/*

 * Copyright (c) 2018, 2019, 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_GC_SHARED_STRINGDEDUP_STRINGDEDUPQUEUE_HPP

#define SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUPQUEUE_HPP

#include "memory/allocation.hpp"

#include "oops/oop.hpp"

class StringDedupUnlinkOrOopsDoClosure;

//

// The deduplication queue acts as the communication channel between 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). Popping from the queue is NOT thread safe and can only be done by the deduplication

// thread outside a safepoint.

//

class StringDedupQueue : public CHeapObj<mtGC> {

private:

  static StringDedupQueue*   _queue;

  static volatile size_t     _claimed_index;

public:

  template <typename Q>

  static void create();

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

  static inline void wait();

  // Wakes up any thread blocked waiting for the queue to become non-empty.

  static inline void cancel_wait();

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

  static inline void push(uint worker_id, oop java_string);

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

  // all queues are empty.

  static inline oop pop();

  static void unlink_or_oops_do(StringDedupUnlinkOrOopsDoClosure* cl);

  static void print_statistics();

  static void verify();

  // GC support

  static void gc_prologue();

  static void gc_epilogue();

protected:

  static StringDedupQueue* const queue();

  // Queue interface.

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

  virtual void wait_impl() = 0;

  // Wakes up any thread blocked waiting for the queue to become non-empty.

  virtual void cancel_wait_impl() = 0;

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

  virtual void push_impl(uint worker_id, oop java_string) = 0;

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

  // all queues are empty.

  virtual oop pop_impl() = 0;

  virtual void unlink_or_oops_do_impl(StringDedupUnlinkOrOopsDoClosure* cl, size_t queue) = 0;

  virtual void print_statistics_impl() = 0;

  virtual void verify_impl() = 0;

  virtual size_t num_queues() const = 0;

  static size_t claim();

};

#endif // SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUPQUEUE_HPP