lib/CodeGen/MachineScheduler.cpp

//===- MachineScheduler.cpp - Machine Instruction Scheduler ---------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// MachineScheduler schedules machine instructions after phi elimination. It
// preserves LiveIntervals so it can be invoked before register allocation.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "misched"

#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/MachineScheduler.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/RegisterClassInfo.h"
#include "llvm/CodeGen/ScheduleHazardRecognizer.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/OwningPtr.h"
#include "llvm/ADT/PriorityQueue.h"

#include <queue>

using namespace llvm;

namespace llvm {
cl::opt<bool> ForceTopDown("misched-topdown", cl::Hidden,
                           cl::desc("Force top-down list scheduling"));
cl::opt<bool> ForceBottomUp("misched-bottomup", cl::Hidden,
                            cl::desc("Force bottom-up list scheduling"));
}

#ifndef NDEBUG
static cl::opt<bool> ViewMISchedDAGs("view-misched-dags", cl::Hidden,
  cl::desc("Pop up a window to show MISched dags after they are processed"));

static cl::opt<unsigned> MISchedCutoff("misched-cutoff", cl::Hidden,
  cl::desc("Stop scheduling after N instructions"), cl::init(~0U));
#else
static bool ViewMISchedDAGs = false;
#endif // NDEBUG

//===----------------------------------------------------------------------===//
// Machine Instruction Scheduling Pass and Registry
//===----------------------------------------------------------------------===//

MachineSchedContext::MachineSchedContext():
    MF(0), MLI(0), MDT(0), PassConfig(0), AA(0), LIS(0) {
  RegClassInfo = new RegisterClassInfo();
}

MachineSchedContext::~MachineSchedContext() {
  delete RegClassInfo;
}

namespace {
/// MachineScheduler runs after coalescing and before register allocation.
class MachineScheduler : public MachineSchedContext,
                         public MachineFunctionPass {
public:
  MachineScheduler();

  virtual void getAnalysisUsage(AnalysisUsage &AU) const;

  virtual void releaseMemory() {}

  virtual bool runOnMachineFunction(MachineFunction&);

  virtual void print(raw_ostream &O, const Module* = 0) const;

  static char ID; // Class identification, replacement for typeinfo
};
} // namespace

char MachineScheduler::ID = 0;

char &llvm::MachineSchedulerID = MachineScheduler::ID;

INITIALIZE_PASS_BEGIN(MachineScheduler, "misched",
                      "Machine Instruction Scheduler", false, false)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
INITIALIZE_PASS_END(MachineScheduler, "misched",
                    "Machine Instruction Scheduler", false, false)

MachineScheduler::MachineScheduler()
: MachineFunctionPass(ID) {
  initializeMachineSchedulerPass(*PassRegistry::getPassRegistry());
}

void MachineScheduler::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesCFG();
  AU.addRequiredID(MachineDominatorsID);
  AU.addRequired<MachineLoopInfo>();
  AU.addRequired<AliasAnalysis>();
  AU.addRequired<TargetPassConfig>();
  AU.addRequired<SlotIndexes>();
  AU.addPreserved<SlotIndexes>();
  AU.addRequired<LiveIntervals>();
  AU.addPreserved<LiveIntervals>();
  MachineFunctionPass::getAnalysisUsage(AU);
}

MachinePassRegistry MachineSchedRegistry::Registry;

/// A dummy default scheduler factory indicates whether the scheduler
/// is overridden on the command line.
static ScheduleDAGInstrs *useDefaultMachineSched(MachineSchedContext *C) {
  return 0;
}

/// MachineSchedOpt allows command line selection of the scheduler.
static cl::opt<MachineSchedRegistry::ScheduleDAGCtor, false,
               RegisterPassParser<MachineSchedRegistry> >
MachineSchedOpt("misched",
                cl::init(&useDefaultMachineSched), cl::Hidden,
                cl::desc("Machine instruction scheduler to use"));

static MachineSchedRegistry
DefaultSchedRegistry("default", "Use the target's default scheduler choice.",
                     useDefaultMachineSched);

/// Forward declare the standard machine scheduler. This will be used as the
/// default scheduler if the target does not set a default.
static ScheduleDAGInstrs *createConvergingSched(MachineSchedContext *C);


/// Decrement this iterator until reaching the top or a non-debug instr.
static MachineBasicBlock::iterator
priorNonDebug(MachineBasicBlock::iterator I, MachineBasicBlock::iterator Beg) {
  assert(I != Beg && "reached the top of the region, cannot decrement");
  while (--I != Beg) {
    if (!I->isDebugValue())
      break;
  }
  return I;
}

/// If this iterator is a debug value, increment until reaching the End or a
/// non-debug instruction.
static MachineBasicBlock::iterator
nextIfDebug(MachineBasicBlock::iterator I, MachineBasicBlock::iterator End) {
  for(; I != End; ++I) {
    if (!I->isDebugValue())
      break;
  }
  return I;
}

/// Top-level MachineScheduler pass driver.
///
/// Visit blocks in function order. Divide each block into scheduling regions
/// and visit them bottom-up. Visiting regions bottom-up is not required, but is
/// consistent with the DAG builder, which traverses the interior of the
/// scheduling regions bottom-up.
///
/// This design avoids exposing scheduling boundaries to the DAG builder,
/// simplifying the DAG builder's support for "special" target instructions.
/// At the same time the design allows target schedulers to operate across
/// scheduling boundaries, for example to bundle the boudary instructions
/// without reordering them. This creates complexity, because the target
/// scheduler must update the RegionBegin and RegionEnd positions cached by
/// ScheduleDAGInstrs whenever adding or removing instructions. A much simpler
/// design would be to split blocks at scheduling boundaries, but LLVM has a
/// general bias against block splitting purely for implementation simplicity.
bool MachineScheduler::runOnMachineFunction(MachineFunction &mf) {
  DEBUG(dbgs() << "Before MISsched:\n"; mf.print(dbgs()));

  // Initialize the context of the pass.
  MF = &mf;
  MLI = &getAnalysis<MachineLoopInfo>();
  MDT = &getAnalysis<MachineDominatorTree>();
  PassConfig = &getAnalysis<TargetPassConfig>();
  AA = &getAnalysis<AliasAnalysis>();

  LIS = &getAnalysis<LiveIntervals>();
  const TargetInstrInfo *TII = MF->getTarget().getInstrInfo();

  RegClassInfo->runOnMachineFunction(*MF);

  // Select the scheduler, or set the default.
  MachineSchedRegistry::ScheduleDAGCtor Ctor = MachineSchedOpt;
  if (Ctor == useDefaultMachineSched) {
    // Get the default scheduler set by the target.
    Ctor = MachineSchedRegistry::getDefault();
    if (!Ctor) {
      Ctor = createConvergingSched;
      MachineSchedRegistry::setDefault(Ctor);
    }
  }
  // Instantiate the selected scheduler.
  OwningPtr<ScheduleDAGInstrs> Scheduler(Ctor(this));

  // Visit all machine basic blocks.
  //
  // TODO: Visit blocks in global postorder or postorder within the bottom-up
  // loop tree. Then we can optionally compute global RegPressure.
  for (MachineFunction::iterator MBB = MF->begin(), MBBEnd = MF->end();
       MBB != MBBEnd; ++MBB) {

    Scheduler->startBlock(MBB);

    // Break the block into scheduling regions [I, RegionEnd), and schedule each
    // region as soon as it is discovered. RegionEnd points the scheduling
    // boundary at the bottom of the region. The DAG does not include RegionEnd,
    // but the region does (i.e. the next RegionEnd is above the previous
    // RegionBegin). If the current block has no terminator then RegionEnd ==
    // MBB->end() for the bottom region.
    //
    // The Scheduler may insert instructions during either schedule() or
    // exitRegion(), even for empty regions. So the local iterators 'I' and
    // 'RegionEnd' are invalid across these calls.
    unsigned RemainingCount = MBB->size();
    for(MachineBasicBlock::iterator RegionEnd = MBB->end();
        RegionEnd != MBB->begin(); RegionEnd = Scheduler->begin()) {

      // Avoid decrementing RegionEnd for blocks with no terminator.
      if (RegionEnd != MBB->end()
          || TII->isSchedulingBoundary(llvm::prior(RegionEnd), MBB, *MF)) {
        --RegionEnd;
        // Count the boundary instruction.
        --RemainingCount;
      }

      // The next region starts above the previous region. Look backward in the
      // instruction stream until we find the nearest boundary.
      MachineBasicBlock::iterator I = RegionEnd;
      for(;I != MBB->begin(); --I, --RemainingCount) {
        if (TII->isSchedulingBoundary(llvm::prior(I), MBB, *MF))
          break;
      }
      // Notify the scheduler of the region, even if we may skip scheduling
      // it. Perhaps it still needs to be bundled.
      Scheduler->enterRegion(MBB, I, RegionEnd, RemainingCount);

      // Skip empty scheduling regions (0 or 1 schedulable instructions).
      if (I == RegionEnd || I == llvm::prior(RegionEnd)) {
        // Close the current region. Bundle the terminator if needed.
        // This invalidates 'RegionEnd' and 'I'.
        Scheduler->exitRegion();
        continue;
      }
      DEBUG(dbgs() << "********** MI Scheduling **********\n");
      DEBUG(dbgs() << MF->getName()
            << ":BB#" << MBB->getNumber() << "\n  From: " << *I << "    To: ";
            if (RegionEnd != MBB->end()) dbgs() << *RegionEnd;
            else dbgs() << "End";
            dbgs() << " Remaining: " << RemainingCount << "\n");

      // Schedule a region: possibly reorder instructions.
      // This invalidates 'RegionEnd' and 'I'.
      Scheduler->schedule();

      // Close the current region.
      Scheduler->exitRegion();

      // Scheduling has invalidated the current iterator 'I'. Ask the
      // scheduler for the top of it's scheduled region.
      RegionEnd = Scheduler->begin();
    }
    assert(RemainingCount == 0 && "Instruction count mismatch!");
    Scheduler->finishBlock();
  }
  Scheduler->finalizeSchedule();
  DEBUG(LIS->print(dbgs()));
  return true;
}

void MachineScheduler::print(raw_ostream &O, const Module* m) const {
  // unimplemented
}

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
void ReadyQueue::dump() {
  dbgs() << Name << ": ";
  for (unsigned i = 0, e = Queue.size(); i < e; ++i)
    dbgs() << Queue[i]->NodeNum << " ";
  dbgs() << "\n";
}
#endif

//===----------------------------------------------------------------------===//
// ScheduleDAGMI - Base class for MachineInstr scheduling with LiveIntervals
// preservation.
//===----------------------------------------------------------------------===//

/// ReleaseSucc - Decrement the NumPredsLeft count of a successor. When
/// NumPredsLeft reaches zero, release the successor node.
///
/// FIXME: Adjust SuccSU height based on MinLatency.
void ScheduleDAGMI::releaseSucc(SUnit *SU, SDep *SuccEdge) {
  SUnit *SuccSU = SuccEdge->getSUnit();

#ifndef NDEBUG
  if (SuccSU->NumPredsLeft == 0) {
    dbgs() << "*** Scheduling failed! ***\n";
    SuccSU->dump(this);
    dbgs() << " has been released too many times!\n";
    llvm_unreachable(0);
  }
#endif
  --SuccSU->NumPredsLeft;
  if (SuccSU->NumPredsLeft == 0 && SuccSU != &ExitSU)
    SchedImpl->releaseTopNode(SuccSU);
}

/// releaseSuccessors - Call releaseSucc on each of SU's successors.
void ScheduleDAGMI::releaseSuccessors(SUnit *SU) {
  for (SUnit::succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
       I != E; ++I) {
    releaseSucc(SU, &*I);
  }
}

/// ReleasePred - Decrement the NumSuccsLeft count of a predecessor. When
/// NumSuccsLeft reaches zero, release the predecessor node.
///
/// FIXME: Adjust PredSU height based on MinLatency.
void ScheduleDAGMI::releasePred(SUnit *SU, SDep *PredEdge) {
  SUnit *PredSU = PredEdge->getSUnit();

#ifndef NDEBUG
  if (PredSU->NumSuccsLeft == 0) {
    dbgs() << "*** Scheduling failed! ***\n";
    PredSU->dump(this);
    dbgs() << " has been released too many times!\n";
    llvm_unreachable(0);
  }
#endif
  --PredSU->NumSuccsLeft;
  if (PredSU->NumSuccsLeft == 0 && PredSU != &EntrySU)
    SchedImpl->releaseBottomNode(PredSU);
}

/// releasePredecessors - Call releasePred on each of SU's predecessors.
void ScheduleDAGMI::releasePredecessors(SUnit *SU) {
  for (SUnit::pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
       I != E; ++I) {
    releasePred(SU, &*I);
  }
}

void ScheduleDAGMI::moveInstruction(MachineInstr *MI,
                                    MachineBasicBlock::iterator InsertPos) {
  // Advance RegionBegin if the first instruction moves down.
  if (&*RegionBegin == MI)
    ++RegionBegin;

  // Update the instruction stream.
  BB->splice(InsertPos, BB, MI);

  // Update LiveIntervals
  LIS->handleMove(MI);

  // Recede RegionBegin if an instruction moves above the first.
  if (RegionBegin == InsertPos)
    RegionBegin = MI;
}

bool ScheduleDAGMI::checkSchedLimit() {
#ifndef NDEBUG
  if (NumInstrsScheduled == MISchedCutoff && MISchedCutoff != ~0U) {
    CurrentTop = CurrentBottom;
    return false;
  }
  ++NumInstrsScheduled;
#endif
  return true;
}

/// enterRegion - Called back from MachineScheduler::runOnMachineFunction after
/// crossing a scheduling boundary. [begin, end) includes all instructions in
/// the region, including the boundary itself and single-instruction regions
/// that don't get scheduled.
void ScheduleDAGMI::enterRegion(MachineBasicBlock *bb,
                                MachineBasicBlock::iterator begin,
                                MachineBasicBlock::iterator end,
                                unsigned endcount)
{
  ScheduleDAGInstrs::enterRegion(bb, begin, end, endcount);

  // For convenience remember the end of the liveness region.
  LiveRegionEnd =
    (RegionEnd == bb->end()) ? RegionEnd : llvm::next(RegionEnd);
}

// Setup the register pressure trackers for the top scheduled top and bottom
// scheduled regions.
void ScheduleDAGMI::initRegPressure() {
  TopRPTracker.init(&MF, RegClassInfo, LIS, BB, RegionBegin);
  BotRPTracker.init(&MF, RegClassInfo, LIS, BB, LiveRegionEnd);

  // Close the RPTracker to finalize live ins.
  RPTracker.closeRegion();

  DEBUG(RPTracker.getPressure().dump(TRI));

  // Initialize the live ins and live outs.
  TopRPTracker.addLiveRegs(RPTracker.getPressure().LiveInRegs);
  BotRPTracker.addLiveRegs(RPTracker.getPressure().LiveOutRegs);

  // Close one end of the tracker so we can call
  // getMaxUpward/DownwardPressureDelta before advancing across any
  // instructions. This converts currently live regs into live ins/outs.
  TopRPTracker.closeTop();
  BotRPTracker.closeBottom();

  // Account for liveness generated by the region boundary.
  if (LiveRegionEnd != RegionEnd)
    BotRPTracker.recede();

  assert(BotRPTracker.getPos() == RegionEnd && "Can't find the region bottom");

  // Cache the list of excess pressure sets in this region. This will also track
  // the max pressure in the scheduled code for these sets.
  RegionCriticalPSets.clear();
  std::vector<unsigned> RegionPressure = RPTracker.getPressure().MaxSetPressure;
  for (unsigned i = 0, e = RegionPressure.size(); i < e; ++i) {
    unsigned Limit = TRI->getRegPressureSetLimit(i);
    DEBUG(dbgs() << TRI->getRegPressureSetName(i)
          << "Limit " << Limit
          << " Actual " << RegionPressure[i] << "\n");
    if (RegionPressure[i] > Limit)
      RegionCriticalPSets.push_back(PressureElement(i, 0));
  }
  DEBUG(dbgs() << "Excess PSets: ";
        for (unsigned i = 0, e = RegionCriticalPSets.size(); i != e; ++i)
          dbgs() << TRI->getRegPressureSetName(
            RegionCriticalPSets[i].PSetID) << " ";
        dbgs() << "\n");
}

// FIXME: When the pressure tracker deals in pressure differences then we won't
// iterate over all RegionCriticalPSets[i].
void ScheduleDAGMI::
updateScheduledPressure(std::vector<unsigned> NewMaxPressure) {
  for (unsigned i = 0, e = RegionCriticalPSets.size(); i < e; ++i) {
    unsigned ID = RegionCriticalPSets[i].PSetID;
    int &MaxUnits = RegionCriticalPSets[i].UnitIncrease;
    if ((int)NewMaxPressure[ID] > MaxUnits)
      MaxUnits = NewMaxPressure[ID];
  }
}

// Release all DAG roots for scheduling.
void ScheduleDAGMI::releaseRoots() {
  SmallVector<SUnit*, 16> BotRoots;

  for (std::vector<SUnit>::iterator
         I = SUnits.begin(), E = SUnits.end(); I != E; ++I) {
    // A SUnit is ready to top schedule if it has no predecessors.
    if (I->Preds.empty())
      SchedImpl->releaseTopNode(&(*I));
    // A SUnit is ready to bottom schedule if it has no successors.
    if (I->Succs.empty())
      BotRoots.push_back(&(*I));
  }
  // Release bottom roots in reverse order so the higher priority nodes appear
  // first. This is more natural and slightly more efficient.
  for (SmallVectorImpl<SUnit*>::const_reverse_iterator
         I = BotRoots.rbegin(), E = BotRoots.rend(); I != E; ++I)
    SchedImpl->releaseBottomNode(*I);
}

/// schedule - Called back from MachineScheduler::runOnMachineFunction
/// after setting up the current scheduling region. [RegionBegin, RegionEnd)
/// only includes instructions that have DAG nodes, not scheduling boundaries.
///
/// This is a skeletal driver, with all the functionality pushed into helpers,
/// so that it can be easilly extended by experimental schedulers. Generally,
/// implementing MachineSchedStrategy should be sufficient to implement a new
/// scheduling algorithm. However, if a scheduler further subclasses
/// ScheduleDAGMI then it will want to override this virtual method in order to
/// update any specialized state.
void ScheduleDAGMI::schedule() {
  buildDAGWithRegPressure();

  postprocessDAG();

  DEBUG(for (unsigned su = 0, e = SUnits.size(); su != e; ++su)
          SUnits[su].dumpAll(this));

  if (ViewMISchedDAGs) viewGraph();

  initQueues();

  bool IsTopNode = false;
  while (SUnit *SU = SchedImpl->pickNode(IsTopNode)) {
    if (!checkSchedLimit())
      break;

    scheduleMI(SU, IsTopNode);

    updateQueues(SU, IsTopNode);
  }
  assert(CurrentTop == CurrentBottom && "Nonempty unscheduled zone.");

  placeDebugValues();
}

/// Build the DAG and setup three register pressure trackers.
void ScheduleDAGMI::buildDAGWithRegPressure() {
  // Initialize the register pressure tracker used by buildSchedGraph.
  RPTracker.init(&MF, RegClassInfo, LIS, BB, LiveRegionEnd);

  // Account for liveness generate by the region boundary.
  if (LiveRegionEnd != RegionEnd)
    RPTracker.recede();

  // Build the DAG, and compute current register pressure.
  buildSchedGraph(AA, &RPTracker);
  if (ViewMISchedDAGs) viewGraph();

  // Initialize top/bottom trackers after computing region pressure.
  initRegPressure();
}

/// Apply each ScheduleDAGMutation step in order.
void ScheduleDAGMI::postprocessDAG() {
  for (unsigned i = 0, e = Mutations.size(); i < e; ++i) {
    Mutations[i]->apply(this);
  }
}

/// Identify DAG roots and setup scheduler queues.
void ScheduleDAGMI::initQueues() {
  // Initialize the strategy before modifying the DAG.
  SchedImpl->initialize(this);

  // Release edges from the special Entry node or to the special Exit node.
  releaseSuccessors(&EntrySU);
  releasePredecessors(&ExitSU);

  // Release all DAG roots for scheduling.
  releaseRoots();

  CurrentTop = nextIfDebug(RegionBegin, RegionEnd);
  CurrentBottom = RegionEnd;
}

/// Move an instruction and update register pressure.
void ScheduleDAGMI::scheduleMI(SUnit *SU, bool IsTopNode) {
  // Move the instruction to its new location in the instruction stream.
  MachineInstr *MI = SU->getInstr();

  if (IsTopNode) {
    assert(SU->isTopReady() && "node still has unscheduled dependencies");
    if (&*CurrentTop == MI)
      CurrentTop = nextIfDebug(++CurrentTop, CurrentBottom);
    else {
      moveInstruction(MI, CurrentTop);
      TopRPTracker.setPos(MI);
    }

    // Update top scheduled pressure.
    TopRPTracker.advance();
    assert(TopRPTracker.getPos() == CurrentTop && "out of sync");
    updateScheduledPressure(TopRPTracker.getPressure().MaxSetPressure);
  }
  else {
    assert(SU->isBottomReady() && "node still has unscheduled dependencies");
    MachineBasicBlock::iterator priorII =
      priorNonDebug(CurrentBottom, CurrentTop);
    if (&*priorII == MI)
      CurrentBottom = priorII;
    else {
      if (&*CurrentTop == MI) {
        CurrentTop = nextIfDebug(++CurrentTop, priorII);
        TopRPTracker.setPos(CurrentTop);
      }
      moveInstruction(MI, CurrentBottom);
      CurrentBottom = MI;
    }
    // Update bottom scheduled pressure.
    BotRPTracker.recede();
    assert(BotRPTracker.getPos() == CurrentBottom && "out of sync");
    updateScheduledPressure(BotRPTracker.getPressure().MaxSetPressure);
  }
}

/// Update scheduler queues after scheduling an instruction.
void ScheduleDAGMI::updateQueues(SUnit *SU, bool IsTopNode) {
  // Release dependent instructions for scheduling.
  if (IsTopNode)
    releaseSuccessors(SU);
  else
    releasePredecessors(SU);

  SU->isScheduled = true;

  // Notify the scheduling strategy after updating the DAG.
  SchedImpl->schedNode(SU, IsTopNode);
}

/// Reinsert any remaining debug_values, just like the PostRA scheduler.
void ScheduleDAGMI::placeDebugValues() {
  // If first instruction was a DBG_VALUE then put it back.
  if (FirstDbgValue) {
    BB->splice(RegionBegin, BB, FirstDbgValue);
    RegionBegin = FirstDbgValue;
  }

  for (std::vector<std::pair<MachineInstr *, MachineInstr *> >::iterator
         DI = DbgValues.end(), DE = DbgValues.begin(); DI != DE; --DI) {
    std::pair<MachineInstr *, MachineInstr *> P = *prior(DI);
    MachineInstr *DbgValue = P.first;
    MachineBasicBlock::iterator OrigPrevMI = P.second;
    BB->splice(++OrigPrevMI, BB, DbgValue);
    if (OrigPrevMI == llvm::prior(RegionEnd))
      RegionEnd = DbgValue;
  }
  DbgValues.clear();
  FirstDbgValue = NULL;
}

//===----------------------------------------------------------------------===//
// ConvergingScheduler - Implementation of the standard MachineSchedStrategy.
//===----------------------------------------------------------------------===//

namespace {
/// ConvergingScheduler shrinks the unscheduled zone using heuristics to balance
/// the schedule.
class ConvergingScheduler : public MachineSchedStrategy {

  /// Store the state used by ConvergingScheduler heuristics, required for the
  /// lifetime of one invocation of pickNode().
  struct SchedCandidate {
    // The best SUnit candidate.
    SUnit *SU;

    // Register pressure values for the best candidate.
    RegPressureDelta RPDelta;

    SchedCandidate(): SU(NULL) {}
  };
  /// Represent the type of SchedCandidate found within a single queue.
  enum CandResult {
    NoCand, NodeOrder, SingleExcess, SingleCritical, SingleMax, MultiPressure };

  /// Each Scheduling boundary is associated with ready queues. It tracks the
  /// current cycle in whichever direction at has moved, and maintains the state
  /// of "hazards" and other interlocks at the current cycle.
  struct SchedBoundary {
    ScheduleDAGMI *DAG;

    ReadyQueue Available;
    ReadyQueue Pending;
    bool CheckPending;

    ScheduleHazardRecognizer *HazardRec;

    unsigned CurrCycle;
    unsigned IssueCount;

    /// MinReadyCycle - Cycle of the soonest available instruction.
    unsigned MinReadyCycle;

    // Remember the greatest min operand latency.
    unsigned MaxMinLatency;

    /// Pending queues extend the ready queues with the same ID and the
    /// PendingFlag set.
    SchedBoundary(unsigned ID, const Twine &Name):
      DAG(0), Available(ID, Name+".A"),
      Pending(ID << ConvergingScheduler::LogMaxQID, Name+".P"),
      CheckPending(false), HazardRec(0), CurrCycle(0), IssueCount(0),
      MinReadyCycle(UINT_MAX), MaxMinLatency(0) {}

    ~SchedBoundary() { delete HazardRec; }

    bool isTop() const {
      return Available.getID() == ConvergingScheduler::TopQID;
    }

    bool checkHazard(SUnit *SU);

    void releaseNode(SUnit *SU, unsigned ReadyCycle);

    void bumpCycle();

    void bumpNode(SUnit *SU);

    void releasePending();

    void removeReady(SUnit *SU);

    SUnit *pickOnlyChoice();
  };

  ScheduleDAGMI *DAG;
  const TargetRegisterInfo *TRI;

  // State of the top and bottom scheduled instruction boundaries.
  SchedBoundary Top;
  SchedBoundary Bot;

public:
  /// SUnit::NodeQueueId: 0 (none), 1 (top), 2 (bot), 3 (both)
  enum {
    TopQID = 1,
    BotQID = 2,
    LogMaxQID = 2
  };

  ConvergingScheduler():
    DAG(0), TRI(0), Top(TopQID, "TopQ"), Bot(BotQID, "BotQ") {}

  virtual void initialize(ScheduleDAGMI *dag);

  virtual SUnit *pickNode(bool &IsTopNode);

  virtual void schedNode(SUnit *SU, bool IsTopNode);

  virtual void releaseTopNode(SUnit *SU);

  virtual void releaseBottomNode(SUnit *SU);

protected:
  SUnit *pickNodeBidrectional(bool &IsTopNode);

  CandResult pickNodeFromQueue(ReadyQueue &Q,
                               const RegPressureTracker &RPTracker,
                               SchedCandidate &Candidate);
#ifndef NDEBUG
  void traceCandidate(const char *Label, const ReadyQueue &Q, SUnit *SU,
                      PressureElement P = PressureElement());
#endif
};
} // namespace

void ConvergingScheduler::initialize(ScheduleDAGMI *dag) {
  DAG = dag;
  TRI = DAG->TRI;
  Top.DAG = dag;
  Bot.DAG = dag;

  // Initialize the HazardRecognizers.
  const TargetMachine &TM = DAG->MF.getTarget();
  const InstrItineraryData *Itin = TM.getInstrItineraryData();
  Top.HazardRec = TM.getInstrInfo()->CreateTargetMIHazardRecognizer(Itin, DAG);
  Bot.HazardRec = TM.getInstrInfo()->CreateTargetMIHazardRecognizer(Itin, DAG);

  assert((!ForceTopDown || !ForceBottomUp) &&
         "-misched-topdown incompatible with -misched-bottomup");
}

void ConvergingScheduler::releaseTopNode(SUnit *SU) {
  if (SU->isScheduled)
    return;

  for (SUnit::succ_iterator I = SU->Preds.begin(), E = SU->Preds.end();
       I != E; ++I) {
    unsigned PredReadyCycle = I->getSUnit()->TopReadyCycle;
    unsigned MinLatency = I->getMinLatency();
#ifndef NDEBUG
    Top.MaxMinLatency = std::max(MinLatency, Top.MaxMinLatency);
#endif
    if (SU->TopReadyCycle < PredReadyCycle + MinLatency)
      SU->TopReadyCycle = PredReadyCycle + MinLatency;
  }
  Top.releaseNode(SU, SU->TopReadyCycle);
}

void ConvergingScheduler::releaseBottomNode(SUnit *SU) {
  if (SU->isScheduled)
    return;

  assert(SU->getInstr() && "Scheduled SUnit must have instr");

  for (SUnit::succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
       I != E; ++I) {
    unsigned SuccReadyCycle = I->getSUnit()->BotReadyCycle;
    unsigned MinLatency = I->getMinLatency();
#ifndef NDEBUG
    Bot.MaxMinLatency = std::max(MinLatency, Bot.MaxMinLatency);
#endif
    if (SU->BotReadyCycle < SuccReadyCycle + MinLatency)
      SU->BotReadyCycle = SuccReadyCycle + MinLatency;
  }
  Bot.releaseNode(SU, SU->BotReadyCycle);
}

/// Does this SU have a hazard within the current instruction group.
///
/// The scheduler supports two modes of hazard recognition. The first is the
/// ScheduleHazardRecognizer API. It is a fully general hazard recognizer that
/// supports highly complicated in-order reservation tables
/// (ScoreboardHazardRecognizer) and arbitraty target-specific logic.
///
/// The second is a streamlined mechanism that checks for hazards based on
/// simple counters that the scheduler itself maintains. It explicitly checks
/// for instruction dispatch limitations, including the number of micro-ops that
/// can dispatch per cycle.
///
/// TODO: Also check whether the SU must start a new group.
bool ConvergingScheduler::SchedBoundary::checkHazard(SUnit *SU) {
  if (HazardRec->isEnabled())
    return HazardRec->getHazardType(SU) != ScheduleHazardRecognizer::NoHazard;

  if (IssueCount + DAG->getNumMicroOps(SU->getInstr()) > DAG->getIssueWidth())
    return true;

  return false;
}

void ConvergingScheduler::SchedBoundary::releaseNode(SUnit *SU,
                                                     unsigned ReadyCycle) {
  if (ReadyCycle < MinReadyCycle)
    MinReadyCycle = ReadyCycle;

  // Check for interlocks first. For the purpose of other heuristics, an
  // instruction that cannot issue appears as if it's not in the ReadyQueue.
  if (ReadyCycle > CurrCycle || checkHazard(SU))
    Pending.push(SU);
  else
    Available.push(SU);
}

/// Move the boundary of scheduled code by one cycle.
void ConvergingScheduler::SchedBoundary::bumpCycle() {
  unsigned Width = DAG->getIssueWidth();
  IssueCount = (IssueCount <= Width) ? 0 : IssueCount - Width;

  assert(MinReadyCycle < UINT_MAX && "MinReadyCycle uninitialized");
  unsigned NextCycle = std::max(CurrCycle + 1, MinReadyCycle);

  if (!HazardRec->isEnabled()) {
    // Bypass HazardRec virtual calls.
    CurrCycle = NextCycle;
  }
  else {
    // Bypass getHazardType calls in case of long latency.
    for (; CurrCycle != NextCycle; ++CurrCycle) {
      if (isTop())
        HazardRec->AdvanceCycle();
      else
        HazardRec->RecedeCycle();
    }
  }
  CheckPending = true;

  DEBUG(dbgs() << "*** " << Available.getName() << " cycle "
        << CurrCycle << '\n');
}

/// Move the boundary of scheduled code by one SUnit.
void ConvergingScheduler::SchedBoundary::bumpNode(SUnit *SU) {
  // Update the reservation table.
  if (HazardRec->isEnabled()) {
    if (!isTop() && SU->isCall) {
      // Calls are scheduled with their preceding instructions. For bottom-up
      // scheduling, clear the pipeline state before emitting.
      HazardRec->Reset();
    }
    HazardRec->EmitInstruction(SU);
  }
  // Check the instruction group dispatch limit.
  // TODO: Check if this SU must end a dispatch group.
  IssueCount += DAG->getNumMicroOps(SU->getInstr());
  if (IssueCount >= DAG->getIssueWidth()) {
    DEBUG(dbgs() << "*** Max instrs at cycle " << CurrCycle << '\n');
    bumpCycle();
  }
}

/// Release pending ready nodes in to the available queue. This makes them
/// visible to heuristics.
void ConvergingScheduler::SchedBoundary::releasePending() {
  // If the available queue is empty, it is safe to reset MinReadyCycle.
  if (Available.empty())
    MinReadyCycle = UINT_MAX;

  // Check to see if any of the pending instructions are ready to issue.  If
  // so, add them to the available queue.
  for (unsigned i = 0, e = Pending.size(); i != e; ++i) {
    SUnit *SU = *(Pending.begin()+i);
    unsigned ReadyCycle = isTop() ? SU->TopReadyCycle : SU->BotReadyCycle;

    if (ReadyCycle < MinReadyCycle)
      MinReadyCycle = ReadyCycle;

    if (ReadyCycle > CurrCycle)
      continue;

    if (checkHazard(SU))
      continue;

    Available.push(SU);
    Pending.remove(Pending.begin()+i);
    --i; --e;
  }
  CheckPending = false;
}

/// Remove SU from the ready set for this boundary.
void ConvergingScheduler::SchedBoundary::removeReady(SUnit *SU) {
  if (Available.isInQueue(SU))
    Available.remove(Available.find(SU));
  else {
    assert(Pending.isInQueue(SU) && "bad ready count");
    Pending.remove(Pending.find(SU));
  }
}

/// If this queue only has one ready candidate, return it. As a side effect,
/// advance the cycle until at least one node is ready. If multiple instructions
/// are ready, return NULL.
SUnit *ConvergingScheduler::SchedBoundary::pickOnlyChoice() {
  if (CheckPending)
    releasePending();

  for (unsigned i = 0; Available.empty(); ++i) {
    assert(i <= (HazardRec->getMaxLookAhead() + MaxMinLatency) &&
           "permanent hazard"); (void)i;
    bumpCycle();
    releasePending();
  }
  if (Available.size() == 1)
    return *Available.begin();
  return NULL;
}

#ifndef NDEBUG
void ConvergingScheduler::traceCandidate(const char *Label, const ReadyQueue &Q,
                                         SUnit *SU, PressureElement P) {
  dbgs() << Label << " " << Q.getName() << " ";
  if (P.isValid())
    dbgs() << TRI->getRegPressureSetName(P.PSetID) << ":" << P.UnitIncrease
           << " ";
  else
    dbgs() << "     ";
  SU->dump(DAG);
}
#endif

/// pickNodeFromQueue helper that returns true if the LHS reg pressure effect is
/// more desirable than RHS from scheduling standpoint.
static bool compareRPDelta(const RegPressureDelta &LHS,
                           const RegPressureDelta &RHS) {
  // Compare each component of pressure in decreasing order of importance
  // without checking if any are valid. Invalid PressureElements are assumed to
  // have UnitIncrease==0, so are neutral.

  // Avoid increasing the max critical pressure in the scheduled region.
  if (LHS.Excess.UnitIncrease != RHS.Excess.UnitIncrease)
    return LHS.Excess.UnitIncrease < RHS.Excess.UnitIncrease;

  // Avoid increasing the max critical pressure in the scheduled region.
  if (LHS.CriticalMax.UnitIncrease != RHS.CriticalMax.UnitIncrease)
    return LHS.CriticalMax.UnitIncrease < RHS.CriticalMax.UnitIncrease;

  // Avoid increasing the max pressure of the entire region.
  if (LHS.CurrentMax.UnitIncrease != RHS.CurrentMax.UnitIncrease)
    return LHS.CurrentMax.UnitIncrease < RHS.CurrentMax.UnitIncrease;

  return false;
}

/// Pick the best candidate from the top queue.
///
/// TODO: getMaxPressureDelta results can be mostly cached for each SUnit during
/// DAG building. To adjust for the current scheduling location we need to
/// maintain the number of vreg uses remaining to be top-scheduled.
ConvergingScheduler::CandResult ConvergingScheduler::
pickNodeFromQueue(ReadyQueue &Q, const RegPressureTracker &RPTracker,
                  SchedCandidate &Candidate) {
  DEBUG(Q.dump());

  // getMaxPressureDelta temporarily modifies the tracker.
  RegPressureTracker &TempTracker = const_cast<RegPressureTracker&>(RPTracker);

  // BestSU remains NULL if no top candidates beat the best existing candidate.
  CandResult FoundCandidate = NoCand;
  for (ReadyQueue::iterator I = Q.begin(), E = Q.end(); I != E; ++I) {
    RegPressureDelta RPDelta;
    TempTracker.getMaxPressureDelta((*I)->getInstr(), RPDelta,
                                    DAG->getRegionCriticalPSets(),
                                    DAG->getRegPressure().MaxSetPressure);

    // Initialize the candidate if needed.
    if (!Candidate.SU) {
      Candidate.SU = *I;
      Candidate.RPDelta = RPDelta;
      FoundCandidate = NodeOrder;
      continue;
    }
    // Avoid exceeding the target's limit.
    if (RPDelta.Excess.UnitIncrease < Candidate.RPDelta.Excess.UnitIncrease) {
      DEBUG(traceCandidate("ECAND", Q, *I, RPDelta.Excess));
      Candidate.SU = *I;
      Candidate.RPDelta = RPDelta;
      FoundCandidate = SingleExcess;
      continue;
    }
    if (RPDelta.Excess.UnitIncrease > Candidate.RPDelta.Excess.UnitIncrease)
      continue;
    if (FoundCandidate == SingleExcess)
      FoundCandidate = MultiPressure;

    // Avoid increasing the max critical pressure in the scheduled region.
    if (RPDelta.CriticalMax.UnitIncrease
        < Candidate.RPDelta.CriticalMax.UnitIncrease) {
      DEBUG(traceCandidate("PCAND", Q, *I, RPDelta.CriticalMax));
      Candidate.SU = *I;
      Candidate.RPDelta = RPDelta;
      FoundCandidate = SingleCritical;
      continue;
    }
    if (RPDelta.CriticalMax.UnitIncrease
        > Candidate.RPDelta.CriticalMax.UnitIncrease)
      continue;
    if (FoundCandidate == SingleCritical)
      FoundCandidate = MultiPressure;

    // Avoid increasing the max pressure of the entire region.
    if (RPDelta.CurrentMax.UnitIncrease
        < Candidate.RPDelta.CurrentMax.UnitIncrease) {
      DEBUG(traceCandidate("MCAND", Q, *I, RPDelta.CurrentMax));
      Candidate.SU = *I;
      Candidate.RPDelta = RPDelta;
      FoundCandidate = SingleMax;
      continue;
    }
    if (RPDelta.CurrentMax.UnitIncrease
        > Candidate.RPDelta.CurrentMax.UnitIncrease)
      continue;
    if (FoundCandidate == SingleMax)
      FoundCandidate = MultiPressure;

    // Fall through to original instruction order.
    // Only consider node order if Candidate was chosen from this Q.
    if (FoundCandidate == NoCand)
      continue;

    if ((Q.getID() == TopQID && (*I)->NodeNum < Candidate.SU->NodeNum)
        || (Q.getID() == BotQID && (*I)->NodeNum > Candidate.SU->NodeNum)) {
      DEBUG(traceCandidate("NCAND", Q, *I));
      Candidate.SU = *I;
      Candidate.RPDelta = RPDelta;
      FoundCandidate = NodeOrder;
    }
  }
  return FoundCandidate;
}

/// Pick the best candidate node from either the top or bottom queue.
SUnit *ConvergingScheduler::pickNodeBidrectional(bool &IsTopNode) {
  // Schedule as far as possible in the direction of no choice. This is most
  // efficient, but also provides the best heuristics for CriticalPSets.
  if (SUnit *SU = Bot.pickOnlyChoice()) {
    IsTopNode = false;
    return SU;
  }
  if (SUnit *SU = Top.pickOnlyChoice()) {
    IsTopNode = true;
    return SU;
  }
  SchedCandidate BotCand;
  // Prefer bottom scheduling when heuristics are silent.
  CandResult BotResult = pickNodeFromQueue(Bot.Available,
                                           DAG->getBotRPTracker(), BotCand);
  assert(BotResult != NoCand && "failed to find the first candidate");

  // If either Q has a single candidate that provides the least increase in
  // Excess pressure, we can immediately schedule from that Q.
  //
  // RegionCriticalPSets summarizes the pressure within the scheduled region and
  // affects picking from either Q. If scheduling in one direction must
  // increase pressure for one of the excess PSets, then schedule in that
  // direction first to provide more freedom in the other direction.
  if (BotResult == SingleExcess || BotResult == SingleCritical) {
    IsTopNode = false;
    return BotCand.SU;
  }
  // Check if the top Q has a better candidate.
  SchedCandidate TopCand;
  CandResult TopResult = pickNodeFromQueue(Top.Available,
                                           DAG->getTopRPTracker(), TopCand);
  assert(TopResult != NoCand && "failed to find the first candidate");

  if (TopResult == SingleExcess || TopResult == SingleCritical) {
    IsTopNode = true;
    return TopCand.SU;
  }
  // If either Q has a single candidate that minimizes pressure above the
  // original region's pressure pick it.
  if (BotResult == SingleMax) {
    IsTopNode = false;
    return BotCand.SU;
  }
  if (TopResult == SingleMax) {
    IsTopNode = true;
    return TopCand.SU;
  }
  // Check for a salient pressure difference and pick the best from either side.
  if (compareRPDelta(TopCand.RPDelta, BotCand.RPDelta)) {
    IsTopNode = true;
    return TopCand.SU;
  }
  // Otherwise prefer the bottom candidate in node order.
  IsTopNode = false;
  return BotCand.SU;
}

/// Pick the best node to balance the schedule. Implements MachineSchedStrategy.
SUnit *ConvergingScheduler::pickNode(bool &IsTopNode) {
  if (DAG->top() == DAG->bottom()) {
    assert(Top.Available.empty() && Top.Pending.empty() &&
           Bot.Available.empty() && Bot.Pending.empty() && "ReadyQ garbage");
    return NULL;
  }
  SUnit *SU;
  if (ForceTopDown) {
    SU = Top.pickOnlyChoice();
    if (!SU) {
      SchedCandidate TopCand;
      CandResult TopResult =
        pickNodeFromQueue(Top.Available, DAG->getTopRPTracker(), TopCand);
      assert(TopResult != NoCand && "failed to find the first candidate");
      (void)TopResult;
      SU = TopCand.SU;
    }
    IsTopNode = true;
  }
  else if (ForceBottomUp) {
    SU = Bot.pickOnlyChoice();
    if (!SU) {
      SchedCandidate BotCand;
      CandResult BotResult =
        pickNodeFromQueue(Bot.Available, DAG->getBotRPTracker(), BotCand);
      assert(BotResult != NoCand && "failed to find the first candidate");
      (void)BotResult;
      SU = BotCand.SU;
    }
    IsTopNode = false;
  }
  else {
    SU = pickNodeBidrectional(IsTopNode);
  }
  if (SU->isTopReady())
    Top.removeReady(SU);
  if (SU->isBottomReady())
    Bot.removeReady(SU);

  DEBUG(dbgs() << "*** " << (IsTopNode ? "Top" : "Bottom")
        << " Scheduling Instruction in cycle "
        << (IsTopNode ? Top.CurrCycle : Bot.CurrCycle) << '\n';
        SU->dump(DAG));
  return SU;
}

/// Update the scheduler's state after scheduling a node. This is the same node
/// that was just returned by pickNode(). However, ScheduleDAGMI needs to update
/// it's state based on the current cycle before MachineSchedStrategy does.
void ConvergingScheduler::schedNode(SUnit *SU, bool IsTopNode) {
  if (IsTopNode) {
    SU->TopReadyCycle = Top.CurrCycle;
    Top.bumpNode(SU);
  }
  else {
    SU->BotReadyCycle = Bot.CurrCycle;
    Bot.bumpNode(SU);
  }
}

/// Create the standard converging machine scheduler. This will be used as the
/// default scheduler if the target does not set a default.
static ScheduleDAGInstrs *createConvergingSched(MachineSchedContext *C) {
  assert((!ForceTopDown || !ForceBottomUp) &&
         "-misched-topdown incompatible with -misched-bottomup");
  return new ScheduleDAGMI(C, new ConvergingScheduler());
}
static MachineSchedRegistry
ConvergingSchedRegistry("converge", "Standard converging scheduler.",
                        createConvergingSched);

//===----------------------------------------------------------------------===//
// Machine Instruction Shuffler for Correctness Testing
//===----------------------------------------------------------------------===//

#ifndef NDEBUG
namespace {
/// Apply a less-than relation on the node order, which corresponds to the
/// instruction order prior to scheduling. IsReverse implements greater-than.
template<bool IsReverse>
struct SUnitOrder {
  bool operator()(SUnit *A, SUnit *B) const {
    if (IsReverse)
      return A->NodeNum > B->NodeNum;
    else
      return A->NodeNum < B->NodeNum;
  }
};

/// Reorder instructions as much as possible.
class InstructionShuffler : public MachineSchedStrategy {
  bool IsAlternating;
  bool IsTopDown;

  // Using a less-than relation (SUnitOrder<false>) for the TopQ priority
  // gives nodes with a higher number higher priority causing the latest
  // instructions to be scheduled first.
  PriorityQueue<SUnit*, std::vector<SUnit*>, SUnitOrder<false> >
    TopQ;
  // When scheduling bottom-up, use greater-than as the queue priority.
  PriorityQueue<SUnit*, std::vector<SUnit*>, SUnitOrder<true> >
    BottomQ;
public:
  InstructionShuffler(bool alternate, bool topdown)
    : IsAlternating(alternate), IsTopDown(topdown) {}

  virtual void initialize(ScheduleDAGMI *) {
    TopQ.clear();
    BottomQ.clear();
  }

  /// Implement MachineSchedStrategy interface.
  /// -----------------------------------------

  virtual SUnit *pickNode(bool &IsTopNode) {
    SUnit *SU;
    if (IsTopDown) {
      do {
        if (TopQ.empty()) return NULL;
        SU = TopQ.top();
        TopQ.pop();
      } while (SU->isScheduled);
      IsTopNode = true;
    }
    else {
      do {
        if (BottomQ.empty()) return NULL;
        SU = BottomQ.top();
        BottomQ.pop();
      } while (SU->isScheduled);
      IsTopNode = false;
    }
    if (IsAlternating)
      IsTopDown = !IsTopDown;
    return SU;
  }

  virtual void schedNode(SUnit *SU, bool IsTopNode) {}

  virtual void releaseTopNode(SUnit *SU) {
    TopQ.push(SU);
  }
  virtual void releaseBottomNode(SUnit *SU) {
    BottomQ.push(SU);
  }
};
} // namespace

static ScheduleDAGInstrs *createInstructionShuffler(MachineSchedContext *C) {
  bool Alternate = !ForceTopDown && !ForceBottomUp;
  bool TopDown = !ForceBottomUp;
  assert((TopDown || !ForceTopDown) &&
         "-misched-topdown incompatible with -misched-bottomup");
  return new ScheduleDAGMI(C, new InstructionShuffler(Alternate, TopDown));
}
static MachineSchedRegistry ShufflerRegistry(
  "shuffle", "Shuffle machine instructions alternating directions",
  createInstructionShuffler);
#endif // !NDEBUG