gecko-dev/js/public/HeapAPI.h

/* -*- 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_HeapAPI_h
#define js_HeapAPI_h

#include <limits.h>

#include "jspubtd.h"

#include "js/Utility.h"

/* These values are private to the JS engine. */
namespace js {

// Whether the current thread is permitted access to any part of the specified
// runtime or zone.
JS_FRIEND_API(bool)
CurrentThreadCanAccessRuntime(JSRuntime *rt);

JS_FRIEND_API(bool)
CurrentThreadCanAccessZone(JS::Zone *zone);

namespace gc {

struct Cell;

const size_t ArenaShift = 12;
const size_t ArenaSize = size_t(1) << ArenaShift;
const size_t ArenaMask = ArenaSize - 1;

const size_t ChunkShift = 20;
const size_t ChunkSize = size_t(1) << ChunkShift;
const size_t ChunkMask = ChunkSize - 1;

const size_t CellShift = 3;
const size_t CellSize = size_t(1) << CellShift;
const size_t CellMask = CellSize - 1;

/* These are magic constants derived from actual offsets in gc/Heap.h. */
const size_t ChunkMarkBitmapOffset = 1032352;
const size_t ChunkMarkBitmapBits = 129024;
const size_t ChunkRuntimeOffset = ChunkSize - sizeof(void*);
const size_t ChunkLocationOffset = ChunkSize - 2 * sizeof(void*) - sizeof(uint64_t);

/*
 * Live objects are marked black. How many other additional colors are available
 * depends on the size of the GCThing. Objects marked gray are eligible for
 * cycle collection.
 */
static const uint32_t BLACK = 0;
static const uint32_t GRAY = 1;

/*
 * Constants used to indicate whether a chunk is part of the tenured heap or the
 * nusery.
 */
const uint32_t ChunkLocationNursery = 0;
const uint32_t ChunkLocationTenuredHeap = 1;

#ifdef JS_DEBUG
/* When downcasting, ensure we are actually the right type. */
extern JS_FRIEND_API(void)
AssertGCThingHasType(js::gc::Cell *cell, JSGCTraceKind kind);
#else
inline void
AssertGCThingHasType(js::gc::Cell *cell, JSGCTraceKind kind) {}
#endif

} /* namespace gc */
} /* namespace js */

namespace JS {
struct Zone;

/*
 * We cannot expose the class hierarchy: the implementation is hidden. Instead
 * we provide cast functions with strong debug-mode assertions.
 */
static MOZ_ALWAYS_INLINE js::gc::Cell *
AsCell(JSObject *obj)
{
    js::gc::Cell *cell = reinterpret_cast<js::gc::Cell *>(obj);
    AssertGCThingHasType(cell, JSTRACE_OBJECT);
    return cell;
}

static MOZ_ALWAYS_INLINE js::gc::Cell *
AsCell(JSFunction *fun)
{
    js::gc::Cell *cell = reinterpret_cast<js::gc::Cell *>(fun);
    AssertGCThingHasType(cell, JSTRACE_OBJECT);
    return cell;
}

static MOZ_ALWAYS_INLINE js::gc::Cell *
AsCell(JSString *str)
{
    js::gc::Cell *cell = reinterpret_cast<js::gc::Cell *>(str);
    AssertGCThingHasType(cell, JSTRACE_STRING);
    return cell;
}

static MOZ_ALWAYS_INLINE js::gc::Cell *
AsCell(JSFlatString *flat)
{
    js::gc::Cell *cell = reinterpret_cast<js::gc::Cell *>(flat);
    AssertGCThingHasType(cell, JSTRACE_STRING);
    return cell;
}

static MOZ_ALWAYS_INLINE js::gc::Cell *
AsCell(JSScript *script)
{
    js::gc::Cell *cell = reinterpret_cast<js::gc::Cell *>(script);
    AssertGCThingHasType(cell, JSTRACE_SCRIPT);
    return cell;
}

namespace shadow {

struct ArenaHeader
{
    JS::Zone *zone;
};

struct Zone
{
  protected:
    JSRuntime *const runtime_;
    JSTracer *const barrierTracer_;     // A pointer to the JSRuntime's |gcMarker|.

  public:
    bool needsBarrier_;

    Zone(JSRuntime *runtime, JSTracer *barrierTracerArg)
      : runtime_(runtime),
        barrierTracer_(barrierTracerArg),
        needsBarrier_(false)
    {}

    bool needsBarrier() const {
        return needsBarrier_;
    }

    JSTracer *barrierTracer() {
        MOZ_ASSERT(needsBarrier_);
        MOZ_ASSERT(js::CurrentThreadCanAccessRuntime(runtime_));
        return barrierTracer_;
    }

    JSRuntime *runtimeFromMainThread() const {
        MOZ_ASSERT(js::CurrentThreadCanAccessRuntime(runtime_));
        return runtime_;
    }

    // Note: Unrestricted access to the zone's runtime from an arbitrary
    // thread can easily lead to races. Use this method very carefully.
    JSRuntime *runtimeFromAnyThread() const {
        return runtime_;
    }

    static JS::shadow::Zone *asShadowZone(JS::Zone *zone) {
        return reinterpret_cast<JS::shadow::Zone*>(zone);
    }
};

} /* namespace shadow */
} /* namespace JS */

namespace js {
namespace gc {

static MOZ_ALWAYS_INLINE uintptr_t *
GetGCThingMarkBitmap(const void *thing)
{
    MOZ_ASSERT(thing);
    uintptr_t addr = uintptr_t(thing);
    addr &= ~js::gc::ChunkMask;
    addr |= js::gc::ChunkMarkBitmapOffset;
    return reinterpret_cast<uintptr_t *>(addr);
}

static MOZ_ALWAYS_INLINE JS::shadow::Runtime *
GetGCThingRuntime(const void *thing)
{
    MOZ_ASSERT(thing);
    uintptr_t addr = uintptr_t(thing);
    addr &= ~js::gc::ChunkMask;
    addr |= js::gc::ChunkRuntimeOffset;
    return *reinterpret_cast<JS::shadow::Runtime **>(addr);
}

static MOZ_ALWAYS_INLINE void
GetGCThingMarkWordAndMask(const void *thing, uint32_t color,
                          uintptr_t **wordp, uintptr_t *maskp)
{
    uintptr_t addr = uintptr_t(thing);
    size_t bit = (addr & js::gc::ChunkMask) / js::gc::CellSize + color;
    MOZ_ASSERT(bit < js::gc::ChunkMarkBitmapBits);
    uintptr_t *bitmap = GetGCThingMarkBitmap(thing);
    const uintptr_t nbits = sizeof(*bitmap) * CHAR_BIT;
    *maskp = uintptr_t(1) << (bit % nbits);
    *wordp = &bitmap[bit / nbits];
}

static MOZ_ALWAYS_INLINE JS::shadow::ArenaHeader *
GetGCThingArena(void *thing)
{
    uintptr_t addr = uintptr_t(thing);
    addr &= ~js::gc::ArenaMask;
    return reinterpret_cast<JS::shadow::ArenaHeader *>(addr);
}

MOZ_ALWAYS_INLINE bool
IsInsideNursery(const js::gc::Cell *cell)
{
#ifdef JSGC_GENERATIONAL
    if (!cell)
        return false;
    uintptr_t addr = uintptr_t(cell);
    addr &= ~js::gc::ChunkMask;
    addr |= js::gc::ChunkLocationOffset;
    uint32_t location = *reinterpret_cast<uint32_t *>(addr);
    JS_ASSERT(location == gc::ChunkLocationNursery ||
              location == gc::ChunkLocationTenuredHeap);
    return location == gc::ChunkLocationNursery;
#else
    return false;
#endif
}

} /* namespace gc */

} /* namespace js */

namespace JS {

static MOZ_ALWAYS_INLINE Zone *
GetGCThingZone(void *thing)
{
    MOZ_ASSERT(thing);
    return js::gc::GetGCThingArena(thing)->zone;
}

static MOZ_ALWAYS_INLINE Zone *
GetObjectZone(JSObject *obj)
{
    return GetGCThingZone(obj);
}

static MOZ_ALWAYS_INLINE bool
GCThingIsMarkedGray(void *thing)
{
#ifdef JSGC_GENERATIONAL
    /*
     * GC things residing in the nursery cannot be gray: they have no mark bits.
     * All live objects in the nursery are moved to tenured at the beginning of
     * each GC slice, so the gray marker never sees nursery things.
     */
    if (js::gc::IsInsideNursery((js::gc::Cell *)thing))
        return false;
#endif
    uintptr_t *word, mask;
    js::gc::GetGCThingMarkWordAndMask(thing, js::gc::GRAY, &word, &mask);
    return *word & mask;
}

static MOZ_ALWAYS_INLINE bool
IsIncrementalBarrierNeededOnGCThing(shadow::Runtime *rt, void *thing, JSGCTraceKind kind)
{
    if (!rt->needsBarrier_)
        return false;
    JS::Zone *zone = GetGCThingZone(thing);
    return reinterpret_cast<shadow::Zone *>(zone)->needsBarrier_;
}

} /* namespace JS */

#endif /* js_HeapAPI_h */