Target/X86/X86FixupLEAs.cpp

//===-- X86FixupLEAs.cpp - use or replace LEA instructions -----------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines the pass that finds instructions that can be
// re-written as LEA instructions in order to reduce pipeline delays.
// It replaces LEAs with ADD/INC/DEC when that is better for size/speed.
//
//===----------------------------------------------------------------------===//

#include "X86.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/CodeGen/LazyMachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineSizeOpts.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetSchedule.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;

#define FIXUPLEA_DESC "X86 LEA Fixup"
#define FIXUPLEA_NAME "x86-fixup-LEAs"

#define DEBUG_TYPE FIXUPLEA_NAME

STATISTIC(NumLEAs, "Number of LEA instructions created");

namespace {
class FixupLEAPass : public MachineFunctionPass {
  enum RegUsageState { RU_NotUsed, RU_Write, RU_Read };

  /// Given a machine register, look for the instruction
  /// which writes it in the current basic block. If found,
  /// try to replace it with an equivalent LEA instruction.
  /// If replacement succeeds, then also process the newly created
  /// instruction.
  void seekLEAFixup(MachineOperand &p, MachineBasicBlock::iterator &I,
                    MachineBasicBlock &MBB);

  /// Given a memory access or LEA instruction
  /// whose address mode uses a base and/or index register, look for
  /// an opportunity to replace the instruction which sets the base or index
  /// register with an equivalent LEA instruction.
  void processInstruction(MachineBasicBlock::iterator &I,
                          MachineBasicBlock &MBB);

  /// Given a LEA instruction which is unprofitable
  /// on SlowLEA targets try to replace it with an equivalent ADD instruction.
  void processInstructionForSlowLEA(MachineBasicBlock::iterator &I,
                                    MachineBasicBlock &MBB);

  /// Given a LEA instruction which is unprofitable
  /// on SNB+ try to replace it with other instructions.
  /// According to Intel's Optimization Reference Manual:
  /// " For LEA instructions with three source operands and some specific
  ///   situations, instruction latency has increased to 3 cycles, and must
  ///   dispatch via port 1:
  /// - LEA that has all three source operands: base, index, and offset
  /// - LEA that uses base and index registers where the base is EBP, RBP,
  ///   or R13
  /// - LEA that uses RIP relative addressing mode
  /// - LEA that uses 16-bit addressing mode "
  /// This function currently handles the first 2 cases only.
  void processInstrForSlow3OpLEA(MachineBasicBlock::iterator &I,
                                 MachineBasicBlock &MBB, bool OptIncDec);

  /// Look for LEAs that are really two address LEAs that we might be able to
  /// turn into regular ADD instructions.
  bool optTwoAddrLEA(MachineBasicBlock::iterator &I,
                     MachineBasicBlock &MBB, bool OptIncDec,
                     bool UseLEAForSP) const;

  /// Determine if an instruction references a machine register
  /// and, if so, whether it reads or writes the register.
  RegUsageState usesRegister(MachineOperand &p, MachineBasicBlock::iterator I);

  /// Step backwards through a basic block, looking
  /// for an instruction which writes a register within
  /// a maximum of INSTR_DISTANCE_THRESHOLD instruction latency cycles.
  MachineBasicBlock::iterator searchBackwards(MachineOperand &p,
                                              MachineBasicBlock::iterator &I,
                                              MachineBasicBlock &MBB);

  /// if an instruction can be converted to an
  /// equivalent LEA, insert the new instruction into the basic block
  /// and return a pointer to it. Otherwise, return zero.
  MachineInstr *postRAConvertToLEA(MachineBasicBlock &MBB,
                                   MachineBasicBlock::iterator &MBBI) const;

public:
  static char ID;

  StringRef getPassName() const override { return FIXUPLEA_DESC; }

  FixupLEAPass() : MachineFunctionPass(ID) { }

  /// Loop over all of the basic blocks,
  /// replacing instructions by equivalent LEA instructions
  /// if needed and when possible.
  bool runOnMachineFunction(MachineFunction &MF) override;

  // This pass runs after regalloc and doesn't support VReg operands.
  MachineFunctionProperties getRequiredProperties() const override {
    return MachineFunctionProperties().set(
        MachineFunctionProperties::Property::NoVRegs);
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<ProfileSummaryInfoWrapperPass>();
    AU.addRequired<LazyMachineBlockFrequencyInfoPass>();
    MachineFunctionPass::getAnalysisUsage(AU);
  }

private:
  TargetSchedModel TSM;
  const X86InstrInfo *TII = nullptr;
  const X86RegisterInfo *TRI = nullptr;
};
}

char FixupLEAPass::ID = 0;

INITIALIZE_PASS(FixupLEAPass, FIXUPLEA_NAME, FIXUPLEA_DESC, false, false)

MachineInstr *
FixupLEAPass::postRAConvertToLEA(MachineBasicBlock &MBB,
                                 MachineBasicBlock::iterator &MBBI) const {
  MachineInstr &MI = *MBBI;
  switch (MI.getOpcode()) {
  case X86::MOV32rr:
  case X86::MOV64rr: {
    const MachineOperand &Src = MI.getOperand(1);
    const MachineOperand &Dest = MI.getOperand(0);
    MachineInstr *NewMI =
        BuildMI(MBB, MBBI, MI.getDebugLoc(),
                TII->get(MI.getOpcode() == X86::MOV32rr ? X86::LEA32r
                                                        : X86::LEA64r))
            .add(Dest)
            .add(Src)
            .addImm(1)
            .addReg(0)
            .addImm(0)
            .addReg(0);
    return NewMI;
  }
  }

  if (!MI.isConvertibleTo3Addr())
    return nullptr;

  switch (MI.getOpcode()) {
  default:
    // Only convert instructions that we've verified are safe.
    return nullptr;
  case X86::ADD64ri32:
  case X86::ADD64ri8:
  case X86::ADD64ri32_DB:
  case X86::ADD64ri8_DB:
  case X86::ADD32ri:
  case X86::ADD32ri8:
  case X86::ADD32ri_DB:
  case X86::ADD32ri8_DB:
    if (!MI.getOperand(2).isImm()) {
      // convertToThreeAddress will call getImm()
      // which requires isImm() to be true
      return nullptr;
    }
    break;
  case X86::SHL64ri:
  case X86::SHL32ri:
  case X86::INC64r:
  case X86::INC32r:
  case X86::DEC64r:
  case X86::DEC32r:
  case X86::ADD64rr:
  case X86::ADD64rr_DB:
  case X86::ADD32rr:
  case X86::ADD32rr_DB:
    // These instructions are all fine to convert.
    break;
  }
  MachineFunction::iterator MFI = MBB.getIterator();
  return TII->convertToThreeAddress(MFI, MI, nullptr);
}

FunctionPass *llvm::createX86FixupLEAs() { return new FixupLEAPass(); }

static bool isLEA(unsigned Opcode) {
  return Opcode == X86::LEA32r || Opcode == X86::LEA64r ||
         Opcode == X86::LEA64_32r;
}

bool FixupLEAPass::runOnMachineFunction(MachineFunction &MF) {
  if (skipFunction(MF.getFunction()))
    return false;

  const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
  bool IsSlowLEA = ST.slowLEA();
  bool IsSlow3OpsLEA = ST.slow3OpsLEA();
  bool LEAUsesAG = ST.LEAusesAG();

  bool OptIncDec = !ST.slowIncDec() || MF.getFunction().hasOptSize();
  bool UseLEAForSP = ST.useLeaForSP();

  TSM.init(&ST);
  TII = ST.getInstrInfo();
  TRI = ST.getRegisterInfo();
  auto *PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
  auto *MBFI = (PSI && PSI->hasProfileSummary())
                   ? &getAnalysis<LazyMachineBlockFrequencyInfoPass>().getBFI()
                   : nullptr;

  LLVM_DEBUG(dbgs() << "Start X86FixupLEAs\n";);
  for (MachineBasicBlock &MBB : MF) {
    // First pass. Try to remove or optimize existing LEAs.
    bool OptIncDecPerBB =
        OptIncDec || llvm::shouldOptimizeForSize(&MBB, PSI, MBFI);
    for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ++I) {
      if (!isLEA(I->getOpcode()))
        continue;

      if (optTwoAddrLEA(I, MBB, OptIncDecPerBB, UseLEAForSP))
        continue;

      if (IsSlowLEA)
        processInstructionForSlowLEA(I, MBB);
      else if (IsSlow3OpsLEA)
        processInstrForSlow3OpLEA(I, MBB, OptIncDecPerBB);
    }

    // Second pass for creating LEAs. This may reverse some of the
    // transformations above.
    if (LEAUsesAG) {
      for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ++I)
        processInstruction(I, MBB);
    }
  }

  LLVM_DEBUG(dbgs() << "End X86FixupLEAs\n";);

  return true;
}

FixupLEAPass::RegUsageState
FixupLEAPass::usesRegister(MachineOperand &p, MachineBasicBlock::iterator I) {
  RegUsageState RegUsage = RU_NotUsed;
  MachineInstr &MI = *I;

  for (unsigned i = 0; i < MI.getNumOperands(); ++i) {
    MachineOperand &opnd = MI.getOperand(i);
    if (opnd.isReg() && opnd.getReg() == p.getReg()) {
      if (opnd.isDef())
        return RU_Write;
      RegUsage = RU_Read;
    }
  }
  return RegUsage;
}

/// getPreviousInstr - Given a reference to an instruction in a basic
/// block, return a reference to the previous instruction in the block,
/// wrapping around to the last instruction of the block if the block
/// branches to itself.
static inline bool getPreviousInstr(MachineBasicBlock::iterator &I,
                                    MachineBasicBlock &MBB) {
  if (I == MBB.begin()) {
    if (MBB.isPredecessor(&MBB)) {
      I = --MBB.end();
      return true;
    } else
      return false;
  }
  --I;
  return true;
}

MachineBasicBlock::iterator
FixupLEAPass::searchBackwards(MachineOperand &p, MachineBasicBlock::iterator &I,
                              MachineBasicBlock &MBB) {
  int InstrDistance = 1;
  MachineBasicBlock::iterator CurInst;
  static const int INSTR_DISTANCE_THRESHOLD = 5;

  CurInst = I;
  bool Found;
  Found = getPreviousInstr(CurInst, MBB);
  while (Found && I != CurInst) {
    if (CurInst->isCall() || CurInst->isInlineAsm())
      break;
    if (InstrDistance > INSTR_DISTANCE_THRESHOLD)
      break; // too far back to make a difference
    if (usesRegister(p, CurInst) == RU_Write) {
      return CurInst;
    }
    InstrDistance += TSM.computeInstrLatency(&*CurInst);
    Found = getPreviousInstr(CurInst, MBB);
  }
  return MachineBasicBlock::iterator();
}

static inline bool isInefficientLEAReg(unsigned Reg) {
  return Reg == X86::EBP || Reg == X86::RBP ||
         Reg == X86::R13D || Reg == X86::R13;
}

/// Returns true if this LEA uses base an index registers, and the base register
/// is known to be inefficient for the subtarget.
// TODO: use a variant scheduling class to model the latency profile
// of LEA instructions, and implement this logic as a scheduling predicate.
static inline bool hasInefficientLEABaseReg(const MachineOperand &Base,
                                            const MachineOperand &Index) {
  return Base.isReg() && isInefficientLEAReg(Base.getReg()) && Index.isReg() &&
         Index.getReg() != X86::NoRegister;
}

static inline bool hasLEAOffset(const MachineOperand &Offset) {
  return (Offset.isImm() && Offset.getImm() != 0) || Offset.isGlobal();
}

static inline unsigned getADDrrFromLEA(unsigned LEAOpcode) {
  switch (LEAOpcode) {
  default:
    llvm_unreachable("Unexpected LEA instruction");
  case X86::LEA32r:
  case X86::LEA64_32r:
    return X86::ADD32rr;
  case X86::LEA64r:
    return X86::ADD64rr;
  }
}

static inline unsigned getADDriFromLEA(unsigned LEAOpcode,
                                       const MachineOperand &Offset) {
  bool IsInt8 = Offset.isImm() && isInt<8>(Offset.getImm());
  switch (LEAOpcode) {
  default:
    llvm_unreachable("Unexpected LEA instruction");
  case X86::LEA32r:
  case X86::LEA64_32r:
    return IsInt8 ? X86::ADD32ri8 : X86::ADD32ri;
  case X86::LEA64r:
    return IsInt8 ? X86::ADD64ri8 : X86::ADD64ri32;
  }
}

static inline unsigned getINCDECFromLEA(unsigned LEAOpcode, bool IsINC) {
  switch (LEAOpcode) {
  default:
    llvm_unreachable("Unexpected LEA instruction");
  case X86::LEA32r:
  case X86::LEA64_32r:
    return IsINC ? X86::INC32r : X86::DEC32r;
  case X86::LEA64r:
    return IsINC ? X86::INC64r : X86::DEC64r;
  }
}

bool FixupLEAPass::optTwoAddrLEA(MachineBasicBlock::iterator &I,
                                 MachineBasicBlock &MBB, bool OptIncDec,
                                 bool UseLEAForSP) const {
  MachineInstr &MI = *I;

  const MachineOperand &Base =    MI.getOperand(1 + X86::AddrBaseReg);
  const MachineOperand &Scale =   MI.getOperand(1 + X86::AddrScaleAmt);
  const MachineOperand &Index =   MI.getOperand(1 + X86::AddrIndexReg);
  const MachineOperand &Disp =    MI.getOperand(1 + X86::AddrDisp);
  const MachineOperand &Segment = MI.getOperand(1 + X86::AddrSegmentReg);

  if (Segment.getReg() != 0 || !Disp.isImm() || Scale.getImm() > 1 ||
      !TII->isSafeToClobberEFLAGS(MBB, I))
    return false;

  Register DestReg = MI.getOperand(0).getReg();
  Register BaseReg = Base.getReg();
  Register IndexReg = Index.getReg();

  // Don't change stack adjustment LEAs.
  if (UseLEAForSP && (DestReg == X86::ESP || DestReg == X86::RSP))
    return false;

  // LEA64_32 has 64-bit operands but 32-bit result.
  if (MI.getOpcode() == X86::LEA64_32r) {
    if (BaseReg != 0)
      BaseReg = TRI->getSubReg(BaseReg, X86::sub_32bit);
    if (IndexReg != 0)
      IndexReg = TRI->getSubReg(IndexReg, X86::sub_32bit);
  }

  MachineInstr *NewMI = nullptr;

  // Look for lea(%reg1, %reg2), %reg1 or lea(%reg2, %reg1), %reg1
  // which can be turned into add %reg2, %reg1
  if (BaseReg != 0 && IndexReg != 0 && Disp.getImm() == 0 &&
      (DestReg == BaseReg || DestReg == IndexReg)) {
    unsigned NewOpcode = getADDrrFromLEA(MI.getOpcode());
    if (DestReg != BaseReg)
      std::swap(BaseReg, IndexReg);

    if (MI.getOpcode() == X86::LEA64_32r) {
      // TODO: Do we need the super register implicit use?
      NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpcode), DestReg)
        .addReg(BaseReg).addReg(IndexReg)
        .addReg(Base.getReg(), RegState::Implicit)
        .addReg(Index.getReg(), RegState::Implicit);
    } else {
      NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpcode), DestReg)
        .addReg(BaseReg).addReg(IndexReg);
    }
  } else if (DestReg == BaseReg && IndexReg == 0) {
    // This is an LEA with only a base register and a displacement,
    // We can use ADDri or INC/DEC.

    // Does this LEA have one these forms:
    // lea  %reg, 1(%reg)
    // lea  %reg, -1(%reg)
    if (OptIncDec && (Disp.getImm() == 1 || Disp.getImm() == -1)) {
      bool IsINC = Disp.getImm() == 1;
      unsigned NewOpcode = getINCDECFromLEA(MI.getOpcode(), IsINC);

      if (MI.getOpcode() == X86::LEA64_32r) {
        // TODO: Do we need the super register implicit use?
        NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpcode), DestReg)
          .addReg(BaseReg).addReg(Base.getReg(), RegState::Implicit);
      } else {
        NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpcode), DestReg)
          .addReg(BaseReg);
      }
    } else {
      unsigned NewOpcode = getADDriFromLEA(MI.getOpcode(), Disp);
      if (MI.getOpcode() == X86::LEA64_32r) {
        // TODO: Do we need the super register implicit use?
        NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpcode), DestReg)
          .addReg(BaseReg).addImm(Disp.getImm())
          .addReg(Base.getReg(), RegState::Implicit);
      } else {
        NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpcode), DestReg)
          .addReg(BaseReg).addImm(Disp.getImm());
      }
    }
  } else
    return false;

  MBB.erase(I);
  I = NewMI;
  return true;
}

void FixupLEAPass::processInstruction(MachineBasicBlock::iterator &I,
                                      MachineBasicBlock &MBB) {
  // Process a load, store, or LEA instruction.
  MachineInstr &MI = *I;
  const MCInstrDesc &Desc = MI.getDesc();
  int AddrOffset = X86II::getMemoryOperandNo(Desc.TSFlags);
  if (AddrOffset >= 0) {
    AddrOffset += X86II::getOperandBias(Desc);
    MachineOperand &p = MI.getOperand(AddrOffset + X86::AddrBaseReg);
    if (p.isReg() && p.getReg() != X86::ESP) {
      seekLEAFixup(p, I, MBB);
    }
    MachineOperand &q = MI.getOperand(AddrOffset + X86::AddrIndexReg);
    if (q.isReg() && q.getReg() != X86::ESP) {
      seekLEAFixup(q, I, MBB);
    }
  }
}

void FixupLEAPass::seekLEAFixup(MachineOperand &p,
                                MachineBasicBlock::iterator &I,
                                MachineBasicBlock &MBB) {
  MachineBasicBlock::iterator MBI = searchBackwards(p, I, MBB);
  if (MBI != MachineBasicBlock::iterator()) {
    MachineInstr *NewMI = postRAConvertToLEA(MBB, MBI);
    if (NewMI) {
      ++NumLEAs;
      LLVM_DEBUG(dbgs() << "FixLEA: Candidate to replace:"; MBI->dump(););
      // now to replace with an equivalent LEA...
      LLVM_DEBUG(dbgs() << "FixLEA: Replaced by: "; NewMI->dump(););
      MBB.erase(MBI);
      MachineBasicBlock::iterator J =
          static_cast<MachineBasicBlock::iterator>(NewMI);
      processInstruction(J, MBB);
    }
  }
}

void FixupLEAPass::processInstructionForSlowLEA(MachineBasicBlock::iterator &I,
                                                MachineBasicBlock &MBB) {
  MachineInstr &MI = *I;
  const unsigned Opcode = MI.getOpcode();

  const MachineOperand &Dst =     MI.getOperand(0);
  const MachineOperand &Base =    MI.getOperand(1 + X86::AddrBaseReg);
  const MachineOperand &Scale =   MI.getOperand(1 + X86::AddrScaleAmt);
  const MachineOperand &Index =   MI.getOperand(1 + X86::AddrIndexReg);
  const MachineOperand &Offset =  MI.getOperand(1 + X86::AddrDisp);
  const MachineOperand &Segment = MI.getOperand(1 + X86::AddrSegmentReg);

  if (Segment.getReg() != 0 || !Offset.isImm() ||
      !TII->isSafeToClobberEFLAGS(MBB, I))
    return;
  const Register DstR = Dst.getReg();
  const Register SrcR1 = Base.getReg();
  const Register SrcR2 = Index.getReg();
  if ((SrcR1 == 0 || SrcR1 != DstR) && (SrcR2 == 0 || SrcR2 != DstR))
    return;
  if (Scale.getImm() > 1)
    return;
  LLVM_DEBUG(dbgs() << "FixLEA: Candidate to replace:"; I->dump(););
  LLVM_DEBUG(dbgs() << "FixLEA: Replaced by: ";);
  MachineInstr *NewMI = nullptr;
  // Make ADD instruction for two registers writing to LEA's destination
  if (SrcR1 != 0 && SrcR2 != 0) {
    const MCInstrDesc &ADDrr = TII->get(getADDrrFromLEA(Opcode));
    const MachineOperand &Src = SrcR1 == DstR ? Index : Base;
    NewMI =
        BuildMI(MBB, I, MI.getDebugLoc(), ADDrr, DstR).addReg(DstR).add(Src);
    LLVM_DEBUG(NewMI->dump(););
  }
  // Make ADD instruction for immediate
  if (Offset.getImm() != 0) {
    const MCInstrDesc &ADDri =
        TII->get(getADDriFromLEA(Opcode, Offset));
    const MachineOperand &SrcR = SrcR1 == DstR ? Base : Index;
    NewMI = BuildMI(MBB, I, MI.getDebugLoc(), ADDri, DstR)
                .add(SrcR)
                .addImm(Offset.getImm());
    LLVM_DEBUG(NewMI->dump(););
  }
  if (NewMI) {
    MBB.erase(I);
    I = NewMI;
  }
}

void FixupLEAPass::processInstrForSlow3OpLEA(MachineBasicBlock::iterator &I,
                                             MachineBasicBlock &MBB,
                                             bool OptIncDec) {
  MachineInstr &MI = *I;
  const unsigned LEAOpcode = MI.getOpcode();

  const MachineOperand &Dest =    MI.getOperand(0);
  const MachineOperand &Base =    MI.getOperand(1 + X86::AddrBaseReg);
  const MachineOperand &Scale =   MI.getOperand(1 + X86::AddrScaleAmt);
  const MachineOperand &Index =   MI.getOperand(1 + X86::AddrIndexReg);
  const MachineOperand &Offset =  MI.getOperand(1 + X86::AddrDisp);
  const MachineOperand &Segment = MI.getOperand(1 + X86::AddrSegmentReg);

  if (!(TII->isThreeOperandsLEA(MI) || hasInefficientLEABaseReg(Base, Index)) ||
      !TII->isSafeToClobberEFLAGS(MBB, MI) ||
      Segment.getReg() != X86::NoRegister)
    return;

  Register DestReg = Dest.getReg();
  Register BaseReg = Base.getReg();
  Register IndexReg = Index.getReg();

  if (MI.getOpcode() == X86::LEA64_32r) {
    if (BaseReg != 0)
      BaseReg = TRI->getSubReg(BaseReg, X86::sub_32bit);
    if (IndexReg != 0)
      IndexReg = TRI->getSubReg(IndexReg, X86::sub_32bit);
  }

  bool IsScale1 = Scale.getImm() == 1;
  bool IsInefficientBase = isInefficientLEAReg(BaseReg);
  bool IsInefficientIndex = isInefficientLEAReg(IndexReg);

  // Skip these cases since it takes more than 2 instructions
  // to replace the LEA instruction.
  if (IsInefficientBase && DestReg == BaseReg && !IsScale1)
    return;

  LLVM_DEBUG(dbgs() << "FixLEA: Candidate to replace:"; MI.dump(););
  LLVM_DEBUG(dbgs() << "FixLEA: Replaced by: ";);

  MachineInstr *NewMI = nullptr;

  // First try to replace LEA with one or two (for the 3-op LEA case)
  // add instructions:
  // 1.lea (%base,%index,1), %base => add %index,%base
  // 2.lea (%base,%index,1), %index => add %base,%index
  if (IsScale1 && (DestReg == BaseReg || DestReg == IndexReg)) {
    unsigned NewOpc = getADDrrFromLEA(MI.getOpcode());
    if (DestReg != BaseReg)
      std::swap(BaseReg, IndexReg);

    if (MI.getOpcode() == X86::LEA64_32r) {
      // TODO: Do we need the super register implicit use?
      NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpc), DestReg)
                  .addReg(BaseReg)
                  .addReg(IndexReg)
                  .addReg(Base.getReg(), RegState::Implicit)
                  .addReg(Index.getReg(), RegState::Implicit);
    } else {
      NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpc), DestReg)
                  .addReg(BaseReg)
                  .addReg(IndexReg);
    }
  } else if (!IsInefficientBase || (!IsInefficientIndex && IsScale1)) {
    // If the base is inefficient try switching the index and base operands,
    // otherwise just break the 3-Ops LEA inst into 2-Ops LEA + ADD instruction:
    // lea offset(%base,%index,scale),%dst =>
    // lea (%base,%index,scale); add offset,%dst
    NewMI = BuildMI(MBB, MI, MI.getDebugLoc(), TII->get(LEAOpcode))
                .add(Dest)
                .add(IsInefficientBase ? Index : Base)
                .add(Scale)
                .add(IsInefficientBase ? Base : Index)
                .addImm(0)
                .add(Segment);
    LLVM_DEBUG(NewMI->dump(););
  }

  // If either replacement succeeded above, add the offset if needed, then
  // replace the instruction.
  if (NewMI) {
    // Create ADD instruction for the Offset in case of 3-Ops LEA.
    if (hasLEAOffset(Offset)) {
      if (OptIncDec && Offset.isImm() &&
          (Offset.getImm() == 1 || Offset.getImm() == -1)) {
        unsigned NewOpc =
            getINCDECFromLEA(MI.getOpcode(), Offset.getImm() == 1);
        NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpc), DestReg)
                    .addReg(DestReg);
        LLVM_DEBUG(NewMI->dump(););
      } else {
        unsigned NewOpc = getADDriFromLEA(MI.getOpcode(), Offset);
        NewMI = BuildMI(MBB, I, MI.getDebugLoc(), TII->get(NewOpc), DestReg)
                    .addReg(DestReg)
                    .add(Offset);
        LLVM_DEBUG(NewMI->dump(););
      }
    }

    MBB.erase(I);
    I = NewMI;
    return;
  }

  // Handle the rest of the cases with inefficient base register:
  assert(DestReg != BaseReg && "DestReg == BaseReg should be handled already!");
  assert(IsInefficientBase && "efficient base should be handled already!");

  // FIXME: Handle LEA64_32r.
  if (LEAOpcode == X86::LEA64_32r)
    return;

  // lea (%base,%index,1), %dst => mov %base,%dst; add %index,%dst
  if (IsScale1 && !hasLEAOffset(Offset)) {
    bool BIK = Base.isKill() && BaseReg != IndexReg;
    TII->copyPhysReg(MBB, MI, MI.getDebugLoc(), DestReg, BaseReg, BIK);
    LLVM_DEBUG(MI.getPrevNode()->dump(););

    unsigned NewOpc = getADDrrFromLEA(MI.getOpcode());
    NewMI = BuildMI(MBB, MI, MI.getDebugLoc(), TII->get(NewOpc), DestReg)
                .addReg(DestReg)
                .add(Index);
    LLVM_DEBUG(NewMI->dump(););

    MBB.erase(I);
    I = NewMI;
    return;
  }

  // lea offset(%base,%index,scale), %dst =>
  // lea offset( ,%index,scale), %dst; add %base,%dst
  NewMI = BuildMI(MBB, MI, MI.getDebugLoc(), TII->get(LEAOpcode))
              .add(Dest)
              .addReg(0)
              .add(Scale)
              .add(Index)
              .add(Offset)
              .add(Segment);
  LLVM_DEBUG(NewMI->dump(););

  unsigned NewOpc = getADDrrFromLEA(MI.getOpcode());
  NewMI = BuildMI(MBB, MI, MI.getDebugLoc(), TII->get(NewOpc), DestReg)
              .addReg(DestReg)
              .add(Base);
  LLVM_DEBUG(NewMI->dump(););

  MBB.erase(I);
  I = NewMI;
}