/*

 * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * 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

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 */

#ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPQUEUE_HPP

#define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPQUEUE_HPP

#include "memory/allocation.hpp"

#include "oops/oop.hpp"

#include "utilities/stack.hpp"

class G1StringDedupUnlinkOrOopsDoClosure;

//

// The deduplication queue acts as the communication channel between the stop-the-world

// mark/evacuation phase and the concurrent deduplication phase. Deduplication candidates

// found during mark/evacuation are placed on this queue for later processing in the

// deduplication thread. A queue entry is an oop pointing to a String object (as opposed

// to entries in the deduplication hashtable which points to character arrays).

//

// While users of the queue treat it as a single queue, it is implemented as a set of

// queues, one queue per GC worker thread, to allow lock-free and cache-friendly enqueue

// operations by the GC workers.

//

// The oops in the queue are treated as weak pointers, meaning the objects they point to

// can become unreachable and pruned (cleared) before being popped by the deduplication

// thread.

//

// Pushing to the queue is thread safe (this relies on each thread using a unique worker

// id), but only allowed during a safepoint. Popping from the queue is NOT thread safe

// and can only be done by the deduplication thread outside a safepoint.

//

// The StringDedupQueue_lock is only used for blocking and waking up the deduplication

// thread in case the queue is empty or becomes non-empty, respectively. This lock does

// not otherwise protect the queue content.

//

class G1StringDedupQueue : public CHeapObj<mtGC> {

private:

  typedef Stack<oop, mtGC> G1StringDedupWorkerQueue;

  static G1StringDedupQueue* _queue;

  static const size_t        _max_size;

  static const size_t        _max_cache_size;

  G1StringDedupWorkerQueue*  _queues;

  size_t                     _nqueues;

  size_t                     _cursor;

  volatile bool              _empty;

  // Statistics counter, only used for logging.

  uintx                      _dropped;

  G1StringDedupQueue();

  ~G1StringDedupQueue();

  static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, size_t queue);

public:

  static void create();

  // Blocks and waits for the queue to become non-empty.

  static void wait();

  // Pushes a deduplication candidate onto a specific GC worker queue.

  static void push(uint worker_id, oop java_string);

  // Pops a deduplication candidate from any queue, returns NULL if

  // all queues are empty.

  static oop pop();

  static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl);

  static void print_statistics(outputStream* st);

  static void verify();

};

#endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPQUEUE_HPP