The Tor Browser: js/public/RootingAPI.h@925c144e1f1f (annotated)

js/public/RootingAPI.h@925c144e1f1f (annotated)

js/public/RootingAPI.h

Fri, 16 Jan 2015 18:13:44 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Fri, 16 Jan 2015 18:13:44 +0100
branch: TOR_BUG_9701
changeset 14: 925c144e1f1f
permissions: -rw-r--r--

Integrate suggestion from review to improve consistency with existing code.

 /* -*- 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/GuardObjects.h"
 #include "mozilla/LinkedList.h"
 #include "mozilla/NullPtr.h"
 #include "mozilla/TypeTraits.h"
 #include "jspubtd.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 {
 class ScriptSourceObject;
 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 {};
 /*
  * 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 */
 } /* 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;
 };
 /*
  * 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();
     }
     bool operator==(const Heap<T> &other) { return ptr == other.ptr; }
     bool operator!=(const Heap<T> &other) { return ptr != other.ptr; }
     bool operator==(const T &other) const { return ptr == other; }
     bool operator!=(const T &other) const { return ptr != other; }
     operator T() const { return ptr; }
     T operator->() const { return ptr; }
     const T *address() const { return &ptr; }
     const T &get() const { return ptr; }
     T *unsafeGet() { return &ptr; }
     Heap<T> &operator=(T p) {
         set(p);
         return *this;
     }
     Heap<T> &operator=(const Heap<T>& other) {
         set(other.get());
         return *this;
     }
     void set(T newPtr) {
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(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;
         }
     }
     /*
      * 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) {
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(newPtr));
         ptr = newPtr;
         if (js::GCMethods<T>::needsPostBarrier(ptr))
             post();
     }
     void post() {
 #ifdef JSGC_GENERATIONAL
         MOZ_ASSERT(js::GCMethods<T>::needsPostBarrier(ptr));
         js::GCMethods<T>::postBarrier(&ptr);
 #endif
     }
     void relocate() {
 #ifdef JSGC_GENERATIONAL
         js::GCMethods<T>::relocate(&ptr);
 #endif
     }
     enum {
         crashOnTouchPointer = 1
     };
     T ptr;
 };
 #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 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);
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(newPtr));
         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. */
     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. */
     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);
     }
     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);
     const T *address() const { return ptr; }
     const T& get() const { return *ptr; }
     /*
      * Return a reference so passing a Handle<T> to something that
      * takes a |const T&| is not a GC hazard.
      */
     operator const T&() const { return get(); }
     T operator->() const { return get(); }
     bool operator!=(const T &other) const { return *ptr != other; }
     bool operator==(const T &other) const { return *ptr == other; }
     /* Change this handle to point to the same rooted location RHS does. */
     void repoint(const Handle &rhs) { ptr = rhs.address(); }
   private:
     Handle() {}
     enum Disambiguator { DeliberatelyChoosingThisOverload = 42 };
     enum CallerIdentity { ImUsingThisOnlyInFromFromMarkedLocation = 17 };
     MOZ_CONSTEXPR Handle(const T *p, Disambiguator, CallerIdentity) : ptr(p) {}
     const T *ptr;
     template <typename S> void operator=(S) MOZ_DELETE;
     void operator=(Handle) MOZ_DELETE;
 };
 /*
  * 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 MutableHandle(Rooted<T> *root);
     inline MutableHandle(PersistentRooted<T> *root);
   private:
     // Disallow true nullptr and emulated nullptr (gcc 4.4/4.5, __null, appears
     // as int/long [32/64-bit]) for overloading purposes.
     template<typename N>
     MutableHandle(N,
                   typename mozilla::EnableIf<mozilla::IsNullPointer<N>::value ||
                                              mozilla::IsSame<N, int>::value ||
                                              mozilla::IsSame<N, long>::value,
                                              int>::Type dummy = 0)
     MOZ_DELETE;
   public:
     void set(T v) {
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(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;
     }
     T *address() const { return ptr; }
     const T& get() const { return *ptr; }
     /*
      * Return a reference so passing a MutableHandle<T> to something that takes
      * a |const T&| is not a GC hazard.
      */
     operator const T&() const { return get(); }
     T operator->() const { return get(); }
   private:
     MutableHandle() {}
     T *ptr;
     template <typename S> void operator=(S v) MOZ_DELETE;
     void operator=(MutableHandle other) MOZ_DELETE;
 };
 #ifdef JSGC_GENERATIONAL
 JS_FRIEND_API(void) HeapCellPostBarrier(js::gc::Cell **cellp);
 JS_FRIEND_API(void) HeapCellRelocate(js::gc::Cell **cellp);
 #endif
 } /* 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().
      */
     InternalHandle(T *field)
       : holder(reinterpret_cast<void * const *>(&js::NullPtr::constNullValue)),
         offset(uintptr_t(field))
     {}
     void operator=(InternalHandle<T*> other) MOZ_DELETE;
 };
 /*
  * By default, pointers 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<T *>
 {
     static ThingRootKind rootKind() { return T::rootKind(); }
 };
 template <typename T>
 struct GCMethods<T *>
 {
     static T *initial() { return nullptr; }
     static ThingRootKind kind() { return RootKind<T *>::rootKind(); }
     static bool poisoned(T *v) { return JS::IsPoisonedPtr(v); }
     static bool needsPostBarrier(T *v) { return false; }
 #ifdef JSGC_GENERATIONAL
     static void postBarrier(T **vp) {}
     static void relocate(T **vp) {}
 #endif
 };
 template <>
 struct GCMethods<JSObject *>
 {
     static JSObject *initial() { return nullptr; }
     static ThingRootKind kind() { return RootKind<JSObject *>::rootKind(); }
     static bool poisoned(JSObject *v) { return JS::IsPoisonedPtr(v); }
     static bool needsPostBarrier(JSObject *v) { return v; }
 #ifdef JSGC_GENERATIONAL
     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));
     }
 #endif
 };
 template <>
 struct GCMethods<JSFunction *>
 {
     static JSFunction *initial() { return nullptr; }
     static ThingRootKind kind() { return RootKind<JSObject *>::rootKind(); }
     static bool poisoned(JSFunction *v) { return JS::IsPoisonedPtr(v); }
     static bool needsPostBarrier(JSFunction *v) { return v; }
 #ifdef JSGC_GENERATIONAL
     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));
     }
 #endif
 };
 #ifdef JS_DEBUG
 /* This helper allows us to assert that Rooted<T> is scoped within a request. */
 extern JS_PUBLIC_API(bool)
 IsInRequest(JSContext *cx);
 #endif
 } /* 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) {
 #ifdef JSGC_TRACK_EXACT_ROOTS
         js::ThingRootKind kind = js::GCMethods<T>::kind();
         this->stack = &cx->thingGCRooters[kind];
         this->prev = *stack;
         *stack = reinterpret_cast<Rooted<void*>*>(this);
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(ptr));
 #endif
     }
   public:
     Rooted(JSContext *cx
            MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
       : ptr(js::GCMethods<T>::initial())
     {
         MOZ_GUARD_OBJECT_NOTIFIER_INIT;
 #ifdef JS_DEBUG
         MOZ_ASSERT(js::IsInRequest(cx));
 #endif
         init(js::ContextFriendFields::get(cx));
     }
     Rooted(JSContext *cx, T initial
            MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
       : ptr(initial)
     {
         MOZ_GUARD_OBJECT_NOTIFIER_INIT;
 #ifdef JS_DEBUG
         MOZ_ASSERT(js::IsInRequest(cx));
 #endif
         init(js::ContextFriendFields::get(cx));
     }
     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);
     }
     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);
     }
     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));
     }
     // Note that we need to let the compiler generate the default destructor in
     // non-exact-rooting builds because of a bug in the instrumented PGO builds
     // using MSVC, see bug 915735 for more details.
 #ifdef JSGC_TRACK_EXACT_ROOTS
     ~Rooted() {
         MOZ_ASSERT(*stack == reinterpret_cast<Rooted<void*>*>(this));
         *stack = prev;
     }
 #endif
 #ifdef JSGC_TRACK_EXACT_ROOTS
     Rooted<T> *previous() { return prev; }
 #endif
     /*
      * Important: Return a reference here so passing a Rooted<T> to
      * something that takes a |const T&| is not a GC hazard.
      */
     operator const T&() const { return ptr; }
     T operator->() const { return ptr; }
     T *address() { return &ptr; }
     const T *address() const { return &ptr; }
     T &get() { return ptr; }
     const T &get() const { return ptr; }
     T &operator=(T value) {
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(value));
         ptr = value;
         return ptr;
     }
     T &operator=(const Rooted &value) {
         ptr = value;
         return ptr;
     }
     void set(T value) {
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(value));
         ptr = value;
     }
     bool operator!=(const T &other) const { return ptr != other; }
     bool operator==(const T &other) const { return ptr == other; }
   private:
 #ifdef JSGC_TRACK_EXACT_ROOTS
     Rooted<void*> **stack, *prev;
 #endif
     /*
      * |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 &) MOZ_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;
 };
 /*
  * RootedGeneric<T> allows a class to instantiate its own Rooted type by
  * including the following two methods:
  *
  *    static inline js::ThingRootKind rootKind() { return js::THING_ROOT_CUSTOM; }
  *    void trace(JSTracer *trc);
  *
  * The trace() method must trace all of the class's fields.
  *
  * Implementation:
  *
  * RootedGeneric<T> works by placing a pointer to its 'rooter' field into the
  * usual list of rooters when it is instantiated. When marking, it backs up
  * from this pointer to find a vtable containing a type-appropriate trace()
  * method.
  */
 template <typename GCType>
 class JS_PUBLIC_API(RootedGeneric)
 {
   public:
     JS::Rooted<GCType> rooter;
     RootedGeneric(js::ContextFriendFields *cx)
         : rooter(cx)
     {
     }
     RootedGeneric(js::ContextFriendFields *cx, const GCType &initial)
         : rooter(cx, initial)
     {
     }
     virtual inline void trace(JSTracer *trc);
     operator const GCType&() const { return rooter.get(); }
     GCType operator->() const { return rooter.get(); }
 };
 template <typename GCType>
 inline void RootedGeneric<GCType>::trace(JSTracer *trc)
 {
     rooter->trace(trc);
 }
 // We will instantiate RootedGeneric<void*> in RootMarking.cpp, and MSVC will
 // notice that void*s have no trace() method defined on them and complain (even
 // though it's never called.) MSVC's complaint is not unreasonable, so
 // specialize for void*.
 template <>
 inline void RootedGeneric<void*>::trace(JSTracer *trc)
 {
     MOZ_ASSUME_UNREACHABLE("RootedGeneric<void*>::trace()");
 }
 /* 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;
     }
     operator T() const { return ptr; }
     T operator->() const { return ptr; }
     T *address() { return &ptr; }
     const T *address() const { return &ptr; }
     T &get() { return ptr; }
     const T &get() const { return ptr; }
     FakeRooted<T> &operator=(T value) {
         MOZ_ASSERT(!GCMethods<T>::poisoned(value));
         ptr = value;
         return *this;
     }
     FakeRooted<T> &operator=(const FakeRooted<T> &other) {
         MOZ_ASSERT(!GCMethods<T>::poisoned(other.ptr));
         ptr = other.ptr;
         return *this;
     }
     bool operator!=(const T &other) const { return ptr != other; }
     bool operator==(const T &other) const { return ptr == other; }
   private:
     T ptr;
     MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
     FakeRooted(const FakeRooted &) MOZ_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:
     FakeMutableHandle(T *t) {
         ptr = t;
     }
     FakeMutableHandle(FakeRooted<T> *root) {
         ptr = root->address();
     }
     void set(T v) {
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(v));
         *ptr = v;
     }
     T *address() const { return ptr; }
     T get() const { return *ptr; }
     operator T() const { return get(); }
     T operator->() const { return get(); }
   private:
     FakeMutableHandle() {}
     T *ptr;
     template <typename S>
     void operator=(S v) MOZ_DELETE;
     void operator=(const FakeMutableHandle<T>& other) MOZ_DELETE;
 };
 /*
  * 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 T> class MaybeRooted<T, NoGC>
 {
   public:
     typedef T HandleType;
     typedef FakeRooted<T> RootType;
     typedef FakeMutableHandle<T> MutableHandleType;
     static inline JS::Handle<T> toHandle(HandleType v) {
         MOZ_ASSUME_UNREACHABLE("Bad conversion");
     }
     static inline JS::MutableHandle<T> toMutableHandle(MutableHandleType v) {
         MOZ_ASSUME_UNREACHABLE("Bad conversion");
     }
 };
 } /* 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
  * constructed, not one of its more restricted superclasses.
  *
  * 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 : private mozilla::LinkedListElement<PersistentRooted<T> > {
     friend class mozilla::LinkedList<PersistentRooted>;
     friend class mozilla::LinkedListElement<PersistentRooted>;
     friend class js::gc::PersistentRootedMarker<T>;
     void registerWithRuntime(JSRuntime *rt) {
         JS::shadow::Runtime *srt = JS::shadow::Runtime::asShadowRuntime(rt);
         srt->getPersistentRootedList<T>().insertBack(this);
     }
   public:
     PersistentRooted(JSContext *cx) : ptr(js::GCMethods<T>::initial())
     {
         registerWithRuntime(js::GetRuntime(cx));
     }
     PersistentRooted(JSContext *cx, T initial) : ptr(initial)
     {
         registerWithRuntime(js::GetRuntime(cx));
     }
     PersistentRooted(JSRuntime *rt) : ptr(js::GCMethods<T>::initial())
     {
         registerWithRuntime(rt);
     }
     PersistentRooted(JSRuntime *rt, T initial) : ptr(initial)
     {
         registerWithRuntime(rt);
     }
     PersistentRooted(PersistentRooted &rhs) : 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.
          */
         rhs.setNext(this);
     }
     /*
      * Important: Return a reference here so passing a Rooted<T> to
      * something that takes a |const T&| is not a GC hazard.
      */
     operator const T&() const { return ptr; }
     T operator->() const { return ptr; }
     T *address() { return &ptr; }
     const T *address() const { return &ptr; }
     T &get() { return ptr; }
     const T &get() const { return ptr; }
     T &operator=(T value) {
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(value));
         ptr = value;
         return ptr;
     }
     T &operator=(const PersistentRooted &value) {
         ptr = value;
         return ptr;
     }
     void set(T value) {
         MOZ_ASSERT(!js::GCMethods<T>::poisoned(value));
         ptr = value;
     }
     bool operator!=(const T &other) const { return ptr != other; }
     bool operator==(const T &other) const { return ptr == other; }
   private:
     T ptr;
 };
 } /* namespace JS */
 namespace js {
 /* Base class for automatic read-only object rooting during compilation. */
 class CompilerRootNode
 {
   protected:
     CompilerRootNode(js::gc::Cell *ptr) : next(nullptr), ptr_(ptr) {}
   public:
     void **address() { return (void **)&ptr_; }
   public:
     CompilerRootNode *next;
   protected:
     js::gc::Cell *ptr_;
 };
 }  /* namespace js */
 #endif  /* js_RootingAPI_h */

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js/public/RootingAPI.h