C++ SmallSetVector::pop_back方法代码示例

/// Sort the blocks, taking special care to make sure that loops are not
/// interrupted by blocks not dominated by their header.
/// TODO: There are many opportunities for improving the heuristics here.
/// Explore them.
static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI,
                       const MachineDominatorTree &MDT) {
  // Prepare for a topological sort: Record the number of predecessors each
  // block has, ignoring loop backedges.
  MF.RenumberBlocks();
  SmallVector<unsigned, 16> NumPredsLeft(MF.getNumBlockIDs(), 0);
  for (MachineBasicBlock &MBB : MF) {
    unsigned N = MBB.pred_size();
    if (MachineLoop *L = MLI.getLoopFor(&MBB))
      if (L->getHeader() == &MBB)
        for (const MachineBasicBlock *Pred : MBB.predecessors())
          if (L->contains(Pred))
            --N;
    NumPredsLeft[MBB.getNumber()] = N;
  }

  // Topological sort the CFG, with additional constraints:
  //  - Between a loop header and the last block in the loop, there can be
  //    no blocks not dominated by the loop header.
  //  - It's desirable to preserve the original block order when possible.
  // We use two ready lists; Preferred and Ready. Preferred has recently
  // processed sucessors, to help preserve block sequences from the original
  // order. Ready has the remaining ready blocks.
  PriorityQueue<MachineBasicBlock *, std::vector<MachineBasicBlock *>,
                CompareBlockNumbers>
      Preferred;
  PriorityQueue<MachineBasicBlock *, std::vector<MachineBasicBlock *>,
                CompareBlockNumbersBackwards>
      Ready;
  SmallVector<Entry, 4> Loops;
  for (MachineBasicBlock *MBB = &MF.front();;) {
    const MachineLoop *L = MLI.getLoopFor(MBB);
    if (L) {
      // If MBB is a loop header, add it to the active loop list. We can't put
      // any blocks that it doesn't dominate until we see the end of the loop.
      if (L->getHeader() == MBB)
        Loops.push_back(Entry(L));
      // For each active loop the block is in, decrement the count. If MBB is
      // the last block in an active loop, take it off the list and pick up any
      // blocks deferred because the header didn't dominate them.
      for (Entry &E : Loops)
        if (E.Loop->contains(MBB) && --E.NumBlocksLeft == 0)
          for (auto DeferredBlock : E.Deferred)
            Ready.push(DeferredBlock);
      while (!Loops.empty() && Loops.back().NumBlocksLeft == 0)
        Loops.pop_back();
    }
    // The main topological sort logic.
    for (MachineBasicBlock *Succ : MBB->successors()) {
      // Ignore backedges.
      if (MachineLoop *SuccL = MLI.getLoopFor(Succ))
        if (SuccL->getHeader() == Succ && SuccL->contains(MBB))
          continue;
      // Decrement the predecessor count. If it's now zero, it's ready.
      if (--NumPredsLeft[Succ->getNumber()] == 0)
        Preferred.push(Succ);
    }
    // Determine the block to follow MBB. First try to find a preferred block,
    // to preserve the original block order when possible.
    MachineBasicBlock *Next = nullptr;
    while (!Preferred.empty()) {
      Next = Preferred.top();
      Preferred.pop();
      // If X isn't dominated by the top active loop header, defer it until that
      // loop is done.
      if (!Loops.empty() &&
          !MDT.dominates(Loops.back().Loop->getHeader(), Next)) {
        Loops.back().Deferred.push_back(Next);
        Next = nullptr;
        continue;
      }
      // If Next was originally ordered before MBB, and it isn't because it was
      // loop-rotated above the header, it's not preferred.
      if (Next->getNumber() < MBB->getNumber() &&
          (!L || !L->contains(Next) ||
           L->getHeader()->getNumber() < Next->getNumber())) {
        Ready.push(Next);
        Next = nullptr;
        continue;
      }
      break;
    }
    // If we didn't find a suitable block in the Preferred list, check the
    // general Ready list.
    if (!Next) {
      // If there are no more blocks to process, we're done.
      if (Ready.empty()) {
        MaybeUpdateTerminator(MBB);
        break;
      }
      for (;;) {
        Next = Ready.top();
        Ready.pop();
        // If Next isn't dominated by the top active loop header, defer it until
        // that loop is done.
        if (!Loops.empty() &&
//.........这里部分代码省略.........

/// Sort the blocks in RPO, taking special care to make sure that loops are
/// contiguous even in the case of split backedges.
///
/// TODO: Determine whether RPO is actually worthwhile, or whether we should
/// move to just a stable-topological-sort-based approach that would preserve
/// more of the original order.
static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI) {
  // Note that we do our own RPO rather than using
  // "llvm/ADT/PostOrderIterator.h" because we want control over the order that
  // successors are visited in (see above). Also, we can sort the blocks in the
  // MachineFunction as we go.
  SmallPtrSet<MachineBasicBlock *, 16> Visited;
  SmallVector<POStackEntry, 16> Stack;

  MachineBasicBlock *EntryBlock = &*MF.begin();
  Visited.insert(EntryBlock);
  Stack.push_back(POStackEntry(EntryBlock, MF, MLI));

  for (;;) {
    POStackEntry &Entry = Stack.back();
    SmallVectorImpl<MachineBasicBlock *> &Succs = Entry.Succs;
    if (!Succs.empty()) {
      MachineBasicBlock *Succ = Succs.pop_back_val();
      if (Visited.insert(Succ).second)
        Stack.push_back(POStackEntry(Succ, MF, MLI));
      continue;
    }

    // Put the block in its position in the MachineFunction.
    MachineBasicBlock &MBB = *Entry.MBB;
    MBB.moveBefore(&*MF.begin());

    // Branch instructions may utilize a fallthrough, so update them if a
    // fallthrough has been added or removed.
    if (!MBB.empty() && MBB.back().isTerminator() && !MBB.back().isBranch() &&
        !MBB.back().isBarrier())
      report_fatal_error(
          "Non-branch terminator with fallthrough cannot yet be rewritten");
    if (MBB.empty() || !MBB.back().isTerminator() || MBB.back().isBranch())
      MBB.updateTerminator();

    Stack.pop_back();
    if (Stack.empty())
      break;
  }

  // Now that we've sorted the blocks in RPO, renumber them.
  MF.RenumberBlocks();

#ifndef NDEBUG
  SmallSetVector<MachineLoop *, 8> OnStack;

  // Insert a sentinel representing the degenerate loop that starts at the
  // function entry block and includes the entire function as a "loop" that
  // executes once.
  OnStack.insert(nullptr);

  for (auto &MBB : MF) {
    assert(MBB.getNumber() >= 0 && "Renumbered blocks should be non-negative.");

    MachineLoop *Loop = MLI.getLoopFor(&MBB);
    if (Loop && &MBB == Loop->getHeader()) {
      // Loop header. The loop predecessor should be sorted above, and the other
      // predecessors should be backedges below.
      for (auto Pred : MBB.predecessors())
        assert(
            (Pred->getNumber() < MBB.getNumber() || Loop->contains(Pred)) &&
            "Loop header predecessors must be loop predecessors or backedges");
      assert(OnStack.insert(Loop) && "Loops should be declared at most once.");
    } else {
      // Not a loop header. All predecessors should be sorted above.
      for (auto Pred : MBB.predecessors())
        assert(Pred->getNumber() < MBB.getNumber() &&
               "Non-loop-header predecessors should be topologically sorted");
      assert(OnStack.count(MLI.getLoopFor(&MBB)) &&
             "Blocks must be nested in their loops");
    }
    while (OnStack.size() > 1 && &MBB == LoopBottom(OnStack.back()))
      OnStack.pop_back();
  }
  assert(OnStack.pop_back_val() == nullptr &&
         "The function entry block shouldn't actually be a loop header");
  assert(OnStack.empty() &&
         "Control flow stack pushes and pops should be balanced.");
#endif
}