view ppc_patches/0208_rt-improve_handling_of_memory_ordering_in_taskqueue_hpp.patch @ 4881:edf2bb42e70f

Recent changes to patch queue
author goetz
date Fri, 28 Jun 2013 16:31:09 +0200
parents df79d76c17ab
children 9c922da99034
line wrap: on
line source
# HG changeset patch
# Parent 4a3d3fbe571cd7845a4dc62f71f264bb7f1d95e3
8006971 rt: Improve handling of memory ordering in taskqueue.hpp.

The current implementation does not work on PPC.
 - fix problem ordering accesses to _bottom:
   Pop_global not always reads the most recent value written
   by pop_local.
 - Add workaround for AIX.  On Aix the second call to _age.top()
   in pop_local is optimized away in some cases.  Depends on where
   and how it is inlined.
diff -r 4a3d3fbe571c src/os_cpu/aix_ppc/vm/taskqueue_aix_ppc.hpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/os_cpu/aix_ppc/vm/taskqueue_aix_ppc.hpp	Mon Apr 08 15:54:55 2013 +0200
@@ -0,0 +1,815 @@
+/*
+ * 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
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+// Don't fix!!! Must be the same define as in utilities/taskqueue.hpp!
+#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_aix_ppc
+# include "orderAccess_aix_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;
+
+private:
+  // The first free element after the last one pushed (mod N).
+  // We keep _bottom private and DO NOT USE IT DIRECTLY.
+  volatile uint _private_bottom;
+
+protected:
+  // Access to _bottom must be ordered. Use OrderAccess.
+  inline uint get_bottom() const {
+    return OrderAccess::load_acquire((volatile juint*)&_private_bottom);
+  }
+
+  inline void set_bottom(uint new_bottom) {
+    OrderAccess::release_store(&_private_bottom, new_bottom);
+  }
+
+  enum { MOD_N_MASK = N - 1 };
+
+  // Simple field access here, so no OrderAccess and no volatiles.
+  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; }
+
+    idx_t top() const { return _fields._top; }
+    idx_t tag() const { 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;
+    }
+
+    bool operator ==(const Age& other) const { return _data == other._data; }
+
+  public:
+    struct fields {
+      idx_t _top;
+      idx_t _tag;
+    };
+    union {
+      size_t _data;
+      fields _fields;
+    };
+  };
+
+
+private:
+  // Keep _age private and DO NOT USE IT DIRECTLY.
+  volatile Age _private_age;
+
+protected:
+  inline idx_t get_age_top() const {
+    return OrderAccess::load_acquire((volatile idx_t*) &(_private_age._fields._top));
+  }
+
+  inline Age get_age() {
+    size_t res = OrderAccess::load_ptr_acquire((volatile intptr_t*) &_private_age);
+    return *(Age*)(&res);
+  }
+
+  inline void set_age(Age a) {
+    OrderAccess::release_store_ptr((volatile intptr_t*) &_private_age, *(size_t*)(&a));
+  }
+
+  Age cmpxchg_age(const Age new_age, const Age old_age) volatile {
+    return (Age)(size_t)Atomic::cmpxchg_ptr((intptr_t)new_age._data,
+                                            (volatile intptr_t*) &(_private_age._data),
+                                            (intptr_t)old_age._data);
+  }
+
+  // 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() : _private_bottom(0), _private_age() {}
+
+  // Return true if the TaskQueue contains/does not contain any tasks.
+  // Use getters/setters with OrderAccess when accessing _bottom and _age.
+  bool peek() {
+    return get_bottom() != get_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(get_bottom(), get_age_top());
+  }
+
+  uint dirty_size() const {
+    return dirty_size(get_bottom(), get_age_top());
+  }
+
+  void set_empty() {
+    set_bottom(0);
+    set_age(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> {
+protected:
+  typedef typename TaskQueueSuper<N, F>::Age Age;
+  typedef typename TaskQueueSuper<N, F>::idx_t idx_t;
+
+  using TaskQueueSuper<N, F>::increment_index;
+  using TaskQueueSuper<N, F>::decrement_index;
+  using TaskQueueSuper<N, F>::dirty_size;
+  using TaskQueueSuper<N, F>::get_age_top;
+  using TaskQueueSuper<N, F>::get_age;
+
+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 = NEW_C_HEAP_ARRAY(E, N, F);
+}
+
+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 = this->get_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 = this->get_bottom();
+    // g++ complains if the volatile result of the assignment is unused.
+    const_cast<E&>(_elems[localBot] = t);
+    this->set_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 = this->cmpxchg_age(newAge, oldAge);
+    if (tempAge == oldAge) {
+      // We win.
+      assert(dirty_size(localBot, get_age_top()) != N - 1, "sanity");
+      TASKQUEUE_STATS_ONLY(stats.record_pop_slow());
+      return true;
+    }
+  }
+  // We loose; 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.
+  this->set_age(newAge);
+  assert(dirty_size(localBot, get_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 = get_age();
+
+  // Add fence for architectures with weak memory model.
+  // The fence has a cumulative effect on get_age and get_bottom.
+  // This way it is guaranteed that bottom is not older than age,
+  // what is crucial for the correctness of the algorithm.
+#if (defined ARM || defined IA64 || defined PPC64)
+  OrderAccess::fence();
+#endif
+
+  uint localBot = this->get_bottom();
+  uint n_elems = size(localBot, oldAge.top());
+  if (n_elems == 0) {
+    return false;
+  }
+
+  const_cast<E&>(t = _elems[oldAge.top()]);
+  Age newAge(oldAge);
+  newAge.increment();
+  Age resAge = this->cmpxchg_age(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 = this->get_bottom();
+  assert(localBot < N, "_bottom out of range.");
+  idx_t top = get_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.
+    const_cast<E&>(_elems[localBot] = t);
+    this->set_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 = this->get_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, get_age_top());
+  assert(dirty_n_elems != N - 1, "Shouldn't be possible...");
+  if (dirty_n_elems == 0) return false;
+  localBot = decrement_index(localBot);
+  this->set_bottom(localBot);
+  // This is necessary to prevent any read below from being reordered
+  // before the store just above.
+  OrderAccess::fence();
+  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 = get_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, get_age());
+  }
+}
+
+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
diff -r 4a3d3fbe571c src/share/vm/utilities/taskqueue.hpp
--- a/src/share/vm/utilities/taskqueue.hpp	Mon Apr 08 15:52:47 2013 +0200
+++ b/src/share/vm/utilities/taskqueue.hpp	Mon Apr 08 15:54:55 2013 +0200
@@ -22,6 +22,11 @@
  *
  */
 
+// Use special implementation on Aix as xlC optimizes volatile loads.
+#ifdef TARGET_OS_ARCH_aix_ppc
+# include "taskqueue_aix_ppc.hpp"
+#endif
+
 #ifndef SHARE_VM_UTILITIES_TASKQUEUE_HPP
 #define SHARE_VM_UTILITIES_TASKQUEUE_HPP
 
@@ -393,7 +398,16 @@
 template<class E, MEMFLAGS F, unsigned int N>
 bool GenericTaskQueue<E, F, N>::pop_global(E& t) {
   Age oldAge = _age.get();
-  uint localBot = _bottom;
+
+  // Architectures with weak memory model require a fence here. The
+  // fence has a cumulative effect on getting age and getting bottom.
+  // This way it is guaranteed that bottom is not older than age,
+  // what is crucial for the correctness of the algorithm.
+#if (defined ARM || defined IA64 || defined PPC64)
+  OrderAccess::fence();
+#endif
+
+  uint localBot = OrderAccess::load_acquire((volatile juint*)&_bottom);
   uint n_elems = size(localBot, oldAge.top());
   if (n_elems == 0) {
     return false;