RegAllocLinearScan.cpp

  unsigned LastCandidate = RegsWeights.size() >= 3 ? 3 : 1;
  while (LastCandidate > 1) {
    if (weightsAreClose(RegsWeights[LastCandidate-1].second, minWeight))
      break;
    --LastCandidate;
  }

  DOUT << "\t\tregister(s) with min weight(s): ";
  DEBUG(for (unsigned i = 0; i != LastCandidate; ++i)
          DOUT << tri_->getName(RegsWeights[i].first)
               << " (" << RegsWeights[i].second << ")\n");

  // If the current has the minimum weight, we need to spill it and
  // add any added intervals back to unhandled, and restart
  // linearscan.
  if (cur->weight != HUGE_VALF && cur->weight <= minWeight) {
    DOUT << "\t\t\tspilling(c): " << *cur << '\n';
    float SSWeight;
    SmallVector<LiveInterval*, 8> spillIs;
    std::vector<LiveInterval*> added =
      li_->addIntervalsForSpills(*cur, spillIs, loopInfo, *vrm_, SSWeight);
    std::sort(added.begin(), added.end(), LISorter());
    addStackInterval(cur, ls_, li_, SSWeight, *vrm_);
    if (added.empty())
      return;  // Early exit if all spills were folded.

    // Merge added with unhandled.  Note that we have already sorted
    // intervals returned by addIntervalsForSpills by their starting
    // point.
    // This also update the NextReloadMap. That is, it adds mapping from a
    // register defined by a reload from SS to the next reload from SS in the
    // same basic block.
    MachineBasicBlock *LastReloadMBB = 0;
    LiveInterval *LastReload = 0;
    int LastReloadSS = VirtRegMap::NO_STACK_SLOT;
    for (unsigned i = 0, e = added.size(); i != e; ++i) {
      LiveInterval *ReloadLi = added[i];
      if (ReloadLi->weight == HUGE_VALF &&
          li_->getApproximateInstructionCount(*ReloadLi) == 0) {
        unsigned ReloadIdx = ReloadLi->beginNumber();
        MachineBasicBlock *ReloadMBB = li_->getMBBFromIndex(ReloadIdx);
        int ReloadSS = vrm_->getStackSlot(ReloadLi->reg);
        if (LastReloadMBB == ReloadMBB && LastReloadSS == ReloadSS) {
          // Last reload of same SS is in the same MBB. We want to try to
          // allocate both reloads the same register and make sure the reg
          // isn't clobbered in between if at all possible.
          assert(LastReload->beginNumber() < ReloadIdx);
          NextReloadMap.insert(std::make_pair(LastReload->reg, ReloadLi->reg));
        }
        LastReloadMBB = ReloadMBB;
        LastReload = ReloadLi;
        LastReloadSS = ReloadSS;
      }
      unhandled_.push(ReloadLi);
    }
    return;
  }

  ++NumBacktracks;

  // Push the current interval back to unhandled since we are going
  // to re-run at least this iteration. Since we didn't modify it it
  // should go back right in the front of the list
  unhandled_.push(cur);

  assert(TargetRegisterInfo::isPhysicalRegister(minReg) &&
         "did not choose a register to spill?");

  // We spill all intervals aliasing the register with
  // minimum weight, rollback to the interval with the earliest
  // start point and let the linear scan algorithm run again
  SmallVector<LiveInterval*, 8> spillIs;

  // Determine which intervals have to be spilled.
  findIntervalsToSpill(cur, RegsWeights, LastCandidate, spillIs);

  // Set of spilled vregs (used later to rollback properly)
  SmallSet<unsigned, 8> spilled;

  // The earliest start of a Spilled interval indicates up to where
  // in handled we need to roll back
  unsigned earliestStart = cur->beginNumber();

  // Spill live intervals of virtual regs mapped to the physical register we
  // want to clear (and its aliases).  We only spill those that overlap with the
  // current interval as the rest do not affect its allocation. we also keep
  // track of the earliest start of all spilled live intervals since this will
  // mark our rollback point.
  std::vector<LiveInterval*> added;
  while (!spillIs.empty()) {
    LiveInterval *sli = spillIs.back();
    spillIs.pop_back();
    DOUT << "\t\t\tspilling(a): " << *sli << '\n';
    earliestStart = std::min(earliestStart, sli->beginNumber());
    float SSWeight;
    std::vector<LiveInterval*> newIs =
      li_->addIntervalsForSpills(*sli, spillIs, loopInfo, *vrm_, SSWeight);
    addStackInterval(sli, ls_, li_, SSWeight, *vrm_);
    std::copy(newIs.begin(), newIs.end(), std::back_inserter(added));
    spilled.insert(sli->reg);
  }

  DOUT << "\t\trolling back to: " << earliestStart << '\n';

  // Scan handled in reverse order up to the earliest start of a
  // spilled live interval and undo each one, restoring the state of
  // unhandled.
  while (!handled_.empty()) {
    LiveInterval* i = handled_.back();
    // If this interval starts before t we are done.
    if (i->beginNumber() < earliestStart)
      break;
    DOUT << "\t\t\tundo changes for: " << *i << '\n';
    handled_.pop_back();

    // When undoing a live interval allocation we must know if it is active or
    // inactive to properly update the PhysRegTracker and the VirtRegMap.
    IntervalPtrs::iterator it;
    if ((it = FindIntervalInVector(active_, i)) != active_.end()) {
      active_.erase(it);
      assert(!TargetRegisterInfo::isPhysicalRegister(i->reg));
      if (!spilled.count(i->reg))
        unhandled_.push(i);
      prt_->delRegUse(vrm_->getPhys(i->reg));
      vrm_->clearVirt(i->reg);
    } else if ((it = FindIntervalInVector(inactive_, i)) != inactive_.end()) {
      inactive_.erase(it);
      assert(!TargetRegisterInfo::isPhysicalRegister(i->reg));
      if (!spilled.count(i->reg))
        unhandled_.push(i);
      vrm_->clearVirt(i->reg);
    } else {
      assert(TargetRegisterInfo::isVirtualRegister(i->reg) &&
             "Can only allocate virtual registers!");
      vrm_->clearVirt(i->reg);
      unhandled_.push(i);
    }

    DenseMap<unsigned, unsigned>::iterator ii = DowngradeMap.find(i->reg);
    if (ii == DowngradeMap.end())
      // It interval has a preference, it must be defined by a copy. Clear the
      // preference now since the source interval allocation may have been
      // undone as well.
      i->preference = 0;
    else {
      UpgradeRegister(ii->second);
    }
  }

  // Rewind the iterators in the active, inactive, and fixed lists back to the
  // point we reverted to.
  RevertVectorIteratorsTo(active_, earliestStart);
  RevertVectorIteratorsTo(inactive_, earliestStart);
  RevertVectorIteratorsTo(fixed_, earliestStart);

  // Scan the rest and undo each interval that expired after t and
  // insert it in active (the next iteration of the algorithm will
  // put it in inactive if required)
  for (unsigned i = 0, e = handled_.size(); i != e; ++i) {
    LiveInterval *HI = handled_[i];
    if (!HI->expiredAt(earliestStart) &&
        HI->expiredAt(cur->beginNumber())) {
      DOUT << "\t\t\tundo changes for: " << *HI << '\n';
      active_.push_back(std::make_pair(HI, HI->begin()));
      assert(!TargetRegisterInfo::isPhysicalRegister(HI->reg));
      prt_->addRegUse(vrm_->getPhys(HI->reg));
    }
  }

  // Merge added with unhandled.
  // This also update the NextReloadMap. That is, it adds mapping from a
  // register defined by a reload from SS to the next reload from SS in the
  // same basic block.
  MachineBasicBlock *LastReloadMBB = 0;
  LiveInterval *LastReload = 0;
  int LastReloadSS = VirtRegMap::NO_STACK_SLOT;
  std::sort(added.begin(), added.end(), LISorter());
  for (unsigned i = 0, e = added.size(); i != e; ++i) {
    LiveInterval *ReloadLi = added[i];
    if (ReloadLi->weight == HUGE_VALF &&
        li_->getApproximateInstructionCount(*ReloadLi) == 0) {
      unsigned ReloadIdx = ReloadLi->beginNumber();
      MachineBasicBlock *ReloadMBB = li_->getMBBFromIndex(ReloadIdx);
      int ReloadSS = vrm_->getStackSlot(ReloadLi->reg);
      if (LastReloadMBB == ReloadMBB && LastReloadSS == ReloadSS) {
        // Last reload of same SS is in the same MBB. We want to try to
        // allocate both reloads the same register and make sure the reg
        // isn't clobbered in between if at all possible.
        assert(LastReload->beginNumber() < ReloadIdx);
        NextReloadMap.insert(std::make_pair(LastReload->reg, ReloadLi->reg));
      }
      LastReloadMBB = ReloadMBB;
      LastReload = ReloadLi;
      LastReloadSS = ReloadSS;
    }
    unhandled_.push(ReloadLi);
  }
}

unsigned RALinScan::getFreePhysReg(const TargetRegisterClass *RC,
                                   unsigned MaxInactiveCount,
                                   SmallVector<unsigned, 256> &inactiveCounts,
                                   bool SkipDGRegs) {
  unsigned FreeReg = 0;
  unsigned FreeRegInactiveCount = 0;

  TargetRegisterClass::iterator I = RC->allocation_order_begin(*mf_);
  TargetRegisterClass::iterator E = RC->allocation_order_end(*mf_);
  assert(I != E && "No allocatable register in this register class!");

  // Scan for the first available register.
  for (; I != E; ++I) {
    unsigned Reg = *I;
    // Ignore "downgraded" registers.
    if (SkipDGRegs && DowngradedRegs.count(Reg))
      continue;
    if (prt_->isRegAvail(Reg)) {
      FreeReg = Reg;
      if (FreeReg < inactiveCounts.size())
        FreeRegInactiveCount = inactiveCounts[FreeReg];
      else
        FreeRegInactiveCount = 0;
      break;
    }
  }

  // If there are no free regs, or if this reg has the max inactive count,
  // return this register.
  if (FreeReg == 0 || FreeRegInactiveCount == MaxInactiveCount)
    return FreeReg;
  
  // Continue scanning the registers, looking for the one with the highest
  // inactive count.  Alkis found that this reduced register pressure very
  // slightly on X86 (in rev 1.94 of this file), though this should probably be
  // reevaluated now.
  for (; I != E; ++I) {
    unsigned Reg = *I;
    // Ignore "downgraded" registers.
    if (SkipDGRegs && DowngradedRegs.count(Reg))
      continue;
    if (prt_->isRegAvail(Reg) && Reg < inactiveCounts.size() &&
        FreeRegInactiveCount < inactiveCounts[Reg]) {
      FreeReg = Reg;
      FreeRegInactiveCount = inactiveCounts[Reg];
      if (FreeRegInactiveCount == MaxInactiveCount)
        break;    // We found the one with the max inactive count.
    }
  }

  return FreeReg;
}

/// getFreePhysReg - return a free physical register for this virtual register
/// interval if we have one, otherwise return 0.
unsigned RALinScan::getFreePhysReg(LiveInterval *cur) {
  SmallVector<unsigned, 256> inactiveCounts;
  unsigned MaxInactiveCount = 0;
  
  const TargetRegisterClass *RC = mri_->getRegClass(cur->reg);
  const TargetRegisterClass *RCLeader = RelatedRegClasses.getLeaderValue(RC);
 
  for (IntervalPtrs::iterator i = inactive_.begin(), e = inactive_.end();
       i != e; ++i) {
    unsigned reg = i->first->reg;
    assert(TargetRegisterInfo::isVirtualRegister(reg) &&
           "Can only allocate virtual registers!");

    // If this is not in a related reg class to the register we're allocating, 
    // don't check it.
    const TargetRegisterClass *RegRC = mri_->getRegClass(reg);
    if (RelatedRegClasses.getLeaderValue(RegRC) == RCLeader) {
      reg = vrm_->getPhys(reg);
      if (inactiveCounts.size() <= reg)
        inactiveCounts.resize(reg+1);
      ++inactiveCounts[reg];
      MaxInactiveCount = std::max(MaxInactiveCount, inactiveCounts[reg]);
    }
  }

  // If copy coalescer has assigned a "preferred" register, check if it's
  // available first.
  if (cur->preference) {
    DOUT << "(preferred: " << tri_->getName(cur->preference) << ") ";
    if (prt_->isRegAvail(cur->preference) && 
        RC->contains(cur->preference))
      return cur->preference;
  }

  if (!DowngradedRegs.empty()) {
    unsigned FreeReg = getFreePhysReg(RC, MaxInactiveCount, inactiveCounts,
                                      true);
    if (FreeReg)
      return FreeReg;
  }
  return getFreePhysReg(RC, MaxInactiveCount, inactiveCounts, false);
}

FunctionPass* llvm::createLinearScanRegisterAllocator() {
  return new RALinScan();
}