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//===- X86OptimizeLEAs.cpp - optimize usage of LEA instructions -----------===//
// 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 file defines the pass that performs some optimizations with LEA
// instructions in order to improve performance and code size.
// Currently, it does two things:
// 1) If there are two LEA instructions calculating addresses which only differ
// by displacement inside a basic block, one of them is removed.
// 2) Address calculations in load and store instructions are replaced by
// existing LEA def registers where possible.
#include "MCTargetDesc/X86BaseInfo.h"
#include "X86.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseMapInfo.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/CodeGen/LazyMachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineSizeOpts.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Function.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <cstdint>
#include <iterator>
using namespace llvm;
#define DEBUG_TYPE "x86-optimize-LEAs"
static cl::opt<bool>
DisableX86LEAOpt("disable-x86-lea-opt", cl::Hidden,
cl::desc("X86: Disable LEA optimizations."),
STATISTIC(NumSubstLEAs, "Number of LEA instruction substitutions");
STATISTIC(NumRedundantLEAs, "Number of redundant LEA instructions removed");
/// Returns true if two machine operands are identical and they are not
/// physical registers.
static inline bool isIdenticalOp(const MachineOperand &MO1,
const MachineOperand &MO2);
/// Returns true if two address displacement operands are of the same
/// type and use the same symbol/index/address regardless of the offset.
static bool isSimilarDispOp(const MachineOperand &MO1,
const MachineOperand &MO2);
/// Returns true if the instruction is LEA.
static inline bool isLEA(const MachineInstr &MI);
namespace {
/// A key based on instruction's memory operands.
class MemOpKey {
MemOpKey(const MachineOperand *Base, const MachineOperand *Scale,
const MachineOperand *Index, const MachineOperand *Segment,
const MachineOperand *Disp)
: Disp(Disp) {
Operands[0] = Base;
Operands[1] = Scale;
Operands[2] = Index;
Operands[3] = Segment;
bool operator==(const MemOpKey &Other) const {
// Addresses' bases, scales, indices and segments must be identical.
for (int i = 0; i < 4; ++i)
if (!isIdenticalOp(*Operands[i], *Other.Operands[i]))
return false;
// Addresses' displacements don't have to be exactly the same. It only
// matters that they use the same symbol/index/address. Immediates' or
// offsets' differences will be taken care of during instruction
// substitution.
return isSimilarDispOp(*Disp, *Other.Disp);
// Address' base, scale, index and segment operands.
const MachineOperand *Operands[4];
// Address' displacement operand.
const MachineOperand *Disp;
} // end anonymous namespace
namespace llvm {
/// Provide DenseMapInfo for MemOpKey.
template <> struct DenseMapInfo<MemOpKey> {
using PtrInfo = DenseMapInfo<const MachineOperand *>;
static inline MemOpKey getEmptyKey() {
return MemOpKey(PtrInfo::getEmptyKey(), PtrInfo::getEmptyKey(),
PtrInfo::getEmptyKey(), PtrInfo::getEmptyKey(),
static inline MemOpKey getTombstoneKey() {
return MemOpKey(PtrInfo::getTombstoneKey(), PtrInfo::getTombstoneKey(),
PtrInfo::getTombstoneKey(), PtrInfo::getTombstoneKey(),
static unsigned getHashValue(const MemOpKey &Val) {
// Checking any field of MemOpKey is enough to determine if the key is
// empty or tombstone.
assert(Val.Disp != PtrInfo::getEmptyKey() && "Cannot hash the empty key");
assert(Val.Disp != PtrInfo::getTombstoneKey() &&
"Cannot hash the tombstone key");
hash_code Hash = hash_combine(*Val.Operands[0], *Val.Operands[1],
*Val.Operands[2], *Val.Operands[3]);
// If the address displacement is an immediate, it should not affect the
// hash so that memory operands which differ only be immediate displacement
// would have the same hash. If the address displacement is something else,
// we should reflect symbol/index/address in the hash.
switch (Val.Disp->getType()) {
case MachineOperand::MO_Immediate:
case MachineOperand::MO_ConstantPoolIndex:
case MachineOperand::MO_JumpTableIndex:
Hash = hash_combine(Hash, Val.Disp->getIndex());
case MachineOperand::MO_ExternalSymbol:
Hash = hash_combine(Hash, Val.Disp->getSymbolName());
case MachineOperand::MO_GlobalAddress:
Hash = hash_combine(Hash, Val.Disp->getGlobal());
case MachineOperand::MO_BlockAddress:
Hash = hash_combine(Hash, Val.Disp->getBlockAddress());
case MachineOperand::MO_MCSymbol:
Hash = hash_combine(Hash, Val.Disp->getMCSymbol());
case MachineOperand::MO_MachineBasicBlock:
Hash = hash_combine(Hash, Val.Disp->getMBB());
llvm_unreachable("Invalid address displacement operand");
return (unsigned)Hash;
static bool isEqual(const MemOpKey &LHS, const MemOpKey &RHS) {
// Checking any field of MemOpKey is enough to determine if the key is
// empty or tombstone.
if (RHS.Disp == PtrInfo::getEmptyKey())
return LHS.Disp == PtrInfo::getEmptyKey();
if (RHS.Disp == PtrInfo::getTombstoneKey())
return LHS.Disp == PtrInfo::getTombstoneKey();
return LHS == RHS;
} // end namespace llvm
/// Returns a hash table key based on memory operands of \p MI. The
/// number of the first memory operand of \p MI is specified through \p N.
static inline MemOpKey getMemOpKey(const MachineInstr &MI, unsigned N) {
assert((isLEA(MI) || MI.mayLoadOrStore()) &&
"The instruction must be a LEA, a load or a store");
return MemOpKey(&MI.getOperand(N + X86::AddrBaseReg),
&MI.getOperand(N + X86::AddrScaleAmt),
&MI.getOperand(N + X86::AddrIndexReg),
&MI.getOperand(N + X86::AddrSegmentReg),
&MI.getOperand(N + X86::AddrDisp));
static inline bool isIdenticalOp(const MachineOperand &MO1,
const MachineOperand &MO2) {
return MO1.isIdenticalTo(MO2) &&
(!MO1.isReg() || !Register::isPhysicalRegister(MO1.getReg()));
#ifndef NDEBUG
static bool isValidDispOp(const MachineOperand &MO) {
return MO.isImm() || MO.isCPI() || MO.isJTI() || MO.isSymbol() ||
MO.isGlobal() || MO.isBlockAddress() || MO.isMCSymbol() || MO.isMBB();
static bool isSimilarDispOp(const MachineOperand &MO1,
const MachineOperand &MO2) {
assert(isValidDispOp(MO1) && isValidDispOp(MO2) &&
"Address displacement operand is not valid");
return (MO1.isImm() && MO2.isImm()) ||
(MO1.isCPI() && MO2.isCPI() && MO1.getIndex() == MO2.getIndex()) ||
(MO1.isJTI() && MO2.isJTI() && MO1.getIndex() == MO2.getIndex()) ||
(MO1.isSymbol() && MO2.isSymbol() &&
MO1.getSymbolName() == MO2.getSymbolName()) ||
(MO1.isGlobal() && MO2.isGlobal() &&
MO1.getGlobal() == MO2.getGlobal()) ||
(MO1.isBlockAddress() && MO2.isBlockAddress() &&
MO1.getBlockAddress() == MO2.getBlockAddress()) ||
(MO1.isMCSymbol() && MO2.isMCSymbol() &&
MO1.getMCSymbol() == MO2.getMCSymbol()) ||
(MO1.isMBB() && MO2.isMBB() && MO1.getMBB() == MO2.getMBB());
static inline bool isLEA(const MachineInstr &MI) {
unsigned Opcode = MI.getOpcode();
return Opcode == X86::LEA16r || Opcode == X86::LEA32r ||
Opcode == X86::LEA64r || Opcode == X86::LEA64_32r;
namespace {
class X86OptimizeLEAPass : public MachineFunctionPass {
X86OptimizeLEAPass() : MachineFunctionPass(ID) {}
StringRef getPassName() const override { return "X86 LEA Optimize"; }
/// Loop over all of the basic blocks, replacing address
/// calculations in load and store instructions, if it's already
/// been calculated by LEA. Also, remove redundant LEAs.
bool runOnMachineFunction(MachineFunction &MF) override;
static char ID;
void getAnalysisUsage(AnalysisUsage &AU) const override {
using MemOpMap = DenseMap<MemOpKey, SmallVector<MachineInstr *, 16>>;
/// Returns a distance between two instructions inside one basic block.
/// Negative result means, that instructions occur in reverse order.
int calcInstrDist(const MachineInstr &First, const MachineInstr &Last);
/// Choose the best \p LEA instruction from the \p List to replace
/// address calculation in \p MI instruction. Return the address displacement
/// and the distance between \p MI and the chosen \p BestLEA in
/// \p AddrDispShift and \p Dist.
bool chooseBestLEA(const SmallVectorImpl<MachineInstr *> &List,
const MachineInstr &MI, MachineInstr *&BestLEA,
int64_t &AddrDispShift, int &Dist);
/// Returns the difference between addresses' displacements of \p MI1
/// and \p MI2. The numbers of the first memory operands for the instructions
/// are specified through \p N1 and \p N2.
int64_t getAddrDispShift(const MachineInstr &MI1, unsigned N1,
const MachineInstr &MI2, unsigned N2) const;
/// Returns true if the \p Last LEA instruction can be replaced by the
/// \p First. The difference between displacements of the addresses calculated
/// by these LEAs is returned in \p AddrDispShift. It'll be used for proper
/// replacement of the \p Last LEA's uses with the \p First's def register.
bool isReplaceable(const MachineInstr &First, const MachineInstr &Last,
int64_t &AddrDispShift) const;
/// Find all LEA instructions in the basic block. Also, assign position
/// numbers to all instructions in the basic block to speed up calculation of
/// distance between them.
void findLEAs(const MachineBasicBlock &MBB, MemOpMap &LEAs);
/// Removes redundant address calculations.
bool removeRedundantAddrCalc(MemOpMap &LEAs);
/// Replace debug value MI with a new debug value instruction using register
/// VReg with an appropriate offset and DIExpression to incorporate the
/// address displacement AddrDispShift. Return new debug value instruction.
MachineInstr *replaceDebugValue(MachineInstr &MI, unsigned OldReg,
unsigned NewReg, int64_t AddrDispShift);
/// Removes LEAs which calculate similar addresses.
bool removeRedundantLEAs(MemOpMap &LEAs);
DenseMap<const MachineInstr *, unsigned> InstrPos;
MachineRegisterInfo *MRI = nullptr;
const X86InstrInfo *TII = nullptr;
const X86RegisterInfo *TRI = nullptr;
} // end anonymous namespace
char X86OptimizeLEAPass::ID = 0;
FunctionPass *llvm::createX86OptimizeLEAs() { return new X86OptimizeLEAPass(); }
INITIALIZE_PASS(X86OptimizeLEAPass, DEBUG_TYPE, "X86 optimize LEA pass", false,
int X86OptimizeLEAPass::calcInstrDist(const MachineInstr &First,
const MachineInstr &Last) {
// Both instructions must be in the same basic block and they must be
// presented in InstrPos.
assert(Last.getParent() == First.getParent() &&
"Instructions are in different basic blocks");
assert(InstrPos.find(&First) != InstrPos.end() &&
InstrPos.find(&Last) != InstrPos.end() &&
"Instructions' positions are undefined");
return InstrPos[&Last] - InstrPos[&First];
// Find the best LEA instruction in the List to replace address recalculation in
// MI. Such LEA must meet these requirements:
// 1) The address calculated by the LEA differs only by the displacement from
// the address used in MI.
// 2) The register class of the definition of the LEA is compatible with the
// register class of the address base register of MI.
// 3) Displacement of the new memory operand should fit in 1 byte if possible.
// 4) The LEA should be as close to MI as possible, and prior to it if
// possible.
bool X86OptimizeLEAPass::chooseBestLEA(
const SmallVectorImpl<MachineInstr *> &List, const MachineInstr &MI,
MachineInstr *&BestLEA, int64_t &AddrDispShift, int &Dist) {
const MachineFunction *MF = MI.getParent()->getParent();
const MCInstrDesc &Desc = MI.getDesc();
int MemOpNo = X86II::getMemoryOperandNo(Desc.TSFlags) +
BestLEA = nullptr;
// Loop over all LEA instructions.
for (auto DefMI : List) {
// Get new address displacement.
int64_t AddrDispShiftTemp = getAddrDispShift(MI, MemOpNo, *DefMI, 1);
// Make sure address displacement fits 4 bytes.
if (!isInt<32>(AddrDispShiftTemp))
// Check that LEA def register can be used as MI address base. Some
// instructions can use a limited set of registers as address base, for
// example MOV8mr_NOREX. We could constrain the register class of the LEA
// def to suit MI, however since this case is very rare and hard to
// reproduce in a test it's just more reliable to skip the LEA.
if (TII->getRegClass(Desc, MemOpNo + X86::AddrBaseReg, TRI, *MF) !=
// Choose the closest LEA instruction from the list, prior to MI if
// possible. Note that we took into account resulting address displacement
// as well. Also note that the list is sorted by the order in which the LEAs
// occur, so the break condition is pretty simple.
int DistTemp = calcInstrDist(*DefMI, MI);
assert(DistTemp != 0 &&
"The distance between two different instructions cannot be zero");
if (DistTemp > 0 || BestLEA == nullptr) {
// Do not update return LEA, if the current one provides a displacement
// which fits in 1 byte, while the new candidate does not.
if (BestLEA != nullptr && !isInt<8>(AddrDispShiftTemp) &&
BestLEA = DefMI;
AddrDispShift = AddrDispShiftTemp;
Dist = DistTemp;
// FIXME: Maybe we should not always stop at the first LEA after MI.
if (DistTemp < 0)
return BestLEA != nullptr;
// Get the difference between the addresses' displacements of the two
// instructions \p MI1 and \p MI2. The numbers of the first memory operands are
// passed through \p N1 and \p N2.
int64_t X86OptimizeLEAPass::getAddrDispShift(const MachineInstr &MI1,
unsigned N1,
const MachineInstr &MI2,
unsigned N2) const {
const MachineOperand &Op1 = MI1.getOperand(N1 + X86::AddrDisp);
const MachineOperand &Op2 = MI2.getOperand(N2 + X86::AddrDisp);
assert(isSimilarDispOp(Op1, Op2) &&
"Address displacement operands are not compatible");
// After the assert above we can be sure that both operands are of the same
// valid type and use the same symbol/index/address, thus displacement shift
// calculation is rather simple.
if (Op1.isJTI())
return 0;
return Op1.isImm() ? Op1.getImm() - Op2.getImm()
: Op1.getOffset() - Op2.getOffset();
// Check that the Last LEA can be replaced by the First LEA. To be so,
// these requirements must be met:
// 1) Addresses calculated by LEAs differ only by displacement.
// 2) Def registers of LEAs belong to the same class.
// 3) All uses of the Last LEA def register are replaceable, thus the
// register is used only as address base.
bool X86OptimizeLEAPass::isReplaceable(const MachineInstr &First,
const MachineInstr &Last,
int64_t &AddrDispShift) const {
assert(isLEA(First) && isLEA(Last) &&
"The function works only with LEA instructions");
// Make sure that LEA def registers belong to the same class. There may be
// instructions (like MOV8mr_NOREX) which allow a limited set of registers to
// be used as their operands, so we must be sure that replacing one LEA
// with another won't lead to putting a wrong register in the instruction.
if (MRI->getRegClass(First.getOperand(0).getReg()) !=
return false;
// Get new address displacement.
AddrDispShift = getAddrDispShift(Last, 1, First, 1);
// Loop over all uses of the Last LEA to check that its def register is
// used only as address base for memory accesses. If so, it can be
// replaced, otherwise - no.
for (auto &MO : MRI->use_nodbg_operands(Last.getOperand(0).getReg())) {
MachineInstr &MI = *MO.getParent();
// Get the number of the first memory operand.
const MCInstrDesc &Desc = MI.getDesc();
int MemOpNo = X86II::getMemoryOperandNo(Desc.TSFlags);
// If the use instruction has no memory operand - the LEA is not
// replaceable.
if (MemOpNo < 0)
return false;
MemOpNo += X86II::getOperandBias(Desc);
// If the address base of the use instruction is not the LEA def register -
// the LEA is not replaceable.
if (!isIdenticalOp(MI.getOperand(MemOpNo + X86::AddrBaseReg), MO))
return false;
// If the LEA def register is used as any other operand of the use
// instruction - the LEA is not replaceable.
for (unsigned i = 0; i < MI.getNumOperands(); i++)
if (i != (unsigned)(MemOpNo + X86::AddrBaseReg) &&
isIdenticalOp(MI.getOperand(i), MO))
return false;
// Check that the new address displacement will fit 4 bytes.
if (MI.getOperand(MemOpNo + X86::AddrDisp).isImm() &&
!isInt<32>(MI.getOperand(MemOpNo + X86::AddrDisp).getImm() +
return false;
return true;
void X86OptimizeLEAPass::findLEAs(const MachineBasicBlock &MBB,
MemOpMap &LEAs) {
unsigned Pos = 0;
for (auto &MI : MBB) {
// Assign the position number to the instruction. Note that we are going to
// move some instructions during the optimization however there will never
// be a need to move two instructions before any selected instruction. So to
// avoid multiple positions' updates during moves we just increase position
// counter by two leaving a free space for instructions which will be moved.
InstrPos[&MI] = Pos += 2;
if (isLEA(MI))
LEAs[getMemOpKey(MI, 1)].push_back(const_cast<MachineInstr *>(&MI));
// Try to find load and store instructions which recalculate addresses already
// calculated by some LEA and replace their memory operands with its def
// register.
bool X86OptimizeLEAPass::removeRedundantAddrCalc(MemOpMap &LEAs) {
bool Changed = false;
MachineBasicBlock *MBB = (*LEAs.begin()->second.begin())->getParent();
// Process all instructions in basic block.
for (auto I = MBB->begin(), E = MBB->end(); I != E;) {
MachineInstr &MI = *I++;
// Instruction must be load or store.
if (!MI.mayLoadOrStore())
// Get the number of the first memory operand.
const MCInstrDesc &Desc = MI.getDesc();
int MemOpNo = X86II::getMemoryOperandNo(Desc.TSFlags);
// If instruction has no memory operand - skip it.
if (MemOpNo < 0)
MemOpNo += X86II::getOperandBias(Desc);
// Do not call chooseBestLEA if there was no matching LEA
auto Insns = LEAs.find(getMemOpKey(MI, MemOpNo));
if (Insns == LEAs.end())
// Get the best LEA instruction to replace address calculation.
MachineInstr *DefMI;
int64_t AddrDispShift;
int Dist;
if (!chooseBestLEA(Insns->second, MI, DefMI, AddrDispShift, Dist))
// If LEA occurs before current instruction, we can freely replace
// the instruction. If LEA occurs after, we can lift LEA above the
// instruction and this way to be able to replace it. Since LEA and the
// instruction have similar memory operands (thus, the same def
// instructions for these operands), we can always do that, without
// worries of using registers before their defs.
if (Dist < 0) {
MBB->insert(MachineBasicBlock::iterator(&MI), DefMI);
InstrPos[DefMI] = InstrPos[&MI] - 1;
// Make sure the instructions' position numbers are sane.
assert(((InstrPos[DefMI] == 1 &&
MachineBasicBlock::iterator(DefMI) == MBB->begin()) ||
InstrPos[DefMI] >
InstrPos[&*std::prev(MachineBasicBlock::iterator(DefMI))]) &&
"Instruction positioning is broken");
// Since we can possibly extend register lifetime, clear kill flags.
LLVM_DEBUG(dbgs() << "OptimizeLEAs: Candidate to replace: "; MI.dump(););
// Change instruction operands.
MI.getOperand(MemOpNo + X86::AddrBaseReg)
.ChangeToRegister(DefMI->getOperand(0).getReg(), false);
MI.getOperand(MemOpNo + X86::AddrScaleAmt).ChangeToImmediate(1);
MI.getOperand(MemOpNo + X86::AddrIndexReg)
.ChangeToRegister(X86::NoRegister, false);
MI.getOperand(MemOpNo + X86::AddrDisp).ChangeToImmediate(AddrDispShift);
MI.getOperand(MemOpNo + X86::AddrSegmentReg)
.ChangeToRegister(X86::NoRegister, false);
LLVM_DEBUG(dbgs() << "OptimizeLEAs: Replaced by: "; MI.dump(););
Changed = true;
return Changed;
MachineInstr *X86OptimizeLEAPass::replaceDebugValue(MachineInstr &MI,
unsigned OldReg,
unsigned NewReg,
int64_t AddrDispShift) {
const DIExpression *Expr = MI.getDebugExpression();
if (AddrDispShift != 0) {
if (MI.isNonListDebugValue()) {
Expr =
DIExpression::prepend(Expr, DIExpression::StackValue, AddrDispShift);
} else {
// Update the Expression, appending an offset of `AddrDispShift` to the
// Op corresponding to `OldReg`.
SmallVector<uint64_t, 3> Ops;
DIExpression::appendOffset(Ops, AddrDispShift);
for (MachineOperand &Op : MI.getDebugOperandsForReg(OldReg)) {
unsigned OpIdx = MI.getDebugOperandIndex(&Op);
Expr = DIExpression::appendOpsToArg(Expr, Ops, OpIdx);
// Replace DBG_VALUE instruction with modified version.
MachineBasicBlock *MBB = MI.getParent();
DebugLoc DL = MI.getDebugLoc();
bool IsIndirect = MI.isIndirectDebugValue();
const MDNode *Var = MI.getDebugVariable();
unsigned Opcode = MI.isNonListDebugValue() ? TargetOpcode::DBG_VALUE
: TargetOpcode::DBG_VALUE_LIST;
if (IsIndirect)
assert(MI.getDebugOffset().getImm() == 0 &&
"DBG_VALUE with nonzero offset");
SmallVector<MachineOperand, 4> NewOps;
// If we encounter an operand using the old register, replace it with an
// operand that uses the new register; otherwise keep the old operand.
auto replaceOldReg = [OldReg, NewReg](const MachineOperand &Op) {
if (Op.isReg() && Op.getReg() == OldReg)
return MachineOperand::CreateReg(NewReg, false, false, false, false,
false, false, false, false, 0,
/*IsRenamable*/ true);
return Op;
for (const MachineOperand &Op : MI.debug_operands())
return BuildMI(*MBB, MBB->erase(&MI), DL, TII->get(Opcode), IsIndirect,
NewOps, Var, Expr);
// Try to find similar LEAs in the list and replace one with another.
bool X86OptimizeLEAPass::removeRedundantLEAs(MemOpMap &LEAs) {
bool Changed = false;
// Loop over all entries in the table.
for (auto &E : LEAs) {
auto &List = E.second;
// Loop over all LEA pairs.
auto I1 = List.begin();
while (I1 != List.end()) {
MachineInstr &First = **I1;
auto I2 = std::next(I1);
while (I2 != List.end()) {
MachineInstr &Last = **I2;
int64_t AddrDispShift;
// LEAs should be in occurrence order in the list, so we can freely
// replace later LEAs with earlier ones.
assert(calcInstrDist(First, Last) > 0 &&
"LEAs must be in occurrence order in the list");
// Check that the Last LEA instruction can be replaced by the First.
if (!isReplaceable(First, Last, AddrDispShift)) {
// Loop over all uses of the Last LEA and update their operands. Note
// that the correctness of this has already been checked in the
// isReplaceable function.
Register FirstVReg = First.getOperand(0).getReg();
Register LastVReg = Last.getOperand(0).getReg();
for (auto UI = MRI->use_begin(LastVReg), UE = MRI->use_end();
UI != UE;) {
MachineOperand &MO = *UI++;
MachineInstr &MI = *MO.getParent();
if (MI.isDebugValue()) {
// Replace DBG_VALUE instruction with modified version using the
// register from the replacing LEA and the address displacement
// between the LEA instructions.
replaceDebugValue(MI, LastVReg, FirstVReg, AddrDispShift);
// Get the number of the first memory operand.
const MCInstrDesc &Desc = MI.getDesc();
int MemOpNo =
X86II::getMemoryOperandNo(Desc.TSFlags) +
// Update address base.
// Update address disp.
MachineOperand &Op = MI.getOperand(MemOpNo + X86::AddrDisp);
if (Op.isImm())
Op.setImm(Op.getImm() + AddrDispShift);
else if (!Op.isJTI())
Op.setOffset(Op.getOffset() + AddrDispShift);
// Since we can possibly extend register lifetime, clear kill flags.
LLVM_DEBUG(dbgs() << "OptimizeLEAs: Remove redundant LEA: ";
// By this moment, all of the Last LEA's uses must be replaced. So we
// can freely remove it.
assert(MRI->use_empty(LastVReg) &&
"The LEA's def register must have no uses");
// Erase removed LEA from the list.
I2 = List.erase(I2);
Changed = true;
return Changed;
bool X86OptimizeLEAPass::runOnMachineFunction(MachineFunction &MF) {
bool Changed = false;
if (DisableX86LEAOpt || skipFunction(MF.getFunction()))
return false;
MRI = &MF.getRegInfo();
TII = MF.getSubtarget<X86Subtarget>().getInstrInfo();
TRI = MF.getSubtarget<X86Subtarget>().getRegisterInfo();
auto *PSI =
auto *MBFI = (PSI && PSI->hasProfileSummary()) ?
&getAnalysis<LazyMachineBlockFrequencyInfoPass>().getBFI() :
// Process all basic blocks.
for (auto &MBB : MF) {
MemOpMap LEAs;
// Find all LEA instructions in basic block.
findLEAs(MBB, LEAs);
// If current basic block has no LEAs, move on to the next one.
if (LEAs.empty())
// Remove redundant LEA instructions.
Changed |= removeRedundantLEAs(LEAs);
// Remove redundant address calculations. Do it only for -Os/-Oz since only
// a code size gain is expected from this part of the pass.
bool OptForSize = MF.getFunction().hasOptSize() ||
llvm::shouldOptimizeForSize(&MBB, PSI, MBFI);
if (OptForSize)
Changed |= removeRedundantAddrCalc(LEAs);
return Changed;