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gecko-dev/js/public/RootingAPI.h

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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
* vim: set ts=8 sts=4 et sw=4 tw=99:
* 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
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef js_RootingAPI_h
#define js_RootingAPI_h
#include "mozilla/Attributes.h"
#include "mozilla/DebugOnly.h"
#include "mozilla/GuardObjects.h"
#include "mozilla/LinkedList.h"
#include "mozilla/TypeTraits.h"
#include "jspubtd.h"
#include "js/GCAPI.h"
#include "js/HeapAPI.h"
#include "js/TypeDecls.h"
#include "js/Utility.h"
/*
* Moving GC Stack Rooting
*
* A moving GC may change the physical location of GC allocated things, even
* when they are rooted, updating all pointers to the thing to refer to its new
* location. The GC must therefore know about all live pointers to a thing,
* not just one of them, in order to behave correctly.
*
* The |Rooted| and |Handle| classes below are used to root stack locations
* whose value may be held live across a call that can trigger GC. For a
* code fragment such as:
*
* JSObject* obj = NewObject(cx);
* DoSomething(cx);
* ... = obj->lastProperty();
*
* If |DoSomething()| can trigger a GC, the stack location of |obj| must be
* rooted to ensure that the GC does not move the JSObject referred to by
* |obj| without updating |obj|'s location itself. This rooting must happen
* regardless of whether there are other roots which ensure that the object
* itself will not be collected.
*
* If |DoSomething()| cannot trigger a GC, and the same holds for all other
* calls made between |obj|'s definitions and its last uses, then no rooting
* is required.
*
* SpiderMonkey can trigger a GC at almost any time and in ways that are not
* always clear. For example, the following innocuous-looking actions can
* cause a GC: allocation of any new GC thing; JSObject::hasProperty;
* JS_ReportError and friends; and ToNumber, among many others. The following
* dangerous-looking actions cannot trigger a GC: js_malloc, cx->malloc_,
* rt->malloc_, and friends and JS_ReportOutOfMemory.
*
* The following family of three classes will exactly root a stack location.
* Incorrect usage of these classes will result in a compile error in almost
* all cases. Therefore, it is very hard to be incorrectly rooted if you use
* these classes exclusively. These classes are all templated on the type T of
* the value being rooted.
*
* - Rooted<T> declares a variable of type T, whose value is always rooted.
* Rooted<T> may be automatically coerced to a Handle<T>, below. Rooted<T>
* should be used whenever a local variable's value may be held live across a
* call which can trigger a GC.
*
* - Handle<T> is a const reference to a Rooted<T>. Functions which take GC
* things or values as arguments and need to root those arguments should
* generally use handles for those arguments and avoid any explicit rooting.
* This has two benefits. First, when several such functions call each other
* then redundant rooting of multiple copies of the GC thing can be avoided.
* Second, if the caller does not pass a rooted value a compile error will be
* generated, which is quicker and easier to fix than when relying on a
* separate rooting analysis.
*
* - MutableHandle<T> is a non-const reference to Rooted<T>. It is used in the
* same way as Handle<T> and includes a |set(const T& v)| method to allow
* updating the value of the referenced Rooted<T>. A MutableHandle<T> can be
* created from a Rooted<T> by using |Rooted<T>::operator&()|.
*
* In some cases the small performance overhead of exact rooting (measured to
* be a few nanoseconds on desktop) is too much. In these cases, try the
* following:
*
* - Move all Rooted<T> above inner loops: this allows you to re-use the root
* on each iteration of the loop.
*
* - Pass Handle<T> through your hot call stack to avoid re-rooting costs at
* every invocation.
*
* The following diagram explains the list of supported, implicit type
* conversions between classes of this family:
*
* Rooted<T> ----> Handle<T>
* | ^
* | |
* | |
* +---> MutableHandle<T>
* (via &)
*
* All of these types have an implicit conversion to raw pointers.
*/
namespace js {
template <typename T>
struct GCMethods {};
template <typename T>
class RootedBase {};
template <typename T>
class HandleBase {};
template <typename T>
class MutableHandleBase {};
template <typename T>
class HeapBase {};
template <typename T>
class PersistentRootedBase {};
/*
* js::NullPtr acts like a nullptr pointer in contexts that require a Handle.
*
* Handle provides an implicit constructor for js::NullPtr so that, given:
* foo(Handle<JSObject*> h);
* callers can simply write:
* foo(js::NullPtr());
* which avoids creating a Rooted<JSObject*> just to pass nullptr.
*
* This is the SpiderMonkey internal variant. js::NullPtr should be used in
* preference to JS::NullPtr to avoid the GOT access required for JS_PUBLIC_API
* symbols.
*/
struct NullPtr
{
static void * const constNullValue;
};
namespace gc {
struct Cell;
template<typename T>
struct PersistentRootedMarker;
} /* namespace gc */
#define DECLARE_POINTER_COMPARISON_OPS(T) \
bool operator==(const T& other) const { return get() == other; } \
bool operator!=(const T& other) const { return get() != other; }
// Important: Return a reference so passing a Rooted<T>, etc. to
// something that takes a |const T&| is not a GC hazard.
#define DECLARE_POINTER_CONSTREF_OPS(T) \
operator const T&() const { return get(); } \
const T& operator->() const { return get(); }
// Assignment operators on a base class are hidden by the implicitly defined
// operator= on the derived class. Thus, define the operator= directly on the
// class as we would need to manually pass it through anyway.
#define DECLARE_POINTER_ASSIGN_OPS(Wrapper, T) \
Wrapper<T>& operator=(const T& p) { \
set(p); \
return *this; \
} \
Wrapper<T>& operator=(const Wrapper<T>& other) { \
set(other.get()); \
return *this; \
} \
#define DELETE_ASSIGNMENT_OPS(Wrapper, T) \
template <typename S> Wrapper<T>& operator=(S) = delete; \
Wrapper<T>& operator=(const Wrapper<T>&) = delete;
#define DECLARE_NONPOINTER_ACCESSOR_METHODS(ptr) \
const T* address() const { return &(ptr); } \
const T& get() const { return (ptr); } \
#define DECLARE_NONPOINTER_MUTABLE_ACCESSOR_METHODS(ptr) \
T* address() { return &(ptr); } \
T& get() { return (ptr); } \
} /* namespace js */
namespace JS {
template <typename T> class Rooted;
template <typename T> class PersistentRooted;
/* This is exposing internal state of the GC for inlining purposes. */
JS_FRIEND_API(bool) isGCEnabled();
/*
* JS::NullPtr acts like a nullptr pointer in contexts that require a Handle.
*
* Handle provides an implicit constructor for JS::NullPtr so that, given:
* foo(Handle<JSObject*> h);
* callers can simply write:
* foo(JS::NullPtr());
* which avoids creating a Rooted<JSObject*> just to pass nullptr.
*/
struct JS_PUBLIC_API(NullPtr)
{
static void * const constNullValue;
};
JS_FRIEND_API(void) HeapCellPostBarrier(js::gc::Cell** cellp);
JS_FRIEND_API(void) HeapCellRelocate(js::gc::Cell** cellp);
#ifdef JS_DEBUG
/*
* For generational GC, assert that an object is in the tenured generation as
* opposed to being in the nursery.
*/
extern JS_FRIEND_API(void)
AssertGCThingMustBeTenured(JSObject* obj);
#else
inline void
AssertGCThingMustBeTenured(JSObject* obj) {}
#endif
/*
* The Heap<T> class is a heap-stored reference to a JS GC thing. All members of
* heap classes that refer to GC things should use Heap<T> (or possibly
* TenuredHeap<T>, described below).
*
* Heap<T> is an abstraction that hides some of the complexity required to
* maintain GC invariants for the contained reference. It uses operator
* overloading to provide a normal pointer interface, but notifies the GC every
* time the value it contains is updated. This is necessary for generational GC,
* which keeps track of all pointers into the nursery.
*
* Heap<T> instances must be traced when their containing object is traced to
* keep the pointed-to GC thing alive.
*
* Heap<T> objects should only be used on the heap. GC references stored on the
* C/C++ stack must use Rooted/Handle/MutableHandle instead.
*
* Type T must be one of: JS::Value, jsid, JSObject*, JSString*, JSScript*
*/
template <typename T>
class Heap : public js::HeapBase<T>
{
public:
Heap() {
static_assert(sizeof(T) == sizeof(Heap<T>),
"Heap<T> must be binary compatible with T.");
init(js::GCMethods<T>::initial());
}
explicit Heap(T p) { init(p); }
/*
* For Heap, move semantics are equivalent to copy semantics. In C++, a
* copy constructor taking const-ref is the way to get a single function
* that will be used for both lvalue and rvalue copies, so we can simply
* omit the rvalue variant.
*/
explicit Heap(const Heap<T>& p) { init(p.ptr); }
~Heap() {
if (js::GCMethods<T>::needsPostBarrier(ptr))
relocate();
}
DECLARE_POINTER_CONSTREF_OPS(T);
DECLARE_POINTER_ASSIGN_OPS(Heap, T);
DECLARE_NONPOINTER_ACCESSOR_METHODS(ptr);
T* unsafeGet() { return &ptr; }
/*
* Set the pointer to a value which will cause a crash if it is
* dereferenced.
*/
void setToCrashOnTouch() {
ptr = reinterpret_cast<T>(crashOnTouchPointer);
}
bool isSetToCrashOnTouch() {
return ptr == crashOnTouchPointer;
}
private:
void init(T newPtr) {
ptr = newPtr;
if (js::GCMethods<T>::needsPostBarrier(ptr))
post();
}
void set(T newPtr) {
if (js::GCMethods<T>::needsPostBarrier(newPtr)) {
ptr = newPtr;
post();
} else if (js::GCMethods<T>::needsPostBarrier(ptr)) {
relocate(); /* Called before overwriting ptr. */
ptr = newPtr;
} else {
ptr = newPtr;
}
}
void post() {
MOZ_ASSERT(js::GCMethods<T>::needsPostBarrier(ptr));
js::GCMethods<T>::postBarrier(&ptr);
}
void relocate() {
js::GCMethods<T>::relocate(&ptr);
}
enum {
crashOnTouchPointer = 1
};
T ptr;
};
/*
* The TenuredHeap<T> class is similar to the Heap<T> class above in that it
* encapsulates the GC concerns of an on-heap reference to a JS object. However,
* it has two important differences:
*
* 1) Pointers which are statically known to only reference "tenured" objects
* can avoid the extra overhead of SpiderMonkey's write barriers.
*
* 2) Objects in the "tenured" heap have stronger alignment restrictions than
* those in the "nursery", so it is possible to store flags in the lower
* bits of pointers known to be tenured. TenuredHeap wraps a normal tagged
* pointer with a nice API for accessing the flag bits and adds various
* assertions to ensure that it is not mis-used.
*
* GC things are said to be "tenured" when they are located in the long-lived
* heap: e.g. they have gained tenure as an object by surviving past at least
* one GC. For performance, SpiderMonkey allocates some things which are known
* to normally be long lived directly into the tenured generation; for example,
* global objects. Additionally, SpiderMonkey does not visit individual objects
* when deleting non-tenured objects, so object with finalizers are also always
* tenured; for instance, this includes most DOM objects.
*
* The considerations to keep in mind when using a TenuredHeap<T> vs a normal
* Heap<T> are:
*
* - It is invalid for a TenuredHeap<T> to refer to a non-tenured thing.
* - It is however valid for a Heap<T> to refer to a tenured thing.
* - It is not possible to store flag bits in a Heap<T>.
*/
template <typename T>
class TenuredHeap : public js::HeapBase<T>
{
public:
TenuredHeap() : bits(0) {
static_assert(sizeof(T) == sizeof(TenuredHeap<T>),
"TenuredHeap<T> must be binary compatible with T.");
}
explicit TenuredHeap(T p) : bits(0) { setPtr(p); }
explicit TenuredHeap(const TenuredHeap<T>& p) : bits(0) { setPtr(p.getPtr()); }
bool operator==(const TenuredHeap<T>& other) { return bits == other.bits; }
bool operator!=(const TenuredHeap<T>& other) { return bits != other.bits; }
void setPtr(T newPtr) {
MOZ_ASSERT((reinterpret_cast<uintptr_t>(newPtr) & flagsMask) == 0);
if (newPtr)
AssertGCThingMustBeTenured(newPtr);
bits = (bits & flagsMask) | reinterpret_cast<uintptr_t>(newPtr);
}
void setFlags(uintptr_t flagsToSet) {
MOZ_ASSERT((flagsToSet & ~flagsMask) == 0);
bits |= flagsToSet;
}
void unsetFlags(uintptr_t flagsToUnset) {
MOZ_ASSERT((flagsToUnset & ~flagsMask) == 0);
bits &= ~flagsToUnset;
}
bool hasFlag(uintptr_t flag) const {
MOZ_ASSERT((flag & ~flagsMask) == 0);
return (bits & flag) != 0;
}
T getPtr() const { return reinterpret_cast<T>(bits & ~flagsMask); }
uintptr_t getFlags() const { return bits & flagsMask; }
operator T() const { return getPtr(); }
T operator->() const { return getPtr(); }
TenuredHeap<T>& operator=(T p) {
setPtr(p);
return *this;
}
TenuredHeap<T>& operator=(const TenuredHeap<T>& other) {
bits = other.bits;
return *this;
}
private:
enum {
maskBits = 3,
flagsMask = (1 << maskBits) - 1,
};
uintptr_t bits;
};
/*
* Reference to a T that has been rooted elsewhere. This is most useful
* as a parameter type, which guarantees that the T lvalue is properly
* rooted. See "Move GC Stack Rooting" above.
*
* If you want to add additional methods to Handle for a specific
* specialization, define a HandleBase<T> specialization containing them.
*/
template <typename T>
class MOZ_NONHEAP_CLASS Handle : public js::HandleBase<T>
{
friend class JS::MutableHandle<T>;
public:
/* Creates a handle from a handle of a type convertible to T. */
template <typename S>
Handle(Handle<S> handle,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0)
{
static_assert(sizeof(Handle<T>) == sizeof(T*),
"Handle must be binary compatible with T*.");
ptr = reinterpret_cast<const T*>(handle.address());
}
/* Create a handle for a nullptr pointer. */
MOZ_IMPLICIT Handle(js::NullPtr) {
static_assert(mozilla::IsPointer<T>::value,
"js::NullPtr overload not valid for non-pointer types");
ptr = reinterpret_cast<const T*>(&js::NullPtr::constNullValue);
}
/* Create a handle for a nullptr pointer. */
MOZ_IMPLICIT Handle(JS::NullPtr) {
static_assert(mozilla::IsPointer<T>::value,
"JS::NullPtr overload not valid for non-pointer types");
ptr = reinterpret_cast<const T*>(&JS::NullPtr::constNullValue);
}
MOZ_IMPLICIT Handle(MutableHandle<T> handle) {
ptr = handle.address();
}
/*
* Take care when calling this method!
*
* This creates a Handle from the raw location of a T.
*
* It should be called only if the following conditions hold:
*
* 1) the location of the T is guaranteed to be marked (for some reason
* other than being a Rooted), e.g., if it is guaranteed to be reachable
* from an implicit root.
*
* 2) the contents of the location are immutable, or at least cannot change
* for the lifetime of the handle, as its users may not expect its value
* to change underneath them.
*/
static MOZ_CONSTEXPR Handle fromMarkedLocation(const T* p) {
return Handle(p, DeliberatelyChoosingThisOverload,
ImUsingThisOnlyInFromFromMarkedLocation);
}
/*
* Construct a handle from an explicitly rooted location. This is the
* normal way to create a handle, and normally happens implicitly.
*/
template <typename S>
inline
Handle(const Rooted<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0);
template <typename S>
inline
Handle(const PersistentRooted<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0);
/* Construct a read only handle from a mutable handle. */
template <typename S>
inline
Handle(MutableHandle<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0);
DECLARE_POINTER_COMPARISON_OPS(T);
DECLARE_POINTER_CONSTREF_OPS(T);
DECLARE_NONPOINTER_ACCESSOR_METHODS(*ptr);
private:
Handle() {}
DELETE_ASSIGNMENT_OPS(Handle, T);
enum Disambiguator { DeliberatelyChoosingThisOverload = 42 };
enum CallerIdentity { ImUsingThisOnlyInFromFromMarkedLocation = 17 };
MOZ_CONSTEXPR Handle(const T* p, Disambiguator, CallerIdentity) : ptr(p) {}
const T* ptr;
};
/*
* Similar to a handle, but the underlying storage can be changed. This is
* useful for outparams.
*
* If you want to add additional methods to MutableHandle for a specific
* specialization, define a MutableHandleBase<T> specialization containing
* them.
*/
template <typename T>
class MOZ_STACK_CLASS MutableHandle : public js::MutableHandleBase<T>
{
public:
inline MOZ_IMPLICIT MutableHandle(Rooted<T>* root);
inline MOZ_IMPLICIT MutableHandle(PersistentRooted<T>* root);
private:
// Disallow nullptr for overloading purposes.
MutableHandle(decltype(nullptr)) = delete;
public:
void set(T v) {
*ptr = v;
}
/*
* This may be called only if the location of the T is guaranteed
* to be marked (for some reason other than being a Rooted),
* e.g., if it is guaranteed to be reachable from an implicit root.
*
* Create a MutableHandle from a raw location of a T.
*/
static MutableHandle fromMarkedLocation(T* p) {
MutableHandle h;
h.ptr = p;
return h;
}
DECLARE_POINTER_CONSTREF_OPS(T);
DECLARE_NONPOINTER_ACCESSOR_METHODS(*ptr);
DECLARE_NONPOINTER_MUTABLE_ACCESSOR_METHODS(*ptr);
private:
MutableHandle() {}
DELETE_ASSIGNMENT_OPS(MutableHandle, T);
T* ptr;
};
} /* namespace JS */
namespace js {
/*
* InternalHandle is a handle to an internal pointer into a gcthing. Use
* InternalHandle when you have a pointer to a direct field of a gcthing, or
* when you need a parameter type for something that *may* be a pointer to a
* direct field of a gcthing.
*/
template <typename T>
class InternalHandle {};
template <typename T>
class InternalHandle<T*>
{
void * const* holder;
size_t offset;
public:
/*
* Create an InternalHandle using a Handle to the gcthing containing the
* field in question, and a pointer to the field.
*/
template<typename H>
InternalHandle(const JS::Handle<H>& handle, T* field)
: holder((void**)handle.address()), offset(uintptr_t(field) - uintptr_t(handle.get()))
{}
/*
* Create an InternalHandle to a field within a Rooted<>.
*/
template<typename R>
InternalHandle(const JS::Rooted<R>& root, T* field)
: holder((void**)root.address()), offset(uintptr_t(field) - uintptr_t(root.get()))
{}
InternalHandle(const InternalHandle<T*>& other)
: holder(other.holder), offset(other.offset) {}
T* get() const { return reinterpret_cast<T*>(uintptr_t(*holder) + offset); }
const T& operator*() const { return *get(); }
T* operator->() const { return get(); }
static InternalHandle<T*> fromMarkedLocation(T* fieldPtr) {
return InternalHandle(fieldPtr);
}
private:
/*
* Create an InternalHandle to something that is not a pointer to a
* gcthing, and so does not need to be rooted in the first place. Use these
* InternalHandles to pass pointers into functions that also need to accept
* regular InternalHandles to gcthing fields.
*
* Make this private to prevent accidental misuse; this is only for
* fromMarkedLocation().
*/
explicit InternalHandle(T* field)
: holder(reinterpret_cast<void * const*>(&js::NullPtr::constNullValue)),
offset(uintptr_t(field))
{}
void operator=(InternalHandle<T*> other) = delete;
};
/*
* By default, things should use the inheritance hierarchy to find their
* ThingRootKind. Some pointer types are explicitly set in jspubtd.h so that
* Rooted<T> may be used without the class definition being available.
*/
template <typename T>
struct RootKind
{
static ThingRootKind rootKind() { return T::rootKind(); }
};
template <typename T>
struct RootKind<T*>
{
static ThingRootKind rootKind() { return T::rootKind(); }
};
template <typename T>
struct GCMethods<T*>
{
static T* initial() { return nullptr; }
static bool needsPostBarrier(T* v) { return false; }
static void postBarrier(T** vp) {}
static void relocate(T** vp) {}
};
template <>
struct GCMethods<JSObject*>
{
static JSObject* initial() { return nullptr; }
static gc::Cell* asGCThingOrNull(JSObject* v) {
if (!v)
return nullptr;
MOZ_ASSERT(uintptr_t(v) > 32);
return reinterpret_cast<gc::Cell*>(v);
}
static bool needsPostBarrier(JSObject* v) {
return v != nullptr && gc::IsInsideNursery(reinterpret_cast<gc::Cell*>(v));
}
static void postBarrier(JSObject** vp) {
JS::HeapCellPostBarrier(reinterpret_cast<js::gc::Cell**>(vp));
}
static void relocate(JSObject** vp) {
JS::HeapCellRelocate(reinterpret_cast<js::gc::Cell**>(vp));
}
};
template <>
struct GCMethods<JSFunction*>
{
static JSFunction* initial() { return nullptr; }
static bool needsPostBarrier(JSFunction* v) {
return v != nullptr && gc::IsInsideNursery(reinterpret_cast<gc::Cell*>(v));
}
static void postBarrier(JSFunction** vp) {
JS::HeapCellPostBarrier(reinterpret_cast<js::gc::Cell**>(vp));
}
static void relocate(JSFunction** vp) {
JS::HeapCellRelocate(reinterpret_cast<js::gc::Cell**>(vp));
}
};
} /* namespace js */
namespace JS {
/*
* Local variable of type T whose value is always rooted. This is typically
* used for local variables, or for non-rooted values being passed to a
* function that requires a handle, e.g. Foo(Root<T>(cx, x)).
*
* If you want to add additional methods to Rooted for a specific
* specialization, define a RootedBase<T> specialization containing them.
*/
template <typename T>
class MOZ_STACK_CLASS Rooted : public js::RootedBase<T>
{
/* Note: CX is a subclass of either ContextFriendFields or PerThreadDataFriendFields. */
template <typename CX>
void init(CX* cx) {
js::ThingRootKind kind = js::RootKind<T>::rootKind();
this->stack = &cx->thingGCRooters[kind];
this->prev = *stack;
*stack = reinterpret_cast<Rooted<void*>*>(this);
}
public:
explicit Rooted(JSContext* cx
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(js::GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(js::ContextFriendFields::get(cx));
}
Rooted(JSContext* cx, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(js::ContextFriendFields::get(cx));
}
explicit Rooted(js::ContextFriendFields* cx
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(js::GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(cx);
}
Rooted(js::ContextFriendFields* cx, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(cx);
}
explicit Rooted(js::PerThreadDataFriendFields* pt
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(js::GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(pt);
}
Rooted(js::PerThreadDataFriendFields* pt, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(pt);
}
explicit Rooted(JSRuntime* rt
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(js::GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(js::PerThreadDataFriendFields::getMainThread(rt));
}
Rooted(JSRuntime* rt, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(js::PerThreadDataFriendFields::getMainThread(rt));
}
~Rooted() {
MOZ_ASSERT(*stack == reinterpret_cast<Rooted<void*>*>(this));
*stack = prev;
}
Rooted<T>* previous() { return reinterpret_cast<Rooted<T>*>(prev); }
/*
* This method is public for Rooted so that Codegen.py can use a Rooted
* interchangeably with a MutableHandleValue.
*/
void set(T value) {
ptr = value;
}
DECLARE_POINTER_COMPARISON_OPS(T);
DECLARE_POINTER_CONSTREF_OPS(T);
DECLARE_POINTER_ASSIGN_OPS(Rooted, T);
DECLARE_NONPOINTER_ACCESSOR_METHODS(ptr);
DECLARE_NONPOINTER_MUTABLE_ACCESSOR_METHODS(ptr);
private:
/*
* These need to be templated on void* to avoid aliasing issues between, for
* example, Rooted<JSObject> and Rooted<JSFunction>, which use the same
* stack head pointer for different classes.
*/
Rooted<void*>** stack;
Rooted<void*>* prev;
/*
* |ptr| must be the last field in Rooted because the analysis treats all
* Rooted as Rooted<void*> during the analysis. See bug 829372.
*/
T ptr;
MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
Rooted(const Rooted&) = delete;
};
} /* namespace JS */
namespace js {
/*
* Augment the generic Rooted<T> interface when T = JSObject* with
* class-querying and downcasting operations.
*
* Given a Rooted<JSObject*> obj, one can view
* Handle<StringObject*> h = obj.as<StringObject*>();
* as an optimization of
* Rooted<StringObject*> rooted(cx, &obj->as<StringObject*>());
* Handle<StringObject*> h = rooted;
*/
template <>
class RootedBase<JSObject*>
{
public:
template <class U>
JS::Handle<U*> as() const;
};
/*
* Augment the generic Handle<T> interface when T = JSObject* with
* downcasting operations.
*
* Given a Handle<JSObject*> obj, one can view
* Handle<StringObject*> h = obj.as<StringObject*>();
* as an optimization of
* Rooted<StringObject*> rooted(cx, &obj->as<StringObject*>());
* Handle<StringObject*> h = rooted;
*/
template <>
class HandleBase<JSObject*>
{
public:
template <class U>
JS::Handle<U*> as() const;
};
/* Interface substitute for Rooted<T> which does not root the variable's memory. */
template <typename T>
class FakeRooted : public RootedBase<T>
{
public:
template <typename CX>
FakeRooted(CX* cx
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
}
template <typename CX>
FakeRooted(CX* cx, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
}
DECLARE_POINTER_COMPARISON_OPS(T);
DECLARE_POINTER_CONSTREF_OPS(T);
DECLARE_POINTER_ASSIGN_OPS(FakeRooted, T);
DECLARE_NONPOINTER_ACCESSOR_METHODS(ptr);
DECLARE_NONPOINTER_MUTABLE_ACCESSOR_METHODS(ptr);
private:
T ptr;
void set(const T& value) {
ptr = value;
}
MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
FakeRooted(const FakeRooted&) = delete;
};
/* Interface substitute for MutableHandle<T> which is not required to point to rooted memory. */
template <typename T>
class FakeMutableHandle : public js::MutableHandleBase<T>
{
public:
MOZ_IMPLICIT FakeMutableHandle(T* t) {
ptr = t;
}
MOZ_IMPLICIT FakeMutableHandle(FakeRooted<T>* root) {
ptr = root->address();
}
void set(T v) {
*ptr = v;
}
DECLARE_POINTER_CONSTREF_OPS(T);
DECLARE_NONPOINTER_ACCESSOR_METHODS(*ptr);
DECLARE_NONPOINTER_MUTABLE_ACCESSOR_METHODS(*ptr);
private:
FakeMutableHandle() {}
DELETE_ASSIGNMENT_OPS(FakeMutableHandle, T);
T* ptr;
};
/*
* Types for a variable that either should or shouldn't be rooted, depending on
* the template parameter allowGC. Used for implementing functions that can
* operate on either rooted or unrooted data.
*
* The toHandle() and toMutableHandle() functions are for calling functions
* which require handle types and are only called in the CanGC case. These
* allow the calling code to type check.
*/
enum AllowGC {
NoGC = 0,
CanGC = 1
};
template <typename T, AllowGC allowGC>
class MaybeRooted
{
};
template <typename T> class MaybeRooted<T, CanGC>
{
public:
typedef JS::Handle<T> HandleType;
typedef JS::Rooted<T> RootType;
typedef JS::MutableHandle<T> MutableHandleType;
static inline JS::Handle<T> toHandle(HandleType v) {
return v;
}
static inline JS::MutableHandle<T> toMutableHandle(MutableHandleType v) {
return v;
}
template <typename T2>
static inline JS::Handle<T2*> downcastHandle(HandleType v) {
return v.template as<T2>();
}
};
template <typename T> class MaybeRooted<T, NoGC>
{
public:
typedef T HandleType;
typedef FakeRooted<T> RootType;
typedef FakeMutableHandle<T> MutableHandleType;
static JS::Handle<T> toHandle(HandleType v) {
MOZ_CRASH("Bad conversion");
}
static JS::MutableHandle<T> toMutableHandle(MutableHandleType v) {
MOZ_CRASH("Bad conversion");
}
template <typename T2>
static inline T2* downcastHandle(HandleType v) {
return &v->template as<T2>();
}
};
} /* namespace js */
namespace JS {
template <typename T> template <typename S>
inline
Handle<T>::Handle(const Rooted<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy)
{
ptr = reinterpret_cast<const T*>(root.address());
}
template <typename T> template <typename S>
inline
Handle<T>::Handle(const PersistentRooted<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy)
{
ptr = reinterpret_cast<const T*>(root.address());
}
template <typename T> template <typename S>
inline
Handle<T>::Handle(MutableHandle<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy)
{
ptr = reinterpret_cast<const T*>(root.address());
}
template <typename T>
inline
MutableHandle<T>::MutableHandle(Rooted<T>* root)
{
static_assert(sizeof(MutableHandle<T>) == sizeof(T*),
"MutableHandle must be binary compatible with T*.");
ptr = root->address();
}
template <typename T>
inline
MutableHandle<T>::MutableHandle(PersistentRooted<T>* root)
{
static_assert(sizeof(MutableHandle<T>) == sizeof(T*),
"MutableHandle must be binary compatible with T*.");
ptr = root->address();
}
/*
* A copyable, assignable global GC root type with arbitrary lifetime, an
* infallible constructor, and automatic unrooting on destruction.
*
* These roots can be used in heap-allocated data structures, so they are not
* associated with any particular JSContext or stack. They are registered with
* the JSRuntime itself, without locking, so they require a full JSContext to be
* initialized, not one of its more restricted superclasses. Initialization may
* take place on construction, or in two phases if the no-argument constructor
* is called followed by init().
*
* Note that you must not use an PersistentRooted in an object owned by a JS
* object:
*
* Whenever one object whose lifetime is decided by the GC refers to another
* such object, that edge must be traced only if the owning JS object is traced.
* This applies not only to JS objects (which obviously are managed by the GC)
* but also to C++ objects owned by JS objects.
*
* If you put a PersistentRooted in such a C++ object, that is almost certainly
* a leak. When a GC begins, the referent of the PersistentRooted is treated as
* live, unconditionally (because a PersistentRooted is a *root*), even if the
* JS object that owns it is unreachable. If there is any path from that
* referent back to the JS object, then the C++ object containing the
* PersistentRooted will not be destructed, and the whole blob of objects will
* not be freed, even if there are no references to them from the outside.
*
* In the context of Firefox, this is a severe restriction: almost everything in
* Firefox is owned by some JS object or another, so using PersistentRooted in
* such objects would introduce leaks. For these kinds of edges, Heap<T> or
* TenuredHeap<T> would be better types. It's up to the implementor of the type
* containing Heap<T> or TenuredHeap<T> members to make sure their referents get
* marked when the object itself is marked.
*/
template<typename T>
class PersistentRooted : public js::PersistentRootedBase<T>,
private mozilla::LinkedListElement<PersistentRooted<T>>
{
typedef mozilla::LinkedListElement<PersistentRooted<T>> ListBase;
friend class mozilla::LinkedList<PersistentRooted>;
friend class mozilla::LinkedListElement<PersistentRooted>;
friend struct js::gc::PersistentRootedMarker<T>;
friend void js::gc::FinishPersistentRootedChains(JSRuntime* rt);
void registerWithRuntime(JSRuntime* rt) {
MOZ_ASSERT(!initialized());
JS::shadow::Runtime* srt = JS::shadow::Runtime::asShadowRuntime(rt);
srt->getPersistentRootedList<T>().insertBack(this);
}
public:
PersistentRooted() : ptr(js::GCMethods<T>::initial()) {}
explicit PersistentRooted(JSContext* cx) {
init(cx);
}
PersistentRooted(JSContext* cx, T initial) {
init(cx, initial);
}
explicit PersistentRooted(JSRuntime* rt) {
init(rt);
}
PersistentRooted(JSRuntime* rt, T initial) {
init(rt, initial);
}
PersistentRooted(const PersistentRooted& rhs)
: mozilla::LinkedListElement<PersistentRooted<T>>(),
ptr(rhs.ptr)
{
/*
* Copy construction takes advantage of the fact that the original
* is already inserted, and simply adds itself to whatever list the
* original was on - no JSRuntime pointer needed.
*
* This requires mutating rhs's links, but those should be 'mutable'
* anyway. C++ doesn't let us declare mutable base classes.
*/
const_cast<PersistentRooted&>(rhs).setNext(this);
}
bool initialized() {
return ListBase::isInList();
}
void init(JSContext* cx) {
init(cx, js::GCMethods<T>::initial());
}
void init(JSContext* cx, T initial) {
ptr = initial;
registerWithRuntime(js::GetRuntime(cx));
}
void init(JSRuntime* rt) {
init(rt, js::GCMethods<T>::initial());
}
void init(JSRuntime* rt, T initial) {
ptr = initial;
registerWithRuntime(rt);
}
void reset() {
if (initialized()) {
set(js::GCMethods<T>::initial());
ListBase::remove();
}
}
DECLARE_POINTER_COMPARISON_OPS(T);
DECLARE_POINTER_CONSTREF_OPS(T);
DECLARE_POINTER_ASSIGN_OPS(PersistentRooted, T);
DECLARE_NONPOINTER_ACCESSOR_METHODS(ptr);
DECLARE_NONPOINTER_MUTABLE_ACCESSOR_METHODS(ptr);
private:
void set(T value) {
MOZ_ASSERT(initialized());
ptr = value;
}
T ptr;
};
class JS_PUBLIC_API(ObjectPtr)
{
Heap<JSObject*> value;
public:
ObjectPtr() : value(nullptr) {}
explicit ObjectPtr(JSObject* obj) : value(obj) {}
/* Always call finalize before the destructor. */
~ObjectPtr() { MOZ_ASSERT(!value); }
void finalize(JSRuntime* rt) {
if (IsIncrementalBarrierNeeded(rt))
IncrementalObjectBarrier(value);
value = nullptr;
}
void init(JSObject* obj) { value = obj; }
JSObject* get() const { return value; }
void writeBarrierPre(JSRuntime* rt) {
IncrementalObjectBarrier(value);
}
void updateWeakPointerAfterGC();
ObjectPtr& operator=(JSObject* obj) {
IncrementalObjectBarrier(value);
value = obj;
return *this;
}
void trace(JSTracer* trc, const char* name);
JSObject& operator*() const { return *value; }
JSObject* operator->() const { return value; }
operator JSObject*() const { return value; }
};
} /* namespace JS */
namespace js {
namespace gc {
template <typename T, typename TraceCallbacks>
void
CallTraceCallbackOnNonHeap(T* v, const TraceCallbacks& aCallbacks, const char* aName, void* aClosure)
{
static_assert(sizeof(T) == sizeof(JS::Heap<T>), "T and Heap<T> must be compatible.");
MOZ_ASSERT(v);
mozilla::DebugOnly<Cell*> cell = GCMethods<T>::asGCThingOrNull(*v);
MOZ_ASSERT(cell);
MOZ_ASSERT(!IsInsideNursery(cell));
JS::Heap<T>* asHeapT = reinterpret_cast<JS::Heap<T>*>(v);
aCallbacks.Trace(asHeapT, aName, aClosure);
}
} /* namespace gc */
} /* namespace js */
#undef DELETE_ASSIGNMENT_OPS
#undef DECLARE_NONPOINTER_MUTABLE_ACCESSOR_METHODS
#undef DECLARE_NONPOINTER_ACCESSOR_METHODS
#endif /* js_RootingAPI_h */
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