/*

 * Copyright 2001-2008 Sun Microsystems, Inc.  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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.com if you need additional information or

 * have any questions.

 *

 */

class TaskQueueSuper: public CHeapObj {

protected:

  // The first free element after the last one pushed (mod _n).

  // (For now we'll assume only 32-bit CAS).

  volatile juint _bottom;

  // log2 of the size of the queue.

  enum SomeProtectedConstants {

    Log_n = 14

  };

  // Size of the queue.

  juint n() { return (1 << Log_n); }

  // For computing "x mod n" efficiently.

  juint n_mod_mask() { return n() - 1; }

  struct Age {

    jushort _top;

    jushort _tag;

    jushort tag() const { return _tag; }

    jushort top() const { return _top; }

    Age() { _tag = 0; _top = 0; }

    friend bool operator ==(const Age& a1, const Age& a2) {

      return a1.tag() == a2.tag() && a1.top() == a2.top();

    }

  };

  Age _age;

  // These make sure we do single atomic reads and writes.

  Age get_age() {

    jint res = *(volatile jint*)(&_age);

    return *(Age*)(&res);

  }

  void set_age(Age a) {

    *(volatile jint*)(&_age) = *(int*)(&a);

  }

  jushort get_top() {

    return get_age().top();

  }

  // These both operate mod _n.

  juint increment_index(juint ind) {

    return (ind + 1) & n_mod_mask();

  }

  juint decrement_index(juint ind) {

    return (ind - 1) & n_mod_mask();

  }

  // Returns a number in the range [0.._n).  If the result is "n-1", it

  // should be interpreted as 0.

  juint dirty_size(juint bot, juint top) {

    return ((jint)bot - (jint)top) & n_mod_mask();

  }

  // Returns the size corresponding to the given "bot" and "top".

  juint size(juint bot, juint top) {

    juint sz = dirty_size(bot, top);

    // Has the queue "wrapped", so that bottom is less than top?

    // There's a complicated special case here.  A pair of threads could

    // perform pop_local and pop_global operations concurrently, starting

    // from a state in which _bottom == _top+1.  The pop_local could

    // succeed in decrementing _bottom, and the pop_global in incrementing

    // _top (in which case the pop_global will be awarded the contested

    // queue element.)  The resulting state must be interpreted as an empty

    // queue.  (We only need to worry about one such event: only the queue

    // owner performs pop_local's, and several concurrent threads

    // attempting to perform the pop_global will all perform the same CAS,

    // and only one can succeed.  Any stealing thread that reads after

    // either the increment or decrement will seen an empty queue, and will

    // not join the competitors.  The "sz == -1 || sz == _n-1" state will

    // not be modified  by concurrent queues, so the owner thread can reset

    // the state to _bottom == top so subsequent pushes will be performed

    // normally.

    if (sz == (n()-1)) return 0;

    else return sz;

  }

public:

  TaskQueueSuper() : _bottom(0), _age() {}

  // Return "true" if the TaskQueue contains any tasks.

  bool peek();

  // Return an estimate of the number of elements in the queue.

  // The "careful" version admits the possibility of pop_local/pop_global

  // races.

  juint size() {

    return size(_bottom, get_top());

  }

  juint dirty_size() {

    return dirty_size(_bottom, get_top());

  }

  void set_empty() {

    _bottom = 0;

    _age = Age();

  }

  // Maximum number of elements allowed in the queue.  This is two less

  // than the actual queue size, for somewhat complicated reasons.

  juint max_elems() { return n() - 2; }

};

template<class E> class GenericTaskQueue: public TaskQueueSuper {

private:

  // Slow paths for push, pop_local.  (pop_global has no fast path.)

  bool push_slow(E t, juint dirty_n_elems);

  bool pop_local_slow(juint localBot, Age oldAge);

public:

  // Initializes the queue to empty.

  GenericTaskQueue();

  void initialize();

  // Push the task "t" on the queue.  Returns "false" iff the queue is

  // full.

  inline bool push(E t);

  // If succeeds in claiming a task (from the 'local' end, that is, the

  // most recently pushed task), returns "true" and sets "t" to that task.

  // Otherwise, the queue is empty and returns false.

  inline bool pop_local(E& t);

  // If succeeds in claiming a task (from the 'global' end, that is, the

  // least recently pushed task), returns "true" and sets "t" to that task.

  // Otherwise, the queue is empty and returns false.

  bool pop_global(E& t);

  // Delete any resource associated with the queue.

  ~GenericTaskQueue();

  // apply the closure to all elements in the task queue

  void oops_do(OopClosure* f);

private:

  // Element array.

  volatile E* _elems;

};

template<class E>

GenericTaskQueue<E>::GenericTaskQueue():TaskQueueSuper() {

  assert(sizeof(Age) == sizeof(jint), "Depends on this.");

}

template<class E>

void GenericTaskQueue<E>::initialize() {

  _elems = NEW_C_HEAP_ARRAY(E, n());

  guarantee(_elems != NULL, "Allocation failed.");

}

template<class E>

void GenericTaskQueue<E>::oops_do(OopClosure* f) {

  // tty->print_cr("START OopTaskQueue::oops_do");

  int iters = size();

  juint index = _bottom;

  for (int i = 0; i < iters; ++i) {

    index = decrement_index(index);

    // tty->print_cr("  doing entry %d," INTPTR_T " -> " INTPTR_T,

    //            index, &_elems[index], _elems[index]);

    E* t = (E*)&_elems[index];      // cast away volatility

    oop* p = (oop*)t;

    assert((*t)->is_oop_or_null(), "Not an oop or null");

    f->do_oop(p);

  }

  // tty->print_cr("END OopTaskQueue::oops_do");

}

template<class E>

bool GenericTaskQueue<E>::push_slow(E t, juint dirty_n_elems) {

  if (dirty_n_elems == n() - 1) {

    // Actually means 0, so do the push.

    juint localBot = _bottom;

    _elems[localBot] = t;

    _bottom = increment_index(localBot);

    return true;

  } else

    return false;

}

template<class E>

bool GenericTaskQueue<E>::

pop_local_slow(juint localBot, Age oldAge) {

  // This queue was observed to contain exactly one element; either this

  // thread will claim it, or a competing "pop_global".  In either case,

  // the queue will be logically empty afterwards.  Create a new Age value

  // that represents the empty queue for the given value of "_bottom".  (We

  // must also increment "tag" because of the case where "bottom == 1",

  // "top == 0".  A pop_global could read the queue element in that case,

  // then have the owner thread do a pop followed by another push.  Without

  // the incrementing of "tag", the pop_global's CAS could succeed,

  // allowing it to believe it has claimed the stale element.)

  Age newAge;

  newAge._top = localBot;

  newAge._tag = oldAge.tag() + 1;

  // Perhaps a competing pop_global has already incremented "top", in which

  // case it wins the element.

  if (localBot == oldAge.top()) {

    Age tempAge;

    // No competing pop_global has yet incremented "top"; we'll try to

    // install new_age, thus claiming the element.

    assert(sizeof(Age) == sizeof(jint) && sizeof(jint) == sizeof(juint),

           "Assumption about CAS unit.");

    *(jint*)&tempAge = Atomic::cmpxchg(*(jint*)&newAge, (volatile jint*)&_age, *(jint*)&oldAge);

    if (tempAge == oldAge) {

      // We win.

      assert(dirty_size(localBot, get_top()) != n() - 1,

             "Shouldn't be possible...");

      return true;

    }

  }

  // We fail; a completing pop_global gets the element.  But the queue is

  // empty (and top is greater than bottom.)  Fix this representation of

  // the empty queue to become the canonical one.

  set_age(newAge);

  assert(dirty_size(localBot, get_top()) != n() - 1,

         "Shouldn't be possible...");

  return false;

}

template<class E>

bool GenericTaskQueue<E>::pop_global(E& t) {

  Age newAge;

  Age oldAge = get_age();

  juint localBot = _bottom;

  juint n_elems = size(localBot, oldAge.top());

  if (n_elems == 0) {

    return false;

  }

  t = _elems[oldAge.top()];

  newAge = oldAge;

  newAge._top = increment_index(newAge.top());

  if ( newAge._top == 0 ) newAge._tag++;

  Age resAge;

  *(jint*)&resAge = Atomic::cmpxchg(*(jint*)&newAge, (volatile jint*)&_age, *(jint*)&oldAge);

  // Note that using "_bottom" here might fail, since a pop_local might

  // have decremented it.

  assert(dirty_size(localBot, newAge._top) != n() - 1,

         "Shouldn't be possible...");

  return (resAge == oldAge);

}

template<class E>

GenericTaskQueue<E>::~GenericTaskQueue() {

  FREE_C_HEAP_ARRAY(E, _elems);

}

// Inherits the typedef of "Task" from above.

class TaskQueueSetSuper: public CHeapObj {

protected:

  static int randomParkAndMiller(int* seed0);

public:

  // Returns "true" if some TaskQueue in the set contains a task.

  virtual bool peek() = 0;

};

template<class E> class GenericTaskQueueSet: public TaskQueueSetSuper {

private:

  int _n;

  GenericTaskQueue<E>** _queues;

public:

  GenericTaskQueueSet(int n) : _n(n) {

    typedef GenericTaskQueue<E>* GenericTaskQueuePtr;

    _queues = NEW_C_HEAP_ARRAY(GenericTaskQueuePtr, n);

    guarantee(_queues != NULL, "Allocation failure.");

    for (int i = 0; i < n; i++) {

      _queues[i] = NULL;

    }

  }

  bool steal_1_random(int queue_num, int* seed, E& t);

  bool steal_best_of_2(int queue_num, int* seed, E& t);

  bool steal_best_of_all(int queue_num, int* seed, E& t);

  void register_queue(int i, GenericTaskQueue<E>* q);

  GenericTaskQueue<E>* queue(int n);

  // The thread with queue number "queue_num" (and whose random number seed

  // is at "seed") is trying to steal a task from some other queue.  (It

  // may try several queues, according to some configuration parameter.)

  // If some steal succeeds, returns "true" and sets "t" the stolen task,

  // otherwise returns false.

  bool steal(int queue_num, int* seed, E& t);

  bool peek();

};

template<class E>

void GenericTaskQueueSet<E>::register_queue(int i, GenericTaskQueue<E>* q) {

  assert(0 <= i && i < _n, "index out of range.");

  _queues[i] = q;

}

template<class E>

GenericTaskQueue<E>* GenericTaskQueueSet<E>::queue(int i) {

  return _queues[i];

}

template<class E>

bool GenericTaskQueueSet<E>::steal(int queue_num, int* seed, E& t) {

  for (int i = 0; i < 2 * _n; i++)

    if (steal_best_of_2(queue_num, seed, t))

      return true;

  return false;

}

template<class E>

bool GenericTaskQueueSet<E>::steal_best_of_all(int queue_num, int* seed, E& t) {

  if (_n > 2) {

    int best_k;

    jint best_sz = 0;

    for (int k = 0; k < _n; k++) {

      if (k == queue_num) continue;

      jint sz = _queues[k]->size();

      if (sz > best_sz) {

        best_sz = sz;

        best_k = k;

      }

    }

    return best_sz > 0 && _queues[best_k]->pop_global(t);

  } else if (_n == 2) {

    // Just try the other one.

    int k = (queue_num + 1) % 2;

    return _queues[k]->pop_global(t);

  } else {

    assert(_n == 1, "can't be zero.");

    return false;

  }

}

template<class E>

bool GenericTaskQueueSet<E>::steal_1_random(int queue_num, int* seed, E& t) {

  if (_n > 2) {

    int k = queue_num;

    while (k == queue_num) k = randomParkAndMiller(seed) % _n;

    return _queues[2]->pop_global(t);

  } else if (_n == 2) {

    // Just try the other one.

    int k = (queue_num + 1) % 2;

    return _queues[k]->pop_global(t);

  } else {

    assert(_n == 1, "can't be zero.");

    return false;

  }

}

template<class E>

bool GenericTaskQueueSet<E>::steal_best_of_2(int queue_num, int* seed, E& t) {

  if (_n > 2) {

    int k1 = queue_num;

    while (k1 == queue_num) k1 = randomParkAndMiller(seed) % _n;

    int k2 = queue_num;

    while (k2 == queue_num || k2 == k1) k2 = randomParkAndMiller(seed) % _n;

    // Sample both and try the larger.

    juint sz1 = _queues[k1]->size();

    juint sz2 = _queues[k2]->size();

    if (sz2 > sz1) return _queues[k2]->pop_global(t);

    else return _queues[k1]->pop_global(t);

  } else if (_n == 2) {

    // Just try the other one.

    int k = (queue_num + 1) % 2;

    return _queues[k]->pop_global(t);

  } else {

    assert(_n == 1, "can't be zero.");

    return false;

  }

}

template<class E>

bool GenericTaskQueueSet<E>::peek() {

  // Try all the queues.

  for (int j = 0; j < _n; j++) {

    if (_queues[j]->peek())

      return true;

  }

  return false;

}

// When to terminate from the termination protocol.

class TerminatorTerminator: public CHeapObj {

public:

  virtual bool should_exit_termination() = 0;

};

// A class to aid in the termination of a set of parallel tasks using

// TaskQueueSet's for work stealing.

class ParallelTaskTerminator: public StackObj {

private:

  int _n_threads;

  TaskQueueSetSuper* _queue_set;

  jint _offered_termination;

  bool peek_in_queue_set();

protected:

  virtual void yield();

  void sleep(uint millis);

public:

  // "n_threads" is the number of threads to be terminated.  "queue_set" is a

  // queue sets of work queues of other threads.

  ParallelTaskTerminator(int n_threads, TaskQueueSetSuper* queue_set);

  // The current thread has no work, and is ready to terminate if everyone

  // else is.  If returns "true", all threads are terminated.  If returns

  // "false", available work has been observed in one of the task queues,

  // so the global task is not complete.

  bool offer_termination() {

    return offer_termination(NULL);

  }

  // As above, but it also terminates of the should_exit_termination()

  // method of the terminator parameter returns true. If terminator is

  // NULL, then it is ignored.

  bool offer_termination(TerminatorTerminator* terminator);

  // Reset the terminator, so that it may be reused again.

  // The caller is responsible for ensuring that this is done

  // in an MT-safe manner, once the previous round of use of

  // the terminator is finished.

  void reset_for_reuse();

};

#define SIMPLE_STACK 0

template<class E> inline bool GenericTaskQueue<E>::push(E t) {

#if SIMPLE_STACK

  juint localBot = _bottom;

  if (_bottom < max_elems()) {

    _elems[localBot] = t;

    _bottom = localBot + 1;

    return true;

  } else {

    return false;

  }

#else

  juint localBot = _bottom;

  assert((localBot >= 0) && (localBot < n()), "_bottom out of range.");

  jushort top = get_top();

  juint dirty_n_elems = dirty_size(localBot, top);

  assert((dirty_n_elems >= 0) && (dirty_n_elems < n()),

         "n_elems out of range.");

  if (dirty_n_elems < max_elems()) {

    _elems[localBot] = t;

    _bottom = increment_index(localBot);

    return true;

  } else {

    return push_slow(t, dirty_n_elems);

  }

#endif

}

template<class E> inline bool GenericTaskQueue<E>::pop_local(E& t) {

#if SIMPLE_STACK

  juint localBot = _bottom;

  assert(localBot > 0, "precondition.");

  localBot--;

  t = _elems[localBot];

  _bottom = localBot;

  return true;

#else

  juint localBot = _bottom;

  // This value cannot be n-1.  That can only occur as a result of

  // the assignment to bottom in this method.  If it does, this method

  // resets the size( to 0 before the next call (which is sequential,

  // since this is pop_local.)

  juint dirty_n_elems = dirty_size(localBot, get_top());

  assert(dirty_n_elems != n() - 1, "Shouldn't be possible...");

  if (dirty_n_elems == 0) return false;

  localBot = decrement_index(localBot);

  _bottom = localBot;

  // This is necessary to prevent any read below from being reordered

  // before the store just above.

  OrderAccess::fence();

  t = _elems[localBot];

  // This is a second read of "age"; the "size()" above is the first.

  // If there's still at least one element in the queue, based on the

  // "_bottom" and "age" we've read, then there can be no interference with

  // a "pop_global" operation, and we're done.

  juint tp = get_top();

  if (size(localBot, tp) > 0) {

    assert(dirty_size(localBot, tp) != n() - 1,

           "Shouldn't be possible...");

    return true;

  } else {

    // Otherwise, the queue contained exactly one element; we take the slow

    // path.

    return pop_local_slow(localBot, get_age());

  }

#endif