/* -*- 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 "js/HeapAPI.h"
#include "vm/JSObject.h"
#include "vm/Shape.h"

namespace js {
namespace gc {

struct Cell;

/*
 * 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 {
  /* See comment in js/public/HeapAPI.h. */
  static const uint32_t Relocated = js::gc::Relocated;

#if MOZ_LITTLE_ENDIAN || JS_BITS_PER_WORD == 32
  /*
   * Keep the first 32 bits untouched. Use them to distinguish strings from
   * objects in the nursery.
   */
  uint32_t preserve_;

  /* Set to Relocated when moved. */
  uint32_t magic_;
#elif JS_BITS_PER_WORD == 64
  /*
   * On big-endian, we need to reorder to keep preserve_ lined up with the
   * low 32 bits of the aligned group_ pointer in JSObject.
   */
  uint32_t magic_;
  uint32_t preserve_;
#else
#  error "Unknown endianness or word size"
#endif

  /* The location |this| was moved to. */
  Cell* newLocation_;

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

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

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

  bool isForwarded() const {
    (void)preserve_;  // Suppress warning
    return magic_ == Relocated;
  }

  Cell* forwardingAddress() const {
    MOZ_ASSERT(isForwarded());
    return newLocation_;
  }

  void forwardTo(Cell* cell);

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

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

  static bool isCellForwarded(const Cell* cell) {
    return fromCell(cell)->isForwarded();
  }
};

}  // namespace gc
}  // namespace js

#endif /* gc_RelocationOverlay_h */