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+//===-- X86OptimizeLEAs.cpp - optimize usage of LEA instructions ----------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the pass that performs some optimizations with LEA
+// instructions in order to improve 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 "X86.h"
+#include "X86InstrInfo.h"
+#include "X86Subtarget.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/LiveVariables.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/IR/Function.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetInstrInfo.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "x86-optimize-LEAs"
+
+static cl::opt<bool> EnableX86LEAOpt("enable-x86-lea-opt", cl::Hidden,
+ cl::desc("X86: Enable LEA optimizations."),
+ cl::init(false));
+
+STATISTIC(NumSubstLEAs, "Number of LEA instruction substitutions");
+STATISTIC(NumRedundantLEAs, "Number of redundant LEA instructions removed");
+
+namespace {
+class OptimizeLEAPass : public MachineFunctionPass {
+public:
+ OptimizeLEAPass() : MachineFunctionPass(ID) {}
+
+ const char *getPassName() const override { return "X86 LEA Optimize"; }
+
+ /// \brief 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;
+
+private:
+ /// \brief 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);
+
+ /// \brief 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 choosen \p LEA in \p AddrDispShift
+ /// and \p Dist.
+ bool chooseBestLEA(const SmallVectorImpl<MachineInstr *> &List,
+ const MachineInstr &MI, MachineInstr *&LEA,
+ int64_t &AddrDispShift, int &Dist);
+
+ /// \brief Returns true if two machine operand are identical and they are not
+ /// physical registers.
+ bool isIdenticalOp(const MachineOperand &MO1, const MachineOperand &MO2);
+
+ /// \brief Returns true if the instruction is LEA.
+ bool isLEA(const MachineInstr &MI);
+
+ /// \brief 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);
+
+ /// \brief Returns true if two instructions have memory operands that only
+ /// differ by displacement. The numbers of the first memory operands for both
+ /// instructions are specified through \p N1 and \p N2. The address
+ /// displacement is returned through AddrDispShift.
+ bool isSimilarMemOp(const MachineInstr &MI1, unsigned N1,
+ const MachineInstr &MI2, unsigned N2,
+ int64_t &AddrDispShift);
+
+ /// \brief 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,
+ SmallVectorImpl<MachineInstr *> &List);
+
+ /// \brief Removes redundant address calculations.
+ bool removeRedundantAddrCalc(const SmallVectorImpl<MachineInstr *> &List);
+
+ /// \brief Removes LEAs which calculate similar addresses.
+ bool removeRedundantLEAs(SmallVectorImpl<MachineInstr *> &List);
+
+ DenseMap<const MachineInstr *, unsigned> InstrPos;
+
+ MachineRegisterInfo *MRI;
+ const X86InstrInfo *TII;
+ const X86RegisterInfo *TRI;
+
+ static char ID;
+};
+char OptimizeLEAPass::ID = 0;
+}
+
+FunctionPass *llvm::createX86OptimizeLEAs() { return new OptimizeLEAPass(); }
+
+int OptimizeLEAPass::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 OptimizeLEAPass::chooseBestLEA(const SmallVectorImpl<MachineInstr *> &List,
+ const MachineInstr &MI, MachineInstr *&LEA,
+ int64_t &AddrDispShift, int &Dist) {
+ const MachineFunction *MF = MI.getParent()->getParent();
+ const MCInstrDesc &Desc = MI.getDesc();
+ int MemOpNo = X86II::getMemoryOperandNo(Desc.TSFlags, MI.getOpcode()) +
+ X86II::getOperandBias(Desc);
+
+ LEA = nullptr;
+
+ // Loop over all LEA instructions.
+ for (auto DefMI : List) {
+ int64_t AddrDispShiftTemp = 0;
+
+ // Compare instructions memory operands.
+ if (!isSimilarMemOp(MI, MemOpNo, *DefMI, 1, AddrDispShiftTemp))
+ continue;
+
+ // Make sure address displacement fits 4 bytes.
+ if (!isInt<32>(AddrDispShiftTemp))
+ continue;
+
+ // 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) !=
+ MRI->getRegClass(DefMI->getOperand(0).getReg()))
+ continue;
+
+ // 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 || LEA == 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 (LEA != nullptr && !isInt<8>(AddrDispShiftTemp) &&
+ isInt<8>(AddrDispShift))
+ continue;
+
+ LEA = DefMI;
+ AddrDispShift = AddrDispShiftTemp;
+ Dist = DistTemp;
+ }
+
+ // FIXME: Maybe we should not always stop at the first LEA after MI.
+ if (DistTemp < 0)
+ break;
+ }
+
+ return LEA != nullptr;
+}
+
+bool OptimizeLEAPass::isIdenticalOp(const MachineOperand &MO1,
+ const MachineOperand &MO2) {
+ return MO1.isIdenticalTo(MO2) &&
+ (!MO1.isReg() ||
+ !TargetRegisterInfo::isPhysicalRegister(MO1.getReg()));
+}
+
+bool OptimizeLEAPass::isLEA(const MachineInstr &MI) {
+ unsigned Opcode = MI.getOpcode();
+ return Opcode == X86::LEA16r || Opcode == X86::LEA32r ||
+ Opcode == X86::LEA64r || Opcode == X86::LEA64_32r;
+}
+
+// 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 OptimizeLEAPass::isReplaceable(const MachineInstr &First,
+ const MachineInstr &Last,
+ int64_t &AddrDispShift) {
+ assert(isLEA(First) && isLEA(Last) &&
+ "The function works only with LEA instructions");
+
+ // Compare instructions' memory operands.
+ if (!isSimilarMemOp(Last, 1, First, 1, AddrDispShift))
+ return false;
+
+ // 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()) !=
+ MRI->getRegClass(Last.getOperand(0).getReg()))
+ return false;
+
+ // 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_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, MI.getOpcode());
+
+ // 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() +
+ AddrDispShift))
+ return false;
+ }
+
+ return true;
+}
+
+// Check if MI1 and MI2 have memory operands which represent addresses that
+// differ only by displacement.
+bool OptimizeLEAPass::isSimilarMemOp(const MachineInstr &MI1, unsigned N1,
+ const MachineInstr &MI2, unsigned N2,
+ int64_t &AddrDispShift) {
+ // Address base, scale, index and segment operands must be identical.
+ static const int IdenticalOpNums[] = {X86::AddrBaseReg, X86::AddrScaleAmt,
+ X86::AddrIndexReg, X86::AddrSegmentReg};
+ for (auto &N : IdenticalOpNums)
+ if (!isIdenticalOp(MI1.getOperand(N1 + N), MI2.getOperand(N2 + N)))
+ return false;
+
+ // Address displacement operands may differ by a constant.
+ const MachineOperand *Op1 = &MI1.getOperand(N1 + X86::AddrDisp);
+ const MachineOperand *Op2 = &MI2.getOperand(N2 + X86::AddrDisp);
+ if (!isIdenticalOp(*Op1, *Op2)) {
+ if (Op1->isImm() && Op2->isImm())
+ AddrDispShift = Op1->getImm() - Op2->getImm();
+ else if (Op1->isGlobal() && Op2->isGlobal() &&
+ Op1->getGlobal() == Op2->getGlobal())
+ AddrDispShift = Op1->getOffset() - Op2->getOffset();
+ else
+ return false;
+ }
+
+ return true;
+}
+
+void OptimizeLEAPass::findLEAs(const MachineBasicBlock &MBB,
+ SmallVectorImpl<MachineInstr *> &List) {
+ 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))
+ List.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 OptimizeLEAPass::removeRedundantAddrCalc(
+ const SmallVectorImpl<MachineInstr *> &List) {
+ bool Changed = false;
+
+ assert(List.size() > 0);
+ MachineBasicBlock *MBB = List[0]->getParent();
+
+ // Process all instructions in basic block.
+ for (auto I = MBB->begin(), E = MBB->end(); I != E;) {
+ MachineInstr &MI = *I++;
+ unsigned Opcode = MI.getOpcode();
+
+ // Instruction must be load or store.
+ if (!MI.mayLoadOrStore())
+ continue;
+
+ // Get the number of the first memory operand.
+ const MCInstrDesc &Desc = MI.getDesc();
+ int MemOpNo = X86II::getMemoryOperandNo(Desc.TSFlags, Opcode);
+
+ // If instruction has no memory operand - skip it.
+ if (MemOpNo < 0)
+ continue;
+
+ MemOpNo += X86II::getOperandBias(Desc);
+
+ // Get the best LEA instruction to replace address calculation.
+ MachineInstr *DefMI;
+ int64_t AddrDispShift;
+ int Dist;
+ if (!chooseBestLEA(List, MI, DefMI, AddrDispShift, Dist))
+ continue;
+
+ // 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) {
+ DefMI->removeFromParent();
+ MBB->insert(MachineBasicBlock::iterator(&MI), DefMI);
+ InstrPos[DefMI] = InstrPos[&MI] - 1;
+
+ // Make sure the instructions' position numbers are sane.
+ assert(((InstrPos[DefMI] == 1 && 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.
+ MRI->clearKillFlags(DefMI->getOperand(0).getReg());
+
+ ++NumSubstLEAs;
+ 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);
+
+ DEBUG(dbgs() << "OptimizeLEAs: Replaced by: "; MI.dump(););
+
+ Changed = true;
+ }
+
+ return Changed;
+}
+
+// Try to find similar LEAs in the list and replace one with another.
+bool
+OptimizeLEAPass::removeRedundantLEAs(SmallVectorImpl<MachineInstr *> &List) {
+ bool Changed = false;
+
+ // 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 occurence order in the list, so we can freely
+ // replace later LEAs with earlier ones.
+ assert(calcInstrDist(First, Last) > 0 &&
+ "LEAs must be in occurence order in the list");
+
+ // Check that the Last LEA instruction can be replaced by the First.
+ if (!isReplaceable(First, Last, AddrDispShift)) {
+ ++I2;
+ continue;
+ }
+
+ // 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.
+ for (auto UI = MRI->use_begin(Last.getOperand(0).getReg()),
+ UE = MRI->use_end();
+ UI != UE;) {
+ MachineOperand &MO = *UI++;
+ MachineInstr &MI = *MO.getParent();
+
+ // Get the number of the first memory operand.
+ const MCInstrDesc &Desc = MI.getDesc();
+ int MemOpNo = X86II::getMemoryOperandNo(Desc.TSFlags, MI.getOpcode()) +
+ X86II::getOperandBias(Desc);
+
+ // Update address base.
+ MO.setReg(First.getOperand(0).getReg());
+
+ // Update address disp.
+ MachineOperand *Op = &MI.getOperand(MemOpNo + X86::AddrDisp);
+ if (Op->isImm())
+ Op->setImm(Op->getImm() + AddrDispShift);
+ else if (Op->isGlobal())
+ Op->setOffset(Op->getOffset() + AddrDispShift);
+ else
+ llvm_unreachable("Invalid address displacement operand");
+ }
+
+ // Since we can possibly extend register lifetime, clear kill flags.
+ MRI->clearKillFlags(First.getOperand(0).getReg());
+
+ ++NumRedundantLEAs;
+ DEBUG(dbgs() << "OptimizeLEAs: Remove redundant LEA: "; Last.dump(););
+
+ // By this moment, all of the Last LEA's uses must be replaced. So we can
+ // freely remove it.
+ assert(MRI->use_empty(Last.getOperand(0).getReg()) &&
+ "The LEA's def register must have no uses");
+ Last.eraseFromParent();
+
+ // Erase removed LEA from the list.
+ I2 = List.erase(I2);
+
+ Changed = true;
+ }
+ ++I1;
+ }
+
+ return Changed;
+}
+
+bool OptimizeLEAPass::runOnMachineFunction(MachineFunction &MF) {
+ bool Changed = false;
+
+ // Perform this optimization only if we care about code size.
+ if (!EnableX86LEAOpt || !MF.getFunction()->optForSize())
+ return false;
+
+ MRI = &MF.getRegInfo();
+ TII = MF.getSubtarget<X86Subtarget>().getInstrInfo();
+ TRI = MF.getSubtarget<X86Subtarget>().getRegisterInfo();
+
+ // Process all basic blocks.
+ for (auto &MBB : MF) {
+ SmallVector<MachineInstr *, 16> LEAs;
+ InstrPos.clear();
+
+ // 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())
+ continue;
+
+ // Remove redundant LEA instructions. The optimization may have a negative
+ // effect on performance, so do it only for -Oz.
+ if (MF.getFunction()->optForMinSize())
+ Changed |= removeRedundantLEAs(LEAs);
+
+ // Remove redundant address calculations.
+ Changed |= removeRedundantAddrCalc(LEAs);
+ }
+
+ return Changed;
+}
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