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 /*

  * 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