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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/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/source-location.h"
#include "cppgc/trace-trait.h"
namespace cppgc {
namespace internal {
template <typename T, typename WeaknessPolicy, typename LocationPolicy,
typename CheckingPolicy>
class BasicPersistent;
class VisitorBase;
} // 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 Visitor {
public:
/**
* 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);
Trace(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::IsGarbageCollectedType<T>::value,
"T must be GarbageCollected or GarbageCollectedMixin type");
const T* value = weak_member.GetRawAtomic();
// Bailout assumes that WeakMember emits write barrier.
if (!value) {
return;
}
// TODO(chromium:1056170): DCHECK (or similar) for deleted values as they
// should come in at a different path.
VisitWeak(value, TraceTrait<T>::GetTraceDescriptor(value),
&HandleWeak<WeakMember<T>>, &weak_member);
}
/**
* 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);
}
/**
* 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) {}
protected:
virtual void Visit(const void* self, TraceDescriptor) {}
virtual void VisitWeak(const void* self, TraceDescriptor, WeakCallback,
const void* weak_member) {}
virtual void VisitRoot(const void*, TraceDescriptor) {}
virtual void VisitWeakRoot(const void* self, TraceDescriptor, WeakCallback,
const void* weak_root) {}
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);
// Sentinel values are preserved for weak pointers.
if (*weak == kSentinelPointer) return;
const auto* raw = weak->Get();
if (!info.IsHeapObjectAlive(raw)) {
weak->ClearFromGC();
}
}
Visitor() = default;
template <typename Persistent,
std::enable_if_t<Persistent::IsStrongPersistent::value>* = nullptr>
void TraceRoot(const Persistent& p, const SourceLocation& loc) {
using PointeeType = typename Persistent::PointeeType;
static_assert(sizeof(PointeeType),
"Persistent's pointee type must be fully defined");
static_assert(internal::IsGarbageCollectedType<PointeeType>::value,
"Persistent's pointee type must be GarbageCollected or "
"GarbageCollectedMixin");
if (!p.Get()) {
return;
}
VisitRoot(p.Get(), TraceTrait<PointeeType>::GetTraceDescriptor(p.Get()));
}
template <
typename WeakPersistent,
std::enable_if_t<!WeakPersistent::IsStrongPersistent::value>* = nullptr>
void TraceRoot(const WeakPersistent& p, const SourceLocation& loc) {
using PointeeType = typename WeakPersistent::PointeeType;
static_assert(sizeof(PointeeType),
"Persistent's pointee type must be fully defined");
static_assert(internal::IsGarbageCollectedType<PointeeType>::value,
"Persistent's pointee type must be GarbageCollected or "
"GarbageCollectedMixin");
VisitWeakRoot(p.Get(), TraceTrait<PointeeType>::GetTraceDescriptor(p.Get()),
&HandleWeak<WeakPersistent>, &p);
}
template <typename T>
void Trace(const T* t) {
static_assert(sizeof(T), "Pointee type must be fully defined.");
static_assert(internal::IsGarbageCollectedType<T>::value,
"T must be GarbageCollected or GarbageCollectedMixin type");
if (!t) {
return;
}
Visit(t, TraceTrait<T>::GetTraceDescriptor(t));
}
#if V8_ENABLE_CHECKS
V8_EXPORT void CheckObjectNotInConstruction(const void* address);
#endif // V8_ENABLE_CHECKS
friend class internal::VisitorBase;
template <typename T, typename WeaknessPolicy, typename LocationPolicy,
typename CheckingPolicy>
friend class internal::BasicPersistent;
};
} // namespace cppgc
#endif // INCLUDE_CPPGC_VISITOR_H_
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