src/heap/marking-worklist.h

// Copyright 2019 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef V8_HEAP_MARKING_WORKLIST_H_
#define V8_HEAP_MARKING_WORKLIST_H_

#include <unordered_map>
#include <vector>

#include "src/heap/marking.h"
#include "src/heap/worklist.h"
#include "src/objects/heap-object.h"

namespace v8 {
namespace internal {

using MarkingWorklist = Worklist<HeapObject, 64>;
using EmbedderTracingWorklist = Worklist<HeapObject, 16>;

// The index of the main thread task used by concurrent/parallel GC.
const int kMainThreadTask = 0;

// We piggyback on marking to compute object sizes per native context that is
// needed for the new memory measurement API. The algorithm works as follows:
// 1) At the start of marking we create a marking worklist for each context.
//    The existing shared, on_hold, and embedder worklists continue to work
//    as they did before, but they hold objects that are not attributed to any
//    context yet.
// 2) Each marker has an active worklist where it pushes newly discovered
//    objects. Initially the shared worklist is set as active for all markers.
// 3) When a marker pops an object from the active worklist:
//    a) It checks if the object has a known context (e.g. JSObjects, Maps,
//       Contexts know the context they belong to). If that's the case, then
//       the marker changes its active worklist to the worklist corresponding
//       to the context of the object.
//    b) It account the size of object to the active context.
//    c) It visits all pointers in the object and pushes new objects onto the
//       active worklist.
// 4) When the active worklist becomes empty the marker selects any other
//    non-empty worklist as the active worklist.
// 5) The write barrier pushes onto the shared worklist.
//
// The main invariant for context worklists:
//    If object X is in the worklist of context C, then either
//    a) X has a context and that context is C.
//    b) X is retained by object Y that has context C.
//
// The algorithm allows us to attribute context-independent objects such as
// strings, numbers, FixedArrays to their retaining contexts. The algorithm is
// not precise for context-independent objects that are shared between multiple
// contexts. Such objects may be attributed to any retaining context.

// Named pair of native context address and its marking worklist.
// Since native contexts are allocated in the old generation, their addresses
// a stable across Scavenges and stay valid throughout the marking phase.
struct ContextWorklistPair {
  Address context;
  MarkingWorklist* worklist;
};

// A helper class that owns all marking worklists.
class V8_EXPORT_PRIVATE MarkingWorklistsHolder {
 public:
  // Fake addresses of special contexts used for per-context accounting.
  // - kSharedContext is for objects that are not attributed to any context.
  // - kOtherContext is for objects that are attributed to contexts that are
  //   not being measured.
  static const Address kSharedContext = 0;
  static const Address kOtherContext = 8;

  ~MarkingWorklistsHolder();

  // Calls the specified callback on each element of the deques and replaces
  // the element with the result of the callback. If the callback returns
  // nullptr then the element is removed from the deque.
  // The callback must accept HeapObject and return HeapObject.
  template <typename Callback>
  void Update(Callback callback) {
    shared_.Update(callback);
    on_hold_.Update(callback);
    embedder_.Update(callback);
    for (auto cw : context_worklists_) {
      if (cw.context == kSharedContext) {
        // The shared context was updated above.
        continue;
      }
      cw.worklist->Update(callback);
    }
  }

  MarkingWorklist* shared() { return &shared_; }
  MarkingWorklist* on_hold() { return &on_hold_; }
  EmbedderTracingWorklist* embedder() { return &embedder_; }

  // A list of (context, worklist) pairs that was set up at the start of
  // marking by CreateContextWorklists.
  const std::vector<ContextWorklistPair>& context_worklists() {
    return context_worklists_;
  }
  // This should be invoked at the start of marking with the list of contexts
  // that require object size accounting.
  void CreateContextWorklists(const std::vector<Address>& contexts);
  // This should be invoked at the end of marking. All worklists must be
  // empty at that point.
  void ReleaseContextWorklists();

  void Clear();
  void Print();

 private:
  // Prints the stats about the global pool of the worklist.
  void PrintWorklist(const char* worklist_name, MarkingWorklist* worklist);

  // Worklist used for most objects.
  MarkingWorklist shared_;

  // Concurrent marking uses this worklist to bail out of marking objects
  // in new space's linear allocation area. Used to avoid black allocation
  // for new space. This allow the compiler to remove write barriers
  // for freshly allocatd objects.
  MarkingWorklist on_hold_;

  // Worklist for objects that potentially require embedder tracing, i.e.,
  // these objects need to be handed over to the embedder to find the full
  // transitive closure.
  EmbedderTracingWorklist embedder_;

  // Per-context worklists.
  std::vector<ContextWorklistPair> context_worklists_;
  // This is used only for lifetime management of the per-context worklists.
  std::vector<std::unique_ptr<MarkingWorklist>> worklists_;

  // Worklist used for objects that are attributed to contexts that are
  // not being measured.
  MarkingWorklist other_;
};

// A thread-local view of the marking worklists.
class V8_EXPORT_PRIVATE MarkingWorklists {
 public:
  static const Address kSharedContext = MarkingWorklistsHolder::kSharedContext;
  static const Address kOtherContext = MarkingWorklistsHolder::kOtherContext;

  MarkingWorklists(int task_id, MarkingWorklistsHolder* holder);

  void Push(HeapObject object) {
    bool success = active_->Push(task_id_, object);
    USE(success);
    DCHECK(success);
  }

  bool Pop(HeapObject* object) {
    if (active_->Pop(task_id_, object)) return true;
    if (!is_per_context_mode_) return false;
    // The active worklist is empty. Find any other non-empty worklist and
    // switch the active worklist to it.
    return PopContext(object);
  }

  void PushOnHold(HeapObject object) {
    bool success = on_hold_->Push(task_id_, object);
    USE(success);
    DCHECK(success);
  }

  bool PopOnHold(HeapObject* object) { return on_hold_->Pop(task_id_, object); }

  void PushEmbedder(HeapObject object) {
    bool success = embedder_->Push(task_id_, object);
    USE(success);
    DCHECK(success);
  }

  bool PopEmbedder(HeapObject* object) {
    return embedder_->Pop(task_id_, object);
  }

  void FlushToGlobal();
  bool IsEmpty();
  bool IsEmbedderEmpty();
  void MergeOnHold();
  void ShareWorkIfGlobalPoolIsEmpty();

  // Returns the context of the active worklist.
  Address Context() { return active_context_; }
  // Switches the active worklist to that of the given context.
  Address SwitchToContext(Address context) {
    if (context == active_context_) return context;
    return SwitchToContextSlow(context);
  }
  // Switches the active worklist to the shared worklist.
  void SwitchToShared() {
    active_context_ = kSharedContext;
    active_ = shared_;
  }
  bool IsPerContextMode() { return is_per_context_mode_; }

 private:
  bool PopContext(HeapObject* object);
  Address SwitchToContextSlow(Address context);
  MarkingWorklist* shared_;
  MarkingWorklist* on_hold_;
  EmbedderTracingWorklist* embedder_;
  MarkingWorklist* active_;
  Address active_context_;
  int task_id_;
  bool is_per_context_mode_;
  // Per-context worklists. For simplicity we treat the shared worklist as
  // the worklist of dummy kSharedContext.
  std::vector<ContextWorklistPair> context_worklists_;
  std::unordered_map<Address, MarkingWorklist*> worklist_by_context_;
};

}  // namespace internal
}  // namespace v8

#endif  // V8_HEAP_MARKING_WORKLIST_H_