xref: /external/llvm/lib/CodeGen/LiveInterval.cpp

//===-- LiveInterval.cpp - Live Interval Representation -------------------===//
//
//                     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 LiveRange and LiveInterval classes.  Given some
// numbering of each the machine instructions an interval [i, j) is said to be a
// live interval for register v if there is no instruction with number j' > j
// such that v is live at j' and there is no instruction with number i' < i such
// that v is live at i'. In this implementation intervals can have holes,
// i.e. an interval might look like [1,20), [50,65), [1000,1001).  Each
// individual range is represented as an instance of LiveRange, and the whole
// interval is represented as an instance of LiveInterval.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/LiveInterval.h"
#include "RegisterCoalescer.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include <algorithm>
using namespace llvm;

LiveInterval::iterator LiveInterval::find(SlotIndex Pos) {
  // This algorithm is basically std::upper_bound.
  // Unfortunately, std::upper_bound cannot be used with mixed types until we
  // adopt C++0x. Many libraries can do it, but not all.
  if (empty() || Pos >= endIndex())
    return end();
  iterator I = begin();
  size_t Len = ranges.size();
  do {
    size_t Mid = Len >> 1;
    if (Pos < I[Mid].end)
      Len = Mid;
    else
      I += Mid + 1, Len -= Mid + 1;
  } while (Len);
  return I;
}

VNInfo *LiveInterval::createDeadDef(SlotIndex Def,
                                    VNInfo::Allocator &VNInfoAllocator) {
  assert(!Def.isDead() && "Cannot define a value at the dead slot");
  iterator I = find(Def);
  if (I == end()) {
    VNInfo *VNI = getNextValue(Def, VNInfoAllocator);
    ranges.push_back(LiveRange(Def, Def.getDeadSlot(), VNI));
    return VNI;
  }
  if (SlotIndex::isSameInstr(Def, I->start)) {
    assert(I->valno->def == I->start && "Inconsistent existing value def");

    // It is possible to have both normal and early-clobber defs of the same
    // register on an instruction. It doesn't make a lot of sense, but it is
    // possible to specify in inline assembly.
    //
    // Just convert everything to early-clobber.
    Def = std::min(Def, I->start);
    if (Def != I->start)
      I->start = I->valno->def = Def;
    return I->valno;
  }
  assert(SlotIndex::isEarlierInstr(Def, I->start) && "Already live at def");
  VNInfo *VNI = getNextValue(Def, VNInfoAllocator);
  ranges.insert(I, LiveRange(Def, Def.getDeadSlot(), VNI));
  return VNI;
}

// overlaps - Return true if the intersection of the two live intervals is
// not empty.
//
// An example for overlaps():
//
// 0: A = ...
// 4: B = ...
// 8: C = A + B ;; last use of A
//
// The live intervals should look like:
//
// A = [3, 11)
// B = [7, x)
// C = [11, y)
//
// A->overlaps(C) should return false since we want to be able to join
// A and C.
//
bool LiveInterval::overlapsFrom(const LiveInterval& other,
                                const_iterator StartPos) const {
  assert(!empty() && "empty interval");
  const_iterator i = begin();
  const_iterator ie = end();
  const_iterator j = StartPos;
  const_iterator je = other.end();

  assert((StartPos->start <= i->start || StartPos == other.begin()) &&
         StartPos != other.end() && "Bogus start position hint!");

  if (i->start < j->start) {
    i = std::upper_bound(i, ie, j->start);
    if (i != ranges.begin()) --i;
  } else if (j->start < i->start) {
    ++StartPos;
    if (StartPos != other.end() && StartPos->start <= i->start) {
      assert(StartPos < other.end() && i < end());
      j = std::upper_bound(j, je, i->start);
      if (j != other.ranges.begin()) --j;
    }
  } else {
    return true;
  }

  if (j == je) return false;

  while (i != ie) {
    if (i->start > j->start) {
      std::swap(i, j);
      std::swap(ie, je);
    }

    if (i->end > j->start)
      return true;
    ++i;
  }

  return false;
}

bool LiveInterval::overlaps(const LiveInterval &Other,
                            const CoalescerPair &CP,
                            const SlotIndexes &Indexes) const {
  assert(!empty() && "empty interval");
  if (Other.empty())
    return false;

  // Use binary searches to find initial positions.
  const_iterator I = find(Other.beginIndex());
  const_iterator IE = end();
  if (I == IE)
    return false;
  const_iterator J = Other.find(I->start);
  const_iterator JE = Other.end();
  if (J == JE)
    return false;

  for (;;) {
    // J has just been advanced to satisfy:
    assert(J->end >= I->start);
    // Check for an overlap.
    if (J->start < I->end) {
      // I and J are overlapping. Find the later start.
      SlotIndex Def = std::max(I->start, J->start);
      // Allow the overlap if Def is a coalescable copy.
      if (Def.isBlock() ||
          !CP.isCoalescable(Indexes.getInstructionFromIndex(Def)))
        return true;
    }
    // Advance the iterator that ends first to check for more overlaps.
    if (J->end > I->end) {
      std::swap(I, J);
      std::swap(IE, JE);
    }
    // Advance J until J->end >= I->start.
    do
      if (++J == JE)
        return false;
    while (J->end < I->start);
  }
}

/// overlaps - Return true if the live interval overlaps a range specified
/// by [Start, End).
bool LiveInterval::overlaps(SlotIndex Start, SlotIndex End) const {
  assert(Start < End && "Invalid range");
  const_iterator I = std::lower_bound(begin(), end(), End);
  return I != begin() && (--I)->end > Start;
}


/// ValNo is dead, remove it.  If it is the largest value number, just nuke it
/// (and any other deleted values neighboring it), otherwise mark it as ~1U so
/// it can be nuked later.
void LiveInterval::markValNoForDeletion(VNInfo *ValNo) {
  if (ValNo->id == getNumValNums()-1) {
    do {
      valnos.pop_back();
    } while (!valnos.empty() && valnos.back()->isUnused());
  } else {
    ValNo->markUnused();
  }
}

/// RenumberValues - Renumber all values in order of appearance and delete the
/// remaining unused values.
void LiveInterval::RenumberValues(LiveIntervals &lis) {
  SmallPtrSet<VNInfo*, 8> Seen;
  valnos.clear();
  for (const_iterator I = begin(), E = end(); I != E; ++I) {
    VNInfo *VNI = I->valno;
    if (!Seen.insert(VNI))
      continue;
    assert(!VNI->isUnused() && "Unused valno used by live range");
    VNI->id = (unsigned)valnos.size();
    valnos.push_back(VNI);
  }
}

/// extendIntervalEndTo - This method is used when we want to extend the range
/// specified by I to end at the specified endpoint.  To do this, we should
/// merge and eliminate all ranges that this will overlap with.  The iterator is
/// not invalidated.
void LiveInterval::extendIntervalEndTo(Ranges::iterator I, SlotIndex NewEnd) {
  assert(I != ranges.end() && "Not a valid interval!");
  VNInfo *ValNo = I->valno;

  // Search for the first interval that we can't merge with.
  Ranges::iterator MergeTo = llvm::next(I);
  for (; MergeTo != ranges.end() && NewEnd >= MergeTo->end; ++MergeTo) {
    assert(MergeTo->valno == ValNo && "Cannot merge with differing values!");
  }

  // If NewEnd was in the middle of an interval, make sure to get its endpoint.
  I->end = std::max(NewEnd, prior(MergeTo)->end);

  // If the newly formed range now touches the range after it and if they have
  // the same value number, merge the two ranges into one range.
  if (MergeTo != ranges.end() && MergeTo->start <= I->end &&
      MergeTo->valno == ValNo) {
    I->end = MergeTo->end;
    ++MergeTo;
  }

  // Erase any dead ranges.
  ranges.erase(llvm::next(I), MergeTo);
}


/// extendIntervalStartTo - This method is used when we want to extend the range
/// specified by I to start at the specified endpoint.  To do this, we should
/// merge and eliminate all ranges that this will overlap with.
LiveInterval::Ranges::iterator
LiveInterval::extendIntervalStartTo(Ranges::iterator I, SlotIndex NewStart) {
  assert(I != ranges.end() && "Not a valid interval!");
  VNInfo *ValNo = I->valno;

  // Search for the first interval that we can't merge with.
  Ranges::iterator MergeTo = I;
  do {
    if (MergeTo == ranges.begin()) {
      I->start = NewStart;
      ranges.erase(MergeTo, I);
      return I;
    }
    assert(MergeTo->valno == ValNo && "Cannot merge with differing values!");
    --MergeTo;
  } while (NewStart <= MergeTo->start);

  // If we start in the middle of another interval, just delete a range and
  // extend that interval.
  if (MergeTo->end >= NewStart && MergeTo->valno == ValNo) {
    MergeTo->end = I->end;
  } else {
    // Otherwise, extend the interval right after.
    ++MergeTo;
    MergeTo->start = NewStart;
    MergeTo->end = I->end;
  }

  ranges.erase(llvm::next(MergeTo), llvm::next(I));
  return MergeTo;
}

LiveInterval::iterator
LiveInterval::addRangeFrom(LiveRange LR, iterator From) {
  SlotIndex Start = LR.start, End = LR.end;
  iterator it = std::upper_bound(From, ranges.end(), Start);

  // If the inserted interval starts in the middle or right at the end of
  // another interval, just extend that interval to contain the range of LR.
  if (it != ranges.begin()) {
    iterator B = prior(it);
    if (LR.valno == B->valno) {
      if (B->start <= Start && B->end >= Start) {
        extendIntervalEndTo(B, End);
        return B;
      }
    } else {
      // Check to make sure that we are not overlapping two live ranges with
      // different valno's.
      assert(B->end <= Start &&
             "Cannot overlap two LiveRanges with differing ValID's"
             " (did you def the same reg twice in a MachineInstr?)");
    }
  }

  // Otherwise, if this range ends in the middle of, or right next to, another
  // interval, merge it into that interval.
  if (it != ranges.end()) {
    if (LR.valno == it->valno) {
      if (it->start <= End) {
        it = extendIntervalStartTo(it, Start);

        // If LR is a complete superset of an interval, we may need to grow its
        // endpoint as well.
        if (End > it->end)
          extendIntervalEndTo(it, End);
        return it;
      }
    } else {
      // Check to make sure that we are not overlapping two live ranges with
      // different valno's.
      assert(it->start >= End &&
             "Cannot overlap two LiveRanges with differing ValID's");
    }
  }

  // Otherwise, this is just a new range that doesn't interact with anything.
  // Insert it.
  return ranges.insert(it, LR);
}

/// extendInBlock - If this interval is live before Kill in the basic
/// block that starts at StartIdx, extend it to be live up to Kill and return
/// the value. If there is no live range before Kill, return NULL.
VNInfo *LiveInterval::extendInBlock(SlotIndex StartIdx, SlotIndex Kill) {
  if (empty())
    return 0;
  iterator I = std::upper_bound(begin(), end(), Kill.getPrevSlot());
  if (I == begin())
    return 0;
  --I;
  if (I->end <= StartIdx)
    return 0;
  if (I->end < Kill)
    extendIntervalEndTo(I, Kill);
  return I->valno;
}

/// removeRange - Remove the specified range from this interval.  Note that
/// the range must be in a single LiveRange in its entirety.
void LiveInterval::removeRange(SlotIndex Start, SlotIndex End,
                               bool RemoveDeadValNo) {
  // Find the LiveRange containing this span.
  Ranges::iterator I = find(Start);
  assert(I != ranges.end() && "Range is not in interval!");
  assert(I->containsRange(Start, End) && "Range is not entirely in interval!");

  // If the span we are removing is at the start of the LiveRange, adjust it.
  VNInfo *ValNo = I->valno;
  if (I->start == Start) {
    if (I->end == End) {
      if (RemoveDeadValNo) {
        // Check if val# is dead.
        bool isDead = true;
        for (const_iterator II = begin(), EE = end(); II != EE; ++II)
          if (II != I && II->valno == ValNo) {
            isDead = false;
            break;
          }
        if (isDead) {
          // Now that ValNo is dead, remove it.
          markValNoForDeletion(ValNo);
        }
      }

      ranges.erase(I);  // Removed the whole LiveRange.
    } else
      I->start = End;
    return;
  }

  // Otherwise if the span we are removing is at the end of the LiveRange,
  // adjust the other way.
  if (I->end == End) {
    I->end = Start;
    return;
  }

  // Otherwise, we are splitting the LiveRange into two pieces.
  SlotIndex OldEnd = I->end;
  I->end = Start;   // Trim the old interval.

  // Insert the new one.
  ranges.insert(llvm::next(I), LiveRange(End, OldEnd, ValNo));
}

/// removeValNo - Remove all the ranges defined by the specified value#.
/// Also remove the value# from value# list.
void LiveInterval::removeValNo(VNInfo *ValNo) {
  if (empty()) return;
  Ranges::iterator I = ranges.end();
  Ranges::iterator E = ranges.begin();
  do {
    --I;
    if (I->valno == ValNo)
      ranges.erase(I);
  } while (I != E);
  // Now that ValNo is dead, remove it.
  markValNoForDeletion(ValNo);
}

/// join - Join two live intervals (this, and other) together.  This applies
/// mappings to the value numbers in the LHS/RHS intervals as specified.  If
/// the intervals are not joinable, this aborts.
void LiveInterval::join(LiveInterval &Other,
                        const int *LHSValNoAssignments,
                        const int *RHSValNoAssignments,
                        SmallVector<VNInfo*, 16> &NewVNInfo,
                        MachineRegisterInfo *MRI) {
  verify();

  // Determine if any of our live range values are mapped.  This is uncommon, so
  // we want to avoid the interval scan if not.
  bool MustMapCurValNos = false;
  unsigned NumVals = getNumValNums();
  unsigned NumNewVals = NewVNInfo.size();
  for (unsigned i = 0; i != NumVals; ++i) {
    unsigned LHSValID = LHSValNoAssignments[i];
    if (i != LHSValID ||
        (NewVNInfo[LHSValID] && NewVNInfo[LHSValID] != getValNumInfo(i))) {
      MustMapCurValNos = true;
      break;
    }
  }

  // If we have to apply a mapping to our base interval assignment, rewrite it
  // now.
  if (MustMapCurValNos && !empty()) {
    // Map the first live range.

    iterator OutIt = begin();
    OutIt->valno = NewVNInfo[LHSValNoAssignments[OutIt->valno->id]];
    for (iterator I = llvm::next(OutIt), E = end(); I != E; ++I) {
      VNInfo* nextValNo = NewVNInfo[LHSValNoAssignments[I->valno->id]];
      assert(nextValNo != 0 && "Huh?");

      // If this live range has the same value # as its immediate predecessor,
      // and if they are neighbors, remove one LiveRange.  This happens when we
      // have [0,4:0)[4,7:1) and map 0/1 onto the same value #.
      if (OutIt->valno == nextValNo && OutIt->end == I->start) {
        OutIt->end = I->end;
      } else {
        // Didn't merge. Move OutIt to the next interval,
        ++OutIt;
        OutIt->valno = nextValNo;
        if (OutIt != I) {
          OutIt->start = I->start;
          OutIt->end = I->end;
        }
      }
    }
    // If we merge some live ranges, chop off the end.
    ++OutIt;
    ranges.erase(OutIt, end());
  }

  // Rewrite Other values before changing the VNInfo ids.
  // This can leave Other in an invalid state because we're not coalescing
  // touching segments that now have identical values. That's OK since Other is
  // not supposed to be valid after calling join();
  for (iterator I = Other.begin(), E = Other.end(); I != E; ++I)
    I->valno = NewVNInfo[RHSValNoAssignments[I->valno->id]];

  // Update val# info. Renumber them and make sure they all belong to this
  // LiveInterval now. Also remove dead val#'s.
  unsigned NumValNos = 0;
  for (unsigned i = 0; i < NumNewVals; ++i) {
    VNInfo *VNI = NewVNInfo[i];
    if (VNI) {
      if (NumValNos >= NumVals)
        valnos.push_back(VNI);
      else
        valnos[NumValNos] = VNI;
      VNI->id = NumValNos++;  // Renumber val#.
    }
  }
  if (NumNewVals < NumVals)
    valnos.resize(NumNewVals);  // shrinkify

  // Okay, now insert the RHS live ranges into the LHS.
  LiveRangeUpdater Updater(this);
  for (iterator I = Other.begin(), E = Other.end(); I != E; ++I)
    Updater.add(*I);
}

/// MergeRangesInAsValue - Merge all of the intervals in RHS into this live
/// interval as the specified value number.  The LiveRanges in RHS are
/// allowed to overlap with LiveRanges in the current interval, but only if
/// the overlapping LiveRanges have the specified value number.
void LiveInterval::MergeRangesInAsValue(const LiveInterval &RHS,
                                        VNInfo *LHSValNo) {
  LiveRangeUpdater Updater(this);
  for (const_iterator I = RHS.begin(), E = RHS.end(); I != E; ++I)
    Updater.add(I->start, I->end, LHSValNo);
}

/// MergeValueInAsValue - Merge all of the live ranges of a specific val#
/// in RHS into this live interval as the specified value number.
/// The LiveRanges in RHS are allowed to overlap with LiveRanges in the
/// current interval, it will replace the value numbers of the overlaped
/// live ranges with the specified value number.
void LiveInterval::MergeValueInAsValue(const LiveInterval &RHS,
                                       const VNInfo *RHSValNo,
                                       VNInfo *LHSValNo) {
  LiveRangeUpdater Updater(this);
  for (const_iterator I = RHS.begin(), E = RHS.end(); I != E; ++I)
    if (I->valno == RHSValNo)
      Updater.add(I->start, I->end, LHSValNo);
}

/// MergeValueNumberInto - This method is called when two value nubmers
/// are found to be equivalent.  This eliminates V1, replacing all
/// LiveRanges with the V1 value number with the V2 value number.  This can
/// cause merging of V1/V2 values numbers and compaction of the value space.
VNInfo* LiveInterval::MergeValueNumberInto(VNInfo *V1, VNInfo *V2) {
  assert(V1 != V2 && "Identical value#'s are always equivalent!");

  // This code actually merges the (numerically) larger value number into the
  // smaller value number, which is likely to allow us to compactify the value
  // space.  The only thing we have to be careful of is to preserve the
  // instruction that defines the result value.

  // Make sure V2 is smaller than V1.
  if (V1->id < V2->id) {
    V1->copyFrom(*V2);
    std::swap(V1, V2);
  }

  // Merge V1 live ranges into V2.
  for (iterator I = begin(); I != end(); ) {
    iterator LR = I++;
    if (LR->valno != V1) continue;  // Not a V1 LiveRange.

    // Okay, we found a V1 live range.  If it had a previous, touching, V2 live
    // range, extend it.
    if (LR != begin()) {
      iterator Prev = LR-1;
      if (Prev->valno == V2 && Prev->end == LR->start) {
        Prev->end = LR->end;

        // Erase this live-range.
        ranges.erase(LR);
        I = Prev+1;
        LR = Prev;
      }
    }

    // Okay, now we have a V1 or V2 live range that is maximally merged forward.
    // Ensure that it is a V2 live-range.
    LR->valno = V2;

    // If we can merge it into later V2 live ranges, do so now.  We ignore any
    // following V1 live ranges, as they will be merged in subsequent iterations
    // of the loop.
    if (I != end()) {
      if (I->start == LR->end && I->valno == V2) {
        LR->end = I->end;
        ranges.erase(I);
        I = LR+1;
      }
    }
  }

  // Now that V1 is dead, remove it.
  markValNoForDeletion(V1);

  return V2;
}

unsigned LiveInterval::getSize() const {
  unsigned Sum = 0;
  for (const_iterator I = begin(), E = end(); I != E; ++I)
    Sum += I->start.distance(I->end);
  return Sum;
}

raw_ostream& llvm::operator<<(raw_ostream& os, const LiveRange &LR) {
  return os << '[' << LR.start << ',' << LR.end << ':' << LR.valno->id << ")";
}

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
void LiveRange::dump() const {
  dbgs() << *this << "\n";
}
#endif

void LiveInterval::print(raw_ostream &OS) const {
  if (empty())
    OS << "EMPTY";
  else {
    for (LiveInterval::Ranges::const_iterator I = ranges.begin(),
           E = ranges.end(); I != E; ++I) {
      OS << *I;
      assert(I->valno == getValNumInfo(I->valno->id) && "Bad VNInfo");
    }
  }

  // Print value number info.
  if (getNumValNums()) {
    OS << "  ";
    unsigned vnum = 0;
    for (const_vni_iterator i = vni_begin(), e = vni_end(); i != e;
         ++i, ++vnum) {
      const VNInfo *vni = *i;
      if (vnum) OS << " ";
      OS << vnum << "@";
      if (vni->isUnused()) {
        OS << "x";
      } else {
        OS << vni->def;
        if (vni->isPHIDef())
          OS << "-phi";
      }
    }
  }
}

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
void LiveInterval::dump() const {
  dbgs() << *this << "\n";
}
#endif

#ifndef NDEBUG
void LiveInterval::verify() const {
  for (const_iterator I = begin(), E = end(); I != E; ++I) {
    assert(I->start.isValid());
    assert(I->end.isValid());
    assert(I->start < I->end);
    assert(I->valno != 0);
    assert(I->valno == valnos[I->valno->id]);
    if (llvm::next(I) != E) {
      assert(I->end <= llvm::next(I)->start);
      if (I->end == llvm::next(I)->start)
        assert(I->valno != llvm::next(I)->valno);
    }
  }
}
#endif


void LiveRange::print(raw_ostream &os) const {
  os << *this;
}

//===----------------------------------------------------------------------===//
//                           LiveRangeUpdater class
//===----------------------------------------------------------------------===//
//
// The LiveRangeUpdater class always maintains these invariants:
//
// - When LastStart is invalid, Spills is empty and the iterators are invalid.
//   This is the initial state, and the state created by flush().
//   In this state, isDirty() returns false.
//
// Otherwise, segments are kept in three separate areas:
//
// 1. [begin; WriteI) at the front of LI.
// 2. [ReadI; end) at the back of LI.
// 3. Spills.
//
// - LI.begin() <= WriteI <= ReadI <= LI.end().
// - Segments in all three areas are fully ordered and coalesced.
// - Segments in area 1 precede and can't coalesce with segments in area 2.
// - Segments in Spills precede and can't coalesce with segments in area 2.
// - No coalescing is possible between segments in Spills and segments in area
//   1, and there are no overlapping segments.
//
// The segments in Spills are not ordered with respect to the segments in area
// 1. They need to be merged.
//
// When they exist, Spills.back().start <= LastStart,
//                 and WriteI[-1].start <= LastStart.

void LiveRangeUpdater::print(raw_ostream &OS) const {
  if (!isDirty()) {
    if (LI)
      OS << "Clean " << PrintReg(LI->reg) << " updater: " << *LI << '\n';
    else
      OS << "Null updater.\n";
    return;
  }
  assert(LI && "Can't have null LI in dirty updater.");
  OS << PrintReg(LI->reg) << " updater with gap = " << (ReadI - WriteI)
     << ", last start = " << LastStart
     << ":\n  Area 1:";
  for (LiveInterval::const_iterator I = LI->begin(); I != WriteI; ++I)
    OS << ' ' << *I;
  OS << "\n  Spills:";
  for (unsigned I = 0, E = Spills.size(); I != E; ++I)
    OS << ' ' << Spills[I];
  OS << "\n  Area 2:";
  for (LiveInterval::const_iterator I = ReadI, E = LI->end(); I != E; ++I)
    OS << ' ' << *I;
  OS << '\n';
}

void LiveRangeUpdater::dump() const
{
  print(errs());
}

// Determine if A and B should be coalesced.
static inline bool coalescable(const LiveRange &A, const LiveRange &B) {
  assert(A.start <= B.start && "Unordered live ranges.");
  if (A.end == B.start)
    return A.valno == B.valno;
  if (A.end < B.start)
    return false;
  assert(A.valno == B.valno && "Cannot overlap different values");
  return true;
}

void LiveRangeUpdater::add(LiveRange Seg) {
  assert(LI && "Cannot add to a null destination");

  // Flush the state if Start moves backwards.
  if (!LastStart.isValid() || LastStart > Seg.start) {
    if (isDirty())
      flush();
    // This brings us to an uninitialized state. Reinitialize.
    assert(Spills.empty() && "Leftover spilled segments");
    WriteI = ReadI = LI->begin();
  }

  // Remember start for next time.
  LastStart = Seg.start;

  // Advance ReadI until it ends after Seg.start.
  LiveInterval::iterator E = LI->end();
  if (ReadI != E && ReadI->end <= Seg.start) {
    // First try to close the gap between WriteI and ReadI with spills.
    if (ReadI != WriteI)
      mergeSpills();
    // Then advance ReadI.
    if (ReadI == WriteI)
      ReadI = WriteI = LI->find(Seg.start);
    else
      while (ReadI != E && ReadI->end <= Seg.start)
        *WriteI++ = *ReadI++;
  }

  assert(ReadI == E || ReadI->end > Seg.start);

  // Check if the ReadI segment begins early.
  if (ReadI != E && ReadI->start <= Seg.start) {
    assert(ReadI->valno == Seg.valno && "Cannot overlap different values");
    // Bail if Seg is completely contained in ReadI.
    if (ReadI->end >= Seg.end)
      return;
    // Coalesce into Seg.
    Seg.start = ReadI->start;
    ++ReadI;
  }

  // Coalesce as much as possible from ReadI into Seg.
  while (ReadI != E && coalescable(Seg, *ReadI)) {
    Seg.end = std::max(Seg.end, ReadI->end);
    ++ReadI;
  }

  // Try coalescing Spills.back() into Seg.
  if (!Spills.empty() && coalescable(Spills.back(), Seg)) {
    Seg.start = Spills.back().start;
    Seg.end = std::max(Spills.back().end, Seg.end);
    Spills.pop_back();
  }

  // Try coalescing Seg into WriteI[-1].
  if (WriteI != LI->begin() && coalescable(WriteI[-1], Seg)) {
    WriteI[-1].end = std::max(WriteI[-1].end, Seg.end);
    return;
  }

  // Seg doesn't coalesce with anything, and needs to be inserted somewhere.
  if (WriteI != ReadI) {
    *WriteI++ = Seg;
    return;
  }

  // Finally, append to LI or Spills.
  if (WriteI == E) {
    LI->ranges.push_back(Seg);
    WriteI = ReadI = LI->ranges.end();
  } else
    Spills.push_back(Seg);
}

// Merge as many spilled segments as possible into the gap between WriteI
// and ReadI. Advance WriteI to reflect the inserted instructions.
void LiveRangeUpdater::mergeSpills() {
  // Perform a backwards merge of Spills and [SpillI;WriteI).
  size_t GapSize = ReadI - WriteI;
  size_t NumMoved = std::min(Spills.size(), GapSize);
  LiveInterval::iterator Src = WriteI;
  LiveInterval::iterator Dst = Src + NumMoved;
  LiveInterval::iterator SpillSrc = Spills.end();
  LiveInterval::iterator B = LI->begin();

  // This is the new WriteI position after merging spills.
  WriteI = Dst;

  // Now merge Src and Spills backwards.
  while (Src != Dst) {
    if (Src != B && Src[-1].start > SpillSrc[-1].start)
      *--Dst = *--Src;
    else
      *--Dst = *--SpillSrc;
  }
  assert(NumMoved == size_t(Spills.end() - SpillSrc));
  Spills.erase(SpillSrc, Spills.end());
}

void LiveRangeUpdater::flush() {
  if (!isDirty())
    return;
  // Clear the dirty state.
  LastStart = SlotIndex();

  assert(LI && "Cannot add to a null destination");

  // Nothing to merge?
  if (Spills.empty()) {
    LI->ranges.erase(WriteI, ReadI);
    LI->verify();
    return;
  }

  // Resize the WriteI - ReadI gap to match Spills.
  size_t GapSize = ReadI - WriteI;
  if (GapSize < Spills.size()) {
    // The gap is too small. Make some room.
    size_t WritePos = WriteI - LI->begin();
    LI->ranges.insert(ReadI, Spills.size() - GapSize, LiveRange());
    // This also invalidated ReadI, but it is recomputed below.
    WriteI = LI->ranges.begin() + WritePos;
  } else {
    // Shrink the gap if necessary.
    LI->ranges.erase(WriteI + Spills.size(), ReadI);
  }
  ReadI = WriteI + Spills.size();
  mergeSpills();
  LI->verify();
}

unsigned ConnectedVNInfoEqClasses::Classify(const LiveInterval *LI) {
  // Create initial equivalence classes.
  EqClass.clear();
  EqClass.grow(LI->getNumValNums());

  const VNInfo *used = 0, *unused = 0;

  // Determine connections.
  for (LiveInterval::const_vni_iterator I = LI->vni_begin(), E = LI->vni_end();
       I != E; ++I) {
    const VNInfo *VNI = *I;
    // Group all unused values into one class.
    if (VNI->isUnused()) {
      if (unused)
        EqClass.join(unused->id, VNI->id);
      unused = VNI;
      continue;
    }
    used = VNI;
    if (VNI->isPHIDef()) {
      const MachineBasicBlock *MBB = LIS.getMBBFromIndex(VNI->def);
      assert(MBB && "Phi-def has no defining MBB");
      // Connect to values live out of predecessors.
      for (MachineBasicBlock::const_pred_iterator PI = MBB->pred_begin(),
           PE = MBB->pred_end(); PI != PE; ++PI)
        if (const VNInfo *PVNI = LI->getVNInfoBefore(LIS.getMBBEndIdx(*PI)))
          EqClass.join(VNI->id, PVNI->id);
    } else {
      // Normal value defined by an instruction. Check for two-addr redef.
      // FIXME: This could be coincidental. Should we really check for a tied
      // operand constraint?
      // Note that VNI->def may be a use slot for an early clobber def.
      if (const VNInfo *UVNI = LI->getVNInfoBefore(VNI->def))
        EqClass.join(VNI->id, UVNI->id);
    }
  }

  // Lump all the unused values in with the last used value.
  if (used && unused)
    EqClass.join(used->id, unused->id);

  EqClass.compress();
  return EqClass.getNumClasses();
}

void ConnectedVNInfoEqClasses::Distribute(LiveInterval *LIV[],
                                          MachineRegisterInfo &MRI) {
  assert(LIV[0] && "LIV[0] must be set");
  LiveInterval &LI = *LIV[0];

  // Rewrite instructions.
  for (MachineRegisterInfo::reg_iterator RI = MRI.reg_begin(LI.reg),
       RE = MRI.reg_end(); RI != RE;) {
    MachineOperand &MO = RI.getOperand();
    MachineInstr *MI = MO.getParent();
    ++RI;
    // DBG_VALUE instructions should have been eliminated earlier.
    LiveRangeQuery LRQ(LI, LIS.getInstructionIndex(MI));
    const VNInfo *VNI = MO.readsReg() ? LRQ.valueIn() : LRQ.valueDefined();
    // In the case of an <undef> use that isn't tied to any def, VNI will be
    // NULL. If the use is tied to a def, VNI will be the defined value.
    if (!VNI)
      continue;
    MO.setReg(LIV[getEqClass(VNI)]->reg);
  }

  // Move runs to new intervals.
  LiveInterval::iterator J = LI.begin(), E = LI.end();
  while (J != E && EqClass[J->valno->id] == 0)
    ++J;
  for (LiveInterval::iterator I = J; I != E; ++I) {
    if (unsigned eq = EqClass[I->valno->id]) {
      assert((LIV[eq]->empty() || LIV[eq]->expiredAt(I->start)) &&
             "New intervals should be empty");
      LIV[eq]->ranges.push_back(*I);
    } else
      *J++ = *I;
  }
  LI.ranges.erase(J, E);

  // Transfer VNInfos to their new owners and renumber them.
  unsigned j = 0, e = LI.getNumValNums();
  while (j != e && EqClass[j] == 0)
    ++j;
  for (unsigned i = j; i != e; ++i) {
    VNInfo *VNI = LI.getValNumInfo(i);
    if (unsigned eq = EqClass[i]) {
      VNI->id = LIV[eq]->getNumValNums();
      LIV[eq]->valnos.push_back(VNI);
    } else {
      VNI->id = j;
      LI.valnos[j++] = VNI;
    }
  }
  LI.valnos.resize(j);
}