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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 <type_traits>
#include "cppgc/custom-space.h"
#include "cppgc/ephemeron-pair.h"
#include "cppgc/garbage-collected.h"
#include "cppgc/internal/logging.h"
#include "cppgc/internal/member-storage.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)
template <typename T>
void TraceMultiple(const subtle::UncompressedMember<T>* start, size_t len) {
static_assert(sizeof(T), "Pointee type must be fully defined.");
static_assert(internal::IsGarbageCollectedOrMixinType<T>::value,
"T must be GarbageCollected or GarbageCollectedMixin type");
VisitMultipleUncompressedMember(start, len,
&TraceTrait<T>::GetTraceDescriptor);
}
template <typename T,
std::enable_if_t<!std::is_same_v<
Member<T>, subtle::UncompressedMember<T>>>* = nullptr>
void TraceMultiple(const Member<T>* start, size_t len) {
static_assert(sizeof(T), "Pointee type must be fully defined.");
static_assert(internal::IsGarbageCollectedOrMixinType<T>::value,
"T must be GarbageCollected or GarbageCollectedMixin type");
#if defined(CPPGC_POINTER_COMPRESSION)
static_assert(std::is_same_v<Member<T>, subtle::CompressedMember<T>>,
"Member and CompressedMember must be the same.");
VisitMultipleCompressedMember(start, len,
&TraceTrait<T>::GetTraceDescriptor);
#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);
}
template <typename T>
void TraceMultiple(const T* start, size_t len) {
#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(start);
#endif // V8_ENABLE_CHECKS
for (size_t i = 0; i < len; ++i) {
const T* object = &start[i];
if constexpr (std::is_polymorphic_v<T>) {
// The object's vtable may be uninitialized in which case the object is
// not traced.
if (*reinterpret_cast<const uintptr_t*>(object) == 0) continue;
}
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**) {}
virtual void VisitMultipleUncompressedMember(
const void* start, size_t len,
TraceDescriptorCallback get_trace_descriptor) {
// Default implementation merely delegates to Visit().
const char* it = static_cast<const char*>(start);
const char* end = it + len * internal::kSizeOfUncompressedMember;
for (; it < end; it += internal::kSizeOfUncompressedMember) {
const auto* current = reinterpret_cast<const internal::RawPointer*>(it);
const void* object = current->LoadAtomic();
if (!object) continue;
Visit(object, get_trace_descriptor(object));
}
}
#if defined(CPPGC_POINTER_COMPRESSION)
virtual void VisitMultipleCompressedMember(
const void* start, size_t len,
TraceDescriptorCallback get_trace_descriptor) {
// Default implementation merely delegates to Visit().
const char* it = static_cast<const char*>(start);
const char* end = it + len * internal::kSizeofCompressedMember;
for (; it < end; it += internal::kSizeofCompressedMember) {
const auto* current =
reinterpret_cast<const internal::CompressedPointer*>(it);
const void* object = current->LoadAtomic();
if (!object) continue;
Visit(object, get_trace_descriptor(object));
}
}
#endif // defined(CPPGC_POINTER_COMPRESSION)
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);
if (!info.IsHeapObjectAlive(weak->GetFromGC())) {
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);
if (!info.IsHeapObjectAlive(weak->GetFromGC())) {
weak->ClearFromGC();
}
}
};
} // namespace internal
} // namespace cppgc
#endif // INCLUDE_CPPGC_VISITOR_H_