RegAllocGreedy.cpp

    if (!Cand.LiveBundles.any()) {
      DEBUG(dbgs() << " no bundles.\n");
      continue;
    }

    Cost += calcGlobalSplitCost(Cand);
    DEBUG({
      dbgs() << ", total = " << Cost << " with bundles";
      for (int i = Cand.LiveBundles.find_first(); i>=0;
           i = Cand.LiveBundles.find_next(i))
        dbgs() << " EB#" << i;
      dbgs() << ".\n";
    });
    if (Cost < BestCost) {
      BestCand = NumCands;
      BestCost = Hysteresis * Cost; // Prevent rounding effects.
    }
    ++NumCands;
  }

  if (BestCand == NoCand)
    return 0;

  splitAroundRegion(VirtReg, GlobalCand[BestCand], NewVRegs);
  return 0;
}


//===----------------------------------------------------------------------===//
//                             Local Splitting
//===----------------------------------------------------------------------===//


/// calcGapWeights - Compute the maximum spill weight that needs to be evicted
/// in order to use PhysReg between two entries in SA->UseSlots.
///
/// GapWeight[i] represents the gap between UseSlots[i] and UseSlots[i+1].
///
void RAGreedy::calcGapWeights(unsigned PhysReg,
                              SmallVectorImpl<float> &GapWeight) {
  assert(SA->getUseBlocks().size() == 1 && "Not a local interval");
  const SplitAnalysis::BlockInfo &BI = SA->getUseBlocks().front();
  const SmallVectorImpl<SlotIndex> &Uses = SA->UseSlots;
  const unsigned NumGaps = Uses.size()-1;

  // Start and end points for the interference check.
  SlotIndex StartIdx = BI.LiveIn ? BI.FirstUse.getBaseIndex() : BI.FirstUse;
  SlotIndex StopIdx = BI.LiveOut ? BI.LastUse.getBoundaryIndex() : BI.LastUse;

  GapWeight.assign(NumGaps, 0.0f);

  // Add interference from each overlapping register.
  for (const unsigned *AI = TRI->getOverlaps(PhysReg); *AI; ++AI) {
    if (!query(const_cast<LiveInterval&>(SA->getParent()), *AI)
           .checkInterference())
      continue;

    // We know that VirtReg is a continuous interval from FirstUse to LastUse,
    // so we don't need InterferenceQuery.
    //
    // Interference that overlaps an instruction is counted in both gaps
    // surrounding the instruction. The exception is interference before
    // StartIdx and after StopIdx.
    //
    LiveIntervalUnion::SegmentIter IntI = PhysReg2LiveUnion[*AI].find(StartIdx);
    for (unsigned Gap = 0; IntI.valid() && IntI.start() < StopIdx; ++IntI) {
      // Skip the gaps before IntI.
      while (Uses[Gap+1].getBoundaryIndex() < IntI.start())
        if (++Gap == NumGaps)
          break;
      if (Gap == NumGaps)
        break;

      // Update the gaps covered by IntI.
      const float weight = IntI.value()->weight;
      for (; Gap != NumGaps; ++Gap) {
        GapWeight[Gap] = std::max(GapWeight[Gap], weight);
        if (Uses[Gap+1].getBaseIndex() >= IntI.stop())
          break;
      }
      if (Gap == NumGaps)
        break;
    }
  }
}

/// tryLocalSplit - Try to split VirtReg into smaller intervals inside its only
/// basic block.
///
unsigned RAGreedy::tryLocalSplit(LiveInterval &VirtReg, AllocationOrder &Order,
                                 SmallVectorImpl<LiveInterval*> &NewVRegs) {
  assert(SA->getUseBlocks().size() == 1 && "Not a local interval");
  const SplitAnalysis::BlockInfo &BI = SA->getUseBlocks().front();

  // Note that it is possible to have an interval that is live-in or live-out
  // while only covering a single block - A phi-def can use undef values from
  // predecessors, and the block could be a single-block loop.
  // We don't bother doing anything clever about such a case, we simply assume
  // that the interval is continuous from FirstUse to LastUse. We should make
  // sure that we don't do anything illegal to such an interval, though.

  const SmallVectorImpl<SlotIndex> &Uses = SA->UseSlots;
  if (Uses.size() <= 2)
    return 0;
  const unsigned NumGaps = Uses.size()-1;

  DEBUG({
    dbgs() << "tryLocalSplit: ";
    for (unsigned i = 0, e = Uses.size(); i != e; ++i)
      dbgs() << ' ' << SA->UseSlots[i];
    dbgs() << '\n';
  });

  // Since we allow local split results to be split again, there is a risk of
  // creating infinite loops. It is tempting to require that the new live
  // ranges have less instructions than the original. That would guarantee
  // convergence, but it is too strict. A live range with 3 instructions can be
  // split 2+3 (including the COPY), and we want to allow that.
  //
  // Instead we use these rules:
  //
  // 1. Allow any split for ranges with getStage() < RS_Local. (Except for the
  //    noop split, of course).
  // 2. Require progress be made for ranges with getStage() >= RS_Local. All
  //    the new ranges must have fewer instructions than before the split.
  // 3. New ranges with the same number of instructions are marked RS_Local,
  //    smaller ranges are marked RS_New.
  //
  // These rules allow a 3 -> 2+3 split once, which we need. They also prevent
  // excessive splitting and infinite loops.
  //
  bool ProgressRequired = getStage(VirtReg) >= RS_Local;

  // Best split candidate.
  unsigned BestBefore = NumGaps;
  unsigned BestAfter = 0;
  float BestDiff = 0;

  const float blockFreq = SpillPlacer->getBlockFrequency(BI.MBB->getNumber());
  SmallVector<float, 8> GapWeight;

  Order.rewind();
  while (unsigned PhysReg = Order.next()) {
    // Keep track of the largest spill weight that would need to be evicted in
    // order to make use of PhysReg between UseSlots[i] and UseSlots[i+1].
    calcGapWeights(PhysReg, GapWeight);

    // Try to find the best sequence of gaps to close.
    // The new spill weight must be larger than any gap interference.

    // We will split before Uses[SplitBefore] and after Uses[SplitAfter].
    unsigned SplitBefore = 0, SplitAfter = 1;

    // MaxGap should always be max(GapWeight[SplitBefore..SplitAfter-1]).
    // It is the spill weight that needs to be evicted.
    float MaxGap = GapWeight[0];

    for (;;) {
      // Live before/after split?
      const bool LiveBefore = SplitBefore != 0 || BI.LiveIn;
      const bool LiveAfter = SplitAfter != NumGaps || BI.LiveOut;

      DEBUG(dbgs() << PrintReg(PhysReg, TRI) << ' '
                   << Uses[SplitBefore] << '-' << Uses[SplitAfter]
                   << " i=" << MaxGap);

      // Stop before the interval gets so big we wouldn't be making progress.
      if (!LiveBefore && !LiveAfter) {
        DEBUG(dbgs() << " all\n");
        break;
      }
      // Should the interval be extended or shrunk?
      bool Shrink = true;

      // How many gaps would the new range have?
      unsigned NewGaps = LiveBefore + SplitAfter - SplitBefore + LiveAfter;

      // Legally, without causing looping?
      bool Legal = !ProgressRequired || NewGaps < NumGaps;

      if (Legal && MaxGap < HUGE_VALF) {
        // Estimate the new spill weight. Each instruction reads or writes the
        // register. Conservatively assume there are no read-modify-write
        // instructions.
        //
        // Try to guess the size of the new interval.
        const float EstWeight = normalizeSpillWeight(blockFreq * (NewGaps + 1),
                                 Uses[SplitBefore].distance(Uses[SplitAfter]) +
                                 (LiveBefore + LiveAfter)*SlotIndex::InstrDist);
        // Would this split be possible to allocate?
        // Never allocate all gaps, we wouldn't be making progress.
        DEBUG(dbgs() << " w=" << EstWeight);
        if (EstWeight * Hysteresis >= MaxGap) {
          Shrink = false;
          float Diff = EstWeight - MaxGap;
          if (Diff > BestDiff) {
            DEBUG(dbgs() << " (best)");
            BestDiff = Hysteresis * Diff;
            BestBefore = SplitBefore;
            BestAfter = SplitAfter;
          }
        }
      }

      // Try to shrink.
      if (Shrink) {
        if (++SplitBefore < SplitAfter) {
          DEBUG(dbgs() << " shrink\n");
          // Recompute the max when necessary.
          if (GapWeight[SplitBefore - 1] >= MaxGap) {
            MaxGap = GapWeight[SplitBefore];
            for (unsigned i = SplitBefore + 1; i != SplitAfter; ++i)
              MaxGap = std::max(MaxGap, GapWeight[i]);
          }
          continue;
        }
        MaxGap = 0;
      }

      // Try to extend the interval.
      if (SplitAfter >= NumGaps) {
        DEBUG(dbgs() << " end\n");
        break;
      }

      DEBUG(dbgs() << " extend\n");
      MaxGap = std::max(MaxGap, GapWeight[SplitAfter++]);
    }
  }

  // Didn't find any candidates?
  if (BestBefore == NumGaps)
    return 0;

  DEBUG(dbgs() << "Best local split range: " << Uses[BestBefore]
               << '-' << Uses[BestAfter] << ", " << BestDiff
               << ", " << (BestAfter - BestBefore + 1) << " instrs\n");

  LiveRangeEdit LREdit(VirtReg, NewVRegs, this);
  SE->reset(LREdit);

  SE->openIntv();
  SlotIndex SegStart = SE->enterIntvBefore(Uses[BestBefore]);
  SlotIndex SegStop  = SE->leaveIntvAfter(Uses[BestAfter]);
  SE->useIntv(SegStart, SegStop);
  SmallVector<unsigned, 8> IntvMap;
  SE->finish(&IntvMap);
  DebugVars->splitRegister(VirtReg.reg, LREdit.regs());

  // If the new range has the same number of instructions as before, mark it as
  // RS_Local so the next split will be forced to make progress. Otherwise,
  // leave the new intervals as RS_New so they can compete.
  bool LiveBefore = BestBefore != 0 || BI.LiveIn;
  bool LiveAfter = BestAfter != NumGaps || BI.LiveOut;
  unsigned NewGaps = LiveBefore + BestAfter - BestBefore + LiveAfter;
  if (NewGaps >= NumGaps) {
    DEBUG(dbgs() << "Tagging non-progress ranges: ");
    assert(!ProgressRequired && "Didn't make progress when it was required.");
    for (unsigned i = 0, e = IntvMap.size(); i != e; ++i)
      if (IntvMap[i] == 1) {
        setStage(*LREdit.get(i), RS_Local);
        DEBUG(dbgs() << PrintReg(LREdit.get(i)->reg));
      }
    DEBUG(dbgs() << '\n');
  }
  ++NumLocalSplits;

  return 0;
}

//===----------------------------------------------------------------------===//
//                          Live Range Splitting
//===----------------------------------------------------------------------===//

/// trySplit - Try to split VirtReg or one of its interferences, making it
/// assignable.
/// @return Physreg when VirtReg may be assigned and/or new NewVRegs.
unsigned RAGreedy::trySplit(LiveInterval &VirtReg, AllocationOrder &Order,
                            SmallVectorImpl<LiveInterval*>&NewVRegs) {
  // Local intervals are handled separately.
  if (LIS->intervalIsInOneMBB(VirtReg)) {
    NamedRegionTimer T("Local Splitting", TimerGroupName, TimePassesIsEnabled);
    SA->analyze(&VirtReg);
    return tryLocalSplit(VirtReg, Order, NewVRegs);
  }

  NamedRegionTimer T("Global Splitting", TimerGroupName, TimePassesIsEnabled);

  // Don't iterate global splitting.
  // Move straight to spilling if this range was produced by a global split.
  if (getStage(VirtReg) >= RS_Global)
    return 0;

  SA->analyze(&VirtReg);

  // FIXME: SplitAnalysis may repair broken live ranges coming from the
  // coalescer. That may cause the range to become allocatable which means that
  // tryRegionSplit won't be making progress. This check should be replaced with
  // an assertion when the coalescer is fixed.
  if (SA->didRepairRange()) {
    // VirtReg has changed, so all cached queries are invalid.
    invalidateVirtRegs();
    if (unsigned PhysReg = tryAssign(VirtReg, Order, NewVRegs))
      return PhysReg;
  }

  // First try to split around a region spanning multiple blocks.
  unsigned PhysReg = tryRegionSplit(VirtReg, Order, NewVRegs);
  if (PhysReg || !NewVRegs.empty())
    return PhysReg;

  // Then isolate blocks with multiple uses.
  SplitAnalysis::BlockPtrSet Blocks;
  if (SA->getMultiUseBlocks(Blocks)) {
    LiveRangeEdit LREdit(VirtReg, NewVRegs, this);
    SE->reset(LREdit);
    SE->splitSingleBlocks(Blocks);
    setStage(NewVRegs.begin(), NewVRegs.end(), RS_Global);
    if (VerifyEnabled)
      MF->verify(this, "After splitting live range around basic blocks");
  }

  // Don't assign any physregs.
  return 0;
}


//===----------------------------------------------------------------------===//
//                            Main Entry Point
//===----------------------------------------------------------------------===//

unsigned RAGreedy::selectOrSplit(LiveInterval &VirtReg,
                                 SmallVectorImpl<LiveInterval*> &NewVRegs) {
  // First try assigning a free register.
  AllocationOrder Order(VirtReg.reg, *VRM, RegClassInfo);
  if (unsigned PhysReg = tryAssign(VirtReg, Order, NewVRegs))
    return PhysReg;

  LiveRangeStage Stage = getStage(VirtReg);
  DEBUG(dbgs() << StageName[Stage]
               << " Cascade " << ExtraRegInfo[VirtReg.reg].Cascade << '\n');

  // Try to evict a less worthy live range, but only for ranges from the primary
  // queue. The RS_Second ranges already failed to do this, and they should not
  // get a second chance until they have been split.
  if (Stage != RS_Second)
    if (unsigned PhysReg = tryEvict(VirtReg, Order, NewVRegs))
      return PhysReg;

  assert(NewVRegs.empty() && "Cannot append to existing NewVRegs");

  // The first time we see a live range, don't try to split or spill.
  // Wait until the second time, when all smaller ranges have been allocated.
  // This gives a better picture of the interference to split around.
  if (Stage == RS_First) {
    setStage(VirtReg, RS_Second);
    DEBUG(dbgs() << "wait for second round\n");
    NewVRegs.push_back(&VirtReg);
    return 0;
  }

  // If we couldn't allocate a register from spilling, there is probably some
  // invalid inline assembly. The base class wil report it.
  if (Stage >= RS_Spill || !VirtReg.isSpillable())
    return ~0u;

  // Try splitting VirtReg or interferences.
  unsigned PhysReg = trySplit(VirtReg, Order, NewVRegs);
  if (PhysReg || !NewVRegs.empty())
    return PhysReg;

  // Finally spill VirtReg itself.
  NamedRegionTimer T("Spiller", TimerGroupName, TimePassesIsEnabled);
  LiveRangeEdit LRE(VirtReg, NewVRegs, this);
  spiller().spill(LRE);
  setStage(NewVRegs.begin(), NewVRegs.end(), RS_Spill);

  if (VerifyEnabled)
    MF->verify(this, "After spilling");

  // The live virtual register requesting allocation was spilled, so tell
  // the caller not to allocate anything during this round.
  return 0;
}

bool RAGreedy::runOnMachineFunction(MachineFunction &mf) {
  DEBUG(dbgs() << "********** GREEDY REGISTER ALLOCATION **********\n"
               << "********** Function: "
               << ((Value*)mf.getFunction())->getName() << '\n');

  MF = &mf;
  if (VerifyEnabled)
    MF->verify(this, "Before greedy register allocator");

  RegAllocBase::init(getAnalysis<VirtRegMap>(), getAnalysis<LiveIntervals>());
  Indexes = &getAnalysis<SlotIndexes>();
  DomTree = &getAnalysis<MachineDominatorTree>();
  SpillerInstance.reset(createInlineSpiller(*this, *MF, *VRM));
  Loops = &getAnalysis<MachineLoopInfo>();
  Bundles = &getAnalysis<EdgeBundles>();
  SpillPlacer = &getAnalysis<SpillPlacement>();
  DebugVars = &getAnalysis<LiveDebugVariables>();

  SA.reset(new SplitAnalysis(*VRM, *LIS, *Loops));
  SE.reset(new SplitEditor(*SA, *LIS, *VRM, *DomTree));
  ExtraRegInfo.clear();
  ExtraRegInfo.resize(MRI->getNumVirtRegs());
  NextCascade = 1;
  IntfCache.init(MF, &PhysReg2LiveUnion[0], Indexes, TRI);

  allocatePhysRegs();
  addMBBLiveIns(MF);
  LIS->addKillFlags();

  // Run rewriter
  {
    NamedRegionTimer T("Rewriter", TimerGroupName, TimePassesIsEnabled);
    VRM->rewrite(Indexes);
  }

  // Write out new DBG_VALUE instructions.
  DebugVars->emitDebugValues(VRM);

  // The pass output is in VirtRegMap. Release all the transient data.
  releaseMemory();

  return true;
}