Oilpan - Blink GC

Oilpan is a garbage collection system for Blink objects. This document explores the design, API and usage of the GC.

[TOC]

Overview

The general design of Oilpan is explained in the Oilpan README. This section focuses on the Blink specific extensions to that design.

Threading model

Oilpan assumes heaps are not shared among threads. Blink creates and uses a different heap and root set for each thread. Matching a thread to its relevant heap is maintained by blink::ThreadState.

Any object or Persistent (See Persistent, WeakPersistent, CrossThreadPersistent, CrossThreadWeakPersistent for details) that is allocated on a thread automatically belong to that thread's heap or root set.

Threads that want to allocate Oilpan objects must be attached to Oilpan, by calling either blink::ThreadState::AttachMainThread() or blink::ThreadState::AttachCurrentThread().

Heap partitioning

Blink assigns the following types to custom spaces in Oilpan's heap:

Objects that are a collection backing are allocated in one of the collection backing compactable custom spaces.
Objects that are a Node, a CSSValue or a LayoutObject are allocated in one of the typed custom spaces.

Mode of operation

Blink uses concurrent garbage collection (marking and sweeping), except for during thread termination and heap destruction.
When under memory pressure, Blink triggers a conservative GC.

Oilpan API reference

Base class templates

GarbageCollected

Declared in: third_party/blink/renderer/platform/heap/garbage_collected.h

A class that wants the lifetime of its instances to be managed by Oilpan must inherit from GarbageCollected<T>.

class YourClass : public GarbageCollected<YourClass> {
  // ...
};

Instances of such classes are said to be on Oilpan heap, or on heap for short, while instances of other classes are called off heap. In the rest of this document, the terms on heap, on-heap objects or garbage-collected objects are used to mean the objects on Oilpan heap instead of on normal (default) dynamic allocator's heap space.

Instances of a garbage-collected class can be created/allocated through MakeGarbageCollected<T>. Declaring a class as garbage collected disallows the use of operator new. It is not allowed to free garbage-collected object with delete, as Oilpan is responsible for deallocating the object once it determines the object is unreachable.

Garbage-collected objects may not be allocated on stack. See STACK_ALLOCATED and Heap collections for exceptions to this rule.

Garbage-collected class may need to have a tracing method. See Tracing for details.

Non-trivially destructible classes will be automatically finalized. Non-final classes that are not trivially destructible are required to have a virtual destructor. Trivially destructible classes should not have a destructor. Adding a destructor to such classes would make them non-trivially destructible and would hinder performance. Note that finalization is done at an arbitrary time after the object becomes unreachable. Any destructor executed within the finalization period must not touch any other on-heap object because destructors can be executed in any order.

GarbageCollected<T> or any class deriving from GarbageCollected<T>, directly or indirectly, must be the first element in its base class list (called "leftmost derivation rule"). This rule is needed to assure each on-heap object has its own canonical address.

class A : public GarbageCollected<A>, public P {
  // OK: GarbageCollected<A> is leftmost.
};

class B : public A, public Q {
  // OK: A is leftmost.
};

// class C : public R, public A {
//   BAD: A must be the first base class.
//        If R is also garbage collected, A should be [GarbageCollectedMixin](#GarbageCollectedMixin).
// };

If a non-leftmost base class needs to retain an on-heap object, that base class needs to inherit from GarbageCollectedMixin. It's generally recommended to make any non-leftmost base class inherit from GarbageCollectedMixin because it's dangerous to save a pointer to a non-leftmost non-GarbageCollectedMixin subclass of an on-heap object.

P* raw_pointer;
void someFunction(P* p) {
  ...
  raw_pointer = p;
  ...
}

class A : public GarbageCollected<A>, public P {
public:
  void someMemberFunction()
  {
    someFunction(this); // DANGEROUS, a raw pointer to an on-heap object. Object might be collected, resulting in a dangling pointer and possible memory corruption.
  }
};

GarbageCollectedMixin

Declared in: third_party/blink/renderer/platform/heap/garbage_collected.h

A non-leftmost base class of a garbage-collected class should derive from GarbageCollectedMixin.

A class deriving from GarbageCollectedMixin can be treated similarly as garbage-collected classes. Specifically, it can have Member<T>s and WeakMember<T>s, and a tracing method. A pointer to such a class must be retained in the same smart pointer wrappers as a pointer to a garbage-collected class, such as Member<T> or Persistent<T>. The tracing method of a garbage-collected class, if any, must contain a delegating call for each mixin base class.

class P : public GarbageCollectedMixin {
 public:
  // OK: Needs to trace q_.
  virtual void Trace(Visitor* visitor) const { visitor->Trace(q_); }
 private:
  // OK: Allowed to have Member<T>.
  Member<Q> q_;
};

class A final : public GarbageCollected<A>, public P {
 public:
  // Delegating call for P is needed.
  virtual void Trace(Visitor* visitor) const { P::Trace(visitor); }
};

A class that is a descendant of GarbageCollectedMixin but not a descendant of GarbageCollected<T> cannot be instantiated..

class P : public GarbageCollectedMixin { };
class Q : public GarbageCollectedMixin { };
class R : public Q { };

class A : public GarbageCollected<A>, public P, public R {
  // OK: Resolving pure virtual functions of P and R.
};

class B : public GarbageCollected<B>, public P {
  // OK: Different garbage-collected classes may inherit from the same mixin (P).
};

void foo() {
  MakeGarbageCollected<A>();    // OK: A can be instantiated.
  // MakeGarbageCollected<R>(); // BAD: R has pure virtual functions.
}

Class properties

USING_PRE_FINALIZER

Declared in: third_party/blink/renderer/platform/heap/prefinalizer.h

USING_PRE_FINALIZER(ClassName, FunctionName) in a class declaration declares the class has a pre-finalizer of name FunctionName. A pre-finalizer must have the function signature void() but can have any name.

A pre-finalizer is a user-defined member function of a garbage-collected class that is called when the object is going to be reclaimed. It is invoked before the sweeping phase starts to allow a pre-finalizer to touch any other on-heap objects which is forbidden from destructors. It is useful for doing cleanups that cannot be done with a destructor.

class YourClass : public GarbageCollected<YourClass> {
  USING_PRE_FINALIZER(YourClass, Dispose);
public:
  void Dispose() {
    // OK: Other on-heap objects can be touched in a pre-finalizer.
    other_->Dispose();
  }

  ~YourClass() {
    // BAD: Not allowed.
    // other_->Dispose();
  }

private:
  Member<OtherClass> other_;
};

Sometimes it is necessary to further delegate pre-finalizers up the class hierarchy, similar to how destructors destruct in reverse order wrt. to construction. It is possible to construct such delegations using virtual methods.

class Parent : public GarbageCollected<Parent> {
  USING_PRE_FINALIZER(Parent, Dispose);
 public:
  void Dispose() { DisposeImpl(); }

  virtual void DisposeImpl() {
    // Pre-finalizer for {Parent}.
  }
  // ...
};

class Child : public Parent {
 public:
  void DisposeImpl() {
    // Pre-finalizer for {Child}.
    Parent::DisposeImpl();
  }
  // ...
};

Alternatively, several classes in the same hierarchy may have pre-finalizers and these will also be executed in reverse order wrt. to construction.

class Parent : public GarbageCollected<Parent> {
  USING_PRE_FINALIZER(Parent, Dispose);
 public:
  void Dispose() {
    // Pre-finalizer for {Parent}.
  }
  // ...
};

class Child : public Parent {
  USING_PRE_FINALIZER(Child, Dispose);
 public:
  void Dispose() {
    // Pre-finalizer for {Child}.
  }
  // ...
};

Notes

Pre-finalizers are not allowed to resurrect objects (i.e. they are not allowed to relink dead objects into the object graph).
Pre-finalizers have some implications on the garbage collector's performance: the garbage-collector needs to iterate all registered pre-finalizers at every GC. Therefore, a pre-finalizer should be avoided unless it is really necessary. Especially, avoid defining a pre-finalizer in a class that might be allocated a lot.

DISALLOW_NEW

Declared in: third_party/blink/renderer/platform/wtf/allocator/allocator.h

Class-level annotation declaring the class cannot be separately allocated using operator new or MakeGarbageCollected<T>. It can be used on stack, as an embedded part of some other object, or as a value in a heap collection. If the class has Member<T> references, it needs a Trace() method, which must be called by the embedding object (e.g. the collection or the object that this class is a part of).

Classes with this annotation need a Trace() method, but should not inherit a garbage collected class.

STACK_ALLOCATED

Declared in: third_party/blink/renderer/platform/wtf/allocator/allocator.h

Class level annotation that should be used if the class is only stack allocated. Any fields holding garbage-collected objects should use raw pointers or references.

Classes with this annotation do not need to define a Trace() method as they are on the stack, and are automatically traced and kept alive should a conservative GC be required.

Stack allocated classes must not inherit a on-heap garbage collected class.

Marking a class as STACK_ALLOCATED implicitly implies DISALLOW_NEW, and thus disallow the use of operator new and MakeGarbageCollected<T>.

Handles

Class templates in this section are smart pointers, each carrying a pointer to an on-heap object (think of scoped_refptr<T> for RefCounted<T>). Collectively, they are called handles.

On-heap objects must be retained by any of these, depending on the situation.

Member, WeakMember

Declared in: third_party/blink/renderer/platform/heap/member.h

In a garbage-collected class, on-heap objects must be retained by Member<T> or WeakMember<T>, depending on the desired semantics.

Member<T> represents a strong reference to an object of type T, which means that the referred object is kept alive as long as the owner class instance is alive. Unlike scoped_refptr<T>, it is okay to form a reference cycle with members (in on-heap objects) and raw pointers (on stack).

WeakMember<T> is a weak reference to an object of type T. Unlike Member<T>, WeakMember<T> does not keep the pointed object alive. The pointer in a WeakMember<T> can become nullptr when the object gets garbage-collected. It may take some time for the pointer in a WeakMember<T> to become nullptr after the object actually becomes unreachable, because this rewrite is only done within Oilpan's garbage collection cycle.

class SomeGarbageCollectedClass : public GarbageCollected<SomeGarbageCollectedClass> {
  ...
private:
  Member<AnotherGarbageCollectedClass> another_; // OK, retained by Member<T>.
  WeakMember<AnotherGarbageCollectedClass> anotherWeak_; // OK, weak reference.
};

The use of WeakMember<T> incurs some overhead in garbage collector's performance. Use it sparingly. Usually, weak members are not necessary at all, because reference cycles with members are allowed.

More specifically, WeakMember<T> should be used only if the owner of a weak member can outlive the pointed object. Otherwise, Member<T> should be used.

It is required that every Member<T> and WeakMember<T> in a garbage-collected class be traced. See Tracing.

UntracedMember

Declared in: third_party/blink/renderer/platform/heap/member.h

UntracedMember<T> represents a reference to a garbage collected object which is ignored by Oilpan.

Unlike Member<T>, UntracedMember<T> will not keep an object alive. However, unlike WeakMember<T>, the reference will not be cleared (i.e. set to nullptr) if the referenced object dies. Furthermore, class fields of type UntracedMember<T> should not be traced by the class' tracing method.

Users should avoid using UntracedMember<T> in any case other than when implementing custom weakness semantics.

Persistent, WeakPersistent, CrossThreadPersistent, CrossThreadWeakPersistent

Declared in: third_party/blink/renderer/platform/heap/persistent.h

In a non-garbage-collected class, on-heap objects must be retained by Persistent<T>, WeakPersistent<T>, CrossThreadPersistent<T>, or CrossThreadWeakPersistent<T>, depending on the situations and the desired semantics.

Persistent<T> is the most basic handle in the persistent family, which makes the referred object alive unconditionally, as long as the persistent handle is alive.

WeakPersistent<T> does not keep the referred object alive, and becomes nullptr when the object gets garbage-collected, just like WeakMember<T>.

CrossThreadPersistent<T> and CrossThreadWeakPersistent<T> are cross-thread variants of Persistent<T> and WeakPersistent<T>, respectively, which can point to an object in a different thread (i.e. in a different heap).

#include "third_party/blink/renderer/platform/heap/persistent.h"
...
class NonGarbageCollectedClass {
  ...
private:
  Persistent<SomeGarbageCollectedClass> something_; // OK, the object will be alive while this persistent is alive.
};

Warning: Persistent<T> and CrossThreadPersistent<T> are vulnerable to reference cycles. If a reference cycle is formed with Persistents, Members, RefPtrs and OwnPtrs, all the objects in the cycle will leak, since no object in the cycle can be aware of whether they are ever referred to from outside the cycle.

Be careful not to create a reference cycle when adding a new persistent. If a cycle is inevitable, make sure the cycle is eventually cut by someone outside the cycle.

Persistents have a small unavoidable overhead because they require maintaining a list of all persistents. Therefore, creating or keeping a lot of persistents at once would have performance implications and should be avoided when possible.

Weak variants have overhead just like WeakMember<T>. Use them sparingly.

The need of cross-thread persistents may indicate a poor design in multi-thread object ownership. Think twice if they are really necessary.

Raw pointers

Using raw pointers to garbage-collected objects on-heap is forbidden, as they may cause memory corruptions. On-stack references to garbage-collected object (including function parameters and return types), on the other hand, must use raw pointers. This is the only case where raw pointers to on-heap objects should be used.

void someFunction() {
  SomeGarbageCollectedClass* object = MakeGarbageCollected<SomeGarbageCollectedClass>(); // OK, retained by a pointer.
  ...
}
// OK to leave the object behind. The Blink GC system will free it up when it becomes unused.

Tracing

A garbage-collected class is required to have a tracing method, which lists all the references to on-heap objects it has. The tracing method is called when the garbage collector needs to determine (1) all the on-heap objects referred from a live object, and (2) all the weak handles that may be filled with nullptr later.

The basic form of tracing is illustrated below:

// In a header file:
class SomeGarbageCollectedClass final
    : public GarbageCollected<SomeGarbageCollectedClass> {
 public:
  void Trace(Visitor*) const;

private:
  Member<AnotherGarbageCollectedClass> another_;
};

// In an implementation file:
void SomeGarbageCollectedClass::Trace(Visitor* visitor) const {
  visitor->Trace(another_);
}

Specifically, if a class needs a tracing method, declaring and defining a Trace(Visitor*) const method is required. This method is normally declared in the header file and defined once in the implementation file, but there are variations. A common variation is to declare a virtual Trace() for base classes that will be subclassed.

The function implementation must contain:

For each reference tp an on-heap object object in the class, a tracing call: visitor->Trace(object);.
For each embedded DISALLOW_NEW object object in the class, a tracing call: visitor->Trace(object);.
For each base class of the class BaseClass that is a descendant of GarbageCollected<T> or GarbageCollectedMixin, a delegation call to base class: BaseClass::Trace(visitor). It is recommended that the delegation call, if any, is put at the end of a tracing method.
See Advanced weak handling for implementing non-trivial weakness.

The following example shows more involved usage:

class A : public GarbageCollected<A> {
 public:
  virtual void Trace(Visitor*) const {} // Nothing to trace here.
};

class B : public A {
  // Nothing to trace here; exempted from having a tracing method.
};

class C final : public B {
 public:
  void Trace(Visitor*) const final;

 private:
  Member<X> x_;
  WeakMember<Y> y_;
  HeapVector<Member<Z>> z_;
};

void C::Trace(Visitor* visitor) const {
  visitor->Trace(x_);
  visitor->Trace(y_); // Weak member needs to be traced.
  visitor->Trace(z_); // Heap collection does, too.
  B::Trace(visitor); // Delegate to the parent. In this case it's empty, but this is required.
}

Generally, a Trace() method would be just a sequence of delegations of the form visitor->Trace(object); for fields or BaseClass::Trace(visitor) for base classes. It should avoid more complex logic such as branches or iterations. In case a branch is unavoidable, the branch condition should depend only on const fields of the object. In case an iteration is unavoidable, the number of iterations and the location of the buffer that is iterated should also depend only on const fields of the object. Using non-const fields in Trace() methods may cause data races and other issues in the GC.

Heap collections

Oilpan, like any other managed runtime library, provides basic support for collections that integrate its managed types Member<T> and WeakMember<T>. Do not use heap collection with persistent types (e.g. HeapVector<Persistent<T>>).

Collections compared to other libraries used in Blink:

stdlib	WTF	Oilpan
std::vector	WTF::Vector	blink::HeapVector
std::deque	WTF::Deque	blink::HeapDeque
std::unordered_map	WTF::HashMap	blink::HeapHashMap
std::unordered_set	WTF::HashSet	blink::HeapHashSet
-	WTF::LinkedHashSet	blink::HeapLinkedHashSet
-	WTF::HashCountedSet	blink::HeapHashCountedSet

These heap collections work mostly the same way as their stdlib or WTF collection counterparts but there are some things to keep in mind. Heap collections are regular heap objects and thus must be properly traced from a Trace method. They can also be inlined into other objects for performance reasons and are allowed to be directly used on stack.

class A final : public GarbageCollected<A> {
 public:
  void Trace(Visitor* visitor) const { visitor->Trace(vec_); }
 private:
  HeapVector<Member<B>> vec_;
};

Like any other object, they may be referred to from a non-garbage-collected class using Persistent.

class NonGCed final {
 private:
  Persistent<HeapVector<Member<B>>> vec_;
};

Advanced weak handling

In addition to basic weak handling using WeakMember<T> Oilpan also supports:

Weak collections
Custom weak callbacks

Weak collections

Like regular weakness, collections support weakness by putting references in WeakMember<T>.

In associative containers such as HeapHashMap or HeapHashSet Oilpan distinguishes between pure weakness and mixed weakness:

Pure weakness: All entries in such containers are wrapped in WeakMember<T>. Examples are HeapHashSet<WeakMember<T>> and HeapHashMap<WeakMember<T>, WeakMember<U>>.
Mixed weakness: Only some entries in such containers are wrapped in WeakMember<T>. This can only happen in HeapHashMap. Examples are HeapHashMap<WeakMember<T>, Member<U>>, HeapHashMap<Member<T>, WeakMember<U>>, HeapHashMap<WeakMember<T>, int>, and HeapHashMap<int, WeakMember<T>>. Note that in the last example the type int` is traced even though it does not support tracing.

The semantics then are as follows:

Pure weakness: Oilpan will automatically remove the entries from the container if any of its declared WeakMember<T> fields points to a dead object.
Mixed weakness: Oilpan applies ephemeron semantics meaning that the strong parts of an entry are only treated as strong if the WeakMember<T> fields point to a live object.

Weak references (e.g. WeakMember<T>) are not supported in sequential containers such as HeapVector or HeapDeque.

Custom weak callbacks

In case very specific weakness semantics are required Oilpan allows adding custom weakness callbacks through its tracing method.

There exist two helper methods on blink::Visitor to add such callbacks:

RegisterWeakCallback: Used to add custom weak callbacks of the form void(void*, const blink::LivenessBroker&).
RegisterWeakCallbackMethod: Helper for adding an instance method.

Note that custom weak callbacks should not be used to clear WeakMember<T> fields as such fields are automatically handled by Oilpan. Instead, for managed fields that require custom weakness semantics, users should wrap such fields in UntracedMember<T> indicating that Oilpan is ignoring those fields.

The following example shows how this can be used:

class W final : public GarbageCollected<W> {
 public:
  virtual void Trace(Visitor*) const;
 private:
  void ProcessCustomWeakness(const LivenessBroker&);

  UntracedMember<C> other_;
};

void W::Trace(Visitor* visitor) const {
  visitor->template RegisterCustomWeakMethod<W, &W::ProcessCustomWeakness>(this);
}

void W::ProcessCustomWeakness(const LivenessBroker& info) {
  if (info.IsHeapObjectAlive(other_)) {
    // Do something with other_.
  }
  other_ = nullptr;
}

Note that the custom weakness callback in this example is only executed if W is alive and properly traced. If W itself dies than the callback will not be executed. Operations that must always happen should instead go into destructors or pre-finalizers.

Traits helpers

Declared in: third_party/blink/renderer/platform/heap/heap_traits.h

At times, one may be working on code that needs to deal with both, off heap and on heap, objects. The following helpers can aid in writing code that needs to use different wrappers and containers based on whether a type is managed by Oilpan.

AddMemberIfNeeded

Given a type T, defines a type alias that is either Member<T> or T depending on whether T is a type managed by Oilpan.

class A final : public GarbageCollected<A> {};
class B final {};

AddMemberIfNeeded<B> v1;       // B v1;
AddMemberIfNeeded<int32_t> v2; // int32_t v2;
AddMemberIfNeeded<A> v3;       // Member<A> v3;

VectorOf

Given a type T, defines a type alias that is either HeapVector<T>, HeapVector<Member<T>> or Vector<T> based on the following rules:

T is a garbage collected type managed by Oilpan → HeapVector<Member<T>>
T has a Trace() method but is not managed by Oilpan → HeapVector<T> (this is a rare case; IDL unions and dictionaries fall in this category, for example)
All other cases → Vector<T>

class MyGarbageCollectedClass : public GarbageCollected<MyGarbageCollectedClass> {
  // ...
};

class MyNonGCButTraceableClass {
 public:
  void Trace(Visitor* visitor) const {
    // ...
  }
};

class MyNotGarbageCollectedClass {
  // ...
};

VectorOf<float> v1;                       // Vector<float> v1;
VectorOf<MyNotGarbageCollectedClass> v2;  // Vector<MyNotGarbageCollectedClass> v2;
VectorOf<MyNonGCButTraceableClass> v3;    // HeapVector<MyNonGCButTraceableClass> v3;
VectorOf<MyGarbageCollectedClass> v4;     // HeapVector<Member<MyGarbageCollectedClass>> v4;

VectorOfPairs<T, U>

Similar to VectorOf<T>, but defines a type alias that is either HeapVector<std::pair<V, X>> (where V is either T or Member<T> and X is either U or Member<U>) or Vector<std::pair<T, U>>.

In other words, if either T or U needs to be wrapped in a HeapVector instead of a Vector, VectorOfPairs will use a HeapVector<std::pair<>> and wrap them with Member<> appropriately. Otherwise, a Vector<std::pair<>> will be used.

class MyGarbageCollectedClass : public GarbageCollected<MyGarbageCollectedClass> {
  // ...
};

class MyNonGCButTraceableClass {
 public:
  void Trace(Visitor* visitor) const {
    // ...
  }
};

class MyNotGarbageCollectedClass {
  // ...
};

// Vector<std::pair<double, int8_t>> v1;
VectorOfPairs<double, int8_t> v1;

// Vector<std::pair<MyNotGarbageCollectedClass, String>> v2;
VectorOfPairs<MyNotGarbageCollectedClass, String> v2;

// HeapVector<std::pair<float, MyNonGCButTraceableClass>> v3;
VectorOfPairs<float, MyNonGCButTraceableClass> v3;

// HeapVector<std::pair<MyNonGCButTraceableClass, Member<MyGarbageCollectedClass>>> v4;
VectorOfPairs<MyNonGCButTraceableClass, MyGarbageCollectedClass> v4;

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BlinkGCAPIReference.md

BlinkGCAPIReference.md

Oilpan - Blink GC

Overview

Threading model

Heap partitioning

Mode of operation

Oilpan API reference

Base class templates

GarbageCollected

GarbageCollectedMixin

Class properties

USING_PRE_FINALIZER

DISALLOW_NEW

STACK_ALLOCATED

Handles

Member, WeakMember

UntracedMember

Persistent, WeakPersistent, CrossThreadPersistent, CrossThreadWeakPersistent

Raw pointers

Tracing

Heap collections

Advanced weak handling

Weak collections

Custom weak callbacks

Traits helpers

AddMemberIfNeeded

VectorOf

VectorOfPairs<T, U>

Files

BlinkGCAPIReference.md

Latest commit

History

BlinkGCAPIReference.md

File metadata and controls

Oilpan - Blink GC

Overview

Threading model

Heap partitioning

Mode of operation

Oilpan API reference

Base class templates

GarbageCollected

GarbageCollectedMixin

Class properties

USING_PRE_FINALIZER

DISALLOW_NEW

STACK_ALLOCATED

Handles

Member, WeakMember

UntracedMember

Persistent, WeakPersistent, CrossThreadPersistent, CrossThreadWeakPersistent

Raw pointers

Tracing

Heap collections

Advanced weak handling

Weak collections

Custom weak callbacks

Traits helpers

AddMemberIfNeeded

VectorOf

VectorOfPairs<T, U>