lib/CodeGen/MachineCSE.cpp

//===-- MachineCSE.cpp - Machine Common Subexpression Elimination Pass ----===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs global common subexpression elimination on machine
// instructions using a scoped hash table based value numbering scheme. It
// must be run while the machine function is still in SSA form.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "machine-cse"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/RecyclingAllocator.h"
using namespace llvm;

STATISTIC(NumCoalesces, "Number of copies coalesced");
STATISTIC(NumCSEs,      "Number of common subexpression eliminated");
STATISTIC(NumPhysCSEs,
          "Number of physreg referencing common subexpr eliminated");
STATISTIC(NumCrossBBCSEs,
          "Number of cross-MBB physreg referencing CS eliminated");
STATISTIC(NumCommutes,  "Number of copies coalesced after commuting");

namespace {
  class MachineCSE : public MachineFunctionPass {
    const TargetInstrInfo *TII;
    const TargetRegisterInfo *TRI;
    AliasAnalysis *AA;
    MachineDominatorTree *DT;
    MachineRegisterInfo *MRI;
  public:
    static char ID; // Pass identification
    MachineCSE() : MachineFunctionPass(ID), LookAheadLimit(5), CurrVN(0) {
      initializeMachineCSEPass(*PassRegistry::getPassRegistry());
    }

    virtual bool runOnMachineFunction(MachineFunction &MF);

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
      MachineFunctionPass::getAnalysisUsage(AU);
      AU.addRequired<AliasAnalysis>();
      AU.addPreservedID(MachineLoopInfoID);
      AU.addRequired<MachineDominatorTree>();
      AU.addPreserved<MachineDominatorTree>();
    }

    virtual void releaseMemory() {
      ScopeMap.clear();
      Exps.clear();
      AllocatableRegs.clear();
      ReservedRegs.clear();
    }

  private:
    const unsigned LookAheadLimit;
    typedef RecyclingAllocator<BumpPtrAllocator,
        ScopedHashTableVal<MachineInstr*, unsigned> > AllocatorTy;
    typedef ScopedHashTable<MachineInstr*, unsigned,
        MachineInstrExpressionTrait, AllocatorTy> ScopedHTType;
    typedef ScopedHTType::ScopeTy ScopeType;
    DenseMap<MachineBasicBlock*, ScopeType*> ScopeMap;
    ScopedHTType VNT;
    SmallVector<MachineInstr*, 64> Exps;
    unsigned CurrVN;
    BitVector AllocatableRegs;
    BitVector ReservedRegs;

    bool PerformTrivialCoalescing(MachineInstr *MI, MachineBasicBlock *MBB);
    bool isPhysDefTriviallyDead(unsigned Reg,
                                MachineBasicBlock::const_iterator I,
                                MachineBasicBlock::const_iterator E) const;
    bool hasLivePhysRegDefUses(const MachineInstr *MI,
                               const MachineBasicBlock *MBB,
                               SmallSet<unsigned,8> &PhysRefs,
                               SmallVector<unsigned,2> &PhysDefs) const;
    bool PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
                          SmallSet<unsigned,8> &PhysRefs,
                          SmallVector<unsigned,2> &PhysDefs,
                          bool &NonLocal) const;
    bool isCSECandidate(MachineInstr *MI);
    bool isProfitableToCSE(unsigned CSReg, unsigned Reg,
                           MachineInstr *CSMI, MachineInstr *MI);
    void EnterScope(MachineBasicBlock *MBB);
    void ExitScope(MachineBasicBlock *MBB);
    bool ProcessBlock(MachineBasicBlock *MBB);
    void ExitScopeIfDone(MachineDomTreeNode *Node,
                         DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren);
    bool PerformCSE(MachineDomTreeNode *Node);
  };
} // end anonymous namespace

char MachineCSE::ID = 0;
char &llvm::MachineCSEID = MachineCSE::ID;
INITIALIZE_PASS_BEGIN(MachineCSE, "machine-cse",
                "Machine Common Subexpression Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(MachineCSE, "machine-cse",
                "Machine Common Subexpression Elimination", false, false)

bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
                                          MachineBasicBlock *MBB) {
  bool Changed = false;
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI->getOperand(i);
    if (!MO.isReg() || !MO.isUse())
      continue;
    unsigned Reg = MO.getReg();
    if (!TargetRegisterInfo::isVirtualRegister(Reg))
      continue;
    if (!MRI->hasOneNonDBGUse(Reg))
      // Only coalesce single use copies. This ensure the copy will be
      // deleted.
      continue;
    MachineInstr *DefMI = MRI->getVRegDef(Reg);
    if (DefMI->getParent() != MBB)
      continue;
    if (!DefMI->isCopy())
      continue;
    unsigned SrcReg = DefMI->getOperand(1).getReg();
    if (!TargetRegisterInfo::isVirtualRegister(SrcReg))
      continue;
    if (DefMI->getOperand(0).getSubReg() || DefMI->getOperand(1).getSubReg())
      continue;
    if (!MRI->constrainRegClass(SrcReg, MRI->getRegClass(Reg)))
      continue;
    DEBUG(dbgs() << "Coalescing: " << *DefMI);
    DEBUG(dbgs() << "***     to: " << *MI);
    MO.setReg(SrcReg);
    MRI->clearKillFlags(SrcReg);
    DefMI->eraseFromParent();
    ++NumCoalesces;
    Changed = true;
  }

  return Changed;
}

bool
MachineCSE::isPhysDefTriviallyDead(unsigned Reg,
                                   MachineBasicBlock::const_iterator I,
                                   MachineBasicBlock::const_iterator E) const {
  unsigned LookAheadLeft = LookAheadLimit;
  while (LookAheadLeft) {
    // Skip over dbg_value's.
    while (I != E && I->isDebugValue())
      ++I;

    if (I == E)
      // Reached end of block, register is obviously dead.
      return true;

    bool SeenDef = false;
    for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
      const MachineOperand &MO = I->getOperand(i);
      if (MO.isRegMask() && MO.clobbersPhysReg(Reg))
        SeenDef = true;
      if (!MO.isReg() || !MO.getReg())
        continue;
      if (!TRI->regsOverlap(MO.getReg(), Reg))
        continue;
      if (MO.isUse())
        // Found a use!
        return false;
      SeenDef = true;
    }
    if (SeenDef)
      // See a def of Reg (or an alias) before encountering any use, it's
      // trivially dead.
      return true;

    --LookAheadLeft;
    ++I;
  }
  return false;
}

/// hasLivePhysRegDefUses - Return true if the specified instruction read/write
/// physical registers (except for dead defs of physical registers). It also
/// returns the physical register def by reference if it's the only one and the
/// instruction does not uses a physical register.
bool MachineCSE::hasLivePhysRegDefUses(const MachineInstr *MI,
                                       const MachineBasicBlock *MBB,
                                       SmallSet<unsigned,8> &PhysRefs,
                                       SmallVector<unsigned,2> &PhysDefs) const{
  MachineBasicBlock::const_iterator I = MI; I = llvm::next(I);
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    const MachineOperand &MO = MI->getOperand(i);
    if (!MO.isReg())
      continue;
    unsigned Reg = MO.getReg();
    if (!Reg)
      continue;
    if (TargetRegisterInfo::isVirtualRegister(Reg))
      continue;
    // If the def is dead, it's ok. But the def may not marked "dead". That's
    // common since this pass is run before livevariables. We can scan
    // forward a few instructions and check if it is obviously dead.
    if (MO.isDef() &&
        (MO.isDead() || isPhysDefTriviallyDead(Reg, I, MBB->end())))
      continue;
    for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
      PhysRefs.insert(*AI);
    if (MO.isDef())
      PhysDefs.push_back(Reg);
  }

  return !PhysRefs.empty();
}

bool MachineCSE::PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
                                  SmallSet<unsigned,8> &PhysRefs,
                                  SmallVector<unsigned,2> &PhysDefs,
                                  bool &NonLocal) const {
  // For now conservatively returns false if the common subexpression is
  // not in the same basic block as the given instruction. The only exception
  // is if the common subexpression is in the sole predecessor block.
  const MachineBasicBlock *MBB = MI->getParent();
  const MachineBasicBlock *CSMBB = CSMI->getParent();

  bool CrossMBB = false;
  if (CSMBB != MBB) {
    if (MBB->pred_size() != 1 || *MBB->pred_begin() != CSMBB)
      return false;

    for (unsigned i = 0, e = PhysDefs.size(); i != e; ++i) {
      if (AllocatableRegs.test(PhysDefs[i]) || ReservedRegs.test(PhysDefs[i]))
        // Avoid extending live range of physical registers if they are
        //allocatable or reserved.
        return false;
    }
    CrossMBB = true;
  }
  MachineBasicBlock::const_iterator I = CSMI; I = llvm::next(I);
  MachineBasicBlock::const_iterator E = MI;
  MachineBasicBlock::const_iterator EE = CSMBB->end();
  unsigned LookAheadLeft = LookAheadLimit;
  while (LookAheadLeft) {
    // Skip over dbg_value's.
    while (I != E && I != EE && I->isDebugValue())
      ++I;

    if (I == EE) {
      assert(CrossMBB && "Reaching end-of-MBB without finding MI?");
      (void)CrossMBB;
      CrossMBB = false;
      NonLocal = true;
      I = MBB->begin();
      EE = MBB->end();
      continue;
    }

    if (I == E)
      return true;

    for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
      const MachineOperand &MO = I->getOperand(i);
      // RegMasks go on instructions like calls that clobber lots of physregs.
      // Don't attempt to CSE across such an instruction.
      if (MO.isRegMask())
        return false;
      if (!MO.isReg() || !MO.isDef())
        continue;
      unsigned MOReg = MO.getReg();
      if (TargetRegisterInfo::isVirtualRegister(MOReg))
        continue;
      if (PhysRefs.count(MOReg))
        return false;
    }

    --LookAheadLeft;
    ++I;
  }

  return false;
}

bool MachineCSE::isCSECandidate(MachineInstr *MI) {
  if (MI->isLabel() || MI->isPHI() || MI->isImplicitDef() ||
      MI->isKill() || MI->isInlineAsm() || MI->isDebugValue())
    return false;

  // Ignore copies.
  if (MI->isCopyLike())
    return false;

  // Ignore stuff that we obviously can't move.
  if (MI->mayStore() || MI->isCall() || MI->isTerminator() ||
      MI->hasUnmodeledSideEffects())
    return false;

  if (MI->mayLoad()) {
    // Okay, this instruction does a load. As a refinement, we allow the target
    // to decide whether the loaded value is actually a constant. If so, we can
    // actually use it as a load.
    if (!MI->isInvariantLoad(AA))
      // FIXME: we should be able to hoist loads with no other side effects if
      // there are no other instructions which can change memory in this loop.
      // This is a trivial form of alias analysis.
      return false;
  }
  return true;
}

/// isProfitableToCSE - Return true if it's profitable to eliminate MI with a
/// common expression that defines Reg.
bool MachineCSE::isProfitableToCSE(unsigned CSReg, unsigned Reg,
                                   MachineInstr *CSMI, MachineInstr *MI) {
  // FIXME: Heuristics that works around the lack the live range splitting.

  // If CSReg is used at all uses of Reg, CSE should not increase register
  // pressure of CSReg.
  bool MayIncreasePressure = true;
  if (TargetRegisterInfo::isVirtualRegister(CSReg) &&
      TargetRegisterInfo::isVirtualRegister(Reg)) {
    MayIncreasePressure = false;
    SmallPtrSet<MachineInstr*, 8> CSUses;
    for (MachineRegisterInfo::use_nodbg_iterator I =MRI->use_nodbg_begin(CSReg),
         E = MRI->use_nodbg_end(); I != E; ++I) {
      MachineInstr *Use = &*I;
      CSUses.insert(Use);
    }
    for (MachineRegisterInfo::use_nodbg_iterator I = MRI->use_nodbg_begin(Reg),
         E = MRI->use_nodbg_end(); I != E; ++I) {
      MachineInstr *Use = &*I;
      if (!CSUses.count(Use)) {
        MayIncreasePressure = true;
        break;
      }
    }
  }
  if (!MayIncreasePressure) return true;

  // Heuristics #1: Don't CSE "cheap" computation if the def is not local or in
  // an immediate predecessor. We don't want to increase register pressure and
  // end up causing other computation to be spilled.
  if (MI->isAsCheapAsAMove()) {
    MachineBasicBlock *CSBB = CSMI->getParent();
    MachineBasicBlock *BB = MI->getParent();
    if (CSBB != BB && !CSBB->isSuccessor(BB))
      return false;
  }

  // Heuristics #2: If the expression doesn't not use a vr and the only use
  // of the redundant computation are copies, do not cse.
  bool HasVRegUse = false;
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    const MachineOperand &MO = MI->getOperand(i);
    if (MO.isReg() && MO.isUse() &&
        TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
      HasVRegUse = true;
      break;
    }
  }
  if (!HasVRegUse) {
    bool HasNonCopyUse = false;
    for (MachineRegisterInfo::use_nodbg_iterator I =  MRI->use_nodbg_begin(Reg),
           E = MRI->use_nodbg_end(); I != E; ++I) {
      MachineInstr *Use = &*I;
      // Ignore copies.
      if (!Use->isCopyLike()) {
        HasNonCopyUse = true;
        break;
      }
    }
    if (!HasNonCopyUse)
      return false;
  }

  // Heuristics #3: If the common subexpression is used by PHIs, do not reuse
  // it unless the defined value is already used in the BB of the new use.
  bool HasPHI = false;
  SmallPtrSet<MachineBasicBlock*, 4> CSBBs;
  for (MachineRegisterInfo::use_nodbg_iterator I =  MRI->use_nodbg_begin(CSReg),
       E = MRI->use_nodbg_end(); I != E; ++I) {
    MachineInstr *Use = &*I;
    HasPHI |= Use->isPHI();
    CSBBs.insert(Use->getParent());
  }

  if (!HasPHI)
    return true;
  return CSBBs.count(MI->getParent());
}

void MachineCSE::EnterScope(MachineBasicBlock *MBB) {
  DEBUG(dbgs() << "Entering: " << MBB->getName() << '\n');
  ScopeType *Scope = new ScopeType(VNT);
  ScopeMap[MBB] = Scope;
}

void MachineCSE::ExitScope(MachineBasicBlock *MBB) {
  DEBUG(dbgs() << "Exiting: " << MBB->getName() << '\n');
  DenseMap<MachineBasicBlock*, ScopeType*>::iterator SI = ScopeMap.find(MBB);
  assert(SI != ScopeMap.end());
  ScopeMap.erase(SI);
  delete SI->second;
}

bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
  bool Changed = false;

  SmallVector<std::pair<unsigned, unsigned>, 8> CSEPairs;
  SmallVector<unsigned, 2> ImplicitDefsToUpdate;
  for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ) {
    MachineInstr *MI = &*I;
    ++I;

    if (!isCSECandidate(MI))
      continue;

    bool FoundCSE = VNT.count(MI);
    if (!FoundCSE) {
      // Look for trivial copy coalescing opportunities.
      if (PerformTrivialCoalescing(MI, MBB)) {
        Changed = true;

        // After coalescing MI itself may become a copy.
        if (MI->isCopyLike())
          continue;
        FoundCSE = VNT.count(MI);
      }
    }

    // Commute commutable instructions.
    bool Commuted = false;
    if (!FoundCSE && MI->isCommutable()) {
      MachineInstr *NewMI = TII->commuteInstruction(MI);
      if (NewMI) {
        Commuted = true;
        FoundCSE = VNT.count(NewMI);
        if (NewMI != MI) {
          // New instruction. It doesn't need to be kept.
          NewMI->eraseFromParent();
          Changed = true;
        } else if (!FoundCSE)
          // MI was changed but it didn't help, commute it back!
          (void)TII->commuteInstruction(MI);
      }
    }

    // If the instruction defines physical registers and the values *may* be
    // used, then it's not safe to replace it with a common subexpression.
    // It's also not safe if the instruction uses physical registers.
    bool CrossMBBPhysDef = false;
    SmallSet<unsigned, 8> PhysRefs;
    SmallVector<unsigned, 2> PhysDefs;
    if (FoundCSE && hasLivePhysRegDefUses(MI, MBB, PhysRefs, PhysDefs)) {
      FoundCSE = false;

      // ... Unless the CS is local or is in the sole predecessor block
      // and it also defines the physical register which is not clobbered
      // in between and the physical register uses were not clobbered.
      unsigned CSVN = VNT.lookup(MI);
      MachineInstr *CSMI = Exps[CSVN];
      if (PhysRegDefsReach(CSMI, MI, PhysRefs, PhysDefs, CrossMBBPhysDef))
        FoundCSE = true;
    }

    if (!FoundCSE) {
      VNT.insert(MI, CurrVN++);
      Exps.push_back(MI);
      continue;
    }

    // Found a common subexpression, eliminate it.
    unsigned CSVN = VNT.lookup(MI);
    MachineInstr *CSMI = Exps[CSVN];
    DEBUG(dbgs() << "Examining: " << *MI);
    DEBUG(dbgs() << "*** Found a common subexpression: " << *CSMI);

    // Check if it's profitable to perform this CSE.
    bool DoCSE = true;
    unsigned NumDefs = MI->getDesc().getNumDefs() +
                       MI->getDesc().getNumImplicitDefs();
    
    for (unsigned i = 0, e = MI->getNumOperands(); NumDefs && i != e; ++i) {
      MachineOperand &MO = MI->getOperand(i);
      if (!MO.isReg() || !MO.isDef())
        continue;
      unsigned OldReg = MO.getReg();
      unsigned NewReg = CSMI->getOperand(i).getReg();

      // Go through implicit defs of CSMI and MI, if a def is not dead at MI,
      // we should make sure it is not dead at CSMI.
      if (MO.isImplicit() && !MO.isDead() && CSMI->getOperand(i).isDead())
        ImplicitDefsToUpdate.push_back(i);
      if (OldReg == NewReg) {
        --NumDefs;
        continue;
      }

      assert(TargetRegisterInfo::isVirtualRegister(OldReg) &&
             TargetRegisterInfo::isVirtualRegister(NewReg) &&
             "Do not CSE physical register defs!");

      if (!isProfitableToCSE(NewReg, OldReg, CSMI, MI)) {
        DEBUG(dbgs() << "*** Not profitable, avoid CSE!\n");
        DoCSE = false;
        break;
      }

      // Don't perform CSE if the result of the old instruction cannot exist
      // within the register class of the new instruction.
      const TargetRegisterClass *OldRC = MRI->getRegClass(OldReg);
      if (!MRI->constrainRegClass(NewReg, OldRC)) {
        DEBUG(dbgs() << "*** Not the same register class, avoid CSE!\n");
        DoCSE = false;
        break;
      }

      CSEPairs.push_back(std::make_pair(OldReg, NewReg));
      --NumDefs;
    }

    // Actually perform the elimination.
    if (DoCSE) {
      for (unsigned i = 0, e = CSEPairs.size(); i != e; ++i) {
        MRI->replaceRegWith(CSEPairs[i].first, CSEPairs[i].second);
        MRI->clearKillFlags(CSEPairs[i].second);
      }

      // Go through implicit defs of CSMI and MI, if a def is not dead at MI,
      // we should make sure it is not dead at CSMI.
      for (unsigned i = 0, e = ImplicitDefsToUpdate.size(); i != e; ++i)
        CSMI->getOperand(ImplicitDefsToUpdate[i]).setIsDead(false);

      if (CrossMBBPhysDef) {
        // Add physical register defs now coming in from a predecessor to MBB
        // livein list.
        while (!PhysDefs.empty()) {
          unsigned LiveIn = PhysDefs.pop_back_val();
          if (!MBB->isLiveIn(LiveIn))
            MBB->addLiveIn(LiveIn);
        }
        ++NumCrossBBCSEs;
      }

      MI->eraseFromParent();
      ++NumCSEs;
      if (!PhysRefs.empty())
        ++NumPhysCSEs;
      if (Commuted)
        ++NumCommutes;
      Changed = true;
    } else {
      VNT.insert(MI, CurrVN++);
      Exps.push_back(MI);
    }
    CSEPairs.clear();
    ImplicitDefsToUpdate.clear();
  }

  return Changed;
}

/// ExitScopeIfDone - Destroy scope for the MBB that corresponds to the given
/// dominator tree node if its a leaf or all of its children are done. Walk
/// up the dominator tree to destroy ancestors which are now done.
void
MachineCSE::ExitScopeIfDone(MachineDomTreeNode *Node,
                        DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren) {
  if (OpenChildren[Node])
    return;

  // Pop scope.
  ExitScope(Node->getBlock());

  // Now traverse upwards to pop ancestors whose offsprings are all done.
  while (MachineDomTreeNode *Parent = Node->getIDom()) {
    unsigned Left = --OpenChildren[Parent];
    if (Left != 0)
      break;
    ExitScope(Parent->getBlock());
    Node = Parent;
  }
}

bool MachineCSE::PerformCSE(MachineDomTreeNode *Node) {
  SmallVector<MachineDomTreeNode*, 32> Scopes;
  SmallVector<MachineDomTreeNode*, 8> WorkList;
  DenseMap<MachineDomTreeNode*, unsigned> OpenChildren;

  CurrVN = 0;

  // Perform a DFS walk to determine the order of visit.
  WorkList.push_back(Node);
  do {
    Node = WorkList.pop_back_val();
    Scopes.push_back(Node);
    const std::vector<MachineDomTreeNode*> &Children = Node->getChildren();
    unsigned NumChildren = Children.size();
    OpenChildren[Node] = NumChildren;
    for (unsigned i = 0; i != NumChildren; ++i) {
      MachineDomTreeNode *Child = Children[i];
      WorkList.push_back(Child);
    }
  } while (!WorkList.empty());

  // Now perform CSE.
  bool Changed = false;
  for (unsigned i = 0, e = Scopes.size(); i != e; ++i) {
    MachineDomTreeNode *Node = Scopes[i];
    MachineBasicBlock *MBB = Node->getBlock();
    EnterScope(MBB);
    Changed |= ProcessBlock(MBB);
    // If it's a leaf node, it's done. Traverse upwards to pop ancestors.
    ExitScopeIfDone(Node, OpenChildren);
  }

  return Changed;
}

bool MachineCSE::runOnMachineFunction(MachineFunction &MF) {
  TII = MF.getTarget().getInstrInfo();
  TRI = MF.getTarget().getRegisterInfo();
  MRI = &MF.getRegInfo();
  AA = &getAnalysis<AliasAnalysis>();
  DT = &getAnalysis<MachineDominatorTree>();
  AllocatableRegs = TRI->getAllocatableSet(MF);
  ReservedRegs = TRI->getReservedRegs(MF);
  return PerformCSE(DT->getRootNode());
}