RegAllocLinearScan.cpp

//===-- RegAllocLinearScan.cpp - Linear Scan register allocator -----------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a linear scan register allocator.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "regalloc"
#include "llvm/Function.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/SSARegMap.h"
#include "llvm/Target/MRegisterInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "Support/Debug.h"
#include "Support/Statistic.h"
#include "Support/STLExtras.h"
#include "LiveIntervalAnalysis.h"
#include "PhysRegTracker.h"
#include "VirtRegMap.h"
#include <algorithm>
#include <cmath>
#include <set>
#include <queue>

using namespace llvm;

namespace {

  Statistic<double> efficiency
  ("regalloc", "Ratio of intervals processed over total intervals");

  static unsigned numIterations = 0;
  static unsigned numIntervals = 0;

  class RA : public MachineFunctionPass {
  private:
    MachineFunction* mf_;
    const TargetMachine* tm_;
    const MRegisterInfo* mri_;
    LiveIntervals* li_;
    typedef std::vector<LiveInterval*> IntervalPtrs;
    IntervalPtrs handled_, fixed_, active_, inactive_;
    typedef std::priority_queue<LiveInterval*,
                                IntervalPtrs,
                                greater_ptr<LiveInterval> > IntervalHeap;
    IntervalHeap unhandled_;
    std::auto_ptr<PhysRegTracker> prt_;
    std::auto_ptr<VirtRegMap> vrm_;
    std::auto_ptr<Spiller> spiller_;

    typedef std::vector<float> SpillWeights;
    SpillWeights spillWeights_;

  public:
    virtual const char* getPassName() const {
      return "Linear Scan Register Allocator";
    }

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<LiveIntervals>();
      MachineFunctionPass::getAnalysisUsage(AU);
    }

    /// runOnMachineFunction - register allocate the whole function
    bool runOnMachineFunction(MachineFunction&);

    void releaseMemory();

  private:
    /// linearScan - the linear scan algorithm
    void linearScan();

    /// initIntervalSets - initializa the four interval sets:
    /// unhandled, fixed, active and inactive
    void initIntervalSets();

    /// processActiveIntervals - expire old intervals and move
    /// non-overlapping ones to the incative list
    void processActiveIntervals(LiveInterval* cur);

    /// processInactiveIntervals - expire old intervals and move
    /// overlapping ones to the active list
    void processInactiveIntervals(LiveInterval* cur);

    /// updateSpillWeights - updates the spill weights of the
    /// specifed physical register and its weight
    void updateSpillWeights(unsigned reg, SpillWeights::value_type weight);

    /// assignRegOrStackSlotAtInterval - assign a register if one
    /// is available, or spill.
    void assignRegOrStackSlotAtInterval(LiveInterval* cur);

    ///
    /// register handling helpers
    ///

    /// getFreePhysReg - return a free physical register for this
    /// virtual register interval if we have one, otherwise return
    /// 0
    unsigned getFreePhysReg(LiveInterval* cur);

    /// assignVirt2StackSlot - assigns this virtual register to a
    /// stack slot. returns the stack slot
    int assignVirt2StackSlot(unsigned virtReg);

    template <typename ItTy>
    void printIntervals(const char* const str, ItTy i, ItTy e) const {
      if (str) std::cerr << str << " intervals:\n";
      for (; i != e; ++i) {
        std::cerr << "\t" << **i << " -> ";
        unsigned reg = (*i)->reg;
        if (MRegisterInfo::isVirtualRegister(reg)) {
          reg = vrm_->getPhys(reg);
        }
        std::cerr << mri_->getName(reg) << '\n';
      }
    }
  };
}

void RA::releaseMemory()
{
  while (!unhandled_.empty()) unhandled_.pop();
  fixed_.clear();
  active_.clear();
  inactive_.clear();
  handled_.clear();
}

bool RA::runOnMachineFunction(MachineFunction &fn) {
  mf_ = &fn;
  tm_ = &fn.getTarget();
  mri_ = tm_->getRegisterInfo();
  li_ = &getAnalysis<LiveIntervals>();
  if (!prt_.get()) prt_.reset(new PhysRegTracker(*mri_));
  vrm_.reset(new VirtRegMap(*mf_));
  if (!spiller_.get()) spiller_.reset(createSpiller());

  initIntervalSets();

  linearScan();

  spiller_->runOnMachineFunction(*mf_, *vrm_);

  return true;
}

void RA::linearScan()
{
  // linear scan algorithm
  DEBUG(std::cerr << "********** LINEAR SCAN **********\n");
  DEBUG(std::cerr << "********** Function: "
        << mf_->getFunction()->getName() << '\n');

  // DEBUG(printIntervals("unhandled", unhandled_.begin(), unhandled_.end()));
  DEBUG(printIntervals("fixed", fixed_.begin(), fixed_.end()));
  DEBUG(printIntervals("active", active_.begin(), active_.end()));
  DEBUG(printIntervals("inactive", inactive_.begin(), inactive_.end()));

  while (!unhandled_.empty()) {
    // pick the interval with the earliest start point
    LiveInterval* cur = unhandled_.top();
    unhandled_.pop();
    ++numIterations;
    DEBUG(std::cerr << "\n*** CURRENT ***: " << *cur << '\n');

    processActiveIntervals(cur);
    processInactiveIntervals(cur);

    // if this register is fixed we are done
    if (MRegisterInfo::isPhysicalRegister(cur->reg)) {
      prt_->addRegUse(cur->reg);
      active_.push_back(cur);
      handled_.push_back(cur);
    }
    // otherwise we are allocating a virtual register. try to find
    // a free physical register or spill an interval in order to
    // assign it one (we could spill the current though).
    else {
      assignRegOrStackSlotAtInterval(cur);
    }

    DEBUG(printIntervals("active", active_.begin(), active_.end()));
    DEBUG(printIntervals("inactive", inactive_.begin(), inactive_.end()));
  }
  numIntervals += li_->getNumIntervals();
  efficiency = double(numIterations) / double(numIntervals);

  // expire any remaining active intervals
  for (IntervalPtrs::reverse_iterator
         i = active_.rbegin(); i != active_.rend(); ) {
    unsigned reg = (*i)->reg;
    DEBUG(std::cerr << "\tinterval " << **i << " expired\n");
    if (MRegisterInfo::isVirtualRegister(reg))
      reg = vrm_->getPhys(reg);
    prt_->delRegUse(reg);
    i = IntervalPtrs::reverse_iterator(active_.erase(i.base()-1));
  }

  // expire any remaining inactive intervals
  for (IntervalPtrs::reverse_iterator
         i = inactive_.rbegin(); i != inactive_.rend(); ) {
    DEBUG(std::cerr << "\tinterval " << **i << " expired\n");
    i = IntervalPtrs::reverse_iterator(inactive_.erase(i.base()-1));
  }

  DEBUG(std::cerr << *vrm_);
}

void RA::initIntervalSets()
{
  assert(unhandled_.empty() && fixed_.empty() &&
         active_.empty() && inactive_.empty() &&
         "interval sets should be empty on initialization");

  for (LiveIntervals::iterator i = li_->begin(), e = li_->end(); i != e; ++i){
    unhandled_.push(&i->second);
    if (MRegisterInfo::isPhysicalRegister(i->second.reg))
      fixed_.push_back(&i->second);
  }
}

void RA::processActiveIntervals(IntervalPtrs::value_type cur)
{
  DEBUG(std::cerr << "\tprocessing active intervals:\n");
  for (IntervalPtrs::reverse_iterator
         i = active_.rbegin(); i != active_.rend();) {
    unsigned reg = (*i)->reg;
    // remove expired intervals
    if ((*i)->expiredAt(cur->start())) {
      DEBUG(std::cerr << "\t\tinterval " << **i << " expired\n");
      if (MRegisterInfo::isVirtualRegister(reg))
        reg = vrm_->getPhys(reg);
      prt_->delRegUse(reg);
      // remove from active
      i = IntervalPtrs::reverse_iterator(active_.erase(i.base()-1));
    }
    // move inactive intervals to inactive list
    else if (!(*i)->liveAt(cur->start())) {
      DEBUG(std::cerr << "\t\tinterval " << **i << " inactive\n");
      if (MRegisterInfo::isVirtualRegister(reg))
        reg = vrm_->getPhys(reg);
      prt_->delRegUse(reg);
      // add to inactive
      inactive_.push_back(*i);
      // remove from active
      i = IntervalPtrs::reverse_iterator(active_.erase(i.base()-1));
    }
    else {
      ++i;
    }
  }
}

void RA::processInactiveIntervals(IntervalPtrs::value_type cur)
{
  DEBUG(std::cerr << "\tprocessing inactive intervals:\n");
  for (IntervalPtrs::reverse_iterator
         i = inactive_.rbegin(); i != inactive_.rend();) {
    unsigned reg = (*i)->reg;

    // remove expired intervals
    if ((*i)->expiredAt(cur->start())) {
      DEBUG(std::cerr << "\t\tinterval " << **i << " expired\n");
      // remove from inactive
      i = IntervalPtrs::reverse_iterator(inactive_.erase(i.base()-1));
    }
    // move re-activated intervals in active list
    else if ((*i)->liveAt(cur->start())) {
      DEBUG(std::cerr << "\t\tinterval " << **i << " active\n");
      if (MRegisterInfo::isVirtualRegister(reg))
        reg = vrm_->getPhys(reg);
      prt_->addRegUse(reg);
      // add to active
      active_.push_back(*i);
      // remove from inactive
      i = IntervalPtrs::reverse_iterator(inactive_.erase(i.base()-1));
    }
    else {
      ++i;
    }
  }
}

void RA::updateSpillWeights(unsigned reg, SpillWeights::value_type weight)
{
  spillWeights_[reg] += weight;
  for (const unsigned* as = mri_->getAliasSet(reg); *as; ++as)
    spillWeights_[*as] += weight;
}

void RA::assignRegOrStackSlotAtInterval(LiveInterval* cur)
{
  DEBUG(std::cerr << "\tallocating current interval: ");

  PhysRegTracker backupPrt = *prt_;

  spillWeights_.assign(mri_->getNumRegs(), 0.0);

  // for each interval in active update spill weights
  for (IntervalPtrs::const_iterator i = active_.begin(), e = active_.end();
       i != e; ++i) {
    unsigned reg = (*i)->reg;
    if (MRegisterInfo::isVirtualRegister(reg))
      reg = vrm_->getPhys(reg);
    updateSpillWeights(reg, (*i)->weight);
  }

  // for every interval in inactive we overlap with, mark the
  // register as not free and update spill weights
  for (IntervalPtrs::const_iterator i = inactive_.begin(),
         e = inactive_.end(); i != e; ++i) {
    if (cur->overlaps(**i)) {
      unsigned reg = (*i)->reg;
      if (MRegisterInfo::isVirtualRegister(reg))
        reg = vrm_->getPhys(reg);
      prt_->addRegUse(reg);
      updateSpillWeights(reg, (*i)->weight);
    }
  }

  // for every interval in fixed we overlap with,
  // mark the register as not free and update spill weights
  for (IntervalPtrs::const_iterator i = fixed_.begin(),
         e = fixed_.end(); i != e; ++i) {
    if (cur->overlaps(**i)) {
      unsigned reg = (*i)->reg;
      prt_->addRegUse(reg);
      updateSpillWeights(reg, (*i)->weight);
    }
  }

  unsigned physReg = getFreePhysReg(cur);
  // restore the physical register tracker
  *prt_ = backupPrt;
  // if we find a free register, we are done: assign this virtual to
  // the free physical register and add this interval to the active
  // list.
  if (physReg) {
    DEBUG(std::cerr <<  mri_->getName(physReg) << '\n');
    vrm_->assignVirt2Phys(cur->reg, physReg);
    prt_->addRegUse(physReg);
    active_.push_back(cur);
    handled_.push_back(cur);
    return;
  }
  DEBUG(std::cerr << "no free registers\n");

  DEBUG(std::cerr << "\tassigning stack slot at interval "<< *cur << ":\n");

  float minWeight = HUGE_VAL;
  unsigned minReg = 0;
  const TargetRegisterClass* rc = mf_->getSSARegMap()->getRegClass(cur->reg);
  for (TargetRegisterClass::iterator i = rc->allocation_order_begin(*mf_);
       i != rc->allocation_order_end(*mf_); ++i) {
    unsigned reg = *i;
    if (minWeight > spillWeights_[reg]) {
      minWeight = spillWeights_[reg];
      minReg = reg;
    }
  }
  DEBUG(std::cerr << "\t\tregister with min weight: "
        << mri_->getName(minReg) << " (" << minWeight << ")\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 <= minWeight) {
    DEBUG(std::cerr << "\t\t\tspilling(c): " << *cur << '\n';);
    int slot = vrm_->assignVirt2StackSlot(cur->reg);
    std::vector<LiveInterval*> added =
      li_->addIntervalsForSpills(*cur, *vrm_, slot);
    if (added.empty())
      return;  // Early exit if all spills were folded.

    // Merge added with unhandled.  Note that we know that
    // addIntervalsForSpills returns intervals sorted by their starting
    // point.
    for (unsigned i = 0, e = added.size(); i != e; ++i)
      unhandled_.push(added[i]);
    return;
  }

  // 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);

  // otherwise 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
  std::vector<LiveInterval*> added;
  assert(MRegisterInfo::isPhysicalRegister(minReg) &&
         "did not choose a register to spill?");
  std::vector<bool> toSpill(mri_->getNumRegs(), false);
  // we are going to spill minReg and all its aliases
  toSpill[minReg] = true;
  for (const unsigned* as = mri_->getAliasSet(minReg); *as; ++as)
    toSpill[*as] = true;

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

  // set of spilled vregs (used later to rollback properly)
  std::set<unsigned> spilled;

  // 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
  for (IntervalPtrs::iterator
         i = active_.begin(); i != active_.end(); ++i) {
    unsigned reg = (*i)->reg;
    if (MRegisterInfo::isVirtualRegister(reg) &&
        toSpill[vrm_->getPhys(reg)] &&
        cur->overlaps(**i)) {
      DEBUG(std::cerr << "\t\t\tspilling(a): " << **i << '\n');
      earliestStart = std::min(earliestStart, (*i)->start());
      int slot = vrm_->assignVirt2StackSlot((*i)->reg);
      std::vector<LiveInterval*> newIs =
        li_->addIntervalsForSpills(**i, *vrm_, slot);
      std::copy(newIs.begin(), newIs.end(), std::back_inserter(added));
      spilled.insert(reg);
    }
  }
  for (IntervalPtrs::iterator
         i = inactive_.begin(); i != inactive_.end(); ++i) {
    unsigned reg = (*i)->reg;
    if (MRegisterInfo::isVirtualRegister(reg) &&
        toSpill[vrm_->getPhys(reg)] &&
        cur->overlaps(**i)) {
      DEBUG(std::cerr << "\t\t\tspilling(i): " << **i << '\n');
      earliestStart = std::min(earliestStart, (*i)->start());
      int slot = vrm_->assignVirt2StackSlot((*i)->reg);
      std::vector<LiveInterval*> newIs =
        li_->addIntervalsForSpills(**i, *vrm_, slot);
      std::copy(newIs.begin(), newIs.end(), std::back_inserter(added));
      spilled.insert(reg);
    }
  }

  DEBUG(std::cerr << "\t\trolling back to: " << earliestStart << '\n');
  // scan handled in reverse order up to the earliaset 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->start() < earliestStart)
      break;
    DEBUG(std::cerr << "\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 = find(active_.begin(), active_.end(), i)) != active_.end()) {
      active_.erase(it);
      if (MRegisterInfo::isPhysicalRegister(i->reg)) {
        prt_->delRegUse(i->reg);
        unhandled_.push(i);
      }
      else {
        if (!spilled.count(i->reg))
          unhandled_.push(i);
        prt_->delRegUse(vrm_->getPhys(i->reg));
        vrm_->clearVirt(i->reg);
      }
    }
    else if ((it = find(inactive_.begin(), inactive_.end(), i)) != inactive_.end()) {
      inactive_.erase(it);
      if (MRegisterInfo::isPhysicalRegister(i->reg))
        unhandled_.push(i);
      else {
        if (!spilled.count(i->reg))
          unhandled_.push(i);
        vrm_->clearVirt(i->reg);
      }
    }
    else {
      if (MRegisterInfo::isVirtualRegister(i->reg))
        vrm_->clearVirt(i->reg);
      unhandled_.push(i);
    }
  }

  // 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)
  IntervalPtrs::iterator i = handled_.begin(), e = handled_.end();
  for (; i != e; ++i) {
    if (!(*i)->expiredAt(earliestStart) && (*i)->expiredAt(cur->start())) {
      DEBUG(std::cerr << "\t\t\tundo changes for: " << **i << '\n');
      active_.push_back(*i);
      if (MRegisterInfo::isPhysicalRegister((*i)->reg))
        prt_->addRegUse((*i)->reg);
      else
        prt_->addRegUse(vrm_->getPhys((*i)->reg));
    }
  }

  std::sort(added.begin(), added.end(), less_ptr<LiveInterval>());
  // merge added with unhandled
  for (unsigned i = 0, e = added.size(); i != e; ++i)
    unhandled_.push(added[i]);
}

unsigned RA::getFreePhysReg(LiveInterval* cur)
{
  const TargetRegisterClass* rc = mf_->getSSARegMap()->getRegClass(cur->reg);

  for (TargetRegisterClass::iterator i = rc->allocation_order_begin(*mf_);
       i != rc->allocation_order_end(*mf_); ++i) {
    unsigned reg = *i;
    if (prt_->isRegAvail(reg))
      return reg;
  }
  return 0;
}

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