/// processBlock - Determine how to break up PHIs in the current block.  Each
/// PHI is broken up by some combination of renaming its operands and inserting
/// copies.  This method is responsible for determining which operands receive
/// which treatment.
void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
  LiveIntervals& LI = getAnalysis<LiveIntervals>();
  MachineRegisterInfo& MRI = MBB->getParent()->getRegInfo();
  // Holds names that have been added to a set in any PHI within this block
  // before the current one.
  std::set<unsigned> ProcessedNames;
  // Iterate over all the PHI nodes in this block
  MachineBasicBlock::iterator P = MBB->begin();
  while (P != MBB->end() && P->isPHI()) {
    unsigned DestReg = P->getOperand(0).getReg();
    // Don't both doing PHI elimination for dead PHI's.
    if (P->registerDefIsDead(DestReg)) {

    LiveInterval& PI = LI.getOrCreateInterval(DestReg);
    SlotIndex pIdx = LI.getInstructionIndex(P).getDefIndex();
    VNInfo* PVN = PI.getLiveRangeContaining(pIdx)->valno;
    PhiValueNumber.insert(std::make_pair(DestReg, PVN->id));

    // PHIUnion is the set of incoming registers to the PHI node that
    // are going to be renames rather than having copies inserted.  This set
    // is refinded over the course of this function.  UnionedBlocks is the set
    // of corresponding MBBs.
    std::map<unsigned, MachineBasicBlock*> PHIUnion;
    SmallPtrSet<MachineBasicBlock*, 8> UnionedBlocks;
    // Iterate over the operands of the PHI node
    for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
      unsigned SrcReg = P->getOperand(i-1).getReg();
      // Don't need to try to coalesce a register with itself.
      if (SrcReg == DestReg) {
      // We don't need to insert copies for implicit_defs.
      MachineInstr* DefMI = MRI.getVRegDef(SrcReg);
      if (DefMI->isImplicitDef())
      // Check for trivial interferences via liveness information, allowing us
      // to avoid extra work later.  Any registers that interfere cannot both
      // be in the renaming set, so choose one and add copies for it instead.
      // The conditions are:
      //   1) if the operand is live into the PHI node's block OR
      //   2) if the PHI node is live out of the operand's defining block OR
      //   3) if the operand is itself a PHI node and the original PHI is
      //      live into the operand's defining block OR
      //   4) if the operand is already being renamed for another PHI node
      //      in this block OR
      //   5) if any two operands are defined in the same block, insert copies
      //      for one of them
      if (isLiveIn(SrcReg, P->getParent(), LI) ||
                    MRI.getVRegDef(SrcReg)->getParent(), LI) ||
          ( MRI.getVRegDef(SrcReg)->isPHI() &&
                     MRI.getVRegDef(SrcReg)->getParent(), LI) ) ||
          ProcessedNames.count(SrcReg) ||
          UnionedBlocks.count(MRI.getVRegDef(SrcReg)->getParent())) {
        // Add a copy for the selected register
        MachineBasicBlock* From = P->getOperand(i).getMBB();
        Waiting[From].insert(std::make_pair(SrcReg, DestReg));
      } else {
        // Otherwise, add it to the renaming set
    // Compute the dominator forest for the renaming set.  This is a forest
    // where the nodes are the registers and the edges represent dominance 
    // relations between the defining blocks of the registers
    std::vector<StrongPHIElimination::DomForestNode*> DF = 
                                                computeDomForest(PHIUnion, MRI);
    // Walk DomForest to resolve interferences at an inter-block level.  This
    // will remove registers from the renaming set (and insert copies for them)
    // if interferences are found.
    std::vector<std::pair<unsigned, unsigned> > localInterferences;
    processPHIUnion(P, PHIUnion, DF, localInterferences);
    // If one of the inputs is defined in the same block as the current PHI
    // then we need to check for a local interference between that input and
    // the PHI.
    for (std::map<unsigned, MachineBasicBlock*>::iterator I = PHIUnion.begin(),
         E = PHIUnion.end(); I != E; ++I)
      if (MRI.getVRegDef(I->first)->getParent() == P->getParent())
    // The dominator forest walk may have returned some register pairs whose
    // interference cannot be determined from dominator analysis.  We now 
    // examine these pairs for local interferences.
    for (std::vector<std::pair<unsigned, unsigned> >::iterator I =
        localInterferences.begin(), E = localInterferences.end(); I != E; ++I) {
      std::pair<unsigned, unsigned> p = *I;
      MachineDominatorTree& MDT = getAnalysis<MachineDominatorTree>();
      // Determine the block we need to scan and the relationship between
      // the two registers
      MachineBasicBlock* scan = 0;
      unsigned mode = 0;
      if (MRI.getVRegDef(p.first)->getParent() ==
          MRI.getVRegDef(p.second)->getParent()) {
        scan = MRI.getVRegDef(p.first)->getParent();
        mode = 0; // Same block
      } else if (MDT.dominates(MRI.getVRegDef(p.first)->getParent(),
                               MRI.getVRegDef(p.second)->getParent())) {
        scan = MRI.getVRegDef(p.second)->getParent();
        mode = 1; // First dominates second
      } else {
        scan = MRI.getVRegDef(p.first)->getParent();
        mode = 2; // Second dominates first
      // If there's an interference, we need to insert  copies
      if (interferes(p.first, p.second, scan, LI, mode)) {
        // Insert copies for First
        for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
          if (P->getOperand(i-1).getReg() == p.first) {
            unsigned SrcReg = p.first;
            MachineBasicBlock* From = P->getOperand(i).getMBB();
    // Add the renaming set for this PHI node to our overall renaming information
    for (std::map<unsigned, MachineBasicBlock*>::iterator QI = PHIUnion.begin(),
         QE = PHIUnion.end(); QI != QE; ++QI) {
      DEBUG(dbgs() << "Adding Renaming: " << QI->first << " -> "
                   << P->getOperand(0).getReg() << "\n");
    RenameSets.insert(std::make_pair(P->getOperand(0).getReg(), PHIUnion));
    // Remember which registers are already renamed, so that we don't try to 
    // rename them for another PHI node in this block
    for (std::map<unsigned, MachineBasicBlock*>::iterator I = PHIUnion.begin(),
         E = PHIUnion.end(); I != E; ++I)