Example #1
0
unsigned RegScavenger::findSurvivorReg(MachineBasicBlock::iterator StartMI,
                                       BitVector &Candidates,
                                       unsigned InstrLimit,
                                       MachineBasicBlock::iterator &UseMI) {
  int Survivor = Candidates.find_first();
  assert(Survivor > 0 && "No candidates for scavenging");

  MachineBasicBlock::iterator ME = MBB->getFirstTerminator();
  assert(StartMI != ME && "MI already at terminator");
  MachineBasicBlock::iterator RestorePointMI = StartMI;
  MachineBasicBlock::iterator MI = StartMI;

  bool inVirtLiveRange = false;
  for (++MI; InstrLimit > 0 && MI != ME; ++MI, --InstrLimit) {
    if (MI->isDebugInstr()) {
      ++InstrLimit; // Don't count debug instructions
      continue;
    }
    bool isVirtKillInsn = false;
    bool isVirtDefInsn = false;
    // Remove any candidates touched by instruction.
    for (const MachineOperand &MO : MI->operands()) {
      if (MO.isRegMask())
        Candidates.clearBitsNotInMask(MO.getRegMask());
      if (!MO.isReg() || MO.isUndef() || !MO.getReg())
        continue;
      if (TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
        if (MO.isDef())
          isVirtDefInsn = true;
        else if (MO.isKill())
          isVirtKillInsn = true;
        continue;
      }
      for (MCRegAliasIterator AI(MO.getReg(), TRI, true); AI.isValid(); ++AI)
        Candidates.reset(*AI);
    }
    // If we're not in a virtual reg's live range, this is a valid
    // restore point.
    if (!inVirtLiveRange) RestorePointMI = MI;

    // Update whether we're in the live range of a virtual register
    if (isVirtKillInsn) inVirtLiveRange = false;
    if (isVirtDefInsn) inVirtLiveRange = true;

    // Was our survivor untouched by this instruction?
    if (Candidates.test(Survivor))
      continue;

    // All candidates gone?
    if (Candidates.none())
      break;

    Survivor = Candidates.find_first();
  }
  // If we ran off the end, that's where we want to restore.
  if (MI == ME) RestorePointMI = ME;
  assert(RestorePointMI != StartMI &&
         "No available scavenger restore location!");

  // We ran out of candidates, so stop the search.
  UseMI = RestorePointMI;
  return Survivor;
}