C++ const_pred_iterator::getKind方法代码示例

/// CriticalPathStep - Return the next SUnit after SU on the bottom-up
/// critical path.
static const SDep *CriticalPathStep(const SUnit *SU) {
  const SDep *Next = 0;
  unsigned NextDepth = 0;
  // Find the predecessor edge with the greatest depth.
  for (SUnit::const_pred_iterator P = SU->Preds.begin(), PE = SU->Preds.end();
       P != PE; ++P) {
    const SUnit *PredSU = P->getSUnit();
    unsigned PredLatency = P->getLatency();
    unsigned PredTotalLatency = PredSU->getDepth() + PredLatency;
    // In the case of a latency tie, prefer an anti-dependency edge over
    // other types of edges.
    if (NextDepth < PredTotalLatency ||
        (NextDepth == PredTotalLatency && P->getKind() == SDep::Anti)) {
      NextDepth = PredTotalLatency;
      Next = &*P;
    }
  }
  return Next;
}

//.........这里部分代码省略.........
  // still be considered, though only if no other registers are available.
  std::vector<unsigned> LastNewReg(TRI->getNumRegs(), 0);

  // Attempt to break anti-dependence edges on the critical path. Walk the
  // instructions from the bottom up, tracking information about liveness
  // as we go to help determine which registers are available.
  unsigned Broken = 0;
  unsigned Count = InsertPosIndex - 1;
  for (MachineBasicBlock::iterator I = End, E = Begin;
       I != E; --Count) {
    MachineInstr *MI = --I;
    if (MI->isDebugValue())
      continue;

    // Check if this instruction has a dependence on the critical path that
    // is an anti-dependence that we may be able to break. If it is, set
    // AntiDepReg to the non-zero register associated with the anti-dependence.
    //
    // We limit our attention to the critical path as a heuristic to avoid
    // breaking anti-dependence edges that aren't going to significantly
    // impact the overall schedule. There are a limited number of registers
    // and we want to save them for the important edges.
    //
    // TODO: Instructions with multiple defs could have multiple
    // anti-dependencies. The current code here only knows how to break one
    // edge per instruction. Note that we'd have to be able to break all of
    // the anti-dependencies in an instruction in order to be effective.
    unsigned AntiDepReg = 0;
    if (MI == CriticalPathMI) {
      if (const SDep *Edge = CriticalPathStep(CriticalPathSU)) {
        const SUnit *NextSU = Edge->getSUnit();

        // Only consider anti-dependence edges.
        if (Edge->getKind() == SDep::Anti) {
          AntiDepReg = Edge->getReg();
          assert(AntiDepReg != 0 && "Anti-dependence on reg0?");
          if (!RegClassInfo.isAllocatable(AntiDepReg))
            // Don't break anti-dependencies on non-allocatable registers.
            AntiDepReg = 0;
          else if (KeepRegs.count(AntiDepReg))
            // Don't break anti-dependencies if an use down below requires
            // this exact register.
            AntiDepReg = 0;
          else {
            // If the SUnit has other dependencies on the SUnit that it
            // anti-depends on, don't bother breaking the anti-dependency
            // since those edges would prevent such units from being
            // scheduled past each other regardless.
            //
            // Also, if there are dependencies on other SUnits with the
            // same register as the anti-dependency, don't attempt to
            // break it.
            for (SUnit::const_pred_iterator P = CriticalPathSU->Preds.begin(),
                 PE = CriticalPathSU->Preds.end(); P != PE; ++P)
              if (P->getSUnit() == NextSU ?
                    (P->getKind() != SDep::Anti || P->getReg() != AntiDepReg) :
                    (P->getKind() == SDep::Data && P->getReg() == AntiDepReg)) {
                AntiDepReg = 0;
                break;
              }
          }
        }
        CriticalPathSU = NextSU;
        CriticalPathMI = CriticalPathSU->getInstr();
      } else {
        // We've reached the end of the critical path.