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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
#ifndef gc_Barrier_h
#define gc_Barrier_h
#include <type_traits> // std::true_type
#include "NamespaceImports.h"
#include "gc/Cell.h"
#include "gc/GCContext.h"
#include "gc/StoreBuffer.h"
#include "js/ComparisonOperators.h" // JS::detail::DefineComparisonOps
#include "js/experimental/TypedData.h" // js::EnableIfABOVType
#include "js/HeapAPI.h"
#include "js/Id.h"
#include "js/RootingAPI.h"
#include "js/Value.h"
#include "util/Poison.h"
/*
* [SMDOC] GC Barriers
*
* Several kinds of barrier are necessary to allow the GC to function correctly.
* These are triggered by reading or writing to GC pointers in the heap and
* serve to tell the collector about changes to the graph of reachable GC
* things.
*
* Since it would be awkward to change every write to memory into a function
* call, this file contains a bunch of C++ classes and templates that use
* operator overloading to take care of barriers automatically. In most cases,
* all that's necessary is to replace:
*
* Type* field;
*
* with:
*
* HeapPtr<Type> field;
*
* All heap-based GC pointers and tagged pointers must use one of these classes,
* except in a couple of exceptional cases.
*
* These classes are designed to be used by the internals of the JS engine.
* Barriers designed to be used externally are provided in js/RootingAPI.h.
*
* Overview
* ========
*
* This file implements the following concrete classes:
*
* HeapPtr General wrapper for heap-based pointers that provides pre- and
* post-write barriers. Most clients should use this.
*
* GCPtr An optimisation of HeapPtr for objects which are only destroyed
* by GC finalization (this rules out use in Vector, for example).
*
* PreBarriered Provides a pre-barrier but not a post-barrier. Necessary when
* generational GC updates are handled manually, e.g. for hash
* table keys that don't use StableCellHasher.
*
* HeapSlot Provides pre and post-barriers, optimised for use in JSObject
* slots and elements.
*
* WeakHeapPtr Provides read and post-write barriers, for use with weak
* pointers.
*
* UnsafeBarePtr Provides no barriers. Don't add new uses of this, or only if
* you really know what you are doing.
*
* The following classes are implemented in js/RootingAPI.h (in the JS
* namespace):
*
* Heap General wrapper for external clients. Like HeapPtr but also
* handles cycle collector concerns. Most external clients should
* use this.
*
* Heap::Tenured Like Heap but doesn't allow nursery pointers. Allows storing
* flags in unused lower bits of the pointer.
*
* Which class to use?
* -------------------
*
* Answer the following questions to decide which barrier class is right for
* your use case:
*
* Is your code part of the JS engine?
* Yes, it's internal =>
* Is your pointer weak or strong?
* Strong =>
* Do you want automatic handling of nursery pointers?
* Yes, of course =>
* Can your object be destroyed outside of a GC?
* Yes => Use HeapPtr<T>
* No => Use GCPtr<T> (optimization)
* No, I'll do this myself =>
* Do you want pre-barriers so incremental marking works?
* Yes, of course => Use PreBarriered<T>
* No, and I'll fix all the bugs myself => Use UnsafeBarePtr<T>
* Weak => Use WeakHeapPtr<T>
* No, it's external =>
* Can your pointer refer to nursery objects?
* Yes => Use JS::Heap<T>
* Never => Use JS::Heap::Tenured<T> (optimization)
*
* If in doubt, use HeapPtr<T>.
*
* Write barriers
* ==============
*
* A write barrier is a mechanism used by incremental or generational GCs to
* ensure that every value that needs to be marked is marked. In general, the
* write barrier should be invoked whenever a write can cause the set of things
* traced through by the GC to change. This includes:
*
* - writes to object properties
* - writes to array slots
* - writes to fields like JSObject::shape_ that we trace through
* - writes to fields in private data
* - writes to non-markable fields like JSObject::private that point to
* markable data
*
* The last category is the trickiest. Even though the private pointer does not
* point to a GC thing, changing the private pointer may change the set of
* objects that are traced by the GC. Therefore it needs a write barrier.
*
* Every barriered write should have the following form:
*
* <pre-barrier>
* obj->field = value; // do the actual write
* <post-barrier>
*
* The pre-barrier is used for incremental GC and the post-barrier is for
* generational GC.
*
* Pre-write barrier
* -----------------
*
* To understand the pre-barrier, let's consider how incremental GC works. The
* GC itself is divided into "slices". Between each slice, JS code is allowed to
* run. Each slice should be short so that the user doesn't notice the
* interruptions. In our GC, the structure of the slices is as follows:
*
* 1. ... JS work, which leads to a request to do GC ...
* 2. [first GC slice, which performs all root marking and (maybe) more marking]
* 3. ... more JS work is allowed to run ...
* 4. [GC mark slice, which runs entirely in
* GCRuntime::markUntilBudgetExhausted]
* 5. ... more JS work ...
* 6. [GC mark slice, which runs entirely in
* GCRuntime::markUntilBudgetExhausted]
* 7. ... more JS work ...
* 8. [GC marking finishes; sweeping done non-incrementally; GC is done]
* 9. ... JS continues uninterrupted now that GC is finishes ...
*
* Of course, there may be a different number of slices depending on how much
* marking is to be done.
*
* The danger inherent in this scheme is that the JS code in steps 3, 5, and 7
* might change the heap in a way that causes the GC to collect an object that
* is actually reachable. The write barrier prevents this from happening. We use
* a variant of incremental GC called "snapshot at the beginning." This approach
* guarantees the invariant that if an object is reachable in step 2, then we
* will mark it eventually. The name comes from the idea that we take a
* theoretical "snapshot" of all reachable objects in step 2; all objects in
* that snapshot should eventually be marked. (Note that the write barrier
* verifier code takes an actual snapshot.)
*
* The basic correctness invariant of a snapshot-at-the-beginning collector is
* that any object reachable at the end of the GC (step 9) must either:
* (1) have been reachable at the beginning (step 2) and thus in the snapshot
* (2) or must have been newly allocated, in steps 3, 5, or 7.
* To deal with case (2), any objects allocated during an incremental GC are
* automatically marked black.
*
* This strategy is actually somewhat conservative: if an object becomes
* unreachable between steps 2 and 8, it would be safe to collect it. We won't,
* mainly for simplicity. (Also, note that the snapshot is entirely
* theoretical. We don't actually do anything special in step 2 that we wouldn't
* do in a non-incremental GC.
*
* It's the pre-barrier's job to maintain the snapshot invariant. Consider the
* write "obj->field = value". Let the prior value of obj->field be
* value0. Since it's possible that value0 may have been what obj->field
* contained in step 2, when the snapshot was taken, the barrier marks
* value0. Note that it only does this if we're in the middle of an incremental
* GC. Since this is rare, the cost of the write barrier is usually just an
* extra branch.
*
* In practice, we implement the pre-barrier differently based on the type of
* value0. E.g., see JSObject::preWriteBarrier, which is used if obj->field is
* a JSObject*. It takes value0 as a parameter.
*
* Post-write barrier
* ------------------
*
* For generational GC, we want to be able to quickly collect the nursery in a
* minor collection. Part of the way this is achieved is to only mark the
* nursery itself; tenured things, which may form the majority of the heap, are
* not traced through or marked. This leads to the problem of what to do about
* tenured objects that have pointers into the nursery: if such things are not
* marked, they may be discarded while there are still live objects which
* reference them. The solution is to maintain information about these pointers,
* and mark their targets when we start a minor collection.
*
* The pointers can be thought of as edges in an object graph, and the set of
* edges from the tenured generation into the nursery is known as the remembered
* set. Post barriers are used to track this remembered set.
*
* Whenever a slot which could contain such a pointer is written, we check
* whether the pointed-to thing is in the nursery (if storeBuffer() returns a
* buffer). If so we add the cell into the store buffer, which is the
* collector's representation of the remembered set. This means that when we
* come to do a minor collection we can examine the contents of the store buffer
* and mark any edge targets that are in the nursery.
*
* Read barriers
* =============
*
* Weak pointer read barrier
* -------------------------
*
* Weak pointers must have a read barrier to prevent the referent from being
* collected if it is read after the start of an incremental GC.
*
* The problem happens when, during an incremental GC, some code reads a weak
* pointer and writes it somewhere on the heap that has been marked black in a
* previous slice. Since the weak pointer will not otherwise be marked and will
* be swept and finalized in the last slice, this will leave the pointer just
* written dangling after the GC. To solve this, we immediately mark black all
* weak pointers that get read between slices so that it is safe to store them
* in an already marked part of the heap, e.g. in Rooted.
*
* Cycle collector read barrier
* ----------------------------
*
* Heap pointers external to the engine may be marked gray. The JS API has an
* invariant that no gray pointers may be passed, and this maintained by a read
* barrier that calls ExposeGCThingToActiveJS on such pointers. This is
* implemented by JS::Heap<T> in js/RootingAPI.h.
*
* Implementation Details
* ======================
*
* One additional note: not all object writes need to be pre-barriered. Writes
* to newly allocated objects do not need a pre-barrier. In these cases, we use
* the "obj->field.init(value)" method instead of "obj->field = value". We use
* the init naming idiom in many places to signify that a field is being
* assigned for the first time.
*
* This file implements the following hierarchy of classes:
*
* BarrieredBase base class of all barriers
* | |
* | WriteBarriered base class which provides common write operations
* | | | | |
* | | | | PreBarriered provides pre-barriers only
* | | | |
* | | | GCPtr provides pre- and post-barriers
* | | |
* | | HeapPtr provides pre- and post-barriers; is relocatable
* | | and deletable for use inside C++ managed memory
* | |
* | HeapSlot similar to GCPtr, but tailored to slots storage
* |
* ReadBarriered base class which provides common read operations
* |
* WeakHeapPtr provides read barriers only
*
*
* The implementation of the barrier logic is implemented in the
* Cell/TenuredCell base classes, which are called via:
*
* WriteBarriered<T>::pre
* -> InternalBarrierMethods<T*>::preBarrier
* -> Cell::preWriteBarrier
* -> InternalBarrierMethods<Value>::preBarrier
* -> InternalBarrierMethods<jsid>::preBarrier
* -> InternalBarrierMethods<T*>::preBarrier
* -> Cell::preWriteBarrier
*
* GCPtr<T>::post and HeapPtr<T>::post
* -> InternalBarrierMethods<T*>::postBarrier
* -> gc::PostWriteBarrierImpl
* -> InternalBarrierMethods<Value>::postBarrier
* -> StoreBuffer::put
*
* Barriers for use outside of the JS engine call into the same barrier
* implementations at InternalBarrierMethods<T>::post via an indirect call to
* Heap(.+)WriteBarriers.
*
* These clases are designed to be used to wrap GC thing pointers or values that
* act like them (i.e. JS::Value and jsid). It is possible to use them for
* other types by supplying the necessary barrier implementations but this
* is not usually necessary and should be done with caution.
*/
namespace js {
class NativeObject;
namespace gc {
inline void ValueReadBarrier(const Value& v) {
MOZ_ASSERT(v.isGCThing());
ReadBarrierImpl(v.toGCThing());
}
inline void ValuePreWriteBarrier(const Value& v) {
MOZ_ASSERT(v.isGCThing());
PreWriteBarrierImpl(v.toGCThing());
}
inline void IdPreWriteBarrier(jsid id) {
MOZ_ASSERT(id.isGCThing());
PreWriteBarrierImpl(&id.toGCThing()->asTenured());
}
inline void CellPtrPreWriteBarrier(JS::GCCellPtr thing) {
MOZ_ASSERT(thing);
PreWriteBarrierImpl(thing.asCell());
}
inline void WasmAnyRefPreWriteBarrier(const wasm::AnyRef& v) {
MOZ_ASSERT(v.isGCThing());
PreWriteBarrierImpl(v.toGCThing());
}
} // namespace gc
#ifdef DEBUG
bool CurrentThreadIsTouchingGrayThings();
bool IsMarkedBlack(JSObject* obj);
#endif
template <typename T, typename Enable = void>
struct InternalBarrierMethods {};
template <typename T>
struct InternalBarrierMethods<T*> {
static_assert(std::is_base_of_v<gc::Cell, T>, "Expected a GC thing type");
static bool isMarkable(const T* v) { return v != nullptr; }
static void preBarrier(T* v) { gc::PreWriteBarrier(v); }
static void postBarrier(T** vp, T* prev, T* next) {
gc::PostWriteBarrier(vp, prev, next);
}
static void readBarrier(T* v) { gc::ReadBarrier(v); }
#ifdef DEBUG
static void assertThingIsNotGray(T* v) { return T::assertThingIsNotGray(v); }
#endif
};
namespace gc {
MOZ_ALWAYS_INLINE void ValuePostWriteBarrier(Value* vp, const Value& prev,
const Value& next) {
MOZ_ASSERT(!CurrentThreadIsIonCompiling());
MOZ_ASSERT(vp);
// If the target needs an entry, add it.
js::gc::StoreBuffer* sb;
if (next.isGCThing() && (sb = next.toGCThing()->storeBuffer())) {
// If we know that the prev has already inserted an entry, we can
// skip doing the lookup to add the new entry. Note that we cannot
// safely assert the presence of the entry because it may have been
// added via a different store buffer.
if (prev.isGCThing() && prev.toGCThing()->storeBuffer()) {
return;
}
sb->putValue(vp);
return;
}
// Remove the prev entry if the new value does not need it.
if (prev.isGCThing() && (sb = prev.toGCThing()->storeBuffer())) {
sb->unputValue(vp);
}
}
} // namespace gc
template <>
struct InternalBarrierMethods<Value> {
static bool isMarkable(const Value& v) { return v.isGCThing(); }
static void preBarrier(const Value& v) {
if (v.isGCThing()) {
gc::ValuePreWriteBarrier(v);
}
}
static MOZ_ALWAYS_INLINE void postBarrier(Value* vp, const Value& prev,
const Value& next) {
gc::ValuePostWriteBarrier(vp, prev, next);
}
static void readBarrier(const Value& v) {
if (v.isGCThing()) {
gc::ValueReadBarrier(v);
}
}
#ifdef DEBUG
static void assertThingIsNotGray(const Value& v) {
JS::AssertValueIsNotGray(v);
}
#endif
};
template <>
struct InternalBarrierMethods<jsid> {
static bool isMarkable(jsid id) { return id.isGCThing(); }
static void preBarrier(jsid id) {
if (id.isGCThing()) {
gc::IdPreWriteBarrier(id);
}
}
static void postBarrier(jsid* idp, jsid prev, jsid next) {}
#ifdef DEBUG
static void assertThingIsNotGray(jsid id) { JS::AssertIdIsNotGray(id); }
#endif
};
// Specialization for JS::ArrayBufferOrView subclasses.
template <typename T>
struct InternalBarrierMethods<T, EnableIfABOVType<T>> {
using BM = BarrierMethods<T>;
static bool isMarkable(const T& thing) { return bool(thing); }
static void preBarrier(const T& thing) {
gc::PreWriteBarrier(thing.asObjectUnbarriered());
}
static void postBarrier(T* tp, const T& prev, const T& next) {
BM::postWriteBarrier(tp, prev, next);
}
static void readBarrier(const T& thing) { BM::readBarrier(thing); }
#ifdef DEBUG
static void assertThingIsNotGray(const T& thing) {
JSObject* obj = thing.asObjectUnbarriered();
if (obj) {
JS::AssertValueIsNotGray(JS::ObjectValue(*obj));
}
}
#endif
};
template <typename T>
static inline void AssertTargetIsNotGray(const T& v) {
#ifdef DEBUG
if (!CurrentThreadIsTouchingGrayThings()) {
InternalBarrierMethods<T>::assertThingIsNotGray(v);
}
#endif
}
// Base class of all barrier types.
//
// This is marked non-memmovable since post barriers added by derived classes
// can add pointers to class instances to the store buffer.
template <typename T>
class MOZ_NON_MEMMOVABLE BarrieredBase {
protected:
// BarrieredBase is not directly instantiable.
explicit BarrieredBase(const T& v) : value(v) {}
// BarrieredBase subclasses cannot be copy constructed by default.
BarrieredBase(const BarrieredBase<T>& other) = default;
// Storage for all barrier classes. |value| must be a GC thing reference
// type: either a direct pointer to a GC thing or a supported tagged
// pointer that can reference GC things, such as JS::Value or jsid. Nested
// barrier types are NOT supported. See assertTypeConstraints.
T value;
public:
using ElementType = T;
// Note: this is public because C++ cannot friend to a specific template
// instantiation. Friending to the generic template leads to a number of
// unintended consequences, including template resolution ambiguity and a
// circular dependency with Tracing.h.
T* unbarrieredAddress() const { return const_cast<T*>(&value); }
};
// Base class for barriered pointer types that intercept only writes.
template <class T>
class WriteBarriered : public BarrieredBase<T>,
public WrappedPtrOperations<T, WriteBarriered<T>> {
protected:
using BarrieredBase<T>::value;
// WriteBarriered is not directly instantiable.
explicit WriteBarriered(const T& v) : BarrieredBase<T>(v) {}
public:
DECLARE_POINTER_CONSTREF_OPS(T);
// Use this if the automatic coercion to T isn't working.
const T& get() const { return this->value; }
// Use this if you want to change the value without invoking barriers.
// Obviously this is dangerous unless you know the barrier is not needed.
void unbarrieredSet(const T& v) { this->value = v; }
// For users who need to manually barrier the raw types.
static void preWriteBarrier(const T& v) {
InternalBarrierMethods<T>::preBarrier(v);
}
protected:
void pre() { InternalBarrierMethods<T>::preBarrier(this->value); }
MOZ_ALWAYS_INLINE void post(const T& prev, const T& next) {
InternalBarrierMethods<T>::postBarrier(&this->value, prev, next);
}
};
#define DECLARE_POINTER_ASSIGN_AND_MOVE_OPS(Wrapper, T) \
DECLARE_POINTER_ASSIGN_OPS(Wrapper, T) \
Wrapper<T>& operator=(Wrapper<T>&& other) noexcept { \
setUnchecked(other.release()); \
return *this; \
}
/*
* PreBarriered only automatically handles pre-barriers. Post-barriers must be
* manually implemented when using this class. GCPtr and HeapPtr should be used
* in all cases that do not require explicit low-level control of moving
* behavior.
*
* This class is useful for example for HashMap keys where automatically
* updating a moved nursery pointer would break the hash table.
*/
template <class T>
class PreBarriered : public WriteBarriered<T> {
public:
PreBarriered() : WriteBarriered<T>(JS::SafelyInitialized<T>::create()) {}
/*
* Allow implicit construction for use in generic contexts.
*/
MOZ_IMPLICIT PreBarriered(const T& v) : WriteBarriered<T>(v) {}
explicit PreBarriered(const PreBarriered<T>& other)
: WriteBarriered<T>(other.value) {}
PreBarriered(PreBarriered<T>&& other) noexcept
: WriteBarriered<T>(other.release()) {}
~PreBarriered() { this->pre(); }
void init(const T& v) { this->value = v; }
/* Use to set the pointer to nullptr. */
void clear() { set(JS::SafelyInitialized<T>::create()); }
DECLARE_POINTER_ASSIGN_AND_MOVE_OPS(PreBarriered, T);
void set(const T& v) {
AssertTargetIsNotGray(v);
setUnchecked(v);
}
private:
void setUnchecked(const T& v) {
this->pre();
this->value = v;
}
T release() {
T tmp = this->value;
this->value = JS::SafelyInitialized<T>::create();
return tmp;
}
};
} // namespace js
namespace JS::detail {
template <typename T>
struct DefineComparisonOps<js::PreBarriered<T>> : std::true_type {
static const T& get(const js::PreBarriered<T>& v) { return v.get(); }
};
} // namespace JS::detail
namespace js {
/*
* A pre- and post-barriered heap pointer, for use inside the JS engine.
*
* It must only be stored in memory that has GC lifetime. GCPtr must not be
* used in contexts where it may be implicitly moved or deleted, e.g. most
* containers.
*
* The post-barriers implemented by this class are faster than those
* implemented by js::HeapPtr<T> or JS::Heap<T> at the cost of not
* automatically handling deletion or movement.
*/
template <class T>
class GCPtr : public WriteBarriered<T> {
public:
GCPtr() : WriteBarriered<T>(JS::SafelyInitialized<T>::create()) {}
explicit GCPtr(const T& v) : WriteBarriered<T>(v) {
this->post(JS::SafelyInitialized<T>::create(), v);
}
explicit GCPtr(const GCPtr<T>& v) : WriteBarriered<T>(v) {
this->post(JS::SafelyInitialized<T>::create(), v);
}
#ifdef DEBUG
~GCPtr() {
// No barriers are necessary as this only happens when the GC is sweeping or
// before this has been initialized (see above comment).
//
// If this assertion fails you may need to make the containing object use a
// HeapPtr instead, as this can be deleted from outside of GC.
MOZ_ASSERT(CurrentThreadIsGCSweeping() || CurrentThreadIsGCFinalizing() ||
this->value == JS::SafelyInitialized<T>::create());
Poison(this, JS_FREED_HEAP_PTR_PATTERN, sizeof(*this),
MemCheckKind::MakeNoAccess);
}
#endif
/*
* Unlike HeapPtr<T>, GCPtr<T> must be managed with GC lifetimes.
* Specifically, the memory used by the pointer itself must be live until
* at least the next minor GC. For that reason, move semantics are invalid
* and are deleted here. Please note that not all containers support move
* semantics, so this does not completely prevent invalid uses.
*/
GCPtr(GCPtr<T>&&) = delete;
GCPtr<T>& operator=(GCPtr<T>&&) = delete;
void init(const T& v) {
AssertTargetIsNotGray(v);
this->value = v;
this->post(JS::SafelyInitialized<T>::create(), v);
}
DECLARE_POINTER_ASSIGN_OPS(GCPtr, T);
void set(const T& v) {
AssertTargetIsNotGray(v);
setUnchecked(v);
}
private:
void setUnchecked(const T& v) {
this->pre();
T tmp = this->value;
this->value = v;
this->post(tmp, this->value);
}
};
} // namespace js
namespace JS::detail {
template <typename T>
struct DefineComparisonOps<js::GCPtr<T>> : std::true_type {
static const T& get(const js::GCPtr<T>& v) { return v.get(); }
};
} // namespace JS::detail
namespace js {
/*
* A pre- and post-barriered heap pointer, for use inside the JS engine. These
* heap pointers can be stored in C++ containers like GCVector and GCHashMap.
*
* The GC sometimes keeps pointers to pointers to GC things --- for example, to
* track references into the nursery. However, C++ containers like GCVector and
* GCHashMap usually reserve the right to relocate their elements any time
* they're modified, invalidating all pointers to the elements. HeapPtr
* has a move constructor which knows how to keep the GC up to date if it is
* moved to a new location.
*
* However, because of this additional communication with the GC, HeapPtr
* is somewhat slower, so it should only be used in contexts where this ability
* is necessary.
*
* Obviously, JSObjects, JSStrings, and the like get tenured and compacted, so
* whatever pointers they contain get relocated, in the sense used here.
* However, since the GC itself is moving those values, it takes care of its
* internal pointers to those pointers itself. HeapPtr is only necessary
* when the relocation would otherwise occur without the GC's knowledge.
*/
template <class T>
class HeapPtr : public WriteBarriered<T> {
public:
HeapPtr() : WriteBarriered<T>(JS::SafelyInitialized<T>::create()) {}
// Implicitly adding barriers is a reasonable default.
MOZ_IMPLICIT HeapPtr(const T& v) : WriteBarriered<T>(v) {
this->post(JS::SafelyInitialized<T>::create(), this->value);
}
MOZ_IMPLICIT HeapPtr(const HeapPtr<T>& other) : WriteBarriered<T>(other) {
this->post(JS::SafelyInitialized<T>::create(), this->value);
}
HeapPtr(HeapPtr<T>&& other) noexcept : WriteBarriered<T>(other.release()) {
this->post(JS::SafelyInitialized<T>::create(), this->value);
}
~HeapPtr() {
this->pre();
this->post(this->value, JS::SafelyInitialized<T>::create());
}
void init(const T& v) {
MOZ_ASSERT(this->value == JS::SafelyInitialized<T>::create());
AssertTargetIsNotGray(v);
this->value = v;
this->post(JS::SafelyInitialized<T>::create(), this->value);
}
DECLARE_POINTER_ASSIGN_AND_MOVE_OPS(HeapPtr, T);
void set(const T& v) {
AssertTargetIsNotGray(v);
setUnchecked(v);
}
/* Make this friend so it can access pre() and post(). */
template <class T1, class T2>
friend inline void BarrieredSetPair(Zone* zone, HeapPtr<T1*>& v1, T1* val1,
HeapPtr<T2*>& v2, T2* val2);
protected:
void setUnchecked(const T& v) {
this->pre();
postBarrieredSet(v);
}
void postBarrieredSet(const T& v) {
T tmp = this->value;
this->value = v;
this->post(tmp, this->value);
}
T release() {
T tmp = this->value;
postBarrieredSet(JS::SafelyInitialized<T>::create());
return tmp;
}
};
/*
* A pre-barriered heap pointer, for use inside the JS engine.
*
* Similar to GCPtr, but used for a pointer to a malloc-allocated structure
* containing GC thing pointers.
*
* It must only be stored in memory that has GC lifetime. It must not be used in
* contexts where it may be implicitly moved or deleted, e.g. most containers.
*
* A post-barrier is unnecessary since malloc-allocated structures cannot be in
* the nursery.
*/
template <class T>
class GCStructPtr : public BarrieredBase<T> {
public:
// This is sometimes used to hold tagged pointers.
static constexpr uintptr_t MaxTaggedPointer = 0x2;
GCStructPtr() : BarrieredBase<T>(JS::SafelyInitialized<T>::create()) {}
// Implicitly adding barriers is a reasonable default.
MOZ_IMPLICIT GCStructPtr(const T& v) : BarrieredBase<T>(v) {}
GCStructPtr(const GCStructPtr<T>& other) : BarrieredBase<T>(other) {}
GCStructPtr(GCStructPtr<T>&& other) noexcept
: BarrieredBase<T>(other.release()) {}
~GCStructPtr() {
// No barriers are necessary as this only happens when the GC is sweeping.
MOZ_ASSERT_IF(isTraceable(),
CurrentThreadIsGCSweeping() || CurrentThreadIsGCFinalizing());
}
void init(const T& v) {
MOZ_ASSERT(this->get() == JS::SafelyInitialized<T>());
AssertTargetIsNotGray(v);
this->value = v;
}
void set(JS::Zone* zone, const T& v) {
pre(zone);
this->value = v;
}
T get() const { return this->value; }
operator T() const { return get(); }
T operator->() const { return get(); }
protected:
bool isTraceable() const { return uintptr_t(get()) > MaxTaggedPointer; }
void pre(JS::Zone* zone) {
if (isTraceable()) {
PreWriteBarrier(zone, get());
}
}
};
} // namespace js
namespace JS::detail {
template <typename T>
struct DefineComparisonOps<js::HeapPtr<T>> : std::true_type {
static const T& get(const js::HeapPtr<T>& v) { return v.get(); }
};
} // namespace JS::detail
namespace js {
// Base class for barriered pointer types that intercept reads and writes.
template <typename T>
class ReadBarriered : public BarrieredBase<T> {
protected:
// ReadBarriered is not directly instantiable.
explicit ReadBarriered(const T& v) : BarrieredBase<T>(v) {}
void read() const { InternalBarrierMethods<T>::readBarrier(this->value); }
void post(const T& prev, const T& next) {
InternalBarrierMethods<T>::postBarrier(&this->value, prev, next);
}
};
// Incremental GC requires that weak pointers have read barriers. See the block
// comment at the top of Barrier.h for a complete discussion of why.
//
// Note that this class also has post-barriers, so is safe to use with nursery
// pointers. However, when used as a hashtable key, care must still be taken to
// insert manual post-barriers on the table for rekeying if the key is based in
// any way on the address of the object.
template <typename T>
class WeakHeapPtr : public ReadBarriered<T>,
public WrappedPtrOperations<T, WeakHeapPtr<T>> {
protected:
using ReadBarriered<T>::value;
public:
WeakHeapPtr() : ReadBarriered<T>(JS::SafelyInitialized<T>::create()) {}
// It is okay to add barriers implicitly.
MOZ_IMPLICIT WeakHeapPtr(const T& v) : ReadBarriered<T>(v) {
this->post(JS::SafelyInitialized<T>::create(), v);
}
// The copy constructor creates a new weak edge but the wrapped pointer does
// not escape, so no read barrier is necessary.
explicit WeakHeapPtr(const WeakHeapPtr& other) : ReadBarriered<T>(other) {
this->post(JS::SafelyInitialized<T>::create(), value);
}
// Move retains the lifetime status of the source edge, so does not fire
// the read barrier of the defunct edge.
WeakHeapPtr(WeakHeapPtr&& other) noexcept
: ReadBarriered<T>(other.release()) {
this->post(JS::SafelyInitialized<T>::create(), value);
}
~WeakHeapPtr() {
this->post(this->value, JS::SafelyInitialized<T>::create());
}
WeakHeapPtr& operator=(const WeakHeapPtr& v) {
AssertTargetIsNotGray(v.value);
T prior = this->value;
this->value = v.value;
this->post(prior, v.value);
return *this;
}
const T& get() const {
if (InternalBarrierMethods<T>::isMarkable(this->value)) {
this->read();
}
return this->value;
}
const T& unbarrieredGet() const { return this->value; }
explicit operator bool() const { return bool(this->value); }
operator const T&() const { return get(); }
const T& operator->() const { return get(); }
void set(const T& v) {
AssertTargetIsNotGray(v);
setUnchecked(v);
}
void unbarrieredSet(const T& v) {
AssertTargetIsNotGray(v);
this->value = v;
}
private:
void setUnchecked(const T& v) {
T tmp = this->value;
this->value = v;
this->post(tmp, v);
}
T release() {
T tmp = value;
set(JS::SafelyInitialized<T>::create());
return tmp;
}
};
// A wrapper for a bare pointer, with no barriers.
//
// This should only be necessary in a limited number of cases. Please don't add
// more uses of this if at all possible.
template <typename T>
class UnsafeBarePtr : public BarrieredBase<T> {
public:
UnsafeBarePtr() : BarrieredBase<T>(JS::SafelyInitialized<T>::create()) {}
MOZ_IMPLICIT UnsafeBarePtr(T v) : BarrieredBase<T>(v) {}
const T& get() const { return this->value; }
void set(T newValue) { this->value = newValue; }
DECLARE_POINTER_CONSTREF_OPS(T);
};
} // namespace js
namespace JS::detail {
template <typename T>
struct DefineComparisonOps<js::WeakHeapPtr<T>> : std::true_type {
static const T& get(const js::WeakHeapPtr<T>& v) {
return v.unbarrieredGet();
}
};
} // namespace JS::detail
namespace js {
// A pre- and post-barriered Value that is specialized to be aware that it
// resides in a slots or elements vector. This allows it to be relocated in
// memory, but with substantially less overhead than a HeapPtr.
class HeapSlot : public WriteBarriered<Value> {
public:
enum Kind { Slot = 0, Element = 1 };
void init(NativeObject* owner, Kind kind, uint32_t slot, const Value& v) {
value = v;
post(owner, kind, slot, v);
}
void initAsUndefined() { value.setUndefined(); }
void destroy() { pre(); }
void setUndefinedUnchecked() {
pre();
value.setUndefined();
}
#ifdef DEBUG
bool preconditionForSet(NativeObject* owner, Kind kind, uint32_t slot) const;
void assertPreconditionForPostWriteBarrier(NativeObject* obj, Kind kind,
uint32_t slot,
const Value& target) const;
#endif
MOZ_ALWAYS_INLINE void set(NativeObject* owner, Kind kind, uint32_t slot,
const Value& v) {
MOZ_ASSERT(preconditionForSet(owner, kind, slot));
pre();
value = v;
post(owner, kind, slot, v);
}
private:
void post(NativeObject* owner, Kind kind, uint32_t slot,
const Value& target) {
#ifdef DEBUG
assertPreconditionForPostWriteBarrier(owner, kind, slot, target);
#endif
if (this->value.isGCThing()) {
gc::Cell* cell = this->value.toGCThing();
if (cell->storeBuffer()) {
cell->storeBuffer()->putSlot(owner, kind, slot, 1);
}
}
}
};
} // namespace js
namespace JS::detail {
template <>
struct DefineComparisonOps<js::HeapSlot> : std::true_type {
static const Value& get(const js::HeapSlot& v) { return v.get(); }
};
} // namespace JS::detail
namespace js {
class HeapSlotArray {
HeapSlot* array;
public:
explicit HeapSlotArray(HeapSlot* array) : array(array) {}
HeapSlot* begin() const { return array; }
operator const Value*() const {
static_assert(sizeof(GCPtr<Value>) == sizeof(Value));
static_assert(sizeof(HeapSlot) == sizeof(Value));
return reinterpret_cast<const Value*>(array);
}
operator HeapSlot*() const { return begin(); }
HeapSlotArray operator+(int offset) const {
return HeapSlotArray(array + offset);
}
HeapSlotArray operator+(uint32_t offset) const {
return HeapSlotArray(array + offset);
}
};
/*
* This is a hack for RegExpStatics::updateFromMatch. It allows us to do two
* barriers with only one branch to check if we're in an incremental GC.
*/
template <class T1, class T2>
static inline void BarrieredSetPair(Zone* zone, HeapPtr<T1*>& v1, T1* val1,
HeapPtr<T2*>& v2, T2* val2) {
AssertTargetIsNotGray(val1);
AssertTargetIsNotGray(val2);
if (T1::needPreWriteBarrier(zone)) {
v1.pre();
v2.pre();
}
v1.postBarrieredSet(val1);
v2.postBarrieredSet(val2);
}
/*
* ImmutableTenuredPtr is designed for one very narrow case: replacing
* immutable raw pointers to GC-managed things, implicitly converting to a
* handle type for ease of use. Pointers encapsulated by this type must:
*
* be immutable (no incremental write barriers),
* never point into the nursery (no generational write barriers), and
* be traced via MarkRuntime (we use fromMarkedLocation).
*
* In short: you *really* need to know what you're doing before you use this
* class!
*/
template <typename T>
class MOZ_HEAP_CLASS ImmutableTenuredPtr {
T value;
public:
operator T() const { return value; }
T operator->() const { return value; }
// `ImmutableTenuredPtr<T>` is implicitly convertible to `Handle<T>`.
//
// In case you need to convert to `Handle<U>` where `U` is base class of `T`,
// convert this to `Handle<T>` by `toHandle()` and then use implicit
// conversion from `Handle<T>` to `Handle<U>`.
operator Handle<T>() const { return toHandle(); }
Handle<T> toHandle() const { return Handle<T>::fromMarkedLocation(&value); }
void init(T ptr) {
MOZ_ASSERT(ptr->isTenured());
AssertTargetIsNotGray(ptr);
value = ptr;
}
T get() const { return value; }
const T* address() { return &value; }
};
// Template to remove any barrier wrapper and get the underlying type.
template <typename T>
struct RemoveBarrier {
using Type = T;
};
template <typename T>
struct RemoveBarrier<HeapPtr<T>> {
using Type = T;
};
template <typename T>
struct RemoveBarrier<GCPtr<T>> {
using Type = T;
};
template <typename T>
struct RemoveBarrier<PreBarriered<T>> {
using Type = T;
};
template <typename T>
struct RemoveBarrier<WeakHeapPtr<T>> {
using Type = T;
};
#if MOZ_IS_GCC
template struct JS_PUBLIC_API StableCellHasher<JSObject*>;
template struct JS_PUBLIC_API StableCellHasher<JSScript*>;
#endif
template <typename T>
struct StableCellHasher<PreBarriered<T>> {
using Key = PreBarriered<T>;
using Lookup = T;
static bool maybeGetHash(const Lookup& l, HashNumber* hashOut) {
return StableCellHasher<T>::maybeGetHash(l, hashOut);
}
static bool ensureHash(const Lookup& l, HashNumber* hashOut) {
return StableCellHasher<T>::ensureHash(l, hashOut);
}
static HashNumber hash(const Lookup& l) {
return StableCellHasher<T>::hash(l);
}
static bool match(const Key& k, const Lookup& l) {
return StableCellHasher<T>::match(k, l);
}
};
template <typename T>
struct StableCellHasher<HeapPtr<T>> {
using Key = HeapPtr<T>;
using Lookup = T;
static bool maybeGetHash(const Lookup& l, HashNumber* hashOut) {
return StableCellHasher<T>::maybeGetHash(l, hashOut);
}
static bool ensureHash(const Lookup& l, HashNumber* hashOut) {
return StableCellHasher<T>::ensureHash(l, hashOut);
}
static HashNumber hash(const Lookup& l) {
return StableCellHasher<T>::hash(l);
}
static bool match(const Key& k, const Lookup& l) {
return StableCellHasher<T>::match(k, l);
}
};
template <typename T>
struct StableCellHasher<WeakHeapPtr<T>> {
using Key = WeakHeapPtr<T>;
using Lookup = T;
static bool maybeGetHash(const Lookup& l, HashNumber* hashOut) {
return StableCellHasher<T>::maybeGetHash(l, hashOut);
}
static bool ensureHash(const Lookup& l, HashNumber* hashOut) {
return StableCellHasher<T>::ensureHash(l, hashOut);
}
static HashNumber hash(const Lookup& l) {
return StableCellHasher<T>::hash(l);
}
static bool match(const Key& k, const Lookup& l) {
return StableCellHasher<T>::match(k.unbarrieredGet(), l);
}
};
/* Useful for hashtables with a HeapPtr as key. */
template <class T>
struct HeapPtrHasher {
using Key = HeapPtr<T>;
using Lookup = T;
static HashNumber hash(Lookup obj) { return DefaultHasher<T>::hash(obj); }
static bool match(const Key& k, Lookup l) { return k.get() == l; }
static void rekey(Key& k, const Key& newKey) { k.unbarrieredSet(newKey); }
};
template <class T>
struct PreBarrieredHasher {
using Key = PreBarriered<T>;
using Lookup = T;
static HashNumber hash(Lookup obj) { return DefaultHasher<T>::hash(obj); }
static bool match(const Key& k, Lookup l) { return k.get() == l; }
static void rekey(Key& k, const Key& newKey) { k.unbarrieredSet(newKey); }
};
/* Useful for hashtables with a WeakHeapPtr as key. */
template <class T>
struct WeakHeapPtrHasher {
using Key = WeakHeapPtr<T>;
using Lookup = T;
static HashNumber hash(Lookup obj) { return DefaultHasher<T>::hash(obj); }
static bool match(const Key& k, Lookup l) { return k.unbarrieredGet() == l; }
static void rekey(Key& k, const Key& newKey) {
k.set(newKey.unbarrieredGet());
}
};
template <class T>
struct UnsafeBarePtrHasher {
using Key = UnsafeBarePtr<T>;
using Lookup = T;
static HashNumber hash(const Lookup& l) { return DefaultHasher<T>::hash(l); }
static bool match(const Key& k, Lookup l) { return k.get() == l; }
static void rekey(Key& k, const Key& newKey) { k.set(newKey.get()); }
};
// Set up descriptive type aliases.
template <class T>
using PreBarrierWrapper = PreBarriered<T>;
template <class T>
using PreAndPostBarrierWrapper = GCPtr<T>;
} // namespace js
namespace mozilla {
template <class T>
struct DefaultHasher<js::HeapPtr<T>> : js::HeapPtrHasher<T> {};
template <class T>
struct DefaultHasher<js::GCPtr<T>> {
// Not implemented. GCPtr can't be used as a hash table key because it has a
// post barrier but doesn't support relocation.
};
template <class T>
struct DefaultHasher<js::PreBarriered<T>> : js::PreBarrieredHasher<T> {};
template <class T>
struct DefaultHasher<js::WeakHeapPtr<T>> : js::WeakHeapPtrHasher<T> {};
template <class T>
struct DefaultHasher<js::UnsafeBarePtr<T>> : js::UnsafeBarePtrHasher<T> {};
} // namespace mozilla
#endif /* gc_Barrier_h */