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 /*

  * Copyright (c) 2001, 2012, 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_UTILITIES_TASKQUEUE_HPP

 #define SHARE_VM_UTILITIES_TASKQUEUE_HPP

 #include "memory/allocation.hpp"

 #include "memory/allocation.inline.hpp"

 #include "runtime/mutex.hpp"

 #include "utilities/stack.hpp"

 #ifdef TARGET_OS_ARCH_linux_x86

 # include "orderAccess_linux_x86.inline.hpp"

 #endif

 #ifdef TARGET_OS_ARCH_linux_sparc

 # include "orderAccess_linux_sparc.inline.hpp"

 #endif

 #ifdef TARGET_OS_ARCH_linux_zero

 # include "orderAccess_linux_zero.inline.hpp"

 #endif

 #ifdef TARGET_OS_ARCH_solaris_x86

 # include "orderAccess_solaris_x86.inline.hpp"

 #endif

 #ifdef TARGET_OS_ARCH_solaris_sparc

 # include "orderAccess_solaris_sparc.inline.hpp"

 #endif

 #ifdef TARGET_OS_ARCH_windows_x86

 # include "orderAccess_windows_x86.inline.hpp"

 #endif

 #ifdef TARGET_OS_ARCH_linux_arm

 # include "orderAccess_linux_arm.inline.hpp"

 #endif

 #ifdef TARGET_OS_ARCH_linux_ppc

 # include "orderAccess_linux_ppc.inline.hpp"

 #endif

 #ifdef TARGET_OS_ARCH_bsd_x86

 # include "orderAccess_bsd_x86.inline.hpp"

 #endif

 #ifdef TARGET_OS_ARCH_bsd_zero

 # include "orderAccess_bsd_zero.inline.hpp"

 #endif

 // Simple TaskQueue stats that are collected by default in debug builds.

 #if !defined(TASKQUEUE_STATS) && defined(ASSERT)

 #define TASKQUEUE_STATS 1

 #elif !defined(TASKQUEUE_STATS)

 #define TASKQUEUE_STATS 0

 #endif

 #if TASKQUEUE_STATS

 #define TASKQUEUE_STATS_ONLY(code) code

 #else

 #define TASKQUEUE_STATS_ONLY(code)

 #endif // TASKQUEUE_STATS

 #if TASKQUEUE_STATS

 class TaskQueueStats {

 public:

   enum StatId {

     push,             // number of taskqueue pushes

     pop,              // number of taskqueue pops

     pop_slow,         // subset of taskqueue pops that were done slow-path

     steal_attempt,    // number of taskqueue steal attempts

     steal,            // number of taskqueue steals

     overflow,         // number of overflow pushes

     overflow_max_len, // max length of overflow stack

     last_stat_id

   };

 public:

   inline TaskQueueStats()       { reset(); }

   inline void record_push()     { ++_stats[push]; }

   inline void record_pop()      { ++_stats[pop]; }

   inline void record_pop_slow() { record_pop(); ++_stats[pop_slow]; }

   inline void record_steal(bool success);

   inline void record_overflow(size_t new_length);

   TaskQueueStats & operator +=(const TaskQueueStats & addend);

   inline size_t get(StatId id) const { return _stats[id]; }

   inline const size_t* get() const   { return _stats; }

   inline void reset();

   // Print the specified line of the header (does not include a line separator).

   static void print_header(unsigned int line, outputStream* const stream = tty,

                            unsigned int width = 10);

   // Print the statistics (does not include a line separator).

   void print(outputStream* const stream = tty, unsigned int width = 10) const;

   DEBUG_ONLY(void verify() const;)

 private:

   size_t                    _stats[last_stat_id];

   static const char * const _names[last_stat_id];

 };

 void TaskQueueStats::record_steal(bool success) {

   ++_stats[steal_attempt];

   if (success) ++_stats[steal];

 }

 void TaskQueueStats::record_overflow(size_t new_len) {

   ++_stats[overflow];

   if (new_len > _stats[overflow_max_len]) _stats[overflow_max_len] = new_len;

 }

 void TaskQueueStats::reset() {

   memset(_stats, 0, sizeof(_stats));

 }

 #endif // TASKQUEUE_STATS

 template <unsigned int N, MEMFLAGS F>

 class TaskQueueSuper: public CHeapObj<F> {

 protected:

   // Internal type for indexing the queue; also used for the tag.

   typedef NOT_LP64(uint16_t) LP64_ONLY(uint32_t) idx_t;

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

   volatile uint _bottom;

   enum { MOD_N_MASK = N - 1 };

   class Age {

   public:

     Age(size_t data = 0)         { _data = data; }

     Age(const Age& age)          { _data = age._data; }

     Age(idx_t top, idx_t tag)    { _fields._top = top; _fields._tag = tag; }

     Age   get()        const volatile { return _data; }

     void  set(Age age) volatile       { _data = age._data; }

     idx_t top()        const volatile { return _fields._top; }

     idx_t tag()        const volatile { return _fields._tag; }

     // Increment top; if it wraps, increment tag also.

     void increment() {

       _fields._top = increment_index(_fields._top);

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

     }

     Age cmpxchg(const Age new_age, const Age old_age) volatile {

       return (size_t) Atomic::cmpxchg_ptr((intptr_t)new_age._data,

                                           (volatile intptr_t *)&_data,

                                           (intptr_t)old_age._data);

     }

     bool operator ==(const Age& other) const { return _data == other._data; }

   private:

     struct fields {

       idx_t _top;

       idx_t _tag;

     };

     union {

       size_t _data;

       fields _fields;

     };

   };

   volatile Age _age;

   // These both operate mod N.

   static uint increment_index(uint ind) {

     return (ind + 1) & MOD_N_MASK;

   }

   static uint decrement_index(uint ind) {

     return (ind - 1) & MOD_N_MASK;

   }

   // Returns a number in the range [0..N).  If the result is "N-1", it should be

   // interpreted as 0.

   uint dirty_size(uint bot, uint top) const {

     return (bot - top) & MOD_N_MASK;

   }

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

   uint size(uint bot, uint top) const {

     uint 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 see 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.

     return (sz == N - 1) ? 0 : sz;

   }

 public:

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

   // Return true if the TaskQueue contains/does not contain any tasks.

   bool peek()     const { return _bottom != _age.top(); }

   bool is_empty() const { return size() == 0; }

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

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

   // races.

   uint size() const {

     return size(_bottom, _age.top());

   }

   uint dirty_size() const {

     return dirty_size(_bottom, _age.top());

   }

   void set_empty() {

     _bottom = 0;

     _age.set(0);

   }

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

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

   uint max_elems() const { return N - 2; }

   // Total size of queue.

   static const uint total_size() { return N; }

   TASKQUEUE_STATS_ONLY(TaskQueueStats stats;)

 };

 template <class E, MEMFLAGS F, unsigned int N = TASKQUEUE_SIZE>

 class GenericTaskQueue: public TaskQueueSuper<N, F> {

   ArrayAllocator<E, F> _array_allocator;

 protected:

   typedef typename TaskQueueSuper<N, F>::Age Age;

   typedef typename TaskQueueSuper<N, F>::idx_t idx_t;

   using TaskQueueSuper<N, F>::_bottom;

   using TaskQueueSuper<N, F>::_age;

   using TaskQueueSuper<N, F>::increment_index;

   using TaskQueueSuper<N, F>::decrement_index;

   using TaskQueueSuper<N, F>::dirty_size;

 public:

   using TaskQueueSuper<N, F>::max_elems;

   using TaskQueueSuper<N, F>::size;

 #if  TASKQUEUE_STATS

   using TaskQueueSuper<N, F>::stats;

 #endif

 private:

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

   bool push_slow(E t, uint dirty_n_elems);

   bool pop_local_slow(uint localBot, Age oldAge);

 public:

   typedef E element_type;

   // 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);

   // Attempts to claim a task from the "local" end of the queue (the most

   // recently pushed).  If successful, returns true and sets t to the task;

   // otherwise, returns false (the queue is empty).

   inline bool pop_local(E& t);

   // Like pop_local(), but uses the "global" end of the queue (the least

   // recently pushed).

   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, MEMFLAGS F, unsigned int N>

 GenericTaskQueue<E, F, N>::GenericTaskQueue() {

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

 }

 template<class E, MEMFLAGS F, unsigned int N>

 void GenericTaskQueue<E, F, N>::initialize() {

   _elems = _array_allocator.allocate(N);

 }

 template<class E, MEMFLAGS F, unsigned int N>

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

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

   uint iters = size();

   uint index = _bottom;

   for (uint 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, MEMFLAGS F, unsigned int N>

 bool GenericTaskQueue<E, F, N>::push_slow(E t, uint dirty_n_elems) {

   if (dirty_n_elems == N - 1) {

     // Actually means 0, so do the push.

     uint localBot = _bottom;

     // g++ complains if the volatile result of the assignment is

     // unused, so we cast the volatile away.  We cannot cast directly

     // to void, because gcc treats that as not using the result of the

     // assignment.  However, casting to E& means that we trigger an

     // unused-value warning.  So, we cast the E& to void.

     (void)const_cast<E&>(_elems[localBot] = t);

     OrderAccess::release_store(&_bottom, increment_index(localBot));

     TASKQUEUE_STATS_ONLY(stats.record_push());

     return true;

   }

   return false;

 }

 // pop_local_slow() is done by the owning thread and is trying to

 // get the last task in the queue.  It will compete with pop_global()

 // that will be used by other threads.  The tag age is incremented

 // whenever the queue goes empty which it will do here if this thread

 // gets the last task or in pop_global() if the queue wraps (top == 0

 // and pop_global() succeeds, see pop_global()).

 template<class E, MEMFLAGS F, unsigned int N>

 bool GenericTaskQueue<E, F, N>::pop_local_slow(uint 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((idx_t)localBot, oldAge.tag() + 1);

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

   // case it wins the element.

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

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

     // install new_age, thus claiming the element.

     Age tempAge = _age.cmpxchg(newAge, oldAge);

     if (tempAge == oldAge) {

       // We win.

       assert(dirty_size(localBot, _age.top()) != N - 1, "sanity");

       TASKQUEUE_STATS_ONLY(stats.record_pop_slow());

       return true;

     }

   }

   // We lose; 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.

   _age.set(newAge);

   assert(dirty_size(localBot, _age.top()) != N - 1, "sanity");

   return false;

 }

 template<class E, MEMFLAGS F, unsigned int N>

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

   Age oldAge = _age.get();

   // Architectures with weak memory model require a barrier here

   // to guarantee that bottom is not older than age,

   // which is crucial for the correctness of the algorithm.

 #if !(defined SPARC || defined IA32 || defined AMD64)

   OrderAccess::fence();

 #endif

   uint localBot = OrderAccess::load_acquire((volatile juint*)&_bottom);

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

   if (n_elems == 0) {

     return false;

   }

   // g++ complains if the volatile result of the assignment is

   // unused, so we cast the volatile away.  We cannot cast directly

   // to void, because gcc treats that as not using the result of the

   // assignment.  However, casting to E& means that we trigger an

   // unused-value warning.  So, we cast the E& to void.

   (void) const_cast<E&>(t = _elems[oldAge.top()]);

   Age newAge(oldAge);

   newAge.increment();

   Age resAge = _age.cmpxchg(newAge, oldAge);

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

   // have decremented it.

   assert(dirty_size(localBot, newAge.top()) != N - 1, "sanity");

   return resAge == oldAge;

 }

 template<class E, MEMFLAGS F, unsigned int N>

 GenericTaskQueue<E, F, N>::~GenericTaskQueue() {

   FREE_C_HEAP_ARRAY(E, _elems, F);

 }

 // OverflowTaskQueue is a TaskQueue that also includes an overflow stack for

 // elements that do not fit in the TaskQueue.

 //

 // This class hides two methods from super classes:

 //

 // push() - push onto the task queue or, if that fails, onto the overflow stack

 // is_empty() - return true if both the TaskQueue and overflow stack are empty

 //

 // Note that size() is not hidden--it returns the number of elements in the

 // TaskQueue, and does not include the size of the overflow stack.  This

 // simplifies replacement of GenericTaskQueues with OverflowTaskQueues.

 template<class E, MEMFLAGS F, unsigned int N = TASKQUEUE_SIZE>

 class OverflowTaskQueue: public GenericTaskQueue<E, F, N>

 {

 public:

   typedef Stack<E, F>               overflow_t;

   typedef GenericTaskQueue<E, F, N> taskqueue_t;

   TASKQUEUE_STATS_ONLY(using taskqueue_t::stats;)

   // Push task t onto the queue or onto the overflow stack.  Return true.

   inline bool push(E t);

   // Attempt to pop from the overflow stack; return true if anything was popped.

   inline bool pop_overflow(E& t);

   inline overflow_t* overflow_stack() { return &_overflow_stack; }

   inline bool taskqueue_empty() const { return taskqueue_t::is_empty(); }

   inline bool overflow_empty()  const { return _overflow_stack.is_empty(); }

   inline bool is_empty()        const {

     return taskqueue_empty() && overflow_empty();

   }

 private:

   overflow_t _overflow_stack;

 };

 template <class E, MEMFLAGS F, unsigned int N>

 bool OverflowTaskQueue<E, F, N>::push(E t)

 {

   if (!taskqueue_t::push(t)) {

     overflow_stack()->push(t);

     TASKQUEUE_STATS_ONLY(stats.record_overflow(overflow_stack()->size()));

   }

   return true;

 }

 template <class E, MEMFLAGS F, unsigned int N>

 bool OverflowTaskQueue<E, F, N>::pop_overflow(E& t)

 {

   if (overflow_empty()) return false;

   t = overflow_stack()->pop();

   return true;

 }

 class TaskQueueSetSuper {

 protected:

   static int randomParkAndMiller(int* seed0);

 public:

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

   virtual bool peek() = 0;

 };

 template <MEMFLAGS F> class TaskQueueSetSuperImpl: public CHeapObj<F>, public TaskQueueSetSuper {

 };

 template<class T, MEMFLAGS F>

 class GenericTaskQueueSet: public TaskQueueSetSuperImpl<F> {

 private:

   uint _n;

   T** _queues;

 public:

   typedef typename T::element_type E;

   GenericTaskQueueSet(int n) : _n(n) {

     typedef T* GenericTaskQueuePtr;

     _queues = NEW_C_HEAP_ARRAY(GenericTaskQueuePtr, n, F);

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

       _queues[i] = NULL;

     }

   }

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

   void register_queue(uint i, T* q);

   T* queue(uint 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" to the stolen task, otherwise returns

   // false.

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

   bool peek();

 };

 template<class T, MEMFLAGS F> void

 GenericTaskQueueSet<T, F>::register_queue(uint i, T* q) {

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

   _queues[i] = q;

 }

 template<class T, MEMFLAGS F> T*

 GenericTaskQueueSet<T, F>::queue(uint i) {

   return _queues[i];

 }

 template<class T, MEMFLAGS F> bool

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

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

     if (steal_best_of_2(queue_num, seed, t)) {

       TASKQUEUE_STATS_ONLY(queue(queue_num)->stats.record_steal(true));

       return true;

     }

   }

   TASKQUEUE_STATS_ONLY(queue(queue_num)->stats.record_steal(false));

   return false;

 }

 template<class T, MEMFLAGS F> bool

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

   if (_n > 2) {

     uint k1 = queue_num;

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

     uint k2 = queue_num;

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

     // Sample both and try the larger.

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

     uint 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.

     uint k = (queue_num + 1) % 2;

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

   } else {

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

     return false;

   }

 }

 template<class T, MEMFLAGS F>

 bool GenericTaskQueueSet<T, F>::peek() {

   // Try all the queues.

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

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

       return true;

   }

   return false;

 }

 // When to terminate from the termination protocol.

 class TerminatorTerminator: public CHeapObj<mtInternal> {

 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.

 #undef TRACESPINNING

 class ParallelTaskTerminator: public StackObj {

 private:

   int _n_threads;

   TaskQueueSetSuper* _queue_set;

   int _offered_termination;

 #ifdef TRACESPINNING

   static uint _total_yields;

   static uint _total_spins;

   static uint _total_peeks;

 #endif

   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 if 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();

   // Same as above but the number of parallel threads is set to the

   // given number.

   void reset_for_reuse(int n_threads);

 #ifdef TRACESPINNING

   static uint total_yields() { return _total_yields; }

   static uint total_spins() { return _total_spins; }

   static uint total_peeks() { return _total_peeks; }

   static void print_termination_counts();

 #endif

 };

 template<class E, MEMFLAGS F, unsigned int N> inline bool

 GenericTaskQueue<E, F, N>::push(E t) {

   uint localBot = _bottom;

   assert(localBot < N, "_bottom out of range.");

   idx_t top = _age.top();

   uint dirty_n_elems = dirty_size(localBot, top);

   assert(dirty_n_elems < N, "n_elems out of range.");

   if (dirty_n_elems < max_elems()) {

     // g++ complains if the volatile result of the assignment is

     // unused, so we cast the volatile away.  We cannot cast directly

     // to void, because gcc treats that as not using the result of the

     // assignment.  However, casting to E& means that we trigger an

     // unused-value warning.  So, we cast the E& to void.

     (void) const_cast<E&>(_elems[localBot] = t);

     OrderAccess::release_store(&_bottom, increment_index(localBot));

     TASKQUEUE_STATS_ONLY(stats.record_push());

     return true;

   } else {

     return push_slow(t, dirty_n_elems);

   }

 }

 template<class E, MEMFLAGS F, unsigned int N> inline bool

 GenericTaskQueue<E, F, N>::pop_local(E& t) {

   uint 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.)

   uint dirty_n_elems = dirty_size(localBot, _age.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();

   // g++ complains if the volatile result of the assignment is

   // unused, so we cast the volatile away.  We cannot cast directly

   // to void, because gcc treats that as not using the result of the

   // assignment.  However, casting to E& means that we trigger an

   // unused-value warning.  So, we cast the E& to void.

   (void) const_cast<E&>(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.

   idx_t tp = _age.top();    // XXX

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

     assert(dirty_size(localBot, tp) != N - 1, "sanity");

     TASKQUEUE_STATS_ONLY(stats.record_pop());

     return true;

   } else {

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

     // path.

     return pop_local_slow(localBot, _age.get());

   }

 }

 typedef GenericTaskQueue<oop, mtGC>             OopTaskQueue;

 typedef GenericTaskQueueSet<OopTaskQueue, mtGC> OopTaskQueueSet;

 #ifdef _MSC_VER

 #pragma warning(push)

 // warning C4522: multiple assignment operators specified

 #pragma warning(disable:4522)

 #endif

 // This is a container class for either an oop* or a narrowOop*.

 // Both are pushed onto a task queue and the consumer will test is_narrow()

 // to determine which should be processed.

 class StarTask {

   void*  _holder;        // either union oop* or narrowOop*

   enum { COMPRESSED_OOP_MASK = 1 };

  public:

   StarTask(narrowOop* p) {

     assert(((uintptr_t)p & COMPRESSED_OOP_MASK) == 0, "Information loss!");

     _holder = (void *)((uintptr_t)p | COMPRESSED_OOP_MASK);

   }

   StarTask(oop* p)       {

     assert(((uintptr_t)p & COMPRESSED_OOP_MASK) == 0, "Information loss!");

     _holder = (void*)p;

   }

   StarTask()             { _holder = NULL; }

   operator oop*()        { return (oop*)_holder; }

   operator narrowOop*()  {

     return (narrowOop*)((uintptr_t)_holder & ~COMPRESSED_OOP_MASK);

   }

   StarTask& operator=(const StarTask& t) {

     _holder = t._holder;

     return *this;

   }

   volatile StarTask& operator=(const volatile StarTask& t) volatile {

     _holder = t._holder;

     return *this;

   }

   bool is_narrow() const {

     return (((uintptr_t)_holder & COMPRESSED_OOP_MASK) != 0);

   }

 };

 class ObjArrayTask

 {

 public:

   ObjArrayTask(oop o = NULL, int idx = 0): _obj(o), _index(idx) { }

   ObjArrayTask(oop o, size_t idx): _obj(o), _index(int(idx)) {

     assert(idx <= size_t(max_jint), "too big");

   }

   ObjArrayTask(const ObjArrayTask& t): _obj(t._obj), _index(t._index) { }

   ObjArrayTask& operator =(const ObjArrayTask& t) {

     _obj = t._obj;

     _index = t._index;

     return *this;

   }

   volatile ObjArrayTask&

   operator =(const volatile ObjArrayTask& t) volatile {

     _obj = t._obj;

     _index = t._index;

     return *this;

   }

   inline oop obj()   const { return _obj; }

   inline int index() const { return _index; }

   DEBUG_ONLY(bool is_valid() const); // Tasks to be pushed/popped must be valid.

 private:

   oop _obj;

   int _index;

 };

 #ifdef _MSC_VER

 #pragma warning(pop)

 #endif

 typedef OverflowTaskQueue<StarTask, mtClass>           OopStarTaskQueue;

 typedef GenericTaskQueueSet<OopStarTaskQueue, mtClass> OopStarTaskQueueSet;

 typedef OverflowTaskQueue<size_t, mtInternal>             RegionTaskQueue;

 typedef GenericTaskQueueSet<RegionTaskQueue, mtClass>     RegionTaskQueueSet;

 #endif // SHARE_VM_UTILITIES_TASKQUEUE_HPP