gecko/js/src/gc/Root.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sw=4 et tw=78:
 *
 * 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 jsgc_root_h__
#define jsgc_root_h__

#ifdef __cplusplus

#include "mozilla/TypeTraits.h"

#include "jsapi.h"

#include "js/TemplateLib.h"
#include "js/Utility.h"

namespace JS {

/*
 * 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 classes below are used to root stack locations whose value may be held
 * live across a call that can trigger GC (i.e. a call which might allocate any
 * GC things). For a code fragment such as:
 *
 * Foo();
 * ... = obj->lastProperty();
 *
 * If Foo() 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 Foo() 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.
 *
 * Several classes are available for rooting stack locations. All are templated
 * on the type T of the value being rooted, for which RootMethods<T> must
 * have an instantiation.
 *
 * - 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 allocate GC things or otherwise 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.
 */

template <typename T> class Rooted;

template <typename T>
struct RootMethods { };

/*
 * Handle provides an implicit constructor for NullPtr so that, given:
 *   foo(Handle<JSObject*> h);
 * callers can simply write:
 *   foo(NullPtr());
 * which avoids creating a Rooted<JSObject*> just to pass NULL.
 */
struct NullPtr
{
    static void * const constNullValue;
};

template <typename T>
class MutableHandle;

template <typename T>
class HandleBase {};

/*
 * 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 Handle : public HandleBase<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)
    {
        ptr = reinterpret_cast<const T *>(handle.address());
    }

    /* Create a handle for a NULL pointer. */
    Handle(NullPtr) {
        typedef typename js::tl::StaticAssert<js::tl::IsPointerType<T>::result>::result _;
        ptr = reinterpret_cast<const T *>(&NullPtr::constNullValue);
    }

    friend class MutableHandle<T>;
    Handle(MutableHandle<T> handle) {
        ptr = handle.address();
    }

    /*
     * 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 Handle from a raw location of a T.
     */
    static Handle fromMarkedLocation(const T *p) {
        Handle h;
        h.ptr = p;
        return h;
    }

    /*
     * 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(Rooted<S> &root,
           typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0);

    const T *address() const { return ptr; }
    T get() const { return *ptr; }

    operator T () const { return get(); }
    T operator ->() const { return get(); }

  private:
    Handle() {}

    const T *ptr;

    template <typename S>
    void operator =(S v) MOZ_DELETE;
};

typedef Handle<JSObject*>    HandleObject;
typedef Handle<JSFunction*>  HandleFunction;
typedef Handle<JSScript*>    HandleScript;
typedef Handle<JSString*>    HandleString;
typedef Handle<jsid>         HandleId;
typedef Handle<Value>        HandleValue;

template <typename T>
class MutableHandleBase {};

/*
 * 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 MutableHandle : public MutableHandleBase<T>
{
  public:
    template <typename S>
    MutableHandle(MutableHandle<S> handle,
                  typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0)
    {
        this->ptr = reinterpret_cast<const T *>(handle.address());
    }

    template <typename S>
    inline
    MutableHandle(Rooted<S> *root,
                  typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0);

    void set(T v)
    {
        JS_ASSERT(!RootMethods<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; }
    T get() const { return *ptr; }

    operator T () const { return get(); }
    T operator ->() const { return get(); }

  private:
    MutableHandle() {}

    T *ptr;
};

typedef MutableHandle<JSObject*>    MutableHandleObject;
typedef MutableHandle<Value>        MutableHandleValue;

/*
 * 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 RootMethods<T *>
{
    static T *initial() { return NULL; }
    static ThingRootKind kind() { return RootKind<T *>::rootKind(); }
    static bool poisoned(T *v) { return IsPoisonedPtr(v); }
};

template <typename T>
class RootedBase {};

/*
 * 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 Rooted : public RootedBase<T>
{
    void init(JSContext *cx_)
    {
#if defined(JSGC_ROOT_ANALYSIS) || defined(JSGC_USE_EXACT_ROOTING)
        ContextFriendFields *cx = ContextFriendFields::get(cx_);

        ThingRootKind kind = RootMethods<T>::kind();
        this->stack = reinterpret_cast<Rooted<T>**>(&cx->thingGCRooters[kind]);
        this->prev = *stack;
        *stack = this;

        JS_ASSERT(!RootMethods<T>::poisoned(ptr));
#endif
    }

  public:
    Rooted(JSContext *cx) : ptr(RootMethods<T>::initial()) { init(cx); }
    Rooted(JSContext *cx, T initial) : ptr(initial) { init(cx); }

    ~Rooted()
    {
#if defined(JSGC_ROOT_ANALYSIS) || defined(JSGC_USE_EXACT_ROOTING)
        JS_ASSERT(*stack == this);
        *stack = prev;
#endif
    }

#if defined(JSGC_ROOT_ANALYSIS) || defined(JSGC_USE_EXACT_ROOTING)
    Rooted<T> *previous() { return prev; }
#endif

    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; }

    T & operator =(T value)
    {
        JS_ASSERT(!RootMethods<T>::poisoned(value));
        ptr = value;
        return ptr;
    }

    T & operator =(const Rooted &value)
    {
        ptr = value;
        return ptr;
    }

  private:

#if defined(JSGC_ROOT_ANALYSIS) || defined(JSGC_USE_EXACT_ROOTING)
    Rooted<T> **stack, *prev;
#endif
    T ptr;

    Rooted() MOZ_DELETE;
    Rooted(const Rooted &) MOZ_DELETE;
};

template<typename T> template <typename S>
inline
Handle<T>::Handle(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
MutableHandle<T>::MutableHandle(Rooted<S> *root,
                                typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy)
{
    ptr = root->address();
}

typedef Rooted<JSObject*>    RootedObject;
typedef Rooted<JSFunction*>  RootedFunction;
typedef Rooted<JSScript*>    RootedScript;
typedef Rooted<JSString*>    RootedString;
typedef Rooted<jsid>         RootedId;
typedef Rooted<Value>        RootedValue;

/*
 * Mark a stack location as a root for the rooting analysis, without actually
 * rooting it in release builds. This should only be used for stack locations
 * of GC things that cannot be relocated by a garbage collection, and that
 * are definitely reachable via another path.
 */
class SkipRoot
{
#if defined(DEBUG) && defined(JSGC_ROOT_ANALYSIS)

    SkipRoot **stack, *prev;
    const uint8_t *start;
    const uint8_t *end;

    template <typename T>
    void init(ContextFriendFields *cx, const T *ptr, size_t count)
    {
        this->stack = &cx->skipGCRooters;
        this->prev = *stack;
        *stack = this;
        this->start = (const uint8_t *) ptr;
        this->end = this->start + (sizeof(T) * count);
    }

  public:
    template <typename T>
    SkipRoot(JSContext *cx, const T *ptr, size_t count = 1
             JS_GUARD_OBJECT_NOTIFIER_PARAM)
    {
        init(ContextFriendFields::get(cx), ptr, count);
        JS_GUARD_OBJECT_NOTIFIER_INIT;
    }

    ~SkipRoot()
    {
        JS_ASSERT(*stack == this);
        *stack = prev;
    }

    SkipRoot *previous() { return prev; }

    bool contains(const uint8_t *v, size_t len) {
        return v >= start && v + len <= end;
    }

#else /* DEBUG && JSGC_ROOT_ANALYSIS */

  public:
    template <typename T>
    SkipRoot(JSContext *cx, const T *ptr, size_t count = 1
              JS_GUARD_OBJECT_NOTIFIER_PARAM)
    {
        JS_GUARD_OBJECT_NOTIFIER_INIT;
    }

#endif /* DEBUG && JSGC_ROOT_ANALYSIS */

    JS_DECL_USE_GUARD_OBJECT_NOTIFIER
};

#ifdef DEBUG
JS_FRIEND_API(bool) IsRootingUnnecessaryForContext(JSContext *cx);
JS_FRIEND_API(void) SetRootingUnnecessaryForContext(JSContext *cx, bool value);
JS_FRIEND_API(bool) RelaxRootChecksForContext(JSContext *cx);
#endif

class AssertRootingUnnecessary {
    JS_DECL_USE_GUARD_OBJECT_NOTIFIER
#ifdef DEBUG
    JSContext *cx;
    bool prev;
#endif
public:
    AssertRootingUnnecessary(JSContext *cx JS_GUARD_OBJECT_NOTIFIER_PARAM)
    {
        JS_GUARD_OBJECT_NOTIFIER_INIT;
#ifdef DEBUG
        this->cx = cx;
        prev = IsRootingUnnecessaryForContext(cx);
        SetRootingUnnecessaryForContext(cx, true);
#endif
    }

    ~AssertRootingUnnecessary() {
#ifdef DEBUG
        SetRootingUnnecessaryForContext(cx, prev);
#endif
    }
};

#if defined(DEBUG) && defined(JS_GC_ZEAL) && defined(JSGC_ROOT_ANALYSIS) && !defined(JS_THREADSAFE)
extern void
CheckStackRoots(JSContext *cx);
#endif

/*
 * Hook for dynamic root analysis. Checks the native stack and poisons
 * references to GC things which have not been rooted.
 */
inline void MaybeCheckStackRoots(JSContext *cx, bool relax = true)
{
#ifdef DEBUG
    JS_ASSERT(!IsRootingUnnecessaryForContext(cx));
# if defined(JS_GC_ZEAL) && defined(JSGC_ROOT_ANALYSIS) && !defined(JS_THREADSAFE)
    if (relax && RelaxRootChecksForContext(cx))
        return;
    CheckStackRoots(cx);
# endif
#endif
}

}  /* namespace JS */

#endif  /* __cplusplus */

#endif  /* jsgc_root_h___ */