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// Copyright 2021 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef THIRD_PARTY_BLINK_RENDERER_PLATFORM_HEAP_TRACE_TRAITS_H_
#define THIRD_PARTY_BLINK_RENDERER_PLATFORM_HEAP_TRACE_TRAITS_H_
#include <tuple>
#include "base/notreached.h"
#include "third_party/blink/renderer/platform/heap/garbage_collected.h"
#include "third_party/blink/renderer/platform/heap/member.h"
#include "third_party/blink/renderer/platform/heap/visitor.h"
#include "third_party/blink/renderer/platform/wtf/hash_table.h"
#include "third_party/blink/renderer/platform/wtf/key_value_pair.h"
#include "third_party/blink/renderer/platform/wtf/type_traits.h"
#include "v8/include/cppgc/trace-trait.h"
namespace blink {
template <typename T>
struct TraceIfNeeded {
STATIC_ONLY(TraceIfNeeded);
static void Trace(Visitor* visitor, const T& t) {
if constexpr (WTF::IsTraceable<T>::value) {
visitor->Trace(t);
}
}
};
// `WTF::IsWeak<typename Traits::TraitType>::value` is always false when used
// from vectors (on and off the GCed heap).
template <WTF::WeakHandlingFlag weakness,
typename T,
typename Traits,
bool = WTF::IsTraceable<typename Traits::TraitType>::value &&
!WTF::IsWeak<typename Traits::TraitType>::value,
WTF::WeakHandlingFlag = WTF::kWeakHandlingTrait<T>>
struct TraceCollectionIfEnabled;
template <WTF::WeakHandlingFlag weakness, typename T, typename Traits>
struct TraceCollectionIfEnabled<weakness,
T,
Traits,
false,
WTF::kNoWeakHandling> {
STATIC_ONLY(TraceCollectionIfEnabled);
static bool IsAlive(const blink::LivenessBroker& info, const T&) {
return true;
}
static void Trace(Visitor*, const void*) {
static_assert(!WTF::IsTraceable<typename Traits::TraitType>::value ||
WTF::IsWeak<typename Traits::TraitType>::value,
"T should not be traced");
}
};
template <typename T, typename Traits>
struct TraceCollectionIfEnabled<WTF::kNoWeakHandling,
T,
Traits,
false,
WTF::kWeakHandling> {
STATIC_ONLY(TraceCollectionIfEnabled);
static void Trace(Visitor* visitor, const void* t) {
WTF::TraceInCollectionTrait<WTF::kNoWeakHandling, T, Traits>::Trace(
visitor, *reinterpret_cast<const T*>(t));
}
};
template <WTF::WeakHandlingFlag weakness,
typename T,
typename Traits,
bool,
WTF::WeakHandlingFlag>
struct TraceCollectionIfEnabled {
STATIC_ONLY(TraceCollectionIfEnabled);
static bool IsAlive(const blink::LivenessBroker& info, const T& traceable) {
return WTF::TraceInCollectionTrait<weakness, T, Traits>::IsAlive(info,
traceable);
}
static void Trace(Visitor* visitor, const void* t) {
static_assert((WTF::IsTraceable<typename Traits::TraitType>::value &&
!WTF::IsWeak<typename Traits::TraitType>::value) ||
weakness == WTF::kWeakHandling,
"Traits should be traced");
WTF::TraceInCollectionTrait<weakness, T, Traits>::Trace(
visitor, *reinterpret_cast<const T*>(t));
}
};
namespace internal {
// Helper for processing ephemerons represented as KeyValuePair. Reorders
// parameters if needed so that KeyType is always weak.
template <typename _KeyType,
typename _ValueType,
typename _KeyTraits,
typename _ValueTraits,
bool = WTF::IsWeak<_ValueType>::value>
struct EphemeronKeyValuePair {
STACK_ALLOCATED();
public:
using KeyType = _KeyType;
using ValueType = _ValueType;
using KeyTraits = _KeyTraits;
using ValueTraits = _ValueTraits;
// Ephemerons have different weakness for KeyType and ValueType. If weakness
// is equal, we either have Strong/Strong, or Weak/Weak, which would indicate
// a full strong or fully weak pair.
static constexpr bool kNeedsEphemeronSemantics =
WTF::IsWeak<KeyType>::value != WTF::IsWeak<ValueType>::value &&
WTF::IsTraceable<ValueType>::value;
static_assert(!WTF::IsWeak<KeyType>::value ||
WTF::IsWeakMemberType<KeyType>::value,
"Weakness must be encoded using WeakMember.");
static_assert(!WTF::IsWeak<ValueType>::value ||
WTF::IsWeakMemberType<ValueType>::value,
"Weakness must be encoded using WeakMember.");
EphemeronKeyValuePair(const KeyType& k, const ValueType& v)
: key(k), value(v) {}
const KeyType& key;
const ValueType& value;
};
template <typename _KeyType,
typename _ValueType,
typename _KeyTraits,
typename _ValueTraits>
struct EphemeronKeyValuePair<_KeyType,
_ValueType,
_KeyTraits,
_ValueTraits,
true> : EphemeronKeyValuePair<_ValueType,
_KeyType,
_ValueTraits,
_KeyTraits,
false> {
EphemeronKeyValuePair(const _KeyType& k, const _ValueType& v)
: EphemeronKeyValuePair<_ValueType,
_KeyType,
_ValueTraits,
_KeyTraits,
false>(v, k) {}
};
template <WTF::WeakHandlingFlag WeakHandling,
typename Key,
typename Value,
typename Traits>
struct KeyValuePairInCollectionTrait {
static bool IsAlive(const blink::LivenessBroker& info,
const WTF::KeyValuePair<Key, Value>& kvp) {
// Needed for Weak/Weak, Strong/Weak (reverse ephemeron), and Weak/Strong
// (ephemeron). Order of invocation does not matter as `IsAlive()` does not
// have any side effects.
return blink::TraceCollectionIfEnabled<
WTF::kWeakHandlingTrait<Key>, Key,
typename Traits::KeyTraits>::IsAlive(info, kvp.key) &&
blink::TraceCollectionIfEnabled<
WTF::kWeakHandlingTrait<Value>, Value,
typename Traits::ValueTraits>::IsAlive(info, kvp.value);
}
static void Trace(blink::Visitor* visitor,
const Key* key,
const Value* value) {
TraceImpl::Trace(visitor, key, value);
}
static void Trace(blink::Visitor* visitor,
const WTF::KeyValuePair<Key, Value>& kvp) {
TraceImpl::Trace(visitor, &kvp.key, &kvp.value);
}
private:
using EphemeronHelper = EphemeronKeyValuePair<Key,
Value,
typename Traits::KeyTraits,
typename Traits::ValueTraits>;
struct WeakTrait {
static void Trace(blink::Visitor* visitor,
const Key* key,
const Value* value) {
// Strongification of ephemerons, i.e., Weak/Strong and Strong/Weak.
// The helper ensures that helper.key always refers to the weak part and
// helper.value always refers to the dependent part.
// We distinguish ephemeron from Weak/Weak and Strong/Strong to allow
// users to override visitation behavior. An example is creating a heap
// snapshot, where it is useful to annotate values as being kept alive
// from keys rather than the table.
EphemeronHelper helper(*key, *value);
if (WeakHandling == WTF::kNoWeakHandling) {
// Strongify the weak part.
blink::TraceCollectionIfEnabled<
WTF::kNoWeakHandling, typename EphemeronHelper::KeyType,
typename EphemeronHelper::KeyTraits>::Trace(visitor, &helper.key);
}
// The following passes on kNoWeakHandling for tracing value as the value
// callback is only invoked to keep value alive iff key is alive,
// following ephemeron semantics.
visitor->TraceEphemeron(helper.key, &helper.value);
}
};
struct StrongTrait {
static void Trace(blink::Visitor* visitor,
const Key* key,
const Value* value) {
// Strongification of non-ephemeron KVP, i.e., Strong/Strong or Weak/Weak.
// Order does not matter here.
blink::TraceCollectionIfEnabled<
WTF::kNoWeakHandling, Key, typename Traits::KeyTraits>::Trace(visitor,
key);
blink::TraceCollectionIfEnabled<
WTF::kNoWeakHandling, Value,
typename Traits::ValueTraits>::Trace(visitor, value);
}
};
using TraceImpl =
typename std::conditional<EphemeronHelper::kNeedsEphemeronSemantics,
WeakTrait,
StrongTrait>::type;
};
} // namespace internal
} // namespace blink
namespace WTF {
// Trait for strong treatment of KeyValuePair. This is used to handle regular
// KVP but also for strongification of otherwise weakly handled KVPs.
template <typename Key, typename Value, typename Traits>
struct TraceInCollectionTrait<kNoWeakHandling, KeyValuePair<Key, Value>, Traits>
: public blink::internal::
KeyValuePairInCollectionTrait<kNoWeakHandling, Key, Value, Traits> {};
template <typename Key, typename Value, typename Traits>
struct TraceInCollectionTrait<kWeakHandling, KeyValuePair<Key, Value>, Traits>
: public blink::internal::
KeyValuePairInCollectionTrait<kWeakHandling, Key, Value, Traits> {};
// Catch-all for types that have a way to trace that don't have special
// handling for weakness in collections. This means that if this type
// contains WeakMember fields, they will simply be zeroed, but the entry
// will not be removed from the collection. This always happens for
// things in vectors, which don't currently support special handling of
// weak elements.
template <typename T, typename Traits>
struct TraceInCollectionTrait<kNoWeakHandling, T, Traits> {
static bool IsAlive(const blink::LivenessBroker& info, const T& t) {
return true;
}
static void Trace(blink::Visitor* visitor, const T& t) {
static_assert(WTF::IsTraceable<typename Traits::TraitType>::value &&
!WTF::IsWeak<typename Traits::TraitType>::value,
"T should be traceable");
visitor->Trace(t);
}
};
template <typename T, typename Traits>
struct TraceInCollectionTrait<kNoWeakHandling, blink::WeakMember<T>, Traits> {
static void Trace(blink::Visitor* visitor, const blink::WeakMember<T>& t) {
// Extract raw pointer to avoid using the WeakMember<> overload in Visitor.
visitor->TraceStrongly(t);
}
};
// Catch-all for types that have HashTrait support for tracing with weakness.
// Empty to enforce specialization.
template <typename T, typename Traits>
struct TraceInCollectionTrait<kWeakHandling, T, Traits> {};
template <typename T, typename Traits>
struct TraceInCollectionTrait<kWeakHandling, blink::WeakMember<T>, Traits> {
static bool IsAlive(const blink::LivenessBroker& info,
const blink::WeakMember<T>& value) {
return info.IsHeapObjectAlive(value);
}
};
} // namespace WTF
namespace cppgc {
// This trace trait for std::pair will clear WeakMember if their referent is
// collected. If you have a collection that contain weakness it does not remove
// entries from the collection that contain nulled WeakMember.
template <typename T, typename U>
struct TraceTrait<std::pair<T, U>> {
STATIC_ONLY(TraceTrait);
public:
static TraceDescriptor GetTraceDescriptor(const void* self) {
// The following code should never be reached as tracing through std::pair
// should always happen eagerly by directly invoking `Trace()` below. This
// happens e.g. when being used in HeapVector<std::pair<...>>.
NOTREACHED();
}
static void Trace(Visitor* visitor, const std::pair<T, U>* pair) {
blink::TraceIfNeeded<U>::Trace(visitor, pair->second);
blink::TraceIfNeeded<T>::Trace(visitor, pair->first);
}
};
} // namespace cppgc
#endif // THIRD_PARTY_BLINK_RENDERER_PLATFORM_HEAP_TRACE_TRAITS_H_
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