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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_VISITOR_H_
#define INCLUDE_CPPGC_VISITOR_H_

#include "cppgc/custom-space.h"
#include "cppgc/ephemeron-pair.h"
#include "cppgc/garbage-collected.h"
#include "cppgc/internal/logging.h"
#include "cppgc/internal/pointer-policies.h"
#include "cppgc/liveness-broker.h"
#include "cppgc/member.h"
#include "cppgc/sentinel-pointer.h"
#include "cppgc/source-location.h"
#include "cppgc/trace-trait.h"
#include "cppgc/type-traits.h"

namespace cppgc {

namespace internal {
template <typename T, typename WeaknessPolicy, typename LocationPolicy,
          typename CheckingPolicy>
class BasicCrossThreadPersistent;
template <typename T, typename WeaknessPolicy, typename LocationPolicy,
          typename CheckingPolicy>
class BasicPersistent;
class ConservativeTracingVisitor;
class VisitorBase;
class VisitorFactory;
}  // namespace internal

using WeakCallback = void (*)(const LivenessBroker&, const void*);

/**
 * Visitor passed to trace methods. All managed pointers must have called the
 * Visitor's trace method on them.
 *
 * \code
 * class Foo final : public GarbageCollected<Foo> {
 *  public:
 *   void Trace(Visitor* visitor) const {
 *     visitor->Trace(foo_);
 *     visitor->Trace(weak_foo_);
 *   }
 *  private:
 *   Member<Foo> foo_;
 *   WeakMember<Foo> weak_foo_;
 * };
 * \endcode
 */
class V8_EXPORT Visitor {
 public:
  class Key {
   private:
    Key() = default;
    friend class internal::VisitorFactory;
  };

  explicit Visitor(Key) {}

  virtual ~Visitor() = default;

  /**
   * Trace method for Member.
   *
   * \param member Member reference retaining an object.
   */
  template <typename T>
  void Trace(const Member<T>& member) {
    const T* value = member.GetRawAtomic();
    CPPGC_DCHECK(value != kSentinelPointer);
    TraceImpl(value);
  }

  /**
   * Trace method for WeakMember.
   *
   * \param weak_member WeakMember reference weakly retaining an object.
   */
  template <typename T>
  void Trace(const WeakMember<T>& weak_member) {
    static_assert(sizeof(T), "Pointee type must be fully defined.");
    static_assert(internal::IsGarbageCollectedOrMixinType<T>::value,
                  "T must be GarbageCollected or GarbageCollectedMixin type");
    static_assert(!internal::IsAllocatedOnCompactableSpace<T>::value,
                  "Weak references to compactable objects are not allowed");

    const T* value = weak_member.GetRawAtomic();

    // Bailout assumes that WeakMember emits write barrier.
    if (!value) {
      return;
    }

    CPPGC_DCHECK(value != kSentinelPointer);
    VisitWeak(value, TraceTrait<T>::GetTraceDescriptor(value),
              &HandleWeak<WeakMember<T>>, &weak_member);
  }

#if defined(CPPGC_POINTER_COMPRESSION)
  /**
   * Trace method for UncompressedMember.
   *
   * \param member UncompressedMember reference retaining an object.
   */
  template <typename T>
  void Trace(const subtle::UncompressedMember<T>& member) {
    const T* value = member.GetRawAtomic();
    CPPGC_DCHECK(value != kSentinelPointer);
    TraceImpl(value);
  }
#endif  // defined(CPPGC_POINTER_COMPRESSION)

  /**
   * Trace method for inlined objects that are not allocated themselves but
   * otherwise follow managed heap layout and have a Trace() method.
   *
   * \param object reference of the inlined object.
   */
  template <typename T>
  void Trace(const T& object) {
#if V8_ENABLE_CHECKS
    // This object is embedded in potentially multiple nested objects. The
    // outermost object must not be in construction as such objects are (a) not
    // processed immediately, and (b) only processed conservatively if not
    // otherwise possible.
    CheckObjectNotInConstruction(&object);
#endif  // V8_ENABLE_CHECKS
    TraceTrait<T>::Trace(this, &object);
  }

  /**
   * Registers a weak callback method on the object of type T. See
   * LivenessBroker for an usage example.
   *
   * \param object of type T specifying a weak callback method.
   */
  template <typename T, void (T::*method)(const LivenessBroker&)>
  void RegisterWeakCallbackMethod(const T* object) {
    RegisterWeakCallback(&WeakCallbackMethodDelegate<T, method>, object);
  }

  /**
   * Trace method for EphemeronPair.
   *
   * \param ephemeron_pair EphemeronPair reference weakly retaining a key object
   * and strongly retaining a value object in case the key object is alive.
   */
  template <typename K, typename V>
  void Trace(const EphemeronPair<K, V>& ephemeron_pair) {
    TraceEphemeron(ephemeron_pair.key, &ephemeron_pair.value);
    RegisterWeakCallbackMethod<EphemeronPair<K, V>,
                               &EphemeronPair<K, V>::ClearValueIfKeyIsDead>(
        &ephemeron_pair);
  }

  /**
   * Trace method for a single ephemeron. Used for tracing a raw ephemeron in
   * which the `key` and `value` are kept separately.
   *
   * \param weak_member_key WeakMember reference weakly retaining a key object.
   * \param member_value Member reference with ephemeron semantics.
   */
  template <typename KeyType, typename ValueType>
  void TraceEphemeron(const WeakMember<KeyType>& weak_member_key,
                      const Member<ValueType>* member_value) {
    const KeyType* key = weak_member_key.GetRawAtomic();
    if (!key) return;

    // `value` must always be non-null.
    CPPGC_DCHECK(member_value);
    const ValueType* value = member_value->GetRawAtomic();
    if (!value) return;

    // KeyType and ValueType may refer to GarbageCollectedMixin.
    TraceDescriptor value_desc =
        TraceTrait<ValueType>::GetTraceDescriptor(value);
    CPPGC_DCHECK(value_desc.base_object_payload);
    const void* key_base_object_payload =
        TraceTrait<KeyType>::GetTraceDescriptor(key).base_object_payload;
    CPPGC_DCHECK(key_base_object_payload);

    VisitEphemeron(key_base_object_payload, value, value_desc);
  }

  /**
   * Trace method for a single ephemeron. Used for tracing a raw ephemeron in
   * which the `key` and `value` are kept separately. Note that this overload
   * is for non-GarbageCollected `value`s that can be traced though.
   *
   * \param key `WeakMember` reference weakly retaining a key object.
   * \param value Reference weakly retaining a value object. Note that
   *   `ValueType` here should not be `Member`. It is expected that
   *   `TraceTrait<ValueType>::GetTraceDescriptor(value)` returns a
   *   `TraceDescriptor` with a null base pointer but a valid trace method.
   */
  template <typename KeyType, typename ValueType>
  void TraceEphemeron(const WeakMember<KeyType>& weak_member_key,
                      const ValueType* value) {
    static_assert(!IsGarbageCollectedOrMixinTypeV<ValueType>,
                  "garbage-collected types must use WeakMember and Member");
    const KeyType* key = weak_member_key.GetRawAtomic();
    if (!key) return;

    // `value` must always be non-null.
    CPPGC_DCHECK(value);
    TraceDescriptor value_desc =
        TraceTrait<ValueType>::GetTraceDescriptor(value);
    // `value_desc.base_object_payload` must be null as this override is only
    // taken for non-garbage-collected values.
    CPPGC_DCHECK(!value_desc.base_object_payload);

    // KeyType might be a GarbageCollectedMixin.
    const void* key_base_object_payload =
        TraceTrait<KeyType>::GetTraceDescriptor(key).base_object_payload;
    CPPGC_DCHECK(key_base_object_payload);

    VisitEphemeron(key_base_object_payload, value, value_desc);
  }

  /**
   * Trace method that strongifies a WeakMember.
   *
   * \param weak_member WeakMember reference retaining an object.
   */
  template <typename T>
  void TraceStrongly(const WeakMember<T>& weak_member) {
    const T* value = weak_member.GetRawAtomic();
    CPPGC_DCHECK(value != kSentinelPointer);
    TraceImpl(value);
  }

  /**
   * Trace method for retaining containers strongly.
   *
   * \param object reference to the container.
   */
  template <typename T>
  void TraceStrongContainer(const T* object) {
    TraceImpl(object);
  }

  /**
   * Trace method for retaining containers weakly. Note that weak containers
   * should emit write barriers.
   *
   * \param object reference to the container.
   * \param callback to be invoked.
   * \param callback_data custom data that is passed to the callback.
   */
  template <typename T>
  void TraceWeakContainer(const T* object, WeakCallback callback,
                          const void* callback_data) {
    if (!object) return;
    VisitWeakContainer(object, TraceTrait<T>::GetTraceDescriptor(object),
                       TraceTrait<T>::GetWeakTraceDescriptor(object), callback,
                       callback_data);
  }

  /**
   * Registers a slot containing a reference to an object allocated on a
   * compactable space. Such references maybe be arbitrarily moved by the GC.
   *
   * \param slot location of reference to object that might be moved by the GC.
   * The slot must contain an uncompressed pointer.
   */
  template <typename T>
  void RegisterMovableReference(const T** slot) {
    static_assert(internal::IsAllocatedOnCompactableSpace<T>::value,
                  "Only references to objects allocated on compactable spaces "
                  "should be registered as movable slots.");
    static_assert(!IsGarbageCollectedMixinTypeV<T>,
                  "Mixin types do not support compaction.");
    HandleMovableReference(reinterpret_cast<const void**>(slot));
  }

  /**
   * Registers a weak callback that is invoked during garbage collection.
   *
   * \param callback to be invoked.
   * \param data custom data that is passed to the callback.
   */
  virtual void RegisterWeakCallback(WeakCallback callback, const void* data) {}

  /**
   * Defers tracing an object from a concurrent thread to the mutator thread.
   * Should be called by Trace methods of types that are not safe to trace
   * concurrently.
   *
   * \param parameter tells the trace callback which object was deferred.
   * \param callback to be invoked for tracing on the mutator thread.
   * \param deferred_size size of deferred object.
   *
   * \returns false if the object does not need to be deferred (i.e. currently
   * traced on the mutator thread) and true otherwise (i.e. currently traced on
   * a concurrent thread).
   */
  virtual V8_WARN_UNUSED_RESULT bool DeferTraceToMutatorThreadIfConcurrent(
      const void* parameter, TraceCallback callback, size_t deferred_size) {
    // By default tracing is not deferred.
    return false;
  }

 protected:
  virtual void Visit(const void* self, TraceDescriptor) {}
  virtual void VisitWeak(const void* self, TraceDescriptor, WeakCallback,
                         const void* weak_member) {}
  virtual void VisitEphemeron(const void* key, const void* value,
                              TraceDescriptor value_desc) {}
  virtual void VisitWeakContainer(const void* self, TraceDescriptor strong_desc,
                                  TraceDescriptor weak_desc,
                                  WeakCallback callback, const void* data) {}
  virtual void HandleMovableReference(const void**) {}

 private:
  template <typename T, void (T::*method)(const LivenessBroker&)>
  static void WeakCallbackMethodDelegate(const LivenessBroker& info,
                                         const void* self) {
    // Callback is registered through a potential const Trace method but needs
    // to be able to modify fields. See HandleWeak.
    (const_cast<T*>(static_cast<const T*>(self))->*method)(info);
  }

  template <typename PointerType>
  static void HandleWeak(const LivenessBroker& info, const void* object) {
    const PointerType* weak = static_cast<const PointerType*>(object);
    auto* raw_ptr = weak->GetFromGC();
    if (!info.IsHeapObjectAlive(raw_ptr)) {
      weak->ClearFromGC();
    }
  }

  template <typename T>
  void TraceImpl(const T* t) {
    static_assert(sizeof(T), "Pointee type must be fully defined.");
    static_assert(internal::IsGarbageCollectedOrMixinType<T>::value,
                  "T must be GarbageCollected or GarbageCollectedMixin type");
    if (!t) {
      return;
    }
    Visit(t, TraceTrait<T>::GetTraceDescriptor(t));
  }

#if V8_ENABLE_CHECKS
  void CheckObjectNotInConstruction(const void* address);
#endif  // V8_ENABLE_CHECKS

  template <typename T, typename WeaknessPolicy, typename LocationPolicy,
            typename CheckingPolicy>
  friend class internal::BasicCrossThreadPersistent;
  template <typename T, typename WeaknessPolicy, typename LocationPolicy,
            typename CheckingPolicy>
  friend class internal::BasicPersistent;
  friend class internal::ConservativeTracingVisitor;
  friend class internal::VisitorBase;
};

namespace internal {

class V8_EXPORT RootVisitor {
 public:
  explicit RootVisitor(Visitor::Key) {}

  virtual ~RootVisitor() = default;

  template <typename AnyStrongPersistentType,
            std::enable_if_t<
                AnyStrongPersistentType::IsStrongPersistent::value>* = nullptr>
  void Trace(const AnyStrongPersistentType& p) {
    using PointeeType = typename AnyStrongPersistentType::PointeeType;
    const void* object = Extract(p);
    if (!object) {
      return;
    }
    VisitRoot(object, TraceTrait<PointeeType>::GetTraceDescriptor(object),
              p.Location());
  }

  template <typename AnyWeakPersistentType,
            std::enable_if_t<
                !AnyWeakPersistentType::IsStrongPersistent::value>* = nullptr>
  void Trace(const AnyWeakPersistentType& p) {
    using PointeeType = typename AnyWeakPersistentType::PointeeType;
    static_assert(!internal::IsAllocatedOnCompactableSpace<PointeeType>::value,
                  "Weak references to compactable objects are not allowed");
    const void* object = Extract(p);
    if (!object) {
      return;
    }
    VisitWeakRoot(object, TraceTrait<PointeeType>::GetTraceDescriptor(object),
                  &HandleWeak<AnyWeakPersistentType>, &p, p.Location());
  }

 protected:
  virtual void VisitRoot(const void*, TraceDescriptor, const SourceLocation&) {}
  virtual void VisitWeakRoot(const void* self, TraceDescriptor, WeakCallback,
                             const void* weak_root, const SourceLocation&) {}

 private:
  template <typename AnyPersistentType>
  static const void* Extract(AnyPersistentType& p) {
    using PointeeType = typename AnyPersistentType::PointeeType;
    static_assert(sizeof(PointeeType),
                  "Persistent's pointee type must be fully defined");
    static_assert(internal::IsGarbageCollectedOrMixinType<PointeeType>::value,
                  "Persistent's pointee type must be GarbageCollected or "
                  "GarbageCollectedMixin");
    return p.GetFromGC();
  }

  template <typename PointerType>
  static void HandleWeak(const LivenessBroker& info, const void* object) {
    const PointerType* weak = static_cast<const PointerType*>(object);
    auto* raw_ptr = weak->GetFromGC();
    if (!info.IsHeapObjectAlive(raw_ptr)) {
      weak->ClearFromGC();
    }
  }
};

}  // namespace internal
}  // namespace cppgc

#endif  // INCLUDE_CPPGC_VISITOR_H_

	// 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_VISITOR_H_
	#define INCLUDE_CPPGC_VISITOR_H_

	#include "cppgc/custom-space.h"
	#include "cppgc/ephemeron-pair.h"
	#include "cppgc/garbage-collected.h"
	#include "cppgc/internal/logging.h"
	#include "cppgc/internal/pointer-policies.h"
	#include "cppgc/liveness-broker.h"
	#include "cppgc/member.h"
	#include "cppgc/sentinel-pointer.h"
	#include "cppgc/source-location.h"
	#include "cppgc/trace-trait.h"
	#include "cppgc/type-traits.h"

	namespace cppgc {

	namespace internal {
	template <typename T, typename WeaknessPolicy, typename LocationPolicy,
	typename CheckingPolicy>
	class BasicCrossThreadPersistent;
	template <typename T, typename WeaknessPolicy, typename LocationPolicy,
	typename CheckingPolicy>
	class BasicPersistent;
	class ConservativeTracingVisitor;
	class VisitorBase;
	class VisitorFactory;
	} // namespace internal

	using WeakCallback = void ()(const LivenessBroker&, const void);

	/**
	* Visitor passed to trace methods. All managed pointers must have called the
	* Visitor's trace method on them.
	*
	* \code
	* class Foo final : public GarbageCollected<Foo> {
	* public:
	* void Trace(Visitor* visitor) const {
	* visitor->Trace(foo_);
	* visitor->Trace(weak_foo_);
	* }
	* private:
	* Member<Foo> foo_;
	* WeakMember<Foo> weak_foo_;
	* };
	* \endcode
	*/
	class V8_EXPORT Visitor {
	public:
	class Key {
	private:
	Key() = default;
	friend class internal::VisitorFactory;
	};

	explicit Visitor(Key) {}

	virtual ~Visitor() = default;

	/**
	* Trace method for Member.
	*
	* \param member Member reference retaining an object.
	*/
	template <typename T>
	void Trace(const Member<T>& member) {
	const T* value = member.GetRawAtomic();
	CPPGC_DCHECK(value != kSentinelPointer);
	TraceImpl(value);
	}

	/**
	* Trace method for WeakMember.
	*
	* \param weak_member WeakMember reference weakly retaining an object.
	*/
	template <typename T>
	void Trace(const WeakMember<T>& weak_member) {
	static_assert(sizeof(T), "Pointee type must be fully defined.");
	static_assert(internal::IsGarbageCollectedOrMixinType<T>::value,
	"T must be GarbageCollected or GarbageCollectedMixin type");
	static_assert(!internal::IsAllocatedOnCompactableSpace<T>::value,
	"Weak references to compactable objects are not allowed");

	const T* value = weak_member.GetRawAtomic();

	// Bailout assumes that WeakMember emits write barrier.
	if (!value) {
	return;
	}

	CPPGC_DCHECK(value != kSentinelPointer);
	VisitWeak(value, TraceTrait<T>::GetTraceDescriptor(value),
	&HandleWeak<WeakMember<T>>, &weak_member);
	}

	#if defined(CPPGC_POINTER_COMPRESSION)
	/**
	* Trace method for UncompressedMember.
	*
	* \param member UncompressedMember reference retaining an object.
	*/
	template <typename T>
	void Trace(const subtle::UncompressedMember<T>& member) {
	const T* value = member.GetRawAtomic();
	CPPGC_DCHECK(value != kSentinelPointer);
	TraceImpl(value);
	}
	#endif // defined(CPPGC_POINTER_COMPRESSION)

	/**
	* Trace method for inlined objects that are not allocated themselves but
	* otherwise follow managed heap layout and have a Trace() method.
	*
	* \param object reference of the inlined object.
	*/
	template <typename T>
	void Trace(const T& object) {
	#if V8_ENABLE_CHECKS
	// This object is embedded in potentially multiple nested objects. The
	// outermost object must not be in construction as such objects are (a) not
	// processed immediately, and (b) only processed conservatively if not
	// otherwise possible.
	CheckObjectNotInConstruction(&object);
	#endif // V8_ENABLE_CHECKS
	TraceTrait<T>::Trace(this, &object);
	}

	/**
	* Registers a weak callback method on the object of type T. See
	* LivenessBroker for an usage example.
	*
	* \param object of type T specifying a weak callback method.
	*/
	template <typename T, void (T::*method)(const LivenessBroker&)>
	void RegisterWeakCallbackMethod(const T* object) {
	RegisterWeakCallback(&WeakCallbackMethodDelegate<T, method>, object);
	}

	/**
	* Trace method for EphemeronPair.
	*
	* \param ephemeron_pair EphemeronPair reference weakly retaining a key object
	* and strongly retaining a value object in case the key object is alive.
	*/
	template <typename K, typename V>
	void Trace(const EphemeronPair<K, V>& ephemeron_pair) {
	TraceEphemeron(ephemeron_pair.key, &ephemeron_pair.value);
	RegisterWeakCallbackMethod<EphemeronPair<K, V>,
	&EphemeronPair<K, V>::ClearValueIfKeyIsDead>(
	&ephemeron_pair);
	}

	/**
	* Trace method for a single ephemeron. Used for tracing a raw ephemeron in
	* which the `key` and `value` are kept separately.
	*
	* \param weak_member_key WeakMember reference weakly retaining a key object.
	* \param member_value Member reference with ephemeron semantics.
	*/
	template <typename KeyType, typename ValueType>
	void TraceEphemeron(const WeakMember<KeyType>& weak_member_key,
	const Member<ValueType>* member_value) {
	const KeyType* key = weak_member_key.GetRawAtomic();
	if (!key) return;

	// `value` must always be non-null.
	CPPGC_DCHECK(member_value);
	const ValueType* value = member_value->GetRawAtomic();
	if (!value) return;

	// KeyType and ValueType may refer to GarbageCollectedMixin.
	TraceDescriptor value_desc =
	TraceTrait<ValueType>::GetTraceDescriptor(value);
	CPPGC_DCHECK(value_desc.base_object_payload);
	const void* key_base_object_payload =
	TraceTrait<KeyType>::GetTraceDescriptor(key).base_object_payload;
	CPPGC_DCHECK(key_base_object_payload);

	VisitEphemeron(key_base_object_payload, value, value_desc);
	}

	/**
	* Trace method for a single ephemeron. Used for tracing a raw ephemeron in
	* which the `key` and `value` are kept separately. Note that this overload
	* is for non-GarbageCollected `value`s that can be traced though.
	*
	* \param key `WeakMember` reference weakly retaining a key object.
	* \param value Reference weakly retaining a value object. Note that
	* `ValueType` here should not be `Member`. It is expected that
	* `TraceTrait<ValueType>::GetTraceDescriptor(value)` returns a
	* `TraceDescriptor` with a null base pointer but a valid trace method.
	*/
	template <typename KeyType, typename ValueType>
	void TraceEphemeron(const WeakMember<KeyType>& weak_member_key,
	const ValueType* value) {
	static_assert(!IsGarbageCollectedOrMixinTypeV<ValueType>,
	"garbage-collected types must use WeakMember and Member");
	const KeyType* key = weak_member_key.GetRawAtomic();
	if (!key) return;

	// `value` must always be non-null.
	CPPGC_DCHECK(value);
	TraceDescriptor value_desc =
	TraceTrait<ValueType>::GetTraceDescriptor(value);
	// `value_desc.base_object_payload` must be null as this override is only
	// taken for non-garbage-collected values.
	CPPGC_DCHECK(!value_desc.base_object_payload);

	// KeyType might be a GarbageCollectedMixin.
	const void* key_base_object_payload =
	TraceTrait<KeyType>::GetTraceDescriptor(key).base_object_payload;
	CPPGC_DCHECK(key_base_object_payload);

	VisitEphemeron(key_base_object_payload, value, value_desc);
	}

	/**
	* Trace method that strongifies a WeakMember.
	*
	* \param weak_member WeakMember reference retaining an object.
	*/
	template <typename T>
	void TraceStrongly(const WeakMember<T>& weak_member) {
	const T* value = weak_member.GetRawAtomic();
	CPPGC_DCHECK(value != kSentinelPointer);
	TraceImpl(value);
	}

	/**
	* Trace method for retaining containers strongly.
	*
	* \param object reference to the container.
	*/
	template <typename T>
	void TraceStrongContainer(const T* object) {
	TraceImpl(object);
	}

	/**
	* Trace method for retaining containers weakly. Note that weak containers
	* should emit write barriers.
	*
	* \param object reference to the container.
	* \param callback to be invoked.
	* \param callback_data custom data that is passed to the callback.
	*/
	template <typename T>
	void TraceWeakContainer(const T* object, WeakCallback callback,
	const void* callback_data) {
	if (!object) return;
	VisitWeakContainer(object, TraceTrait<T>::GetTraceDescriptor(object),
	TraceTrait<T>::GetWeakTraceDescriptor(object), callback,
	callback_data);
	}

	/**
	* Registers a slot containing a reference to an object allocated on a
	* compactable space. Such references maybe be arbitrarily moved by the GC.
	*
	* \param slot location of reference to object that might be moved by the GC.
	* The slot must contain an uncompressed pointer.
	*/
	template <typename T>
	void RegisterMovableReference(const T** slot) {
	static_assert(internal::IsAllocatedOnCompactableSpace<T>::value,
	"Only references to objects allocated on compactable spaces "
	"should be registered as movable slots.");
	static_assert(!IsGarbageCollectedMixinTypeV<T>,
	"Mixin types do not support compaction.");
	HandleMovableReference(reinterpret_cast<const void**>(slot));
	}

	/**
	* Registers a weak callback that is invoked during garbage collection.
	*
	* \param callback to be invoked.
	* \param data custom data that is passed to the callback.
	*/
	virtual void RegisterWeakCallback(WeakCallback callback, const void* data) {}

	/**
	* Defers tracing an object from a concurrent thread to the mutator thread.
	* Should be called by Trace methods of types that are not safe to trace
	* concurrently.
	*
	* \param parameter tells the trace callback which object was deferred.
	* \param callback to be invoked for tracing on the mutator thread.
	* \param deferred_size size of deferred object.
	*
	* \returns false if the object does not need to be deferred (i.e. currently
	* traced on the mutator thread) and true otherwise (i.e. currently traced on
	* a concurrent thread).
	*/
	virtual V8_WARN_UNUSED_RESULT bool DeferTraceToMutatorThreadIfConcurrent(
	const void* parameter, TraceCallback callback, size_t deferred_size) {
	// By default tracing is not deferred.
	return false;
	}

	protected:
	virtual void Visit(const void* self, TraceDescriptor) {}
	virtual void VisitWeak(const void* self, TraceDescriptor, WeakCallback,
	const void* weak_member) {}
	virtual void VisitEphemeron(const void* key, const void* value,
	TraceDescriptor value_desc) {}
	virtual void VisitWeakContainer(const void* self, TraceDescriptor strong_desc,
	TraceDescriptor weak_desc,
	WeakCallback callback, const void* data) {}
	virtual void HandleMovableReference(const void**) {}

	private:
	template <typename T, void (T::*method)(const LivenessBroker&)>
	static void WeakCallbackMethodDelegate(const LivenessBroker& info,
	const void* self) {
	// Callback is registered through a potential const Trace method but needs
	// to be able to modify fields. See HandleWeak.
	(const_cast<T>(static_cast<const T>(self))->*method)(info);
	}

	template <typename PointerType>
	static void HandleWeak(const LivenessBroker& info, const void* object) {
	const PointerType* weak = static_cast<const PointerType*>(object);
	auto* raw_ptr = weak->GetFromGC();
	if (!info.IsHeapObjectAlive(raw_ptr)) {
	weak->ClearFromGC();
	}
	}

	template <typename T>
	void TraceImpl(const T* t) {
	static_assert(sizeof(T), "Pointee type must be fully defined.");
	static_assert(internal::IsGarbageCollectedOrMixinType<T>::value,
	"T must be GarbageCollected or GarbageCollectedMixin type");
	if (!t) {
	return;
	}
	Visit(t, TraceTrait<T>::GetTraceDescriptor(t));
	}

	#if V8_ENABLE_CHECKS
	void CheckObjectNotInConstruction(const void* address);
	#endif // V8_ENABLE_CHECKS

	template <typename T, typename WeaknessPolicy, typename LocationPolicy,
	typename CheckingPolicy>
	friend class internal::BasicCrossThreadPersistent;
	template <typename T, typename WeaknessPolicy, typename LocationPolicy,
	typename CheckingPolicy>
	friend class internal::BasicPersistent;
	friend class internal::ConservativeTracingVisitor;
	friend class internal::VisitorBase;
	};

	namespace internal {

	class V8_EXPORT RootVisitor {
	public:
	explicit RootVisitor(Visitor::Key) {}

	virtual ~RootVisitor() = default;

	template <typename AnyStrongPersistentType,
	std::enable_if_t<
	AnyStrongPersistentType::IsStrongPersistent::value>* = nullptr>
	void Trace(const AnyStrongPersistentType& p) {
	using PointeeType = typename AnyStrongPersistentType::PointeeType;
	const void* object = Extract(p);
	if (!object) {
	return;
	}
	VisitRoot(object, TraceTrait<PointeeType>::GetTraceDescriptor(object),
	p.Location());
	}

	template <typename AnyWeakPersistentType,
	std::enable_if_t<
	!AnyWeakPersistentType::IsStrongPersistent::value>* = nullptr>
	void Trace(const AnyWeakPersistentType& p) {
	using PointeeType = typename AnyWeakPersistentType::PointeeType;
	static_assert(!internal::IsAllocatedOnCompactableSpace<PointeeType>::value,
	"Weak references to compactable objects are not allowed");
	const void* object = Extract(p);
	if (!object) {
	return;
	}
	VisitWeakRoot(object, TraceTrait<PointeeType>::GetTraceDescriptor(object),
	&HandleWeak<AnyWeakPersistentType>, &p, p.Location());
	}

	protected:
	virtual void VisitRoot(const void*, TraceDescriptor, const SourceLocation&) {}
	virtual void VisitWeakRoot(const void* self, TraceDescriptor, WeakCallback,
	const void* weak_root, const SourceLocation&) {}

	private:
	template <typename AnyPersistentType>
	static const void* Extract(AnyPersistentType& p) {
	using PointeeType = typename AnyPersistentType::PointeeType;
	static_assert(sizeof(PointeeType),
	"Persistent's pointee type must be fully defined");
	static_assert(internal::IsGarbageCollectedOrMixinType<PointeeType>::value,
	"Persistent's pointee type must be GarbageCollected or "
	"GarbageCollectedMixin");
	return p.GetFromGC();
	}

	template <typename PointerType>
	static void HandleWeak(const LivenessBroker& info, const void* object) {
	const PointerType* weak = static_cast<const PointerType*>(object);
	auto* raw_ptr = weak->GetFromGC();
	if (!info.IsHeapObjectAlive(raw_ptr)) {
	weak->ClearFromGC();
	}
	}
	};

	} // namespace internal
	} // namespace cppgc

	#endif // INCLUDE_CPPGC_VISITOR_H_