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//===- MachineCSE.cpp - Machine Common Subexpression Elimination Pass -----===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// 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.
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/RecyclingAllocator.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <iterator>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "machine-cse"
STATISTIC(NumCoalesces, "Number of copies coalesced");
STATISTIC(NumCSEs, "Number of common subexpression eliminated");
"Number of physreg referencing common subexpr eliminated");
"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;
static char ID; // Pass identification
MachineCSE() : MachineFunctionPass(ID) {
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
void releaseMemory() override {
using AllocatorTy = RecyclingAllocator<BumpPtrAllocator,
ScopedHashTableVal<MachineInstr *, unsigned>>;
using ScopedHTType =
ScopedHashTable<MachineInstr *, unsigned, MachineInstrExpressionTrait,
using ScopeType = ScopedHTType::ScopeTy;
unsigned LookAheadLimit = 0;
DenseMap<MachineBasicBlock *, ScopeType *> ScopeMap;
ScopedHTType VNT;
SmallVector<MachineInstr *, 64> Exps;
unsigned CurrVN = 0;
bool PerformTrivialCopyPropagation(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,
SmallVectorImpl<unsigned> &PhysDefs,
bool &PhysUseDef) const;
bool PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
SmallSet<unsigned,8> &PhysRefs,
SmallVectorImpl<unsigned> &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;
"Machine Common Subexpression Elimination", false, false)
"Machine Common Subexpression Elimination", false, false)
/// The source register of a COPY machine instruction can be propagated to all
/// its users, and this propagation could increase the probability of finding
/// common subexpressions. If the COPY has only one user, the COPY itself can
/// be removed.
bool MachineCSE::PerformTrivialCopyPropagation(MachineInstr *MI,
MachineBasicBlock *MBB) {
bool Changed = false;
for (MachineOperand &MO : MI->operands()) {
if (!MO.isReg() || !MO.isUse())
unsigned Reg = MO.getReg();
if (!TargetRegisterInfo::isVirtualRegister(Reg))
bool OnlyOneUse = MRI->hasOneNonDBGUse(Reg);
MachineInstr *DefMI = MRI->getVRegDef(Reg);
if (!DefMI->isCopy())
unsigned SrcReg = DefMI->getOperand(1).getReg();
if (!TargetRegisterInfo::isVirtualRegister(SrcReg))
if (DefMI->getOperand(0).getSubReg())
// FIXME: We should trivially coalesce subregister copies to expose CSE
// opportunities on instructions with truncated operands (see
// cse-add-with-overflow.ll). This can be done here as follows:
// if (SrcSubReg)
// RC = TRI->getMatchingSuperRegClass(MRI->getRegClass(SrcReg), RC,
// SrcSubReg);
// MO.substVirtReg(SrcReg, SrcSubReg, *TRI);
// The 2-addr pass has been updated to handle coalesced subregs. However,
// some machine-specific code still can't handle it.
// To handle it properly we also need a way find a constrained subregister
// class given a super-reg class and subreg index.
if (DefMI->getOperand(1).getSubReg())
if (!MRI->constrainRegAttrs(SrcReg, Reg))
LLVM_DEBUG(dbgs() << "Coalescing: " << *DefMI);
LLVM_DEBUG(dbgs() << "*** to: " << *MI);
// Update matching debug values.
// Propagate SrcReg of copies to MI.
// Coalesce single use copies.
if (OnlyOneUse) {
Changed = true;
return Changed;
MachineCSE::isPhysDefTriviallyDead(unsigned Reg,
MachineBasicBlock::const_iterator I,
MachineBasicBlock::const_iterator E) const {
unsigned LookAheadLeft = LookAheadLimit;
while (LookAheadLeft) {
// Skip over dbg_value's.
I = skipDebugInstructionsForward(I, E);
if (I == E)
// Reached end of block, we don't know if register is dead or not.
return false;
bool SeenDef = false;
for (const MachineOperand &MO : I->operands()) {
if (MO.isRegMask() && MO.clobbersPhysReg(Reg))
SeenDef = true;
if (!MO.isReg() || !MO.getReg())
if (!TRI->regsOverlap(MO.getReg(), Reg))
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;
return false;
static bool isCallerPreservedOrConstPhysReg(unsigned Reg,
const MachineFunction &MF,
const TargetRegisterInfo &TRI) {
// MachineRegisterInfo::isConstantPhysReg directly called by
// MachineRegisterInfo::isCallerPreservedOrConstPhysReg expects the
// reserved registers to be frozen. That doesn't cause a problem post-ISel as
// most (if not all) targets freeze reserved registers right after ISel.
// It does cause issues mid-GlobalISel, however, hence the additional
// reservedRegsFrozen check.
const MachineRegisterInfo &MRI = MF.getRegInfo();
return TRI.isCallerPreservedPhysReg(Reg, MF) ||
(MRI.reservedRegsFrozen() && MRI.isConstantPhysReg(Reg));
/// 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,
SmallVectorImpl<unsigned> &PhysDefs,
bool &PhysUseDef) const{
// First, add all uses to PhysRefs.
for (const MachineOperand &MO : MI->operands()) {
if (!MO.isReg() || MO.isDef())
unsigned Reg = MO.getReg();
if (!Reg)
if (TargetRegisterInfo::isVirtualRegister(Reg))
// Reading either caller preserved or constant physregs is ok.
if (!isCallerPreservedOrConstPhysReg(Reg, *MI->getMF(), *TRI))
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
// Next, collect all defs into PhysDefs. If any is already in PhysRefs
// (which currently contains only uses), set the PhysUseDef flag.
PhysUseDef = false;
MachineBasicBlock::const_iterator I = MI; I = std::next(I);
for (const MachineOperand &MO : MI->operands()) {
if (!MO.isReg() || !MO.isDef())
unsigned Reg = MO.getReg();
if (!Reg)
if (TargetRegisterInfo::isVirtualRegister(Reg))
// Check against PhysRefs even if the def is "dead".
if (PhysRefs.count(Reg))
PhysUseDef = true;
// 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.isDead() && !isPhysDefTriviallyDead(Reg, I, MBB->end()))
// Finally, add all defs to PhysRefs as well.
for (unsigned i = 0, e = PhysDefs.size(); i != e; ++i)
for (MCRegAliasIterator AI(PhysDefs[i], TRI, true); AI.isValid(); ++AI)
return !PhysRefs.empty();
bool MachineCSE::PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
SmallSet<unsigned,8> &PhysRefs,
SmallVectorImpl<unsigned> &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 (MRI->isAllocatable(PhysDefs[i]) || MRI->isReserved(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 = std::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->isDebugInstr())
if (I == EE) {
assert(CrossMBB && "Reaching end-of-MBB without finding MI?");
CrossMBB = false;
NonLocal = true;
I = MBB->begin();
EE = MBB->end();
if (I == E)
return true;
for (const MachineOperand &MO : I->operands()) {
// 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())
unsigned MOReg = MO.getReg();
if (TargetRegisterInfo::isVirtualRegister(MOReg))
if (PhysRefs.count(MOReg))
return false;
return false;
bool MachineCSE::isCSECandidate(MachineInstr *MI) {
if (MI->isPosition() || MI->isPHI() || MI->isImplicitDef() || MI->isKill() ||
MI->isInlineAsm() || MI->isDebugInstr())
return false;
// Ignore copies.
if (MI->isCopyLike())
return false;
// Ignore stuff that we obviously can't move.
if (MI->mayStore() || MI->isCall() || MI->isTerminator() ||
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->isDereferenceableInvariantLoad(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;
// Ignore stack guard loads, otherwise the register that holds CSEed value may
// be spilled and get loaded back with corrupted data.
if (MI->getOpcode() == TargetOpcode::LOAD_STACK_GUARD)
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 (MachineInstr &MI : MRI->use_nodbg_instructions(CSReg)) {
for (MachineInstr &MI : MRI->use_nodbg_instructions(Reg)) {
if (!CSUses.count(&MI)) {
MayIncreasePressure = true;
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 (TII->isAsCheapAsAMove(*MI)) {
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 (const MachineOperand &MO : MI->operands()) {
if (MO.isReg() && MO.isUse() &&
TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
HasVRegUse = true;
if (!HasVRegUse) {
bool HasNonCopyUse = false;
for (MachineInstr &MI : MRI->use_nodbg_instructions(Reg)) {
// Ignore copies.
if (!MI.isCopyLike()) {
HasNonCopyUse = true;
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;
for (MachineInstr &UseMI : MRI->use_nodbg_instructions(CSReg)) {
HasPHI |= UseMI.isPHI();
if (UseMI.getParent() == MI->getParent())
return true;
return !HasPHI;
void MachineCSE::EnterScope(MachineBasicBlock *MBB) {
LLVM_DEBUG(dbgs() << "Entering: " << MBB->getName() << '\n');
ScopeType *Scope = new ScopeType(VNT);
ScopeMap[MBB] = Scope;
void MachineCSE::ExitScope(MachineBasicBlock *MBB) {
LLVM_DEBUG(dbgs() << "Exiting: " << MBB->getName() << '\n');
DenseMap<MachineBasicBlock*, ScopeType*>::iterator SI = ScopeMap.find(MBB);
assert(SI != ScopeMap.end());
delete SI->second;
bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
bool Changed = false;
SmallVector<std::pair<unsigned, unsigned>, 8> CSEPairs;
SmallVector<unsigned, 2> ImplicitDefsToUpdate;
SmallVector<unsigned, 2> ImplicitDefs;
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ) {
MachineInstr *MI = &*I;
if (!isCSECandidate(MI))
bool FoundCSE = VNT.count(MI);
if (!FoundCSE) {
// Using trivial copy propagation to find more CSE opportunities.
if (PerformTrivialCopyPropagation(MI, MBB)) {
Changed = true;
// After coalescing MI itself may become a copy.
if (MI->isCopyLike())
// Try again to see if CSE is possible.
FoundCSE = VNT.count(MI);
// Commute commutable instructions.
bool Commuted = false;
if (!FoundCSE && MI->isCommutable()) {
if (MachineInstr *NewMI = TII->commuteInstruction(*MI)) {
Commuted = true;
FoundCSE = VNT.count(NewMI);
if (NewMI != MI) {
// New instruction. It doesn't need to be kept.
Changed = true;
} else if (!FoundCSE)
// MI was changed but it didn't help, commute it back!
// 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;
bool PhysUseDef = false;
if (FoundCSE && hasLivePhysRegDefUses(MI, MBB, PhysRefs,
PhysDefs, PhysUseDef)) {
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.
// This can never be the case if the instruction both uses and
// defines the same physical register, which was detected above.
if (!PhysUseDef) {
unsigned CSVN = VNT.lookup(MI);
MachineInstr *CSMI = Exps[CSVN];
if (PhysRegDefsReach(CSMI, MI, PhysRefs, PhysDefs, CrossMBBPhysDef))
FoundCSE = true;
if (!FoundCSE) {
VNT.insert(MI, CurrVN++);
// Found a common subexpression, eliminate it.
unsigned CSVN = VNT.lookup(MI);
MachineInstr *CSMI = Exps[CSVN];
LLVM_DEBUG(dbgs() << "Examining: " << *MI);
LLVM_DEBUG(dbgs() << "*** Found a common subexpression: " << *CSMI);
// Check if it's profitable to perform this CSE.
bool DoCSE = true;
unsigned NumDefs = MI->getNumDefs();
for (unsigned i = 0, e = MI->getNumOperands(); NumDefs && i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || !MO.isDef())
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())
// Keep track of implicit defs of CSMI and MI, to clear possibly
// made-redundant kill flags.
if (MO.isImplicit() && !MO.isDead() && OldReg == NewReg)
if (OldReg == NewReg) {
assert(TargetRegisterInfo::isVirtualRegister(OldReg) &&
TargetRegisterInfo::isVirtualRegister(NewReg) &&
"Do not CSE physical register defs!");
if (!isProfitableToCSE(NewReg, OldReg, CSMI, MI)) {
LLVM_DEBUG(dbgs() << "*** Not profitable, avoid CSE!\n");
DoCSE = false;
// Don't perform CSE if the result of the new instruction cannot exist
// within the constraints (register class, bank, or low-level type) of
// the old instruction.
if (!MRI->constrainRegAttrs(NewReg, OldReg)) {
dbgs() << "*** Not the same register constraints, avoid CSE!\n");
DoCSE = false;
CSEPairs.push_back(std::make_pair(OldReg, NewReg));
// Actually perform the elimination.
if (DoCSE) {
for (std::pair<unsigned, unsigned> &CSEPair : CSEPairs) {
unsigned OldReg = CSEPair.first;
unsigned NewReg = CSEPair.second;
// OldReg may have been unused but is used now, clear the Dead flag
MachineInstr *Def = MRI->getUniqueVRegDef(NewReg);
assert(Def != nullptr && "CSEd register has no unique definition?");
// Replace with NewReg and clear kill flags which may be wrong now.
MRI->replaceRegWith(OldReg, NewReg);
// 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 ImplicitDefToUpdate : ImplicitDefsToUpdate)
// Go through implicit defs of CSMI and MI, and clear the kill flags on
// their uses in all the instructions between CSMI and MI.
// We might have made some of the kill flags redundant, consider:
// subs ... implicit-def %nzcv <- CSMI
// csinc ... implicit killed %nzcv <- this kill flag isn't valid anymore
// subs ... implicit-def %nzcv <- MI, to be eliminated
// csinc ... implicit killed %nzcv
// Since we eliminated MI, and reused a register imp-def'd by CSMI
// (here %nzcv), that register, if it was killed before MI, should have
// that kill flag removed, because it's lifetime was extended.
if (CSMI->getParent() == MI->getParent()) {
for (MachineBasicBlock::iterator II = CSMI, IE = MI; II != IE; ++II)
for (auto ImplicitDef : ImplicitDefs)
if (MachineOperand *MO = II->findRegisterUseOperand(
ImplicitDef, /*isKill=*/true, TRI))
} else {
// If the instructions aren't in the same BB, bail out and clear the
// kill flag on all uses of the imp-def'd register.
for (auto ImplicitDef : ImplicitDefs)
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))
if (!PhysRefs.empty())
if (Commuted)
Changed = true;
} else {
VNT.insert(MI, CurrVN++);
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.
MachineCSE::ExitScopeIfDone(MachineDomTreeNode *Node,
DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren) {
if (OpenChildren[Node])
// Pop scope.
// Now traverse upwards to pop ancestors whose offsprings are all done.
while (MachineDomTreeNode *Parent = Node->getIDom()) {
unsigned Left = --OpenChildren[Parent];
if (Left != 0)
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.
do {
Node = WorkList.pop_back_val();
const std::vector<MachineDomTreeNode*> &Children = Node->getChildren();
OpenChildren[Node] = Children.size();
for (MachineDomTreeNode *Child : Children)
} while (!WorkList.empty());
// Now perform CSE.
bool Changed = false;
for (MachineDomTreeNode *Node : Scopes) {
MachineBasicBlock *MBB = Node->getBlock();
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) {
if (skipFunction(MF.getFunction()))
return false;
TII = MF.getSubtarget().getInstrInfo();
TRI = MF.getSubtarget().getRegisterInfo();
MRI = &MF.getRegInfo();
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
DT = &getAnalysis<MachineDominatorTree>();
LookAheadLimit = TII->getMachineCSELookAheadLimit();
return PerformCSE(DT->getRootNode());