))
          return false;
        FoundUse = true;
        if (MO.isKill())
          FoundKill = true;
        Uses.push_back(&MO);
      } else if (TRI->regsOverlap(Reg, NewReg) ||
                 TRI->regsOverlap(Reg, OldReg))
        return false;
    }
    if (FoundKill) {
      for (unsigned i = 0, e = Uses.size(); i != e; ++i)
        Uses[i]->setReg(NewReg);
      return true;
    }
  }
  return false;
}

/// UnfoldAndRewriteInstruction - Rewrite specified instruction by unfolding
/// folded memory references and replacing those references with register
/// references instead.
void
StackSlotColoring::UnfoldAndRewriteInstruction(MachineInstr *MI, int OldFI,
                                               unsigned Reg,
                                               const TargetRegisterClass *RC,
                                               SmallSet<unsigned, 4> &Defs,
                                               MachineFunction &MF) {
  MachineBasicBlock *MBB = MI->getParent();
  if (unsigned DstReg = TII->isLoadFromStackSlot(MI, OldFI)) {
    if (PropagateForward(MI, MBB, DstReg, Reg)) {
      DOUT << "Eliminated load: ";
      DEBUG(MI->dump());
      ++NumLoadElim;
    } else {
      TII->copyRegToReg(*MBB, MI, DstReg, Reg, RC, RC);
      ++NumRegRepl;
    }

    if (!Defs.count(Reg)) {
      // If this is the first use of Reg in this MBB and it wasn't previously
      // defined in MBB, add it to livein.
      MBB->addLiveIn(Reg);
      Defs.insert(Reg);
    }
  } else if (unsigned SrcReg = TII->isStoreToStackSlot(MI, OldFI)) {
    if (MI->killsRegister(SrcReg) && PropagateBackward(MI, MBB, SrcReg, Reg)) {
      DOUT << "Eliminated store: ";
      DEBUG(MI->dump());
      ++NumStoreElim;
    } else {
      TII->copyRegToReg(*MBB, MI, Reg, SrcReg, RC, RC);
      ++NumRegRepl;
    }

    // Remember reg has been defined in MBB.
    Defs.insert(Reg);
  } else {
    SmallVector<MachineInstr*, 4> NewMIs;
    bool Success = TII->unfoldMemoryOperand(MF, MI, Reg, false, false, NewMIs);
    Success = Success; // Silence compiler warning.
    assert(Success && "Failed to unfold!");
    MachineInstr *NewMI = NewMIs[0];
    MBB->insert(MI, NewMI);
    ++NumRegRepl;

    if (NewMI->readsRegister(Reg)) {
      if (!Defs.count(Reg))
        // If this is the first use of Reg in this MBB and it wasn't previously
        // defined in MBB, add it to livein.
        MBB->addLiveIn(Reg);
      Defs.insert(Reg);
    }
  }
  MBB->erase(MI);
}

/// RemoveDeadStores - Scan through a basic block and look for loads followed
/// by stores.  If they're both using the same stack slot, then the store is
/// definitely dead.  This could obviously be much more aggressive (consider
/// pairs with instructions between them), but such extensions might have a
/// considerable compile time impact.
bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
  // FIXME: This could be much more aggressive, but we need to investigate
  // the compile time impact of doing so.
  bool changed = false;

  SmallVector<MachineInstr*, 4> toErase;

  for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
       I != E; ++I) {
    if (DCELimit != -1 && (int)NumDead >= DCELimit)
      break;

    MachineBasicBlock::iterator NextMI = next(I);
    if (NextMI == MBB->end()) continue;

    int FirstSS, SecondSS;
    unsigned LoadReg = 0;
    unsigned StoreReg = 0;
    if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
    if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
    if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;

    ++NumDead;
    changed = true;

    if (NextMI->findRegisterUseOperandIdx(LoadReg, true, 0) != -1) {
      ++NumDead;
      toErase.push_back(I);
    }

    toErase.push_back(NextMI);
    ++I;
  }

  for (SmallVector<MachineInstr*, 4>::iterator I = toErase.begin(),
       E = toErase.end(); I != E; ++I)
    (*I)->eraseFromParent();

  return changed;
}


bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
  DOUT << "********** Stack Slot Coloring **********\n";

  MFI = MF.getFrameInfo();
  MRI = &MF.getRegInfo();
  TII = MF.getTarget().getInstrInfo();
  TRI = MF.getTarget().getRegisterInfo();
  LS = &getAnalysis<LiveStacks>();
  VRM = &getAnalysis<VirtRegMap>();
  loopInfo = &getAnalysis<MachineLoopInfo>();

  bool Changed = false;

  unsigned NumSlots = LS->getNumIntervals();
  if (NumSlots < 2) {
    if (NumSlots == 0 || !VRM->HasUnusedRegisters())
      // Nothing to do!
      return false;
  }

  // Gather spill slot references
  ScanForSpillSlotRefs(MF);
  InitializeSlots();
  Changed = ColorSlots(MF);

  NextColor = -1;
  SSIntervals.clear();
  for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
    SSRefs[i].clear();
  SSRefs.clear();
  OrigAlignments.clear();
  OrigSizes.clear();
  AllColors.clear();
  UsedColors.clear();
  for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
    Assignments[i].clear();
  Assignments.clear();

  if (Changed) {
    for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
      Changed |= RemoveDeadStores(I);
  }

  return Changed;
}