/*

 * Copyright (c) 2013, 2018, Red Hat, Inc. All rights reserved.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 *

 */

#ifndef SHARE_VM_GC_SHENANDOAH_SHENANDOAHBROOKSPOINTER_HPP

#define SHARE_VM_GC_SHENANDOAH_SHENANDOAHBROOKSPOINTER_HPP

#include "oops/oop.hpp"

#include "utilities/globalDefinitions.hpp"

class ShenandoahBrooksPointer {

  /*

   * Notes:

   *

   *  a. It is important to have byte_offset and word_offset return constant

   *     expressions, because that will allow to constant-fold forwarding ptr

   *     accesses. This is not a problem in JIT compilers that would generate

   *     the code once, but it is problematic in GC hotpath code.

   *

   *  b. With filler object mechanics, we may need to allocate more space for

   *     the forwarding ptr to meet alignment requirements for objects. This

   *     means *_offset and *_size calls are NOT interchangeable. The accesses

   *     to forwarding ptrs should always be via *_offset. Storage size

   *     calculations should always be via *_size.

   */

public:

  /* Offset from the object start, in HeapWords. */

  static inline int word_offset() {

    return -1; // exactly one HeapWord

  }

  /* Offset from the object start, in bytes. */

  static inline int byte_offset() {

    return -HeapWordSize; // exactly one HeapWord

  }

  /* Allocated size, in HeapWords. */

  static inline uint word_size() {

    return (uint) MinObjAlignment;

  }

  /* Allocated size, in bytes */

  static inline uint byte_size() {

    return (uint) MinObjAlignmentInBytes;

  }

  /* Assert basic stuff once at startup. */

  static void initial_checks() {

    guarantee (MinObjAlignment > 0, "sanity, word_size is correct");

    guarantee (MinObjAlignmentInBytes > 0, "sanity, byte_size is correct");

  }

  /* Initializes Brooks pointer (to self).

   */

  static inline void initialize(oop obj);

  /* Gets forwardee from the given object.

   */

  static inline oop forwardee(oop obj);

  /* Tries to atomically update forwardee in $holder object to $update.

   * Assumes $holder points at itself.

   * Asserts $holder is in from-space.

   * Asserts $update is in to-space.

   */

  static inline oop try_update_forwardee(oop obj, oop update);

  /* Sets raw value for forwardee slot.

   * THIS IS DANGEROUS: USERS HAVE TO INITIALIZE/SET FORWARDEE BACK AFTER THEY ARE DONE.

   */

  static inline void set_raw(oop obj, HeapWord* update);

  /* Returns the raw value from forwardee slot.

   */

  static inline HeapWord* get_raw(oop obj);

  /* Returns the raw value from forwardee slot without any checks.

   * Used for quick verification.

   */

  static inline HeapWord* get_raw_unchecked(oop obj);

private:

  static inline HeapWord** brooks_ptr_addr(oop obj);

};

#endif // SHARE_VM_GC_SHENANDOAH_SHENANDOAHBROOKSPOINTER_HPP