lib/CodeGen/RegisterPressure.cpp

//===-- RegisterPressure.cpp - Dynamic Register Pressure ------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the RegisterPressure class which can be used to track
// MachineInstr level register pressure.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/RegisterPressure.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/RegisterClassInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;

/// Increase pressure for each pressure set provided by TargetRegisterInfo.
static void increaseSetPressure(std::vector<unsigned> &CurrSetPressure,
                                PSetIterator PSetI) {
  unsigned Weight = PSetI.getWeight();
  for (; PSetI.isValid(); ++PSetI)
    CurrSetPressure[*PSetI] += Weight;
}

/// Decrease pressure for each pressure set provided by TargetRegisterInfo.
static void decreaseSetPressure(std::vector<unsigned> &CurrSetPressure,
                                PSetIterator PSetI) {
  unsigned Weight = PSetI.getWeight();
  for (; PSetI.isValid(); ++PSetI) {
    assert(CurrSetPressure[*PSetI] >= Weight && "register pressure underflow");
    CurrSetPressure[*PSetI] -= Weight;
  }
}

LLVM_DUMP_METHOD
void llvm::dumpRegSetPressure(ArrayRef<unsigned> SetPressure,
                              const TargetRegisterInfo *TRI) {
  bool Empty = true;
  for (unsigned i = 0, e = SetPressure.size(); i < e; ++i) {
    if (SetPressure[i] != 0) {
      dbgs() << TRI->getRegPressureSetName(i) << "=" << SetPressure[i] << '\n';
      Empty = false;
    }
  }
  if (Empty)
    dbgs() << "\n";
}

LLVM_DUMP_METHOD
void RegisterPressure::dump(const TargetRegisterInfo *TRI) const {
  dbgs() << "Max Pressure: ";
  dumpRegSetPressure(MaxSetPressure, TRI);
  dbgs() << "Live In: ";
  for (unsigned Reg : LiveInRegs)
    dbgs() << PrintVRegOrUnit(Reg, TRI) << " ";
  dbgs() << '\n';
  dbgs() << "Live Out: ";
  for (unsigned Reg : LiveOutRegs)
    dbgs() << PrintVRegOrUnit(Reg, TRI) << " ";
  dbgs() << '\n';
}

LLVM_DUMP_METHOD
void RegPressureTracker::dump() const {
  if (!isTopClosed() || !isBottomClosed()) {
    dbgs() << "Curr Pressure: ";
    dumpRegSetPressure(CurrSetPressure, TRI);
  }
  P.dump(TRI);
}

void PressureDiff::dump(const TargetRegisterInfo &TRI) const {
  const char *sep = "";
  for (const PressureChange &Change : *this) {
    if (!Change.isValid())
      break;
    dbgs() << sep << TRI.getRegPressureSetName(Change.getPSet())
           << " " << Change.getUnitInc();
    sep = "    ";
  }
  dbgs() << '\n';
}

/// Increase the current pressure as impacted by these registers and bump
/// the high water mark if needed.
void RegPressureTracker::increaseRegPressure(ArrayRef<unsigned> RegUnits) {
  for (unsigned RegUnit : RegUnits) {
    PSetIterator PSetI = MRI->getPressureSets(RegUnit);
    unsigned Weight = PSetI.getWeight();
    for (; PSetI.isValid(); ++PSetI) {
      CurrSetPressure[*PSetI] += Weight;
      if (CurrSetPressure[*PSetI] > P.MaxSetPressure[*PSetI]) {
        P.MaxSetPressure[*PSetI] = CurrSetPressure[*PSetI];
      }
    }
  }
}

/// Simply decrease the current pressure as impacted by these registers.
void RegPressureTracker::decreaseRegPressure(ArrayRef<unsigned> RegUnits) {
  for (unsigned RegUnit : RegUnits)
    decreaseSetPressure(CurrSetPressure, MRI->getPressureSets(RegUnit));
}

/// Clear the result so it can be used for another round of pressure tracking.
void IntervalPressure::reset() {
  TopIdx = BottomIdx = SlotIndex();
  MaxSetPressure.clear();
  LiveInRegs.clear();
  LiveOutRegs.clear();
}

/// Clear the result so it can be used for another round of pressure tracking.
void RegionPressure::reset() {
  TopPos = BottomPos = MachineBasicBlock::const_iterator();
  MaxSetPressure.clear();
  LiveInRegs.clear();
  LiveOutRegs.clear();
}

/// If the current top is not less than or equal to the next index, open it.
/// We happen to need the SlotIndex for the next top for pressure update.
void IntervalPressure::openTop(SlotIndex NextTop) {
  if (TopIdx <= NextTop)
    return;
  TopIdx = SlotIndex();
  LiveInRegs.clear();
}

/// If the current top is the previous instruction (before receding), open it.
void RegionPressure::openTop(MachineBasicBlock::const_iterator PrevTop) {
  if (TopPos != PrevTop)
    return;
  TopPos = MachineBasicBlock::const_iterator();
  LiveInRegs.clear();
}

/// If the current bottom is not greater than the previous index, open it.
void IntervalPressure::openBottom(SlotIndex PrevBottom) {
  if (BottomIdx > PrevBottom)
    return;
  BottomIdx = SlotIndex();
  LiveInRegs.clear();
}

/// If the current bottom is the previous instr (before advancing), open it.
void RegionPressure::openBottom(MachineBasicBlock::const_iterator PrevBottom) {
  if (BottomPos != PrevBottom)
    return;
  BottomPos = MachineBasicBlock::const_iterator();
  LiveInRegs.clear();
}

void LiveRegSet::init(const MachineRegisterInfo &MRI) {
  const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
  unsigned NumRegUnits = TRI.getNumRegs();
  unsigned NumVirtRegs = MRI.getNumVirtRegs();
  Regs.setUniverse(NumRegUnits + NumVirtRegs);
  this->NumRegUnits = NumRegUnits;
}

void LiveRegSet::clear() {
  Regs.clear();
}

static const LiveRange *getLiveRange(const LiveIntervals &LIS, unsigned Reg) {
  if (TargetRegisterInfo::isVirtualRegister(Reg))
    return &LIS.getInterval(Reg);
  return LIS.getCachedRegUnit(Reg);
}

void RegPressureTracker::reset() {
  MBB = nullptr;
  LIS = nullptr;

  CurrSetPressure.clear();
  LiveThruPressure.clear();
  P.MaxSetPressure.clear();

  if (RequireIntervals)
    static_cast<IntervalPressure&>(P).reset();
  else
    static_cast<RegionPressure&>(P).reset();

  LiveRegs.clear();
  UntiedDefs.clear();
}

/// Setup the RegPressureTracker.
///
/// TODO: Add support for pressure without LiveIntervals.
void RegPressureTracker::init(const MachineFunction *mf,
                              const RegisterClassInfo *rci,
                              const LiveIntervals *lis,
                              const MachineBasicBlock *mbb,
                              MachineBasicBlock::const_iterator pos,
                              bool ShouldTrackUntiedDefs)
{
  reset();

  MF = mf;
  TRI = MF->getSubtarget().getRegisterInfo();
  RCI = rci;
  MRI = &MF->getRegInfo();
  MBB = mbb;
  TrackUntiedDefs = ShouldTrackUntiedDefs;

  if (RequireIntervals) {
    assert(lis && "IntervalPressure requires LiveIntervals");
    LIS = lis;
  }

  CurrPos = pos;
  CurrSetPressure.assign(TRI->getNumRegPressureSets(), 0);

  P.MaxSetPressure = CurrSetPressure;

  LiveRegs.init(*MRI);
  if (TrackUntiedDefs)
    UntiedDefs.setUniverse(MRI->getNumVirtRegs());
}

/// Does this pressure result have a valid top position and live ins.
bool RegPressureTracker::isTopClosed() const {
  if (RequireIntervals)
    return static_cast<IntervalPressure&>(P).TopIdx.isValid();
  return (static_cast<RegionPressure&>(P).TopPos ==
          MachineBasicBlock::const_iterator());
}

/// Does this pressure result have a valid bottom position and live outs.
bool RegPressureTracker::isBottomClosed() const {
  if (RequireIntervals)
    return static_cast<IntervalPressure&>(P).BottomIdx.isValid();
  return (static_cast<RegionPressure&>(P).BottomPos ==
          MachineBasicBlock::const_iterator());
}


SlotIndex RegPressureTracker::getCurrSlot() const {
  MachineBasicBlock::const_iterator IdxPos = CurrPos;
  while (IdxPos != MBB->end() && IdxPos->isDebugValue())
    ++IdxPos;
  if (IdxPos == MBB->end())
    return LIS->getMBBEndIdx(MBB);
  return LIS->getInstructionIndex(IdxPos).getRegSlot();
}

/// Set the boundary for the top of the region and summarize live ins.
void RegPressureTracker::closeTop() {
  if (RequireIntervals)
    static_cast<IntervalPressure&>(P).TopIdx = getCurrSlot();
  else
    static_cast<RegionPressure&>(P).TopPos = CurrPos;

  assert(P.LiveInRegs.empty() && "inconsistent max pressure result");
  P.LiveInRegs.reserve(LiveRegs.size());
  LiveRegs.appendTo(P.LiveInRegs);
}

/// Set the boundary for the bottom of the region and summarize live outs.
void RegPressureTracker::closeBottom() {
  if (RequireIntervals)
    static_cast<IntervalPressure&>(P).BottomIdx = getCurrSlot();
  else
    static_cast<RegionPressure&>(P).BottomPos = CurrPos;

  assert(P.LiveOutRegs.empty() && "inconsistent max pressure result");
  P.LiveOutRegs.reserve(LiveRegs.size());
  LiveRegs.appendTo(P.LiveOutRegs);
}

/// Finalize the region boundaries and record live ins and live outs.
void RegPressureTracker::closeRegion() {
  if (!isTopClosed() && !isBottomClosed()) {
    assert(LiveRegs.size() == 0 && "no region boundary");
    return;
  }
  if (!isBottomClosed())
    closeBottom();
  else if (!isTopClosed())
    closeTop();
  // If both top and bottom are closed, do nothing.
}

/// The register tracker is unaware of global liveness so ignores normal
/// live-thru ranges. However, two-address or coalesced chains can also lead
/// to live ranges with no holes. Count these to inform heuristics that we
/// can never drop below this pressure.
void RegPressureTracker::initLiveThru(const RegPressureTracker &RPTracker) {
  LiveThruPressure.assign(TRI->getNumRegPressureSets(), 0);
  assert(isBottomClosed() && "need bottom-up tracking to intialize.");
  for (unsigned Reg : P.LiveOutRegs) {
    if (TargetRegisterInfo::isVirtualRegister(Reg)
        && !RPTracker.hasUntiedDef(Reg)) {
      increaseSetPressure(LiveThruPressure, MRI->getPressureSets(Reg));
    }
  }
}

/// \brief Convenient wrapper for checking membership in RegisterOperands.
/// (std::count() doesn't have an early exit).
static bool containsReg(ArrayRef<unsigned> RegUnits, unsigned RegUnit) {
  return std::find(RegUnits.begin(), RegUnits.end(), RegUnit) != RegUnits.end();
}

namespace {

/// List of register defined and used by a machine instruction.
class RegisterOperands {
public:
  SmallVector<unsigned, 8> Uses;
  SmallVector<unsigned, 8> Defs;
  SmallVector<unsigned, 8> DeadDefs;

  void collect(const MachineInstr &MI, const TargetRegisterInfo &TRI,
               const MachineRegisterInfo &MRI, bool IgnoreDead = false);

  /// Use liveness information to find dead defs not marked with a dead flag
  /// and move them to the DeadDefs vector.
  void detectDeadDefs(const MachineInstr &MI, const LiveIntervals &LIS);
};

/// Collect this instruction's unique uses and defs into SmallVectors for
/// processing defs and uses in order.
///
/// FIXME: always ignore tied opers
class RegisterOperandsCollector {
  RegisterOperands &RegOpers;
  const TargetRegisterInfo &TRI;
  const MachineRegisterInfo &MRI;
  bool IgnoreDead;

  RegisterOperandsCollector(RegisterOperands &RegOpers,
                            const TargetRegisterInfo &TRI,
                            const MachineRegisterInfo &MRI,
                            bool IgnoreDead)
    : RegOpers(RegOpers), TRI(TRI), MRI(MRI), IgnoreDead(IgnoreDead) {}

  void collectInstr(const MachineInstr &MI) const {
    for (ConstMIBundleOperands OperI(&MI); OperI.isValid(); ++OperI)
      collectOperand(*OperI);

    // Remove redundant physreg dead defs.
    SmallVectorImpl<unsigned>::iterator I =
      std::remove_if(RegOpers.DeadDefs.begin(), RegOpers.DeadDefs.end(),
                     std::bind1st(std::ptr_fun(containsReg), RegOpers.Defs));
    RegOpers.DeadDefs.erase(I, RegOpers.DeadDefs.end());
  }

  /// Push this operand's register onto the correct vectors.
  void collectOperand(const MachineOperand &MO) const {
    if (!MO.isReg() || !MO.getReg())
      return;
    unsigned Reg = MO.getReg();
    if (MO.readsReg())
      pushRegUnits(Reg, RegOpers.Uses);
    if (MO.isDef()) {
      if (MO.isDead()) {
        if (!IgnoreDead)
          pushRegUnits(Reg, RegOpers.DeadDefs);
      } else
        pushRegUnits(Reg, RegOpers.Defs);
    }
  }

  void pushRegUnits(unsigned Reg, SmallVectorImpl<unsigned> &RegUnits) const {
    if (TargetRegisterInfo::isVirtualRegister(Reg)) {
      if (containsReg(RegUnits, Reg))
        return;
      RegUnits.push_back(Reg);
    } else if (MRI.isAllocatable(Reg)) {
      for (MCRegUnitIterator Units(Reg, &TRI); Units.isValid(); ++Units) {
        if (containsReg(RegUnits, *Units))
          continue;
        RegUnits.push_back(*Units);
      }
    }
  }

  friend class RegisterOperands;
};

void RegisterOperands::collect(const MachineInstr &MI,
                               const TargetRegisterInfo &TRI,
                               const MachineRegisterInfo &MRI,
                               bool IgnoreDead) {
  RegisterOperandsCollector Collector(*this, TRI, MRI, IgnoreDead);
  Collector.collectInstr(MI);
}

void RegisterOperands::detectDeadDefs(const MachineInstr &MI,
                                      const LiveIntervals &LIS) {
  SlotIndex SlotIdx = LIS.getInstructionIndex(&MI);
  for (SmallVectorImpl<unsigned>::iterator RI = Defs.begin();
       RI != Defs.end(); /*empty*/) {
    unsigned Reg = *RI;
    const LiveRange *LR = getLiveRange(LIS, Reg);
    if (LR != nullptr) {
      LiveQueryResult LRQ = LR->Query(SlotIdx);
      if (LRQ.isDeadDef()) {
        // LiveIntervals knows this is a dead even though it's MachineOperand is
        // not flagged as such.
        DeadDefs.push_back(Reg);
        RI = Defs.erase(RI);
        continue;
      }
    }
    ++RI;
  }
}

} // namespace

/// Initialize an array of N PressureDiffs.
void PressureDiffs::init(unsigned N) {
  Size = N;
  if (N <= Max) {
    memset(PDiffArray, 0, N * sizeof(PressureDiff));
    return;
  }
  Max = Size;
  free(PDiffArray);
  PDiffArray = reinterpret_cast<PressureDiff*>(calloc(N, sizeof(PressureDiff)));
}

/// Add a change in pressure to the pressure diff of a given instruction.
void PressureDiff::addPressureChange(unsigned RegUnit, bool IsDec,
                                     const MachineRegisterInfo *MRI) {
  PSetIterator PSetI = MRI->getPressureSets(RegUnit);
  int Weight = IsDec ? -PSetI.getWeight() : PSetI.getWeight();
  for (; PSetI.isValid(); ++PSetI) {
    // Find an existing entry in the pressure diff for this PSet.
    PressureDiff::iterator I = nonconst_begin(), E = nonconst_end();
    for (; I != E && I->isValid(); ++I) {
      if (I->getPSet() >= *PSetI)
        break;
    }
    // If all pressure sets are more constrained, skip the remaining PSets.
    if (I == E)
      break;
    // Insert this PressureChange.
    if (!I->isValid() || I->getPSet() != *PSetI) {
      PressureChange PTmp = PressureChange(*PSetI);
      for (PressureDiff::iterator J = I; J != E && PTmp.isValid(); ++J)
        std::swap(*J, PTmp);
    }
    // Update the units for this pressure set.
    unsigned NewUnitInc = I->getUnitInc() + Weight;
    if (NewUnitInc != 0) {
      I->setUnitInc(NewUnitInc);
    } else {
      // Remove entry
      PressureDiff::iterator J;
      for (J = std::next(I); J != E && J->isValid(); ++J, ++I)
        *I = *J;
      if (J != E)
        *I = *J;
    }
  }
}

/// Record the pressure difference induced by the given operand list.
static void collectPDiff(PressureDiff &PDiff, RegisterOperands &RegOpers,
                         const MachineRegisterInfo *MRI) {
  assert(!PDiff.begin()->isValid() && "stale PDiff");

  for (unsigned Reg : RegOpers.Defs)
    PDiff.addPressureChange(Reg, true, MRI);

  for (unsigned Reg : RegOpers.Uses)
    PDiff.addPressureChange(Reg, false, MRI);
}

/// Force liveness of registers.
void RegPressureTracker::addLiveRegs(ArrayRef<unsigned> Regs) {
  for (unsigned Reg : Regs) {
    if (LiveRegs.insert(Reg))
      increaseRegPressure(Reg);
  }
}

/// Add Reg to the live in set and increase max pressure.
void RegPressureTracker::discoverLiveIn(unsigned Reg) {
  assert(!LiveRegs.contains(Reg) && "avoid bumping max pressure twice");
  if (containsReg(P.LiveInRegs, Reg))
    return;

  // At live in discovery, unconditionally increase the high water mark.
  P.LiveInRegs.push_back(Reg);
  increaseSetPressure(P.MaxSetPressure, MRI->getPressureSets(Reg));
}

/// Add Reg to the live out set and increase max pressure.
void RegPressureTracker::discoverLiveOut(unsigned Reg) {
  assert(!LiveRegs.contains(Reg) && "avoid bumping max pressure twice");
  if (containsReg(P.LiveOutRegs, Reg))
    return;

  // At live out discovery, unconditionally increase the high water mark.
  P.LiveOutRegs.push_back(Reg);
  increaseSetPressure(P.MaxSetPressure, MRI->getPressureSets(Reg));
}

/// Recede across the previous instruction. If LiveUses is provided, record any
/// RegUnits that are made live by the current instruction's uses. This includes
/// registers that are both defined and used by the instruction.  If a pressure
/// difference pointer is provided record the changes is pressure caused by this
/// instruction independent of liveness.
void RegPressureTracker::recede(SmallVectorImpl<unsigned> *LiveUses,
                                PressureDiff *PDiff) {
  assert(CurrPos != MBB->begin());
  if (!isBottomClosed())
    closeBottom();

  // Open the top of the region using block iterators.
  if (!RequireIntervals && isTopClosed())
    static_cast<RegionPressure&>(P).openTop(CurrPos);

  // Find the previous instruction.
  do
    --CurrPos;
  while (CurrPos != MBB->begin() && CurrPos->isDebugValue());
  assert(!CurrPos->isDebugValue());

  SlotIndex SlotIdx;
  if (RequireIntervals)
    SlotIdx = LIS->getInstructionIndex(CurrPos).getRegSlot();

  // Open the top of the region using slot indexes.
  if (RequireIntervals && isTopClosed())
    static_cast<IntervalPressure&>(P).openTop(SlotIdx);

  const MachineInstr &MI = *CurrPos;
  RegisterOperands RegOpers;
  RegOpers.collect(MI, *TRI, *MRI);
  if (RequireIntervals)
    RegOpers.detectDeadDefs(MI, *LIS);

  if (PDiff)
    collectPDiff(*PDiff, RegOpers, MRI);

  // Boost pressure for all dead defs together.
  increaseRegPressure(RegOpers.DeadDefs);
  decreaseRegPressure(RegOpers.DeadDefs);

  // Kill liveness at live defs.
  // TODO: consider earlyclobbers?
  for (unsigned Reg : RegOpers.Defs) {
    if (LiveRegs.erase(Reg))
      decreaseRegPressure(Reg);
    else
      discoverLiveOut(Reg);
  }

  // Generate liveness for uses.
  for (unsigned Reg : RegOpers.Uses) {
    if (!LiveRegs.contains(Reg)) {
      // Adjust liveouts if LiveIntervals are available.
      if (RequireIntervals) {
        const LiveRange *LR = getLiveRange(*LIS, Reg);
        if (LR) {
          LiveQueryResult LRQ = LR->Query(SlotIdx);
          if (!LRQ.isKill() && !LRQ.valueDefined())
            discoverLiveOut(Reg);
        }
      }
      increaseRegPressure(Reg);
      LiveRegs.insert(Reg);
      if (LiveUses && !containsReg(*LiveUses, Reg))
        LiveUses->push_back(Reg);
    }
  }
  if (TrackUntiedDefs) {
    for (unsigned Reg : RegOpers.Defs) {
      if (TargetRegisterInfo::isVirtualRegister(Reg) && !LiveRegs.contains(Reg))
        UntiedDefs.insert(Reg);
    }
  }
}

/// Advance across the current instruction.
void RegPressureTracker::advance() {
  assert(!TrackUntiedDefs && "unsupported mode");

  assert(CurrPos != MBB->end());
  if (!isTopClosed())
    closeTop();

  SlotIndex SlotIdx;
  if (RequireIntervals)
    SlotIdx = getCurrSlot();

  // Open the bottom of the region using slot indexes.
  if (isBottomClosed()) {
    if (RequireIntervals)
      static_cast<IntervalPressure&>(P).openBottom(SlotIdx);
    else
      static_cast<RegionPressure&>(P).openBottom(CurrPos);
  }

  RegisterOperands RegOpers;
  RegOpers.collect(*CurrPos, *TRI, *MRI);

  for (unsigned Reg : RegOpers.Uses) {
    // Discover live-ins.
    bool isLive = LiveRegs.contains(Reg);
    if (!isLive)
      discoverLiveIn(Reg);
    // Kill liveness at last uses.
    bool lastUse = false;
    if (RequireIntervals) {
      const LiveRange *LR = getLiveRange(*LIS, Reg);
      lastUse = LR && LR->Query(SlotIdx).isKill();
    } else {
      // Allocatable physregs are always single-use before register rewriting.
      lastUse = !TargetRegisterInfo::isVirtualRegister(Reg);
    }
    if (lastUse && isLive) {
      LiveRegs.erase(Reg);
      decreaseRegPressure(Reg);
    } else if (!lastUse && !isLive)
      increaseRegPressure(Reg);
  }

  // Generate liveness for defs.
  for (unsigned Reg : RegOpers.Defs) {
    if (LiveRegs.insert(Reg))
      increaseRegPressure(Reg);
  }

  // Boost pressure for all dead defs together.
  increaseRegPressure(RegOpers.DeadDefs);
  decreaseRegPressure(RegOpers.DeadDefs);

  // Find the next instruction.
  do
    ++CurrPos;
  while (CurrPos != MBB->end() && CurrPos->isDebugValue());
}

/// Find the max change in excess pressure across all sets.
static void computeExcessPressureDelta(ArrayRef<unsigned> OldPressureVec,
                                       ArrayRef<unsigned> NewPressureVec,
                                       RegPressureDelta &Delta,
                                       const RegisterClassInfo *RCI,
                                       ArrayRef<unsigned> LiveThruPressureVec) {
  Delta.Excess = PressureChange();
  for (unsigned i = 0, e = OldPressureVec.size(); i < e; ++i) {
    unsigned POld = OldPressureVec[i];
    unsigned PNew = NewPressureVec[i];
    int PDiff = (int)PNew - (int)POld;
    if (!PDiff) // No change in this set in the common case.
      continue;
    // Only consider change beyond the limit.
    unsigned Limit = RCI->getRegPressureSetLimit(i);
    if (!LiveThruPressureVec.empty())
      Limit += LiveThruPressureVec[i];

    if (Limit > POld) {
      if (Limit > PNew)
        PDiff = 0;            // Under the limit
      else
        PDiff = PNew - Limit; // Just exceeded limit.
    } else if (Limit > PNew)
      PDiff = Limit - POld;   // Just obeyed limit.

    if (PDiff) {
      Delta.Excess = PressureChange(i);
      Delta.Excess.setUnitInc(PDiff);
      break;
    }
  }
}

/// Find the max change in max pressure that either surpasses a critical PSet
/// limit or exceeds the current MaxPressureLimit.
///
/// FIXME: comparing each element of the old and new MaxPressure vectors here is
/// silly. It's done now to demonstrate the concept but will go away with a
/// RegPressureTracker API change to work with pressure differences.
static void computeMaxPressureDelta(ArrayRef<unsigned> OldMaxPressureVec,
                                    ArrayRef<unsigned> NewMaxPressureVec,
                                    ArrayRef<PressureChange> CriticalPSets,
                                    ArrayRef<unsigned> MaxPressureLimit,
                                    RegPressureDelta &Delta) {
  Delta.CriticalMax = PressureChange();
  Delta.CurrentMax = PressureChange();

  unsigned CritIdx = 0, CritEnd = CriticalPSets.size();
  for (unsigned i = 0, e = OldMaxPressureVec.size(); i < e; ++i) {
    unsigned POld = OldMaxPressureVec[i];
    unsigned PNew = NewMaxPressureVec[i];
    if (PNew == POld) // No change in this set in the common case.
      continue;

    if (!Delta.CriticalMax.isValid()) {
      while (CritIdx != CritEnd && CriticalPSets[CritIdx].getPSet() < i)
        ++CritIdx;

      if (CritIdx != CritEnd && CriticalPSets[CritIdx].getPSet() == i) {
        int PDiff = (int)PNew - (int)CriticalPSets[CritIdx].getUnitInc();
        if (PDiff > 0) {
          Delta.CriticalMax = PressureChange(i);
          Delta.CriticalMax.setUnitInc(PDiff);
        }
      }
    }
    // Find the first increase above MaxPressureLimit.
    // (Ignores negative MDiff).
    if (!Delta.CurrentMax.isValid() && PNew > MaxPressureLimit[i]) {
      Delta.CurrentMax = PressureChange(i);
      Delta.CurrentMax.setUnitInc(PNew - POld);
      if (CritIdx == CritEnd || Delta.CriticalMax.isValid())
        break;
    }
  }
}

/// Record the upward impact of a single instruction on current register
/// pressure. Unlike the advance/recede pressure tracking interface, this does
/// not discover live in/outs.
///
/// This is intended for speculative queries. It leaves pressure inconsistent
/// with the current position, so must be restored by the caller.
void RegPressureTracker::bumpUpwardPressure(const MachineInstr *MI) {
  assert(!MI->isDebugValue() && "Expect a nondebug instruction.");

  // Account for register pressure similar to RegPressureTracker::recede().
  RegisterOperands RegOpers;
  RegOpers.collect(*MI, *TRI, *MRI, /*IgnoreDead=*/true);
  assert(RegOpers.DeadDefs.size() == 0);
  if (RequireIntervals)
    RegOpers.detectDeadDefs(*MI, *LIS);

  // Kill liveness at live defs.
  for (unsigned Reg : RegOpers.Defs) {
    if (!containsReg(RegOpers.Uses, Reg))
      decreaseRegPressure(Reg);
  }
  // Generate liveness for uses.
  for (unsigned Reg : RegOpers.Uses) {
    if (!LiveRegs.contains(Reg))
      increaseRegPressure(Reg);
  }
}

/// Consider the pressure increase caused by traversing this instruction
/// bottom-up. Find the pressure set with the most change beyond its pressure
/// limit based on the tracker's current pressure, and return the change in
/// number of register units of that pressure set introduced by this
/// instruction.
///
/// This assumes that the current LiveOut set is sufficient.
///
/// This is expensive for an on-the-fly query because it calls
/// bumpUpwardPressure to recompute the pressure sets based on current
/// liveness. This mainly exists to verify correctness, e.g. with
/// -verify-misched. getUpwardPressureDelta is the fast version of this query
/// that uses the per-SUnit cache of the PressureDiff.
void RegPressureTracker::
getMaxUpwardPressureDelta(const MachineInstr *MI, PressureDiff *PDiff,
                          RegPressureDelta &Delta,
                          ArrayRef<PressureChange> CriticalPSets,
                          ArrayRef<unsigned> MaxPressureLimit) {
  // Snapshot Pressure.
  // FIXME: The snapshot heap space should persist. But I'm planning to
  // summarize the pressure effect so we don't need to snapshot at all.
  std::vector<unsigned> SavedPressure = CurrSetPressure;
  std::vector<unsigned> SavedMaxPressure = P.MaxSetPressure;

  bumpUpwardPressure(MI);

  computeExcessPressureDelta(SavedPressure, CurrSetPressure, Delta, RCI,
                             LiveThruPressure);
  computeMaxPressureDelta(SavedMaxPressure, P.MaxSetPressure, CriticalPSets,
                          MaxPressureLimit, Delta);
  assert(Delta.CriticalMax.getUnitInc() >= 0 &&
         Delta.CurrentMax.getUnitInc() >= 0 && "cannot decrease max pressure");

  // Restore the tracker's state.
  P.MaxSetPressure.swap(SavedMaxPressure);
  CurrSetPressure.swap(SavedPressure);

#ifndef NDEBUG
  if (!PDiff)
    return;

  // Check if the alternate algorithm yields the same result.
  RegPressureDelta Delta2;
  getUpwardPressureDelta(MI, *PDiff, Delta2, CriticalPSets, MaxPressureLimit);
  if (Delta != Delta2) {
    dbgs() << "PDiff: ";
    PDiff->dump(*TRI);
    dbgs() << "DELTA: " << *MI;
    if (Delta.Excess.isValid())
      dbgs() << "Excess1 " << TRI->getRegPressureSetName(Delta.Excess.getPSet())
             << " " << Delta.Excess.getUnitInc() << "\n";
    if (Delta.CriticalMax.isValid())
      dbgs() << "Critic1 " << TRI->getRegPressureSetName(Delta.CriticalMax.getPSet())
             << " " << Delta.CriticalMax.getUnitInc() << "\n";
    if (Delta.CurrentMax.isValid())
      dbgs() << "CurrMx1 " << TRI->getRegPressureSetName(Delta.CurrentMax.getPSet())
             << " " << Delta.CurrentMax.getUnitInc() << "\n";
    if (Delta2.Excess.isValid())
      dbgs() << "Excess2 " << TRI->getRegPressureSetName(Delta2.Excess.getPSet())
             << " " << Delta2.Excess.getUnitInc() << "\n";
    if (Delta2.CriticalMax.isValid())
      dbgs() << "Critic2 " << TRI->getRegPressureSetName(Delta2.CriticalMax.getPSet())
             << " " << Delta2.CriticalMax.getUnitInc() << "\n";
    if (Delta2.CurrentMax.isValid())
      dbgs() << "CurrMx2 " << TRI->getRegPressureSetName(Delta2.CurrentMax.getPSet())
             << " " << Delta2.CurrentMax.getUnitInc() << "\n";
    llvm_unreachable("RegP Delta Mismatch");
  }
#endif
}

/// This is the fast version of querying register pressure that does not
/// directly depend on current liveness.
///
/// @param Delta captures information needed for heuristics.
///
/// @param CriticalPSets Are the pressure sets that are known to exceed some
/// limit within the region, not necessarily at the current position.
///
/// @param MaxPressureLimit Is the max pressure within the region, not
/// necessarily at the current position.
void RegPressureTracker::
getUpwardPressureDelta(const MachineInstr *MI, /*const*/ PressureDiff &PDiff,
                       RegPressureDelta &Delta,
                       ArrayRef<PressureChange> CriticalPSets,
                       ArrayRef<unsigned> MaxPressureLimit) const {
  unsigned CritIdx = 0, CritEnd = CriticalPSets.size();
  for (PressureDiff::const_iterator
         PDiffI = PDiff.begin(), PDiffE = PDiff.end();
       PDiffI != PDiffE && PDiffI->isValid(); ++PDiffI) {

    unsigned PSetID = PDiffI->getPSet();
    unsigned Limit = RCI->getRegPressureSetLimit(PSetID);
    if (!LiveThruPressure.empty())
      Limit += LiveThruPressure[PSetID];

    unsigned POld = CurrSetPressure[PSetID];
    unsigned MOld = P.MaxSetPressure[PSetID];
    unsigned MNew = MOld;
    // Ignore DeadDefs here because they aren't captured by PressureChange.
    unsigned PNew = POld + PDiffI->getUnitInc();
    assert((PDiffI->getUnitInc() >= 0) == (PNew >= POld)
           && "PSet overflow/underflow");
    if (PNew > MOld)
      MNew = PNew;
    // Check if current pressure has exceeded the limit.
    if (!Delta.Excess.isValid()) {
      unsigned ExcessInc = 0;
      if (PNew > Limit)
        ExcessInc = POld > Limit ? PNew - POld : PNew - Limit;
      else if (POld > Limit)
        ExcessInc = Limit - POld;
      if (ExcessInc) {
        Delta.Excess = PressureChange(PSetID);
        Delta.Excess.setUnitInc(ExcessInc);
      }
    }
    // Check if max pressure has exceeded a critical pressure set max.
    if (MNew == MOld)
      continue;
    if (!Delta.CriticalMax.isValid()) {
      while (CritIdx != CritEnd && CriticalPSets[CritIdx].getPSet() < PSetID)
        ++CritIdx;

      if (CritIdx != CritEnd && CriticalPSets[CritIdx].getPSet() == PSetID) {
        int CritInc = (int)MNew - (int)CriticalPSets[CritIdx].getUnitInc();
        if (CritInc > 0 && CritInc <= INT16_MAX) {
          Delta.CriticalMax = PressureChange(PSetID);
          Delta.CriticalMax.setUnitInc(CritInc);
        }
      }
    }
    // Check if max pressure has exceeded the current max.
    if (!Delta.CurrentMax.isValid() && MNew > MaxPressureLimit[PSetID]) {
      Delta.CurrentMax = PressureChange(PSetID);
      Delta.CurrentMax.setUnitInc(MNew - MOld);
    }
  }
}

/// Helper to find a vreg use between two indices [PriorUseIdx, NextUseIdx).
static bool findUseBetween(unsigned Reg, SlotIndex PriorUseIdx,
                           SlotIndex NextUseIdx, const MachineRegisterInfo &MRI,
                           const LiveIntervals *LIS) {
  for (const MachineInstr &MI : MRI.use_nodbg_instructions(Reg)) {
    SlotIndex InstSlot = LIS->getInstructionIndex(&MI).getRegSlot();
    if (InstSlot >= PriorUseIdx && InstSlot < NextUseIdx)
      return true;
  }
  return false;
}

/// Record the downward impact of a single instruction on current register
/// pressure. Unlike the advance/recede pressure tracking interface, this does
/// not discover live in/outs.
///
/// This is intended for speculative queries. It leaves pressure inconsistent
/// with the current position, so must be restored by the caller.
void RegPressureTracker::bumpDownwardPressure(const MachineInstr *MI) {
  assert(!MI->isDebugValue() && "Expect a nondebug instruction.");

  // Account for register pressure similar to RegPressureTracker::recede().
  RegisterOperands RegOpers;
  RegOpers.collect(*MI, *TRI, *MRI);

  // Kill liveness at last uses. Assume allocatable physregs are single-use
  // rather than checking LiveIntervals.
  SlotIndex SlotIdx;
  if (RequireIntervals)
    SlotIdx = LIS->getInstructionIndex(MI).getRegSlot();

  for (unsigned Reg : RegOpers.Uses) {
    if (RequireIntervals) {
      // FIXME: allow the caller to pass in the list of vreg uses that remain
      // to be bottom-scheduled to avoid searching uses at each query.
      SlotIndex CurrIdx = getCurrSlot();
      const LiveRange *LR = getLiveRange(*LIS, Reg);
      if (LR) {
        LiveQueryResult LRQ = LR->Query(SlotIdx);
        if (LRQ.isKill() && !findUseBetween(Reg, CurrIdx, SlotIdx, *MRI, LIS))
          decreaseRegPressure(Reg);
      }
    } else if (!TargetRegisterInfo::isVirtualRegister(Reg)) {
      // Allocatable physregs are always single-use before register rewriting.
      decreaseRegPressure(Reg);
    }
  }

  // Generate liveness for defs.
  increaseRegPressure(RegOpers.Defs);

  // Boost pressure for all dead defs together.
  increaseRegPressure(RegOpers.DeadDefs);
  decreaseRegPressure(RegOpers.DeadDefs);
}

/// Consider the pressure increase caused by traversing this instruction
/// top-down. Find the register class with the most change in its pressure limit
/// based on the tracker's current pressure, and return the number of excess
/// register units of that pressure set introduced by this instruction.
///
/// This assumes that the current LiveIn set is sufficient.
///
/// This is expensive for an on-the-fly query because it calls
/// bumpDownwardPressure to recompute the pressure sets based on current
/// liveness. We don't yet have a fast version of downward pressure tracking
/// analogous to getUpwardPressureDelta.
void RegPressureTracker::
getMaxDownwardPressureDelta(const MachineInstr *MI, RegPressureDelta &Delta,
                            ArrayRef<PressureChange> CriticalPSets,
                            ArrayRef<unsigned> MaxPressureLimit) {
  // Snapshot Pressure.
  std::vector<unsigned> SavedPressure = CurrSetPressure;
  std::vector<unsigned> SavedMaxPressure = P.MaxSetPressure;

  bumpDownwardPressure(MI);

  computeExcessPressureDelta(SavedPressure, CurrSetPressure, Delta, RCI,
                             LiveThruPressure);
  computeMaxPressureDelta(SavedMaxPressure, P.MaxSetPressure, CriticalPSets,
                          MaxPressureLimit, Delta);
  assert(Delta.CriticalMax.getUnitInc() >= 0 &&
         Delta.CurrentMax.getUnitInc() >= 0 && "cannot decrease max pressure");

  // Restore the tracker's state.
  P.MaxSetPressure.swap(SavedMaxPressure);
  CurrSetPressure.swap(SavedPressure);
}

/// Get the pressure of each PSet after traversing this instruction bottom-up.
void RegPressureTracker::
getUpwardPressure(const MachineInstr *MI,
                  std::vector<unsigned> &PressureResult,
                  std::vector<unsigned> &MaxPressureResult) {
  // Snapshot pressure.
  PressureResult = CurrSetPressure;
  MaxPressureResult = P.MaxSetPressure;

  bumpUpwardPressure(MI);

  // Current pressure becomes the result. Restore current pressure.
  P.MaxSetPressure.swap(MaxPressureResult);
  CurrSetPressure.swap(PressureResult);
}

/// Get the pressure of each PSet after traversing this instruction top-down.
void RegPressureTracker::
getDownwardPressure(const MachineInstr *MI,
                    std::vector<unsigned> &PressureResult,
                    std::vector<unsigned> &MaxPressureResult) {
  // Snapshot pressure.
  PressureResult = CurrSetPressure;
  MaxPressureResult = P.MaxSetPressure;

  bumpDownwardPressure(MI);

  // Current pressure becomes the result. Restore current pressure.
  P.MaxSetPressure.swap(MaxPressureResult);
  CurrSetPressure.swap(PressureResult);
}