C++ MachineBasicBlock::terminators方法代码示例

// Branch analysis.
bool WebAssemblyInstrInfo::analyzeBranch(MachineBasicBlock &MBB,
                                         MachineBasicBlock *&TBB,
                                         MachineBasicBlock *&FBB,
                                         SmallVectorImpl<MachineOperand> &Cond,
                                         bool /*AllowModify*/) const {
  bool HaveCond = false;
  for (MachineInstr &MI : MBB.terminators()) {
    switch (MI.getOpcode()) {
    default:
      // Unhandled instruction; bail out.
      return true;
    case WebAssembly::BR_IF:
      if (HaveCond)
        return true;
      // If we're running after CFGStackify, we can't optimize further.
      if (!MI.getOperand(0).isMBB())
        return true;
      Cond.push_back(MachineOperand::CreateImm(true));
      Cond.push_back(MI.getOperand(1));
      TBB = MI.getOperand(0).getMBB();
      HaveCond = true;
      break;
    case WebAssembly::BR_UNLESS:
      if (HaveCond)
        return true;
      // If we're running after CFGStackify, we can't optimize further.
      if (!MI.getOperand(0).isMBB())
        return true;
      Cond.push_back(MachineOperand::CreateImm(false));
      Cond.push_back(MI.getOperand(1));
      TBB = MI.getOperand(0).getMBB();
      HaveCond = true;
      break;
    case WebAssembly::BR:
      // If we're running after CFGStackify, we can't optimize further.
      if (!MI.getOperand(0).isMBB())
        return true;
      if (!HaveCond)
        TBB = MI.getOperand(0).getMBB();
      else
        FBB = MI.getOperand(0).getMBB();
      break;
    }
    if (MI.isBarrier())
      break;
  }

  return false;
}

void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB,
                                         RegScavenger *RS) {
  // Get rid of the easy cases first.
  if (!Save)
    Save = &MBB;
  else
    Save = MDT->findNearestCommonDominator(Save, &MBB);

  if (!Save) {
    LLVM_DEBUG(dbgs() << "Found a block that is not reachable from Entry\n");
    return;
  }

  if (!Restore)
    Restore = &MBB;
  else if (MPDT->getNode(&MBB)) // If the block is not in the post dom tree, it
                                // means the block never returns. If that's the
                                // case, we don't want to call
                                // `findNearestCommonDominator`, which will
                                // return `Restore`.
    Restore = MPDT->findNearestCommonDominator(Restore, &MBB);
  else
    Restore = nullptr; // Abort, we can't find a restore point in this case.

  // Make sure we would be able to insert the restore code before the
  // terminator.
  if (Restore == &MBB) {
    for (const MachineInstr &Terminator : MBB.terminators()) {
      if (!useOrDefCSROrFI(Terminator, RS))
        continue;
      // One of the terminator needs to happen before the restore point.
      if (MBB.succ_empty()) {
        Restore = nullptr; // Abort, we can't find a restore point in this case.
        break;
      }
      // Look for a restore point that post-dominates all the successors.
      // The immediate post-dominator is what we are looking for.
      Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
      break;
    }
  }

  if (!Restore) {
    LLVM_DEBUG(
        dbgs() << "Restore point needs to be spanned on several blocks\n");
    return;
  }

  // Make sure Save and Restore are suitable for shrink-wrapping:
  // 1. all path from Save needs to lead to Restore before exiting.
  // 2. all path to Restore needs to go through Save from Entry.
  // We achieve that by making sure that:
  // A. Save dominates Restore.
  // B. Restore post-dominates Save.
  // C. Save and Restore are in the same loop.
  bool SaveDominatesRestore = false;
  bool RestorePostDominatesSave = false;
  while (Save && Restore &&
         (!(SaveDominatesRestore = MDT->dominates(Save, Restore)) ||
          !(RestorePostDominatesSave = MPDT->dominates(Restore, Save)) ||
          // Post-dominance is not enough in loops to ensure that all uses/defs
          // are after the prologue and before the epilogue at runtime.
          // E.g.,
          // while(1) {
          //  Save
          //  Restore
          //   if (...)
          //     break;
          //  use/def CSRs
          // }
          // All the uses/defs of CSRs are dominated by Save and post-dominated
          // by Restore. However, the CSRs uses are still reachable after
          // Restore and before Save are executed.
          //
          // For now, just push the restore/save points outside of loops.
          // FIXME: Refine the criteria to still find interesting cases
          // for loops.
          MLI->getLoopFor(Save) || MLI->getLoopFor(Restore))) {
    // Fix (A).
    if (!SaveDominatesRestore) {
      Save = MDT->findNearestCommonDominator(Save, Restore);
      continue;
    }
    // Fix (B).
    if (!RestorePostDominatesSave)
      Restore = MPDT->findNearestCommonDominator(Restore, Save);

    // Fix (C).
    if (Save && Restore &&
        (MLI->getLoopFor(Save) || MLI->getLoopFor(Restore))) {
      if (MLI->getLoopDepth(Save) > MLI->getLoopDepth(Restore)) {
        // Push Save outside of this loop if immediate dominator is different
        // from save block. If immediate dominator is not different, bail out.
        Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
        if (!Save)
          break;
      } else {
        // If the loop does not exit, there is no point in looking
        // for a post-dominator outside the loop.
        SmallVector<MachineBasicBlock*, 4> ExitBlocks;
//.........这里部分代码省略.........

void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB) {
  // Get rid of the easy cases first.
  if (!Save)
    Save = &MBB;
  else
    Save = MDT->findNearestCommonDominator(Save, &MBB);

  if (!Save) {
    DEBUG(dbgs() << "Found a block that is not reachable from Entry\n");
    return;
  }

  if (!Restore)
    Restore = &MBB;
  else
    Restore = MPDT->findNearestCommonDominator(Restore, &MBB);

  // Make sure we would be able to insert the restore code before the
  // terminator.
  if (Restore == &MBB) {
    for (const MachineInstr &Terminator : MBB.terminators()) {
      if (!useOrDefCSROrFI(Terminator))
        continue;
      // One of the terminator needs to happen before the restore point.
      if (MBB.succ_empty()) {
        Restore = nullptr;
        break;
      }
      // Look for a restore point that post-dominates all the successors.
      // The immediate post-dominator is what we are looking for.
      Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
      break;
    }
  }

  if (!Restore) {
    DEBUG(dbgs() << "Restore point needs to be spanned on several blocks\n");
    return;
  }

  // Make sure Save and Restore are suitable for shrink-wrapping:
  // 1. all path from Save needs to lead to Restore before exiting.
  // 2. all path to Restore needs to go through Save from Entry.
  // We achieve that by making sure that:
  // A. Save dominates Restore.
  // B. Restore post-dominates Save.
  // C. Save and Restore are in the same loop.
  bool SaveDominatesRestore = false;
  bool RestorePostDominatesSave = false;
  while (Save && Restore &&
         (!(SaveDominatesRestore = MDT->dominates(Save, Restore)) ||
          !(RestorePostDominatesSave = MPDT->dominates(Restore, Save)) ||
          MLI->getLoopFor(Save) != MLI->getLoopFor(Restore))) {
    // Fix (A).
    if (!SaveDominatesRestore) {
      Save = MDT->findNearestCommonDominator(Save, Restore);
      continue;
    }
    // Fix (B).
    if (!RestorePostDominatesSave)
      Restore = MPDT->findNearestCommonDominator(Restore, Save);

    // Fix (C).
    if (Save && Restore && Save != Restore &&
        MLI->getLoopFor(Save) != MLI->getLoopFor(Restore)) {
      if (MLI->getLoopDepth(Save) > MLI->getLoopDepth(Restore))
        // Push Save outside of this loop.
        Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
      else
        // Push Restore outside of this loop.
        Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
    }
  }
}