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//===-- RegisterScavenging.cpp - Machine register scavenging --------------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This file implements the machine register scavenger. It can provide
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// information, such as unused registers, at any point in a machine basic block.
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// It also provides a mechanism to make registers available by evicting them to
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//===----------------------------------------------------------------------===//
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#include "llvm/CodeGen/RegisterScavenging.h"
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#include "llvm/CodeGen/MachineBasicBlock.h"
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#include "llvm/CodeGen/MachineFrameInfo.h"
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#include "llvm/CodeGen/MachineFunction.h"
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#include "llvm/CodeGen/MachineInstr.h"
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#include "llvm/CodeGen/MachineRegisterInfo.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/ErrorHandling.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Target/TargetInstrInfo.h"
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#include "llvm/Target/TargetRegisterInfo.h"
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#include "llvm/Target/TargetSubtargetInfo.h"
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#define DEBUG_TYPE "reg-scavenging"
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/// setUsed - Set the register units of this register as used.
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void RegScavenger::setRegUsed(unsigned Reg) {
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for (MCRegUnitIterator RUI(Reg, TRI); RUI.isValid(); ++RUI)
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RegUnitsAvailable.reset(*RUI);
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void RegScavenger::initRegState() {
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for (SmallVectorImpl<ScavengedInfo>::iterator I = Scavenged.begin(),
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IE = Scavenged.end(); I != IE; ++I) {
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// All register units start out unused.
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RegUnitsAvailable.set();
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// Live-in registers are in use.
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for (MachineBasicBlock::livein_iterator I = MBB->livein_begin(),
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E = MBB->livein_end(); I != E; ++I)
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// Pristine CSRs are also unavailable.
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const MachineFunction &MF = *MBB->getParent();
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BitVector PR = MF.getFrameInfo()->getPristineRegs(MF);
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for (int I = PR.find_first(); I>0; I = PR.find_next(I))
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void RegScavenger::enterBasicBlock(MachineBasicBlock *mbb) {
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MachineFunction &MF = *mbb->getParent();
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TII = MF.getSubtarget().getInstrInfo();
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TRI = MF.getSubtarget().getRegisterInfo();
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MRI = &MF.getRegInfo();
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assert((NumRegUnits == 0 || NumRegUnits == TRI->getNumRegUnits()) &&
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// It is not possible to use the register scavenger after late optimization
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// passes that don't preserve accurate liveness information.
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assert(MRI->tracksLiveness() &&
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"Cannot use register scavenger with inaccurate liveness");
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NumRegUnits = TRI->getNumRegUnits();
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RegUnitsAvailable.resize(NumRegUnits);
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KillRegUnits.resize(NumRegUnits);
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DefRegUnits.resize(NumRegUnits);
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TmpRegUnits.resize(NumRegUnits);
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void RegScavenger::addRegUnits(BitVector &BV, unsigned Reg) {
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for (MCRegUnitIterator RUI(Reg, TRI); RUI.isValid(); ++RUI)
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void RegScavenger::determineKillsAndDefs() {
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assert(Tracking && "Must be tracking to determine kills and defs");
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MachineInstr *MI = MBBI;
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assert(!MI->isDebugValue() && "Debug values have no kills or defs");
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// Find out which registers are early clobbered, killed, defined, and marked
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// def-dead in this instruction.
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KillRegUnits.reset();
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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const MachineOperand &MO = MI->getOperand(i);
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if (MO.isRegMask()) {
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for (unsigned RU = 0, RUEnd = TRI->getNumRegUnits(); RU != RUEnd; ++RU) {
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for (MCRegUnitRootIterator RURI(RU, TRI); RURI.isValid(); ++RURI) {
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if (MO.clobbersPhysReg(*RURI)) {
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KillRegUnits |= TmpRegUnits;
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unsigned Reg = MO.getReg();
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if (!Reg || TargetRegisterInfo::isVirtualRegister(Reg) || isReserved(Reg))
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// Ignore undef uses.
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addRegUnits(KillRegUnits, Reg);
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addRegUnits(KillRegUnits, Reg);
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addRegUnits(DefRegUnits, Reg);
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void RegScavenger::unprocess() {
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assert(Tracking && "Cannot unprocess because we're not tracking");
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MachineInstr *MI = MBBI;
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if (!MI->isDebugValue()) {
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determineKillsAndDefs();
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// Commit the changes.
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setUsed(KillRegUnits);
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setUnused(DefRegUnits);
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if (MBBI == MBB->begin()) {
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MBBI = MachineBasicBlock::iterator(nullptr);
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void RegScavenger::forward() {
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assert(MBBI != MBB->end() && "Already past the end of the basic block!");
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MBBI = std::next(MBBI);
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assert(MBBI != MBB->end() && "Already at the end of the basic block!");
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MachineInstr *MI = MBBI;
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for (SmallVectorImpl<ScavengedInfo>::iterator I = Scavenged.begin(),
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IE = Scavenged.end(); I != IE; ++I) {
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if (I->Restore != MI)
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I->Restore = nullptr;
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if (MI->isDebugValue())
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determineKillsAndDefs();
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// Verify uses and defs.
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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const MachineOperand &MO = MI->getOperand(i);
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unsigned Reg = MO.getReg();
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if (!Reg || TargetRegisterInfo::isVirtualRegister(Reg) || isReserved(Reg))
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if (!isRegUsed(Reg)) {
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// Check if it's partial live: e.g.
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// D0 = insert_subreg D0<undef>, S0
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// The problem is the insert_subreg could be eliminated. The use of
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// D0 is using a partially undef value. This is not *incorrect* since
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// S1 is can be freely clobbered.
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// Ideally we would like a way to model this, but leaving the
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// insert_subreg around causes both correctness and performance issues.
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bool SubUsed = false;
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for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
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if (isRegUsed(*SubRegs)) {
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bool SuperUsed = false;
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for (MCSuperRegIterator SR(Reg, TRI); SR.isValid(); ++SR) {
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if (isRegUsed(*SR)) {
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if (!SubUsed && !SuperUsed) {
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MBB->getParent()->verify(nullptr, "In Register Scavenger");
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llvm_unreachable("Using an undefined register!");
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// FIXME: Enable this once we've figured out how to correctly transfer
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// implicit kills during codegen passes like the coalescer.
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assert((KillRegs.test(Reg) || isUnused(Reg) ||
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isLiveInButUnusedBefore(Reg, MI, MBB, TRI, MRI)) &&
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"Re-defining a live register!");
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// Commit the changes.
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setUnused(KillRegUnits);
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setUsed(DefRegUnits);
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bool RegScavenger::isRegUsed(unsigned Reg, bool includeReserved) const {
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if (includeReserved && isReserved(Reg))
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for (MCRegUnitIterator RUI(Reg, TRI); RUI.isValid(); ++RUI)
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if (!RegUnitsAvailable.test(*RUI))
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unsigned RegScavenger::FindUnusedReg(const TargetRegisterClass *RC) const {
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for (TargetRegisterClass::iterator I = RC->begin(), E = RC->end();
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if (!isRegUsed(*I)) {
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DEBUG(dbgs() << "Scavenger found unused reg: " << TRI->getName(*I) <<
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/// getRegsAvailable - Return all available registers in the register class
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BitVector RegScavenger::getRegsAvailable(const TargetRegisterClass *RC) {
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BitVector Mask(TRI->getNumRegs());
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for (TargetRegisterClass::iterator I = RC->begin(), E = RC->end();
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/// findSurvivorReg - Return the candidate register that is unused for the
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/// longest after StartMII. UseMI is set to the instruction where the search
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/// No more than InstrLimit instructions are inspected.
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unsigned RegScavenger::findSurvivorReg(MachineBasicBlock::iterator StartMI,
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BitVector &Candidates,
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MachineBasicBlock::iterator &UseMI) {
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int Survivor = Candidates.find_first();
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assert(Survivor > 0 && "No candidates for scavenging");
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MachineBasicBlock::iterator ME = MBB->getFirstTerminator();
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assert(StartMI != ME && "MI already at terminator");
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MachineBasicBlock::iterator RestorePointMI = StartMI;
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MachineBasicBlock::iterator MI = StartMI;
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bool inVirtLiveRange = false;
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for (++MI; InstrLimit > 0 && MI != ME; ++MI, --InstrLimit) {
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if (MI->isDebugValue()) {
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++InstrLimit; // Don't count debug instructions
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bool isVirtKillInsn = false;
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bool isVirtDefInsn = false;
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// Remove any candidates touched by instruction.
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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const MachineOperand &MO = MI->getOperand(i);
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Candidates.clearBitsNotInMask(MO.getRegMask());
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if (!MO.isReg() || MO.isUndef() || !MO.getReg())
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if (TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
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isVirtDefInsn = true;
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else if (MO.isKill())
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isVirtKillInsn = true;
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for (MCRegAliasIterator AI(MO.getReg(), TRI, true); AI.isValid(); ++AI)
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Candidates.reset(*AI);
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// If we're not in a virtual reg's live range, this is a valid
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if (!inVirtLiveRange) RestorePointMI = MI;
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// Update whether we're in the live range of a virtual register
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if (isVirtKillInsn) inVirtLiveRange = false;
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if (isVirtDefInsn) inVirtLiveRange = true;
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// Was our survivor untouched by this instruction?
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if (Candidates.test(Survivor))
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// All candidates gone?
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if (Candidates.none())
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Survivor = Candidates.find_first();
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// If we ran off the end, that's where we want to restore.
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if (MI == ME) RestorePointMI = ME;
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assert (RestorePointMI != StartMI &&
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"No available scavenger restore location!");
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// We ran out of candidates, so stop the search.
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UseMI = RestorePointMI;
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static unsigned getFrameIndexOperandNum(MachineInstr *MI) {
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while (!MI->getOperand(i).isFI()) {
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assert(i < MI->getNumOperands() &&
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"Instr doesn't have FrameIndex operand!");
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unsigned RegScavenger::scavengeRegister(const TargetRegisterClass *RC,
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MachineBasicBlock::iterator I,
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// Consider all allocatable registers in the register class initially
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BitVector Candidates =
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TRI->getAllocatableSet(*I->getParent()->getParent(), RC);
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// Exclude all the registers being used by the instruction.
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
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MachineOperand &MO = I->getOperand(i);
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if (MO.isReg() && MO.getReg() != 0 && !(MO.isUse() && MO.isUndef()) &&
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!TargetRegisterInfo::isVirtualRegister(MO.getReg()))
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Candidates.reset(MO.getReg());
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// Try to find a register that's unused if there is one, as then we won't
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BitVector Available = getRegsAvailable(RC);
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Available &= Candidates;
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Candidates = Available;
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// Find the register whose use is furthest away.
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MachineBasicBlock::iterator UseMI;
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unsigned SReg = findSurvivorReg(I, Candidates, 25, UseMI);
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// If we found an unused register there is no reason to spill it.
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if (!isRegUsed(SReg)) {
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DEBUG(dbgs() << "Scavenged register: " << TRI->getName(SReg) << "\n");
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// Find an available scavenging slot.
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for (SI = 0; SI < Scavenged.size(); ++SI)
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if (Scavenged[SI].Reg == 0)
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if (SI == Scavenged.size()) {
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// We need to scavenge a register but have no spill slot, the target
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// must know how to do it (if not, we'll assert below).
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Scavenged.push_back(ScavengedInfo());
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// Avoid infinite regress
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Scavenged[SI].Reg = SReg;
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// If the target knows how to save/restore the register, let it do so;
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// otherwise, use the emergency stack spill slot.
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if (!TRI->saveScavengerRegister(*MBB, I, UseMI, RC, SReg)) {
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// Spill the scavenged register before I.
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assert(Scavenged[SI].FrameIndex >= 0 &&
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"Cannot scavenge register without an emergency spill slot!");
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TII->storeRegToStackSlot(*MBB, I, SReg, true, Scavenged[SI].FrameIndex,
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MachineBasicBlock::iterator II = std::prev(I);
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unsigned FIOperandNum = getFrameIndexOperandNum(II);
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TRI->eliminateFrameIndex(II, SPAdj, FIOperandNum, this);
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// Restore the scavenged register before its use (or first terminator).
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TII->loadRegFromStackSlot(*MBB, UseMI, SReg, Scavenged[SI].FrameIndex,
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II = std::prev(UseMI);
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FIOperandNum = getFrameIndexOperandNum(II);
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TRI->eliminateFrameIndex(II, SPAdj, FIOperandNum, this);
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Scavenged[SI].Restore = std::prev(UseMI);
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// Doing this here leads to infinite regress.
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// Scavenged[SI].Reg = SReg;
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DEBUG(dbgs() << "Scavenged register (with spill): " << TRI->getName(SReg) <<