persistent-node.h

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// Copyright 2020 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 INCLUDE_CPPGC_INTERNAL_PERSISTENT_NODE_H_
#define INCLUDE_CPPGC_INTERNAL_PERSISTENT_NODE_H_
 
#include <array>
#include <memory>
#include <vector>
 
#include "cppgc/internal/logging.h"
#include "cppgc/trace-trait.h"
#include "v8config.h"              // NOLINT(build/include_directory)
 
namespace cppgc {
 
class Visitor;
 
namespace internal {
 
// PersistentNode represesents a variant of two states:
// 1) traceable node with a back pointer to the Persistent object;
// 2) freelist entry.
class PersistentNode final {
 public:
  PersistentNode() = default;
 
  PersistentNode(const PersistentNode&) = delete;
  PersistentNode& operator=(const PersistentNode&) = delete;
 
  void InitializeAsUsedNode(void* owner, TraceCallback trace) {
    owner_ = owner;
    trace_ = trace;
  }
 
  void InitializeAsFreeNode(PersistentNode* next) {
    next_ = next;
    trace_ = nullptr;
  }
 
  void UpdateOwner(void* owner) {
    CPPGC_DCHECK(IsUsed());
    owner_ = owner;
  }
 
  PersistentNode* FreeListNext() const {
    CPPGC_DCHECK(!IsUsed());
    return next_;
  }
 
  void Trace(Visitor* visitor) const {
    CPPGC_DCHECK(IsUsed());
    trace_(visitor, owner_);
  }
 
  bool IsUsed() const { return trace_; }
 
  void* owner() const {
    CPPGC_DCHECK(IsUsed());
    return owner_;
  }
 
 private:
  // PersistentNode acts as a designated union:
  // If trace_ != nullptr, owner_ points to the corresponding Persistent handle.
  // Otherwise, next_ points to the next freed PersistentNode.
  union {
    void* owner_ = nullptr;
    PersistentNode* next_;
  };
  TraceCallback trace_ = nullptr;
};
 
class V8_EXPORT PersistentRegion final {
  using PersistentNodeSlots = std::array<PersistentNode, 256u>;
 
 public:
  PersistentRegion() = default;
  // Clears Persistent fields to avoid stale pointers after heap teardown.
  ~PersistentRegion();
 
  PersistentRegion(const PersistentRegion&) = delete;
  PersistentRegion& operator=(const PersistentRegion&) = delete;
 
  PersistentNode* AllocateNode(void* owner, TraceCallback trace) {
    if (!free_list_head_) {
      EnsureNodeSlots();
    }
    PersistentNode* node = free_list_head_;
    free_list_head_ = free_list_head_->FreeListNext();
    node->InitializeAsUsedNode(owner, trace);
    return node;
  }
 
  void FreeNode(PersistentNode* node) {
    node->InitializeAsFreeNode(free_list_head_);
    free_list_head_ = node;
  }
 
  void Trace(Visitor*);
 
  size_t NodesInUse() const;
 
 private:
  void EnsureNodeSlots();
 
  std::vector<std::unique_ptr<PersistentNodeSlots>> nodes_;
  PersistentNode* free_list_head_ = nullptr;
};
 
}  // namespace internal
 
}  // namespace cppgc
 
#endif  // INCLUDE_CPPGC_INTERNAL_PERSISTENT_NODE_H_