llvm/lib/Target/ARM/Thumb1FrameLowering.cpp

//===-- Thumb1FrameLowering.cpp - Thumb1 Frame Information ----------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains the Thumb1 implementation of TargetFrameLowering class.
//
//===----------------------------------------------------------------------===//

#include "Thumb1FrameLowering.h"
#include "ARMMachineFunctionInfo.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"

using namespace llvm;

Thumb1FrameLowering::Thumb1FrameLowering(const ARMSubtarget &sti)
    : ARMFrameLowering(sti) {}

bool Thumb1FrameLowering::hasReservedCallFrame(const MachineFunction &MF) const{
  const MachineFrameInfo *FFI = MF.getFrameInfo();
  unsigned CFSize = FFI->getMaxCallFrameSize();
  // It's not always a good idea to include the call frame as part of the
  // stack frame. ARM (especially Thumb) has small immediate offset to
  // address the stack frame. So a large call frame can cause poor codegen
  // and may even makes it impossible to scavenge a register.
  if (CFSize >= ((1 << 8) - 1) * 4 / 2) // Half of imm8 * 4
    return false;

  return !MF.getFrameInfo()->hasVarSizedObjects();
}

static void
emitSPUpdate(MachineBasicBlock &MBB,
             MachineBasicBlock::iterator &MBBI,
             const TargetInstrInfo &TII, DebugLoc dl,
             const ThumbRegisterInfo &MRI,
             int NumBytes, unsigned MIFlags = MachineInstr::NoFlags)  {
  emitThumbRegPlusImmediate(MBB, MBBI, dl, ARM::SP, ARM::SP, NumBytes, TII,
                            MRI, MIFlags);
}


void Thumb1FrameLowering::
eliminateCallFramePseudoInstr(MachineFunction &MF, MachineBasicBlock &MBB,
                              MachineBasicBlock::iterator I) const {
  const Thumb1InstrInfo &TII =
      *static_cast<const Thumb1InstrInfo *>(STI.getInstrInfo());
  const ThumbRegisterInfo *RegInfo =
      static_cast<const ThumbRegisterInfo *>(STI.getRegisterInfo());
  if (!hasReservedCallFrame(MF)) {
    // If we have alloca, convert as follows:
    // ADJCALLSTACKDOWN -> sub, sp, sp, amount
    // ADJCALLSTACKUP   -> add, sp, sp, amount
    MachineInstr *Old = I;
    DebugLoc dl = Old->getDebugLoc();
    unsigned Amount = Old->getOperand(0).getImm();
    if (Amount != 0) {
      // We need to keep the stack aligned properly.  To do this, we round the
      // amount of space needed for the outgoing arguments up to the next
      // alignment boundary.
      unsigned Align = getStackAlignment();
      Amount = (Amount+Align-1)/Align*Align;

      // Replace the pseudo instruction with a new instruction...
      unsigned Opc = Old->getOpcode();
      if (Opc == ARM::ADJCALLSTACKDOWN || Opc == ARM::tADJCALLSTACKDOWN) {
        emitSPUpdate(MBB, I, TII, dl, *RegInfo, -Amount);
      } else {
        assert(Opc == ARM::ADJCALLSTACKUP || Opc == ARM::tADJCALLSTACKUP);
        emitSPUpdate(MBB, I, TII, dl, *RegInfo, Amount);
      }
    }
  }
  MBB.erase(I);
}

void Thumb1FrameLowering::emitPrologue(MachineFunction &MF,
                                       MachineBasicBlock &MBB) const {
  assert(&MBB == &MF.front() && "Shrink-wrapping not yet implemented");
  MachineBasicBlock::iterator MBBI = MBB.begin();
  MachineFrameInfo  *MFI = MF.getFrameInfo();
  ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
  MachineModuleInfo &MMI = MF.getMMI();
  const MCRegisterInfo *MRI = MMI.getContext().getRegisterInfo();
  const ThumbRegisterInfo *RegInfo =
      static_cast<const ThumbRegisterInfo *>(STI.getRegisterInfo());
  const Thumb1InstrInfo &TII =
      *static_cast<const Thumb1InstrInfo *>(STI.getInstrInfo());

  unsigned ArgRegsSaveSize = AFI->getArgRegsSaveSize();
  unsigned NumBytes = MFI->getStackSize();
  assert(NumBytes >= ArgRegsSaveSize &&
         "ArgRegsSaveSize is included in NumBytes");
  const std::vector<CalleeSavedInfo> &CSI = MFI->getCalleeSavedInfo();
  DebugLoc dl = MBBI != MBB.end() ? MBBI->getDebugLoc() : DebugLoc();
  unsigned FramePtr = RegInfo->getFrameRegister(MF);
  unsigned BasePtr = RegInfo->getBaseRegister();
  int CFAOffset = 0;

  // Thumb add/sub sp, imm8 instructions implicitly multiply the offset by 4.
  NumBytes = (NumBytes + 3) & ~3;
  MFI->setStackSize(NumBytes);

  // Determine the sizes of each callee-save spill areas and record which frame
  // belongs to which callee-save spill areas.
  unsigned GPRCS1Size = 0, GPRCS2Size = 0, DPRCSSize = 0;
  int FramePtrSpillFI = 0;

  if (ArgRegsSaveSize) {
    emitSPUpdate(MBB, MBBI, TII, dl, *RegInfo, -ArgRegsSaveSize,
                 MachineInstr::FrameSetup);
    CFAOffset -= ArgRegsSaveSize;
    unsigned CFIIndex = MMI.addFrameInst(
        MCCFIInstruction::createDefCfaOffset(nullptr, CFAOffset));
    BuildMI(MBB, MBBI, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
        .addCFIIndex(CFIIndex)
        .setMIFlags(MachineInstr::FrameSetup);
  }

  if (!AFI->hasStackFrame()) {
    if (NumBytes - ArgRegsSaveSize != 0) {
      emitSPUpdate(MBB, MBBI, TII, dl, *RegInfo, -(NumBytes - ArgRegsSaveSize),
                   MachineInstr::FrameSetup);
      CFAOffset -= NumBytes - ArgRegsSaveSize;
      unsigned CFIIndex = MMI.addFrameInst(
          MCCFIInstruction::createDefCfaOffset(nullptr, CFAOffset));
      BuildMI(MBB, MBBI, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex)
          .setMIFlags(MachineInstr::FrameSetup);
    }
    return;
  }

  for (unsigned i = 0, e = CSI.size(); i != e; ++i) {
    unsigned Reg = CSI[i].getReg();
    int FI = CSI[i].getFrameIdx();
    switch (Reg) {
    case ARM::R8:
    case ARM::R9:
    case ARM::R10:
    case ARM::R11:
      if (STI.isTargetMachO()) {
        GPRCS2Size += 4;
        break;
      }
      // fallthrough
    case ARM::R4:
    case ARM::R5:
    case ARM::R6:
    case ARM::R7:
    case ARM::LR:
      if (Reg == FramePtr)
        FramePtrSpillFI = FI;
      GPRCS1Size += 4;
      break;
    default:
      DPRCSSize += 8;
    }
  }

  if (MBBI != MBB.end() && MBBI->getOpcode() == ARM::tPUSH) {
    ++MBBI;
    if (MBBI != MBB.end())
      dl = MBBI->getDebugLoc();
  }

  // Determine starting offsets of spill areas.
  unsigned DPRCSOffset  = NumBytes - ArgRegsSaveSize - (GPRCS1Size + GPRCS2Size + DPRCSSize);
  unsigned GPRCS2Offset = DPRCSOffset + DPRCSSize;
  unsigned GPRCS1Offset = GPRCS2Offset + GPRCS2Size;
  bool HasFP = hasFP(MF);
  if (HasFP)
    AFI->setFramePtrSpillOffset(MFI->getObjectOffset(FramePtrSpillFI) +
                                NumBytes);
  AFI->setGPRCalleeSavedArea1Offset(GPRCS1Offset);
  AFI->setGPRCalleeSavedArea2Offset(GPRCS2Offset);
  AFI->setDPRCalleeSavedAreaOffset(DPRCSOffset);
  NumBytes = DPRCSOffset;

  int FramePtrOffsetInBlock = 0;
  unsigned adjustedGPRCS1Size = GPRCS1Size;
  if (tryFoldSPUpdateIntoPushPop(STI, MF, std::prev(MBBI), NumBytes)) {
    FramePtrOffsetInBlock = NumBytes;
    adjustedGPRCS1Size += NumBytes;
    NumBytes = 0;
  }

  if (adjustedGPRCS1Size) {
    CFAOffset -= adjustedGPRCS1Size;
    unsigned CFIIndex = MMI.addFrameInst(
        MCCFIInstruction::createDefCfaOffset(nullptr, CFAOffset));
    BuildMI(MBB, MBBI, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
        .addCFIIndex(CFIIndex)
        .setMIFlags(MachineInstr::FrameSetup);
  }
  for (std::vector<CalleeSavedInfo>::const_iterator I = CSI.begin(),
         E = CSI.end(); I != E; ++I) {
    unsigned Reg = I->getReg();
    int FI = I->getFrameIdx();
    switch (Reg) {
    case ARM::R8:
    case ARM::R9:
    case ARM::R10:
    case ARM::R11:
    case ARM::R12:
      if (STI.isTargetMachO())
        break;
      // fallthough
    case ARM::R0:
    case ARM::R1:
    case ARM::R2:
    case ARM::R3:
    case ARM::R4:
    case ARM::R5:
    case ARM::R6:
    case ARM::R7:
    case ARM::LR:
      unsigned CFIIndex = MMI.addFrameInst(MCCFIInstruction::createOffset(
          nullptr, MRI->getDwarfRegNum(Reg, true), MFI->getObjectOffset(FI)));
      BuildMI(MBB, MBBI, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex)
          .setMIFlags(MachineInstr::FrameSetup);
      break;
    }
  }


  // Adjust FP so it point to the stack slot that contains the previous FP.
  if (HasFP) {
    FramePtrOffsetInBlock += MFI->getObjectOffset(FramePtrSpillFI)
		                     + GPRCS1Size + ArgRegsSaveSize;
    AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tADDrSPi), FramePtr)
      .addReg(ARM::SP).addImm(FramePtrOffsetInBlock / 4)
      .setMIFlags(MachineInstr::FrameSetup));
    if(FramePtrOffsetInBlock) {
      CFAOffset += FramePtrOffsetInBlock;
      unsigned CFIIndex = MMI.addFrameInst(MCCFIInstruction::createDefCfa(
          nullptr, MRI->getDwarfRegNum(FramePtr, true), CFAOffset));
      BuildMI(MBB, MBBI, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex)
          .setMIFlags(MachineInstr::FrameSetup);
    } else {
      unsigned CFIIndex =
          MMI.addFrameInst(MCCFIInstruction::createDefCfaRegister(
              nullptr, MRI->getDwarfRegNum(FramePtr, true)));
      BuildMI(MBB, MBBI, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex)
          .setMIFlags(MachineInstr::FrameSetup);
    }
    if (NumBytes > 508)
      // If offset is > 508 then sp cannot be adjusted in a single instruction,
      // try restoring from fp instead.
      AFI->setShouldRestoreSPFromFP(true);
  }

  if (NumBytes) {
    // Insert it after all the callee-save spills.
    emitSPUpdate(MBB, MBBI, TII, dl, *RegInfo, -NumBytes,
                 MachineInstr::FrameSetup);
    if (!HasFP) {
      CFAOffset -= NumBytes;
      unsigned CFIIndex = MMI.addFrameInst(
          MCCFIInstruction::createDefCfaOffset(nullptr, CFAOffset));
      BuildMI(MBB, MBBI, dl, TII.get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex)
          .setMIFlags(MachineInstr::FrameSetup);
    }
  }

  if (STI.isTargetELF() && HasFP)
    MFI->setOffsetAdjustment(MFI->getOffsetAdjustment() -
                             AFI->getFramePtrSpillOffset());

  AFI->setGPRCalleeSavedArea1Size(GPRCS1Size);
  AFI->setGPRCalleeSavedArea2Size(GPRCS2Size);
  AFI->setDPRCalleeSavedAreaSize(DPRCSSize);

  // Thumb1 does not currently support dynamic stack realignment.  Report a
  // fatal error rather then silently generate bad code.
  if (RegInfo->needsStackRealignment(MF))
      report_fatal_error("Dynamic stack realignment not supported for thumb1.");

  // If we need a base pointer, set it up here. It's whatever the value
  // of the stack pointer is at this point. Any variable size objects
  // will be allocated after this, so we can still use the base pointer
  // to reference locals.
  if (RegInfo->hasBasePointer(MF))
    AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVr), BasePtr)
                   .addReg(ARM::SP));

  // If the frame has variable sized objects then the epilogue must restore
  // the sp from fp. We can assume there's an FP here since hasFP already
  // checks for hasVarSizedObjects.
  if (MFI->hasVarSizedObjects())
    AFI->setShouldRestoreSPFromFP(true);
}

static bool isCSRestore(MachineInstr *MI, const MCPhysReg *CSRegs) {
  if (MI->getOpcode() == ARM::tLDRspi &&
      MI->getOperand(1).isFI() &&
      isCalleeSavedRegister(MI->getOperand(0).getReg(), CSRegs))
    return true;
  else if (MI->getOpcode() == ARM::tPOP) {
    // The first two operands are predicates. The last two are
    // imp-def and imp-use of SP. Check everything in between.
    for (int i = 2, e = MI->getNumOperands() - 2; i != e; ++i)
      if (!isCalleeSavedRegister(MI->getOperand(i).getReg(), CSRegs))
        return false;
    return true;
  }
  return false;
}

void Thumb1FrameLowering::emitEpilogue(MachineFunction &MF,
                                   MachineBasicBlock &MBB) const {
  MachineBasicBlock::iterator MBBI = MBB.getFirstTerminator();
  DebugLoc dl = MBBI != MBB.end() ? MBBI->getDebugLoc() : DebugLoc();
  MachineFrameInfo *MFI = MF.getFrameInfo();
  ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
  const ThumbRegisterInfo *RegInfo =
      static_cast<const ThumbRegisterInfo *>(STI.getRegisterInfo());
  const Thumb1InstrInfo &TII =
      *static_cast<const Thumb1InstrInfo *>(STI.getInstrInfo());

  unsigned ArgRegsSaveSize = AFI->getArgRegsSaveSize();
  int NumBytes = (int)MFI->getStackSize();
  assert((unsigned)NumBytes >= ArgRegsSaveSize &&
         "ArgRegsSaveSize is included in NumBytes");
  const MCPhysReg *CSRegs = RegInfo->getCalleeSavedRegs(&MF);
  unsigned FramePtr = RegInfo->getFrameRegister(MF);

  if (!AFI->hasStackFrame()) {
    if (NumBytes - ArgRegsSaveSize != 0)
      emitSPUpdate(MBB, MBBI, TII, dl, *RegInfo, NumBytes - ArgRegsSaveSize);
  } else {
    // Unwind MBBI to point to first LDR / VLDRD.
    if (MBBI != MBB.begin()) {
      do
        --MBBI;
      while (MBBI != MBB.begin() && isCSRestore(MBBI, CSRegs));
      if (!isCSRestore(MBBI, CSRegs))
        ++MBBI;
    }

    // Move SP to start of FP callee save spill area.
    NumBytes -= (AFI->getGPRCalleeSavedArea1Size() +
                 AFI->getGPRCalleeSavedArea2Size() +
                 AFI->getDPRCalleeSavedAreaSize() +
                 ArgRegsSaveSize);

    if (AFI->shouldRestoreSPFromFP()) {
      NumBytes = AFI->getFramePtrSpillOffset() - NumBytes;
      // Reset SP based on frame pointer only if the stack frame extends beyond
      // frame pointer stack slot, the target is ELF and the function has FP, or
      // the target uses var sized objects.
      if (NumBytes) {
        assert(!MFI->getPristineRegs(MF).test(ARM::R4) &&
               "No scratch register to restore SP from FP!");
        emitThumbRegPlusImmediate(MBB, MBBI, dl, ARM::R4, FramePtr, -NumBytes,
                                  TII, *RegInfo);
        AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVr),
                               ARM::SP)
          .addReg(ARM::R4));
      } else
        AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVr),
                               ARM::SP)
          .addReg(FramePtr));
    } else {
      if (MBBI != MBB.end() && MBBI->getOpcode() == ARM::tBX_RET &&
          &MBB.front() != MBBI && std::prev(MBBI)->getOpcode() == ARM::tPOP) {
        MachineBasicBlock::iterator PMBBI = std::prev(MBBI);
        if (!tryFoldSPUpdateIntoPushPop(STI, MF, PMBBI, NumBytes))
          emitSPUpdate(MBB, PMBBI, TII, dl, *RegInfo, NumBytes);
      } else if (!tryFoldSPUpdateIntoPushPop(STI, MF, MBBI, NumBytes))
        emitSPUpdate(MBB, MBBI, TII, dl, *RegInfo, NumBytes);
    }
  }

  bool IsV4PopReturn = false;
  for (const CalleeSavedInfo &CSI : MFI->getCalleeSavedInfo())
    if (CSI.getReg() == ARM::LR)
      IsV4PopReturn = true;
  IsV4PopReturn &= STI.hasV4TOps() && !STI.hasV5TOps();

  // Unlike T2 and ARM mode, the T1 pop instruction cannot restore
  // to LR, and we can't pop the value directly to the PC since
  // we need to update the SP after popping the value. So instead
  // we have to emit:
  //   POP {r3}
  //   ADD sp, #offset
  //   BX r3
  // If this would clobber a return value, then generate this sequence instead:
  //   MOV ip, r3
  //   POP {r3}
  //   ADD sp, #offset
  //   MOV lr, r3
  //   MOV r3, ip
  //   BX lr
  if (ArgRegsSaveSize || IsV4PopReturn) {
    // If MBBI is a return instruction, we may be able to directly restore
    // LR in the PC.
    // This is possible if we do not need to emit any SP update.
    // Otherwise, we need a temporary register to pop the value
    // and copy that value into LR.
    MBBI = MBB.getFirstTerminator();
    if (!ArgRegsSaveSize && MBBI != MBB.end() &&
        MBBI->getOpcode() == ARM::tBX_RET) {
      MachineInstrBuilder MIB =
          AddDefaultPred(
              BuildMI(MBB, MBBI, MBBI->getDebugLoc(), TII.get(ARM::tPOP_RET)))
              .addReg(ARM::PC, RegState::Define);
      MIB.copyImplicitOps(&*MBBI);
      // erase the old tBX_RET instruction
      MBB.erase(MBBI);
      return;
    }

    // Look for a temporary register to use.
    // First, compute the liveness information.
    LivePhysRegs UsedRegs(STI.getRegisterInfo());
    UsedRegs.addLiveOuts(&MBB, /*AddPristines*/ true);
    // The semantic of pristines changed recently and now,
    // the callee-saved registers that are touched in the function
    // are not part of the pristines set anymore.
    // Add those callee-saved now.
    const TargetRegisterInfo *TRI = STI.getRegisterInfo();
    const MCPhysReg *CSRegs = TRI->getCalleeSavedRegs(&MF);
    for (unsigned i = 0; CSRegs[i]; ++i)
      UsedRegs.addReg(CSRegs[i]);

    DebugLoc dl = DebugLoc();
    if (MBBI != MBB.end()) {
      dl = MBBI->getDebugLoc();
      auto InstUpToMBBI = MBB.end();
      // The post-decrement is on purpose here.
      // We want to have the liveness right before MBBI.
      while (InstUpToMBBI-- != MBBI)
        UsedRegs.stepBackward(*InstUpToMBBI);
    }

    // Look for a register that can be directly use in the POP.
    unsigned PopReg = 0;
    // And some temporary register, just in case.
    unsigned TemporaryReg = 0;
    BitVector PopFriendly =
        TRI->getAllocatableSet(MF, TRI->getRegClass(ARM::tGPRRegClassID));
    assert(PopFriendly.any() && "No allocatable pop-friendly register?!");
    // Rebuild the GPRs from the high registers because they are removed
    // form the GPR reg class for thumb1.
    BitVector GPRsNoLRSP =
        TRI->getAllocatableSet(MF, TRI->getRegClass(ARM::hGPRRegClassID));
    GPRsNoLRSP |= PopFriendly;
    GPRsNoLRSP.reset(ARM::LR);
    GPRsNoLRSP.reset(ARM::SP);
    GPRsNoLRSP.reset(ARM::PC);
    for (int Register = GPRsNoLRSP.find_first(); Register != -1;
         Register = GPRsNoLRSP.find_next(Register)) {
      if (!UsedRegs.contains(Register)) {
        // Remember the first pop-friendly register and exit.
        if (PopFriendly.test(Register)) {
          PopReg = Register;
          TemporaryReg = 0;
          break;
        }
        // Otherwise, remember that the register will be available to
        // save a pop-friendly register.
        TemporaryReg = Register;
      }
    }

    assert((PopReg || TemporaryReg) && "Cannot get LR");

    if (TemporaryReg) {
      assert(!PopReg && "Unnecessary MOV is about to be inserted");
      PopReg = PopFriendly.find_first();
      AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVr))
                         .addReg(TemporaryReg, RegState::Define)
                         .addReg(PopReg, RegState::Kill));
    }

    assert(PopReg && "Do not know how to get LR");
    AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tPOP)))
        .addReg(PopReg, RegState::Define);

    emitSPUpdate(MBB, MBBI, TII, dl, *RegInfo, ArgRegsSaveSize);

    if (!TemporaryReg && MBBI != MBB.end() &&
        MBBI->getOpcode() == ARM::tBX_RET) {
      MachineInstrBuilder MIB = BuildMI(MBB, MBBI, dl, TII.get(ARM::tBX))
                                    .addReg(PopReg, RegState::Kill);
      AddDefaultPred(MIB);
      MIB.copyImplicitOps(&*MBBI);
      // erase the old tBX_RET instruction
      MBB.erase(MBBI);
      return;
    }

    AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVr))
                       .addReg(ARM::LR, RegState::Define)
                       .addReg(PopReg, RegState::Kill));

    if (TemporaryReg) {
      AddDefaultPred(BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVr))
                         .addReg(PopReg, RegState::Define)
                         .addReg(TemporaryReg, RegState::Kill));
    }
  }
}

bool Thumb1FrameLowering::
spillCalleeSavedRegisters(MachineBasicBlock &MBB,
                          MachineBasicBlock::iterator MI,
                          const std::vector<CalleeSavedInfo> &CSI,
                          const TargetRegisterInfo *TRI) const {
  if (CSI.empty())
    return false;

  DebugLoc DL;
  const TargetInstrInfo &TII = *STI.getInstrInfo();

  if (MI != MBB.end()) DL = MI->getDebugLoc();

  MachineInstrBuilder MIB = BuildMI(MBB, MI, DL, TII.get(ARM::tPUSH));
  AddDefaultPred(MIB);
  for (unsigned i = CSI.size(); i != 0; --i) {
    unsigned Reg = CSI[i-1].getReg();
    bool isKill = true;

    // Add the callee-saved register as live-in unless it's LR and
    // @llvm.returnaddress is called. If LR is returned for @llvm.returnaddress
    // then it's already added to the function and entry block live-in sets.
    if (Reg == ARM::LR) {
      MachineFunction &MF = *MBB.getParent();
      if (MF.getFrameInfo()->isReturnAddressTaken() &&
          MF.getRegInfo().isLiveIn(Reg))
        isKill = false;
    }

    if (isKill)
      MBB.addLiveIn(Reg);

    MIB.addReg(Reg, getKillRegState(isKill));
  }
  MIB.setMIFlags(MachineInstr::FrameSetup);
  return true;
}

bool Thumb1FrameLowering::
restoreCalleeSavedRegisters(MachineBasicBlock &MBB,
                            MachineBasicBlock::iterator MI,
                            const std::vector<CalleeSavedInfo> &CSI,
                            const TargetRegisterInfo *TRI) const {
  if (CSI.empty())
    return false;

  MachineFunction &MF = *MBB.getParent();
  ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
  const TargetInstrInfo &TII = *STI.getInstrInfo();

  bool isVarArg = AFI->getArgRegsSaveSize() > 0;
  DebugLoc DL = MI->getDebugLoc();
  MachineInstrBuilder MIB = BuildMI(MF, DL, TII.get(ARM::tPOP));
  AddDefaultPred(MIB);

  bool NumRegs = false;
  for (unsigned i = CSI.size(); i != 0; --i) {
    unsigned Reg = CSI[i-1].getReg();
    if (Reg == ARM::LR && MBB.succ_empty()) {
      // Special epilogue for vararg functions. See emitEpilogue
      if (isVarArg)
        continue;
      // ARMv4T requires BX, see emitEpilogue
      if (STI.hasV4TOps() && !STI.hasV5TOps())
        continue;
      Reg = ARM::PC;
      (*MIB).setDesc(TII.get(ARM::tPOP_RET));
      if (MI != MBB.end())
        MIB.copyImplicitOps(&*MI);
      MI = MBB.erase(MI);
    }
    MIB.addReg(Reg, getDefRegState(true));
    NumRegs = true;
  }

  // It's illegal to emit pop instruction without operands.
  if (NumRegs)
    MBB.insert(MI, &*MIB);
  else
    MF.DeleteMachineInstr(MIB);

  return true;
}