runtime/vm/gc_marker.cc

// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#include "vm/gc_marker.h"

#include <map>
#include <utility>
#include <vector>

#include "vm/allocation.h"
#include "vm/dart_api_state.h"
#include "vm/isolate.h"
#include "vm/log.h"
#include "vm/pages.h"
#include "vm/raw_object.h"
#include "vm/stack_frame.h"
#include "vm/store_buffer.h"
#include "vm/thread_barrier.h"
#include "vm/thread_pool.h"
#include "vm/visitor.h"
#include "vm/object_id_ring.h"

namespace dart {

DEFINE_FLAG(int, marker_tasks, 2,
            "The number of tasks to spawn during old gen GC marking (0 means "
            "perform all marking on main thread).");
DEFINE_FLAG(bool, log_marker_tasks, false,
            "Log debugging information for old gen GC marking tasks.");

class DelaySet {
 private:
  typedef std::multimap<RawObject*, RawWeakProperty*> Map;
  typedef std::pair<RawObject*, RawWeakProperty*> MapEntry;

 public:
  DelaySet() : mutex_(new Mutex()) {}
  ~DelaySet() { delete mutex_; }

  // After atomically setting the watched bit on a white key (see
  // EnsureWatchedIfWhitewhich; this means the mark bit cannot be set
  // without observing the watched bit), this method atomically
  // inserts raw_weak if its key is *still* white, so that any future
  // call to VisitValuesForKey is guaranteed to include its
  // value. Returns true on success, and false if the key is no longer white.
  bool InsertIfWhite(RawWeakProperty* raw_weak) {
    MutexLocker ml(mutex_);
    RawObject* raw_key = raw_weak->ptr()->key_;
    if (raw_key->IsMarked()) return false;
    // The key was white *after* acquiring the lock. Thus any future call to
    // VisitValuesForKey is guaranteed to include the entry inserted below.
    delay_set_.insert(std::make_pair(raw_key, raw_weak));
    return true;
  }

  void ClearReferences() {
    MutexLocker ml(mutex_);
    for (Map::iterator it = delay_set_.begin(); it != delay_set_.end(); ++it) {
      ASSERT(!it->first->IsMarked());
      WeakProperty::Clear(it->second);
    }
  }

  // Visit all values with a key equal to raw_obj, which must already be marked.
  void VisitValuesForKey(RawObject* raw_obj, ObjectPointerVisitor* visitor) {
    ASSERT(raw_obj->IsMarked());
    // Extract the range into a temporary vector to iterate over it
    // while delay_set_ may be modified.
    std::vector<MapEntry> temp_copy;
    {
      MutexLocker ml(mutex_);
      std::pair<Map::iterator, Map::iterator> ret =
          delay_set_.equal_range(raw_obj);
      temp_copy.insert(temp_copy.end(), ret.first, ret.second);
      delay_set_.erase(ret.first, ret.second);
    }
    for (std::vector<MapEntry>::iterator it = temp_copy.begin();
         it != temp_copy.end(); ++it) {
      it->second->VisitPointers(visitor);
    }
  }

 private:
  Map delay_set_;
  Mutex* mutex_;
};


class SkippedCodeFunctions : public ZoneAllocated {
 public:
  SkippedCodeFunctions() {}

  void Add(RawFunction* func) {
    skipped_code_functions_.Add(func);
  }

  void DetachCode() {
    intptr_t unoptimized_code_count = 0;
    intptr_t current_code_count = 0;
    for (int i = 0; i < skipped_code_functions_.length(); i++) {
      RawFunction* func = skipped_code_functions_[i];
      RawCode* code = func->ptr()->code_;
      if (!code->IsMarked()) {
        // If the code wasn't strongly visited through other references
        // after skipping the function's code pointer, then we disconnect the
        // code from the function.
        func->StorePointer(
            &(func->ptr()->code_),
            StubCode::LazyCompile_entry()->code());
        uword entry_point = StubCode::LazyCompile_entry()->EntryPoint();
        func->ptr()->entry_point_ = entry_point;
        if (FLAG_log_code_drop) {
          // NOTE: This code runs while GC is in progress and runs within
          // a NoHandleScope block. Hence it is not okay to use a regular Zone
          // or Scope handle. We use a direct stack handle so the raw pointer in
          // this handle is not traversed. The use of a handle is mainly to
          // be able to reuse the handle based code and avoid having to add
          // helper functions to the raw object interface.
          String name;
          name = func->ptr()->name_;
          THR_Print("Detaching code: %s\n", name.ToCString());
          current_code_count++;
        }
      }

      code = func->ptr()->unoptimized_code_;
      if (!code->IsMarked()) {
        // If the code wasn't strongly visited through other references
        // after skipping the function's code pointer, then we disconnect the
        // code from the function.
        func->StorePointer(&(func->ptr()->unoptimized_code_), Code::null());
        if (FLAG_log_code_drop) {
          unoptimized_code_count++;
        }
      }
    }
    if (FLAG_log_code_drop) {
      THR_Print("  total detached current: %" Pd "\n", current_code_count);
      THR_Print("  total detached unoptimized: %" Pd "\n",
                unoptimized_code_count);
    }
    // Clean up.
    skipped_code_functions_.Clear();
  }

 private:
  GrowableArray<RawFunction*> skipped_code_functions_;

  DISALLOW_COPY_AND_ASSIGN(SkippedCodeFunctions);
};


class MarkerWorkList : public ValueObject {
 public:
  explicit MarkerWorkList(MarkingStack* marking_stack)
      : marking_stack_(marking_stack) {
    work_ = marking_stack_->PopEmptyBlock();
  }

  ~MarkerWorkList() {
    ASSERT(work_ == NULL);
    ASSERT(marking_stack_ == NULL);
  }

  // Returns NULL if no more work was found.
  RawObject* Pop() {
    ASSERT(work_ != NULL);
    if (work_->IsEmpty()) {
      // TODO(koda): Track over/underflow events and use in heuristics to
      // distribute work and prevent degenerate flip-flopping.
      MarkingStack::Block* new_work = marking_stack_->PopNonEmptyBlock();
      if (new_work == NULL) {
        return NULL;
      }
      marking_stack_->PushBlock(work_);
      work_ = new_work;
    }
    return work_->Pop();
  }

  void Push(RawObject* raw_obj) {
    if (work_->IsFull()) {
      // TODO(koda): Track over/underflow events and use in heuristics to
      // distribute work and prevent degenerate flip-flopping.
      marking_stack_->PushBlock(work_);
      work_ = marking_stack_->PopEmptyBlock();
    }
    work_->Push(raw_obj);
  }

  void Finalize() {
    ASSERT(work_->IsEmpty());
    marking_stack_->PushBlock(work_);
    work_ = NULL;
    // Fail fast on attempts to mark after finalizing.
    marking_stack_ = NULL;
  }

 private:
  MarkingStack::Block* work_;
  MarkingStack* marking_stack_;
};


template<bool sync>
class MarkingVisitorBase : public ObjectPointerVisitor {
 public:
  MarkingVisitorBase(Isolate* isolate,
                 Heap* heap,
                 PageSpace* page_space,
                 MarkingStack* marking_stack,
                 DelaySet* delay_set,
                 SkippedCodeFunctions* skipped_code_functions)
      : ObjectPointerVisitor(isolate),
        thread_(Thread::Current()),
        heap_(heap),
        vm_heap_(Dart::vm_isolate()->heap()),
        class_stats_count_(isolate->class_table()->NumCids()),
        class_stats_size_(isolate->class_table()->NumCids()),
        page_space_(page_space),
        work_list_(marking_stack),
        delay_set_(delay_set),
        visiting_old_object_(NULL),
        skipped_code_functions_(skipped_code_functions),
        marked_bytes_(0) {
    ASSERT(heap_ != vm_heap_);
    ASSERT(thread_->isolate() == isolate);
    class_stats_count_.SetLength(isolate->class_table()->NumCids());
    class_stats_size_.SetLength(isolate->class_table()->NumCids());
    for (intptr_t i = 0; i < class_stats_count_.length(); ++i) {
      class_stats_count_[i] = 0;
      class_stats_size_[i] = 0;
    }
  }

  uintptr_t marked_bytes() const { return marked_bytes_; }

  intptr_t live_count(intptr_t class_id) {
    return class_stats_count_[class_id];
  }

  intptr_t live_size(intptr_t class_id) {
    return class_stats_size_[class_id];
  }

  // Returns true if some non-zero amount of work was performed.
  bool DrainMarkingStack() {
    RawObject* raw_obj = work_list_.Pop();
    if (raw_obj == NULL) {
      ASSERT(visiting_old_object_ == NULL);
      return false;
    }
    do {
      VisitingOldObject(raw_obj);
      const intptr_t class_id = raw_obj->GetClassId();
      if (class_id != kWeakPropertyCid) {
        marked_bytes_ += raw_obj->VisitPointers(this);
      } else {
        RawWeakProperty* raw_weak = reinterpret_cast<RawWeakProperty*>(raw_obj);
        marked_bytes_ += raw_weak->Size();
        ProcessWeakProperty(raw_weak);
      }
      raw_obj = work_list_.Pop();
    } while (raw_obj != NULL);
    VisitingOldObject(NULL);
    return true;
  }

  void VisitPointers(RawObject** first, RawObject** last) {
    for (RawObject** current = first; current <= last; current++) {
      MarkObject(*current, current);
    }
  }

  bool visit_function_code() const {
    return skipped_code_functions_ == NULL;
  }

  virtual void add_skipped_code_function(RawFunction* func) {
    ASSERT(!visit_function_code());
    skipped_code_functions_->Add(func);
  }

  // If unmarked, sets the watch bit and returns true.
  // If marked, does nothing and returns false.
  static bool EnsureWatchedIfWhite(RawObject* obj) {
    if (!sync) {
      if (obj->IsMarked()) return false;
      if (!obj->IsWatched()) obj->SetWatchedBitUnsynchronized();
      return true;
    }
    uword tags = obj->ptr()->tags_;
    uword old_tags;
    do {
      old_tags = tags;
      if (RawObject::MarkBit::decode(tags)) return false;
      if (RawObject::WatchedBit::decode(tags)) return true;
      uword new_tags = RawObject::WatchedBit::update(true, old_tags);
      tags = AtomicOperations::CompareAndSwapWord(
          &obj->ptr()->tags_, old_tags, new_tags);
    } while (tags != old_tags);
    return true;
  }

  void ProcessWeakProperty(RawWeakProperty* raw_weak) {
    // The fate of the weak property is determined by its key.
    RawObject* raw_key = raw_weak->ptr()->key_;
    if (raw_key->IsHeapObject() &&
        raw_key->IsOldObject() &&
        EnsureWatchedIfWhite(raw_key) &&
        delay_set_->InsertIfWhite(raw_weak)) {
      // Key was white.  Delayed the weak property.
    } else {
      // Key is gray or black.  Make the weak property black.
      raw_weak->VisitPointers(this);
    }
  }

  // Called when all marking is complete.
  void Finalize() {
    work_list_.Finalize();
    if (skipped_code_functions_ != NULL) {
      skipped_code_functions_->DetachCode();
    }
  }

  void VisitingOldObject(RawObject* obj) {
    ASSERT((obj == NULL) || obj->IsOldObject());
    visiting_old_object_ = obj;
  }

 private:
  void PushMarked(RawObject* raw_obj) {
    ASSERT(raw_obj->IsHeapObject());
    ASSERT((FLAG_verify_before_gc || FLAG_verify_before_gc) ?
           page_space_->Contains(RawObject::ToAddr(raw_obj)) :
           true);

    // Push the marked object on the marking stack.
    ASSERT(raw_obj->IsMarked());
    const bool is_watched = raw_obj->IsWatched();
    // We acquired the mark bit => no other task is modifying the header.
    // TODO(koda): For concurrent mutator, this needs synchronization. Consider
    // clearing these bits already in the CAS for the mark bit.
    raw_obj->ClearRememberedBitUnsynchronized();
    raw_obj->ClearWatchedBitUnsynchronized();
    if (is_watched) {
      delay_set_->VisitValuesForKey(raw_obj, this);
    }
    work_list_.Push(raw_obj);
  }

  static bool TryAcquireMarkBit(RawObject* raw_obj) {
    if (!sync) {
      if (raw_obj->IsMarked()) return false;
      raw_obj->SetMarkBitUnsynchronized();
      return true;
    }
    return raw_obj->TryAcquireMarkBit();
  }

  void MarkObject(RawObject* raw_obj, RawObject** p) {
    // Fast exit if the raw object is a Smi.
    if (!raw_obj->IsHeapObject()) {
      return;
    }

    // Fast exit if the raw object is marked.
    if (raw_obj->IsMarked()) {
      return;
    }

    // TODO(koda): Investigate performance impact of alternative branching:
    // if (smi or new) <-- can be done as single compare + conditional jump
    //   if (smi) return;
    //   else ...
    // if (marked) return;
    // ...
    if (raw_obj->IsNewObject()) {
      ProcessNewSpaceObject(raw_obj, p);
      return;
    }

    if (!TryAcquireMarkBit(raw_obj)) {
      // Already marked.
      return;
    }
    if (RawObject::IsVariableSizeClassId(raw_obj->GetClassId())) {
      UpdateLiveOld(raw_obj->GetClassId(), raw_obj->Size());
    } else {
      UpdateLiveOld(raw_obj->GetClassId(), 0);
    }

    PushMarked(raw_obj);
  }

  static bool TryAcquireRememberedBit(RawObject* raw_obj) {
    if (!sync) {
      if (raw_obj->IsRemembered()) return false;
      raw_obj->SetRememberedBitUnsynchronized();
      return true;
    }
    return raw_obj->TryAcquireRememberedBit();
  }

  void ProcessNewSpaceObject(RawObject* raw_obj, RawObject** p) {
    // TODO(iposva): Add consistency check.
    if ((visiting_old_object_ != NULL) &&
        TryAcquireRememberedBit(visiting_old_object_)) {
      // NOTE: We pass in the pointer to the address we are visiting
      // allows us to get a distance from the object start. At some
      // point we might want to store exact addresses in store buffers
      // for locations far enough from the header, so that we do not
      // need to walk big objects only to find the single new
      // reference in the last word during scavenge. This doesn't seem
      // to be a problem though currently.
      ASSERT(p != NULL);
      thread_->StoreBufferAddObjectGC(visiting_old_object_);
    }
  }

  void UpdateLiveOld(intptr_t class_id, intptr_t size) {
    // TODO(koda): Support growing the array once mutator runs concurrently.
    ASSERT(class_id < class_stats_count_.length());
    class_stats_count_[class_id] += 1;
    class_stats_size_[class_id] += size;
  }

  Thread* thread_;
  Heap* heap_;
  Heap* vm_heap_;
  GrowableArray<intptr_t> class_stats_count_;
  GrowableArray<intptr_t> class_stats_size_;
  PageSpace* page_space_;
  MarkerWorkList work_list_;
  DelaySet* delay_set_;
  RawObject* visiting_old_object_;
  SkippedCodeFunctions* skipped_code_functions_;
  uintptr_t marked_bytes_;

  DISALLOW_IMPLICIT_CONSTRUCTORS(MarkingVisitorBase);
};


typedef MarkingVisitorBase<false> UnsyncMarkingVisitor;
typedef MarkingVisitorBase<true> SyncMarkingVisitor;


static bool IsUnreachable(const RawObject* raw_obj) {
  if (!raw_obj->IsHeapObject()) {
    return false;
  }
  if (raw_obj == Object::null()) {
    return true;
  }
  if (!raw_obj->IsOldObject()) {
    return false;
  }
  return !raw_obj->IsMarked();
}


class MarkingWeakVisitor : public HandleVisitor {
 public:
  MarkingWeakVisitor() : HandleVisitor(Thread::Current()) {
  }

  void VisitHandle(uword addr) {
    FinalizablePersistentHandle* handle =
        reinterpret_cast<FinalizablePersistentHandle*>(addr);
    RawObject* raw_obj = handle->raw();
    if (IsUnreachable(raw_obj)) {
      handle->UpdateUnreachable(thread()->isolate());
    }
  }

 private:
  DISALLOW_COPY_AND_ASSIGN(MarkingWeakVisitor);
};


void GCMarker::Prologue(Isolate* isolate, bool invoke_api_callbacks) {
  if (invoke_api_callbacks && (isolate->gc_prologue_callback() != NULL)) {
    (isolate->gc_prologue_callback())();
  }
  Thread::PrepareForGC();
  // The store buffers will be rebuilt as part of marking, reset them now.
  isolate->store_buffer()->Reset();
}


void GCMarker::Epilogue(Isolate* isolate, bool invoke_api_callbacks) {
  if (invoke_api_callbacks && (isolate->gc_epilogue_callback() != NULL)) {
    (isolate->gc_epilogue_callback())();
  }
}


void GCMarker::IterateRoots(Isolate* isolate,
                            ObjectPointerVisitor* visitor,
                            bool visit_prologue_weak_persistent_handles,
                            intptr_t slice_index, intptr_t num_slices) {
  ASSERT(0 <= slice_index && slice_index < num_slices);
  if ((slice_index == 0) || (num_slices <= 1)) {
    isolate->VisitObjectPointers(visitor,
                                 visit_prologue_weak_persistent_handles,
                                 StackFrameIterator::kDontValidateFrames);
  }
  if ((slice_index == 1) || (num_slices <= 1)) {
    heap_->new_space()->VisitObjectPointers(visitor);
  }

  // For now, we just distinguish two parts of the root set, so any remaining
  // slices are empty.
}


void GCMarker::IterateWeakRoots(Isolate* isolate,
                                HandleVisitor* visitor,
                                bool visit_prologue_weak_persistent_handles) {
  ApiState* state = isolate->api_state();
  ASSERT(state != NULL);
  isolate->VisitWeakPersistentHandles(visitor,
                                      visit_prologue_weak_persistent_handles);
}


template<class MarkingVisitorType>
void GCMarker::IterateWeakReferences(Isolate* isolate,
                                     MarkingVisitorType* visitor) {
  ApiState* state = isolate->api_state();
  ASSERT(state != NULL);
  while (true) {
    WeakReferenceSet* queue = state->delayed_weak_reference_sets();
    if (queue == NULL) {
      // The delay queue is empty therefore no clean-up is required.
      return;
    }
    state->set_delayed_weak_reference_sets(NULL);
    while (queue != NULL) {
      WeakReferenceSet* reference_set = WeakReferenceSet::Pop(&queue);
      ASSERT(reference_set != NULL);
      intptr_t num_keys = reference_set->num_keys();
      intptr_t num_values = reference_set->num_values();
      if ((num_keys == 1) && (num_values == 1) &&
          reference_set->SingletonKeyEqualsValue()) {
        // We do not have to process sets that have just one key/value pair
        // and the key and value are identical.
        continue;
      }
      bool is_unreachable = true;
      // Test each key object for reachability.  If a key object is
      // reachable, all value objects should be marked.
      for (intptr_t k = 0; k < num_keys; ++k) {
        if (!IsUnreachable(*reference_set->get_key(k))) {
          for (intptr_t v = 0; v < num_values; ++v) {
            visitor->VisitPointer(reference_set->get_value(v));
          }
          is_unreachable = false;
          // Since we have found a key object that is reachable and all
          // value objects have been marked we can break out of iterating
          // this set and move on to the next set.
          break;
        }
      }
      // If all key objects are unreachable put the reference on a
      // delay queue.  This reference will be revisited if another
      // reference is marked.
      if (is_unreachable) {
        state->DelayWeakReferenceSet(reference_set);
      }
    }
    if (!visitor->DrainMarkingStack()) {
      // Break out of the loop if there has been no forward process.
      // All key objects in the weak reference sets are unreachable
      // so we reset the weak reference sets queue.
      state->set_delayed_weak_reference_sets(NULL);
      break;
    }
  }
  ASSERT(state->delayed_weak_reference_sets() == NULL);
  // All weak reference sets are zone allocated and unmarked references which
  // were on the delay queue will be freed when the zone is released in the
  // epilog callback.
}


void GCMarker::ProcessWeakTables(PageSpace* page_space) {
  for (int sel = 0;
       sel < Heap::kNumWeakSelectors;
       sel++) {
    WeakTable* table = heap_->GetWeakTable(
        Heap::kOld, static_cast<Heap::WeakSelector>(sel));
    intptr_t size = table->size();
    for (intptr_t i = 0; i < size; i++) {
      if (table->IsValidEntryAt(i)) {
        RawObject* raw_obj = table->ObjectAt(i);
        ASSERT(raw_obj->IsHeapObject());
        if (!raw_obj->IsMarked()) {
          table->InvalidateAt(i);
        }
      }
    }
  }
}


class ObjectIdRingClearPointerVisitor : public ObjectPointerVisitor {
 public:
  explicit ObjectIdRingClearPointerVisitor(Isolate* isolate) :
      ObjectPointerVisitor(isolate) {}


  void VisitPointers(RawObject** first, RawObject** last) {
    for (RawObject** current = first; current <= last; current++) {
      RawObject* raw_obj = *current;
      ASSERT(raw_obj->IsHeapObject());
      if (raw_obj->IsOldObject() && !raw_obj->IsMarked()) {
        // Object has become garbage. Replace it will null.
        *current = Object::null();
      }
    }
  }
};


void GCMarker::ProcessObjectIdTable(Isolate* isolate) {
  ObjectIdRingClearPointerVisitor visitor(isolate);
  ObjectIdRing* ring = isolate->object_id_ring();
  ASSERT(ring != NULL);
  ring->VisitPointers(&visitor);
}


class MarkTask : public ThreadPool::Task {
 public:
  MarkTask(GCMarker* marker,
           Isolate* isolate,
           Heap* heap,
           PageSpace* page_space,
           MarkingStack* marking_stack,
           DelaySet* delay_set,
           ThreadBarrier* barrier,
           bool collect_code,
           bool visit_prologue_weak_persistent_handles,
           intptr_t task_index,
           intptr_t num_tasks,
           uintptr_t* num_busy)
      : marker_(marker),
        isolate_(isolate),
        heap_(heap),
        page_space_(page_space),
        marking_stack_(marking_stack),
        delay_set_(delay_set),
        barrier_(barrier),
        collect_code_(collect_code),
        visit_prologue_weak_persistent_handles_(
            visit_prologue_weak_persistent_handles),
        task_index_(task_index),
        num_tasks_(num_tasks),
        num_busy_(num_busy) {
  }

  virtual void Run() {
    Thread::EnterIsolateAsHelper(isolate_, true);
    {
      StackZone stack_zone(Thread::Current());
      Zone* zone = stack_zone.GetZone();
      SkippedCodeFunctions* skipped_code_functions =
          collect_code_ ? new(zone) SkippedCodeFunctions() : NULL;
      SyncMarkingVisitor visitor(isolate_, heap_, page_space_, marking_stack_,
                                 delay_set_, skipped_code_functions);
      // Phase 1: Iterate over roots and drain marking stack in tasks.
      marker_->IterateRoots(isolate_, &visitor,
                            visit_prologue_weak_persistent_handles_,
                            task_index_, num_tasks_);
      do {
        visitor.DrainMarkingStack();

        // I can't find more work right now. If no other task is busy,
        // then there will never be more work (NB: 1 is *before* decrement).
        if (AtomicOperations::FetchAndDecrement(num_busy_) == 1) break;

        // Wait for some work to appear.
        // TODO(iposva): Replace busy-waiting with a solution using Monitor,
        // and redraw the boundaries between stack/visitor/task as needed.
        while (marking_stack_->IsEmpty() &&
               AtomicOperations::LoadRelaxed(num_busy_) > 0) {
        }

        // If no tasks are busy, there will never be more work.
        if (AtomicOperations::LoadRelaxed(num_busy_) == 0) break;

        // I saw some work; get busy and compete for it.
        AtomicOperations::FetchAndIncrement(num_busy_);
      } while (true);
      ASSERT(AtomicOperations::LoadRelaxed(num_busy_) == 0);
      barrier_->Sync();

      // Phase 2: Weak processing and follow-up marking on main thread.
      barrier_->Sync();

      // Phase 3: Finalize results from all markers (detach code, etc.).
      if (FLAG_log_marker_tasks) {
        THR_Print("Task %" Pd " marked %" Pd " bytes.\n",
                  task_index_, visitor.marked_bytes());
      }
      marker_->FinalizeResultsFrom(&visitor);
    }
    Thread::ExitIsolateAsHelper(true);

    // This task is done. Notify the original thread.
    barrier_->Exit();
  }

 private:
  GCMarker* marker_;
  Isolate* isolate_;
  Heap* heap_;
  PageSpace* page_space_;
  MarkingStack* marking_stack_;
  DelaySet* delay_set_;
  ThreadBarrier* barrier_;
  bool collect_code_;
  bool visit_prologue_weak_persistent_handles_;
  const intptr_t task_index_;
  const intptr_t num_tasks_;
  uintptr_t* num_busy_;

  DISALLOW_COPY_AND_ASSIGN(MarkTask);
};


template<class MarkingVisitorType>
void GCMarker::FinalizeResultsFrom(MarkingVisitorType* visitor) {
  {
    MutexLocker ml(&stats_mutex_);
    marked_bytes_ += visitor->marked_bytes();
    // Class heap stats are not themselves thread-safe yet, so we update the
    // stats while holding stats_mutex_.
    ClassTable* table = heap_->isolate()->class_table();
    for (intptr_t i = 0; i < table->NumCids(); ++i) {
      const intptr_t count = visitor->live_count(i);
      if (count > 0) {
        const intptr_t size = visitor->live_size(i);
        table->UpdateLiveOld(i, size, count);
      }
    }
  }
  visitor->Finalize();
}


void GCMarker::MarkObjects(Isolate* isolate,
                           PageSpace* page_space,
                           bool invoke_api_callbacks,
                           bool collect_code) {
  Prologue(isolate, invoke_api_callbacks);
  // The API prologue/epilogue may create/destroy zones, so we must not
  // depend on zone allocations surviving beyond the epilogue callback.
  {
    StackZone stack_zone(Thread::Current());
    Zone* zone = stack_zone.GetZone();
    MarkingStack marking_stack;
    DelaySet delay_set;
    const bool visit_prologue_weak_persistent_handles = !invoke_api_callbacks;
    marked_bytes_ = 0;
    const int num_tasks = FLAG_marker_tasks;
    if (num_tasks == 0) {
      // Mark everything on main thread.
      SkippedCodeFunctions* skipped_code_functions =
          collect_code ? new(zone) SkippedCodeFunctions() : NULL;
      UnsyncMarkingVisitor mark(isolate, heap_, page_space, &marking_stack,
                                &delay_set, skipped_code_functions);
      IterateRoots(isolate, &mark, visit_prologue_weak_persistent_handles,
                   0, 1);
      mark.DrainMarkingStack();
      IterateWeakReferences(isolate, &mark);
      MarkingWeakVisitor mark_weak;
      IterateWeakRoots(isolate, &mark_weak,
                       !visit_prologue_weak_persistent_handles);
      // All marking done; detach code, etc.
      FinalizeResultsFrom(&mark);
    } else {
      ThreadBarrier barrier(num_tasks + 1);
      // Used to coordinate draining among tasks; all start out as 'busy'.
      uintptr_t num_busy = num_tasks;
      // Phase 1: Iterate over roots and drain marking stack in tasks.
      for (intptr_t i = 0; i < num_tasks; ++i) {
        MarkTask* mark_task =
            new MarkTask(this, isolate, heap_, page_space, &marking_stack,
                         &delay_set, &barrier, collect_code,
                         visit_prologue_weak_persistent_handles,
                         i, num_tasks, &num_busy);
        ThreadPool* pool = Dart::thread_pool();
        pool->Run(mark_task);
      }
      barrier.Sync();

      // Phase 2: Weak processing and follow-up marking on main thread.
      SkippedCodeFunctions* skipped_code_functions =
          collect_code ? new(zone) SkippedCodeFunctions() : NULL;
      SyncMarkingVisitor mark(isolate, heap_, page_space, &marking_stack,
                              &delay_set, skipped_code_functions);
      IterateWeakReferences(isolate, &mark);
      MarkingWeakVisitor mark_weak;
      IterateWeakRoots(isolate, &mark_weak,
                       !visit_prologue_weak_persistent_handles);
      barrier.Sync();

      // Phase 3: Finalize results from all markers (detach code, etc.).
      if (FLAG_log_marker_tasks) {
        THR_Print("Main thread marked %" Pd " bytes.\n",
                  mark.marked_bytes());
      }
      FinalizeResultsFrom(&mark);
      barrier.Exit();
    }
    delay_set.ClearReferences();
    ProcessWeakTables(page_space);
    ProcessObjectIdTable(isolate);
  }
  Epilogue(isolate, invoke_api_callbacks);
}

}  // namespace dart