RegAllocGreedy.cpp

        // get a conservative estimate.
        //
        // The final distance can still be different if inserting copies
        // triggers a slot index renumbering.
        //
        const float EstWeight = normalizeSpillWeight(blockFreq * EstUses,
                              PrevSlot[SplitBefore].distance(Uses[SplitAfter]));
        // Would this split be possible to allocate?
        // Never allocate all gaps, we wouldn't be making progress.
        float Diff = EstWeight - MaxGap;
        DEBUG(dbgs() << " w=" << EstWeight << " d=" << Diff);
        if (Diff > 0) {
          Shrink = false;
          if (Diff > BestDiff) {
            DEBUG(dbgs() << " (best)");
            BestDiff = Diff;
            BestBefore = SplitBefore;
            BestAfter = SplitAfter;
          }
        }
      }

      // Try to shrink.
      if (Shrink) {
        SplitBefore = nextSplitPoint(SplitBefore);
        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");
      for (unsigned e = nextSplitPoint(SplitAfter + 1) - 1;
           SplitAfter != e; ++SplitAfter)
        MaxGap = std::max(MaxGap, GapWeight[SplitAfter]);
          continue;
    }
  }

  // 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);
  SE->closeIntv();
  SE->finish();
  setStage(NewVRegs.begin(), NewVRegs.end(), RS_Local);
  ++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.
  LiveRangeStage Stage = getStage(VirtReg);
  if (Stage >= RS_Block)
    return 0;

  SA->analyze(&VirtReg);

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

  // Then isolate blocks with multiple uses.
  if (Stage < RS_Block) {
    SplitAnalysis::BlockPtrSet Blocks;
    if (SA->getMultiUseBlocks(Blocks)) {
      LiveRangeEdit LREdit(VirtReg, NewVRegs, this);
      SE->reset(LREdit);
      SE->splitSingleBlocks(Blocks);
      setStage(NewVRegs.begin(), NewVRegs.end(), RS_Block);
      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, ReservedRegs);
  while (unsigned PhysReg = Order.next()) {
    if (!checkPhysRegInterference(VirtReg, PhysReg))
      return PhysReg;
  }

  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.
  LiveRangeStage Stage = getStage(VirtReg);
  if (Stage == RS_Original) {
    LRStage[VirtReg.reg] = RS_Second;
    DEBUG(dbgs() << "wait for second round\n");
    NewVRegs.push_back(&VirtReg);
    return 0;
  }

  assert(Stage < RS_Spill && "Cannot allocate after spilling");

  // 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);

  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>();
  ReservedRegs = TRI->getReservedRegs(*MF);
  SpillerInstance.reset(createInlineSpiller(*this, *MF, *VRM));
  Loops = &getAnalysis<MachineLoopInfo>();
  LoopRanges = &getAnalysis<MachineLoopRanges>();
  Bundles = &getAnalysis<EdgeBundles>();
  SpillPlacer = &getAnalysis<SpillPlacement>();

  SA.reset(new SplitAnalysis(*VRM, *LIS, *Loops));
  SE.reset(new SplitEditor(*SA, *LIS, *VRM, *DomTree));
  LRStage.clear();
  LRStage.resize(MRI->getNumVirtRegs());

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

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

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

  return true;
}