/* -*- 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/. */

/*
 * GC-internal definition of relocation overlay used while moving cells.
 */

#ifndef gc_RelocationOverlay_h
#define gc_RelocationOverlay_h

#include "mozilla/Assertions.h"
#include "mozilla/EndianUtils.h"

#include <stdint.h>

#include "gc/Cell.h"
#include "js/HeapAPI.h"
#include "vm/JSObject.h"
#include "vm/Shape.h"

namespace js {
namespace gc {

/*
 * This structure overlays a Cell that has been moved and provides a way to find
 * its new location. It's used during generational and compacting GC.
 */
class RelocationOverlay : public Cell
{
    // First word of a Cell has additional requirements from GC. The GC flags
    // determine if a Cell is a normal entry or is a RelocationOverlay.
    //                3         0
    //  -------------------------
    //  | NewLocation | GCFlags |
    //  -------------------------
    uintptr_t dataWithTag_;

    /* A list entry to track all relocated things. */
    RelocationOverlay* next_;

  public:
    static const RelocationOverlay* fromCell(const Cell* cell) {
        return static_cast<const RelocationOverlay*>(cell);
    }

    static RelocationOverlay* fromCell(Cell* cell) {
        return static_cast<RelocationOverlay*>(cell);
    }

    Cell* forwardingAddress() const {
        MOZ_ASSERT(isForwarded());
        uintptr_t newLocation = dataWithTag_ & ~Cell::RESERVED_MASK;
        return reinterpret_cast<Cell*>(newLocation);
    }

    void forwardTo(Cell* cell);

    RelocationOverlay*& nextRef() {
        MOZ_ASSERT(isForwarded());
        return next_;
    }

    RelocationOverlay* next() const {
        MOZ_ASSERT(isForwarded());
        return next_;
    }
};

} // namespace gc
} // namespace js

#endif /* gc_RelocationOverlay_h */
