8002356: Add ForkJoin common pool and CountedCompleter

Reviewed-by: chegar, mduigou

make/java/java/FILES_java.gmk

@@ -322,6 +322,7 @@ JAVA_JAVA_java = \
     java/util/concurrent/CopyOnWriteArrayList.java \
     java/util/concurrent/CopyOnWriteArraySet.java \
     java/util/concurrent/CountDownLatch.java \
+    java/util/concurrent/CountedCompleter.java \
     java/util/concurrent/CyclicBarrier.java \
     java/util/concurrent/DelayQueue.java \
     java/util/concurrent/Delayed.java \

src/share/classes/java/util/concurrent/CountedCompleter.java

+/*
+ * 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.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * 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.
+ */
+
+/*
+ * This file is available under and governed by the GNU General Public
+ * License version 2 only, as published by the Free Software Foundation.
+ * However, the following notice accompanied the original version of this
+ * file:
+ *
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package java.util.concurrent;
+
+/**
+ * A {@link ForkJoinTask} with a completion action performed when
+ * triggered and there are no remaining pending
+ * actions. CountedCompleters are in general more robust in the
+ * presence of subtask stalls and blockage than are other forms of
+ * ForkJoinTasks, but are less intuitive to program.  Uses of
+ * CountedCompleter are similar to those of other completion based
+ * components (such as {@link java.nio.channels.CompletionHandler})
+ * except that multiple <em>pending</em> completions may be necessary
+ * to trigger the completion action {@link #onCompletion}, not just one.
+ * Unless initialized otherwise, the {@linkplain #getPendingCount pending
+ * count} starts at zero, but may be (atomically) changed using
+ * methods {@link #setPendingCount}, {@link #addToPendingCount}, and
+ * {@link #compareAndSetPendingCount}. Upon invocation of {@link
+ * #tryComplete}, if the pending action count is nonzero, it is
+ * decremented; otherwise, the completion action is performed, and if
+ * this completer itself has a completer, the process is continued
+ * with its completer.  As is the case with related synchronization
+ * components such as {@link java.util.concurrent.Phaser Phaser} and
+ * {@link java.util.concurrent.Semaphore Semaphore}, these methods
+ * affect only internal counts; they do not establish any further
+ * internal bookkeeping. In particular, the identities of pending
+ * tasks are not maintained. As illustrated below, you can create
+ * subclasses that do record some or all pending tasks or their
+ * results when needed.  As illustrated below, utility methods
+ * supporting customization of completion traversals are also
+ * provided. However, because CountedCompleters provide only basic
+ * synchronization mechanisms, it may be useful to create further
+ * abstract subclasses that maintain linkages, fields, and additional
+ * support methods appropriate for a set of related usages.
+ *
+ * <p>A concrete CountedCompleter class must define method {@link
+ * #compute}, that should in most cases (as illustrated below), invoke
+ * {@code tryComplete()} once before returning. The class may also
+ * optionally override method {@link #onCompletion} to perform an
+ * action upon normal completion, and method {@link
+ * #onExceptionalCompletion} to perform an action upon any exception.
+ *
+ * <p>CountedCompleters most often do not bear results, in which case
+ * they are normally declared as {@code CountedCompleter<Void>}, and
+ * will always return {@code null} as a result value.  In other cases,
+ * you should override method {@link #getRawResult} to provide a
+ * result from {@code join(), invoke()}, and related methods.  In
+ * general, this method should return the value of a field (or a
+ * function of one or more fields) of the CountedCompleter object that
+ * holds the result upon completion. Method {@link #setRawResult} by
+ * default plays no role in CountedCompleters.  It is possible, but
+ * rarely applicable, to override this method to maintain other
+ * objects or fields holding result data.
+ *
+ * <p>A CountedCompleter that does not itself have a completer (i.e.,
+ * one for which {@link #getCompleter} returns {@code null}) can be
+ * used as a regular ForkJoinTask with this added functionality.
+ * However, any completer that in turn has another completer serves
+ * only as an internal helper for other computations, so its own task
+ * status (as reported in methods such as {@link ForkJoinTask#isDone})
+ * is arbitrary; this status changes only upon explicit invocations of
+ * {@link #complete}, {@link ForkJoinTask#cancel}, {@link
+ * ForkJoinTask#completeExceptionally} or upon exceptional completion
+ * of method {@code compute}. Upon any exceptional completion, the
+ * exception may be relayed to a task's completer (and its completer,
+ * and so on), if one exists and it has not otherwise already
+ * completed. Similarly, cancelling an internal CountedCompleter has
+ * only a local effect on that completer, so is not often useful.
+ *
+ * <p><b>Sample Usages.</b>
+ *
+ * <p><b>Parallel recursive decomposition.</b> CountedCompleters may
+ * be arranged in trees similar to those often used with {@link
+ * RecursiveAction}s, although the constructions involved in setting
+ * them up typically vary. Here, the completer of each task is its
+ * parent in the computation tree. Even though they entail a bit more
+ * bookkeeping, CountedCompleters may be better choices when applying
+ * a possibly time-consuming operation (that cannot be further
+ * subdivided) to each element of an array or collection; especially
+ * when the operation takes a significantly different amount of time
+ * to complete for some elements than others, either because of
+ * intrinsic variation (for example I/O) or auxiliary effects such as
+ * garbage collection.  Because CountedCompleters provide their own
+ * continuations, other threads need not block waiting to perform
+ * them.
+ *
+ * <p>For example, here is an initial version of a class that uses
+ * divide-by-two recursive decomposition to divide work into single
+ * pieces (leaf tasks). Even when work is split into individual calls,
+ * tree-based techniques are usually preferable to directly forking
+ * leaf tasks, because they reduce inter-thread communication and
+ * improve load balancing. In the recursive case, the second of each
+ * pair of subtasks to finish triggers completion of its parent
+ * (because no result combination is performed, the default no-op
+ * implementation of method {@code onCompletion} is not overridden). A
+ * static utility method sets up the base task and invokes it
+ * (here, implicitly using the {@link ForkJoinPool#commonPool()}).
+ *
+ * <pre> {@code
+ * class MyOperation<E> { void apply(E e) { ... }  }
+ *
+ * class ForEach<E> extends CountedCompleter<Void> {
+ *
+ *   public static <E> void forEach(E[] array, MyOperation<E> op) {
+ *     new ForEach<E>(null, array, op, 0, array.length).invoke();
+ *   }
+ *
+ *   final E[] array; final MyOperation<E> op; final int lo, hi;
+ *   ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
+ *     super(p);
+ *     this.array = array; this.op = op; this.lo = lo; this.hi = hi;
+ *   }
+ *
+ *   public void compute() { // version 1
+ *     if (hi - lo >= 2) {
+ *       int mid = (lo + hi) >>> 1;
+ *       setPendingCount(2); // must set pending count before fork
+ *       new ForEach(this, array, op, mid, hi).fork(); // right child
+ *       new ForEach(this, array, op, lo, mid).fork(); // left child
+ *     }
+ *     else if (hi > lo)
+ *       op.apply(array[lo]);
+ *     tryComplete();
+ *   }
+ * }}</pre>
+ *
+ * This design can be improved by noticing that in the recursive case,
+ * the task has nothing to do after forking its right task, so can
+ * directly invoke its left task before returning. (This is an analog
+ * of tail recursion removal.)  Also, because the task returns upon
+ * executing its left task (rather than falling through to invoke
+ * {@code tryComplete}) the pending count is set to one:
+ *
+ * <pre> {@code
+ * class ForEach<E> ...
+ *   public void compute() { // version 2
+ *     if (hi - lo >= 2) {
+ *       int mid = (lo + hi) >>> 1;
+ *       setPendingCount(1); // only one pending
+ *       new ForEach(this, array, op, mid, hi).fork(); // right child
+ *       new ForEach(this, array, op, lo, mid).compute(); // direct invoke
+ *     }
+ *     else {
+ *       if (hi > lo)
+ *         op.apply(array[lo]);
+ *       tryComplete();
+ *     }
+ *   }
+ * }</pre>
+ *
+ * As a further improvement, notice that the left task need not even
+ * exist.  Instead of creating a new one, we can iterate using the
+ * original task, and add a pending count for each fork. Additionally,
+ * because no task in this tree implements an {@link #onCompletion}
+ * method, {@code tryComplete()} can be replaced with {@link
+ * #propagateCompletion}.
+ *
+ * <pre> {@code
+ * class ForEach<E> ...
+ *   public void compute() { // version 3
+ *     int l = lo,  h = hi;
+ *     while (h - l >= 2) {
+ *       int mid = (l + h) >>> 1;
+ *       addToPendingCount(1);
+ *       new ForEach(this, array, op, mid, h).fork(); // right child
+ *       h = mid;
+ *     }
+ *     if (h > l)
+ *       op.apply(array[l]);
+ *     propagateCompletion();
+ *   }
+ * }</pre>
+ *
+ * Additional improvements of such classes might entail precomputing
+ * pending counts so that they can be established in constructors,
+ * specializing classes for leaf steps, subdividing by say, four,
+ * instead of two per iteration, and using an adaptive threshold
+ * instead of always subdividing down to single elements.
+ *
+ * <p><b>Searching.</b> A tree of CountedCompleters can search for a
+ * value or property in different parts of a data structure, and
+ * report a result in an {@link
+ * java.util.concurrent.atomic.AtomicReference AtomicReference} as
+ * soon as one is found. The others can poll the result to avoid
+ * unnecessary work. (You could additionally {@linkplain #cancel
+ * cancel} other tasks, but it is usually simpler and more efficient
+ * to just let them notice that the result is set and if so skip
+ * further processing.)  Illustrating again with an array using full
+ * partitioning (again, in practice, leaf tasks will almost always
+ * process more than one element):
+ *
+ * <pre> {@code
+ * class Searcher<E> extends CountedCompleter<E> {
+ *   final E[] array; final AtomicReference<E> result; final int lo, hi;
+ *   Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) {
+ *     super(p);
+ *     this.array = array; this.result = result; this.lo = lo; this.hi = hi;
+ *   }
+ *   public E getRawResult() { return result.get(); }
+ *   public void compute() { // similar to ForEach version 3
+ *     int l = lo,  h = hi;
+ *     while (result.get() == null && h >= l) {
+ *       if (h - l >= 2) {
+ *         int mid = (l + h) >>> 1;
+ *         addToPendingCount(1);
+ *         new Searcher(this, array, result, mid, h).fork();
+ *         h = mid;
+ *       }
+ *       else {
+ *         E x = array[l];
+ *         if (matches(x) && result.compareAndSet(null, x))
+ *           quietlyCompleteRoot(); // root task is now joinable
+ *         break;
+ *       }
+ *     }
+ *     tryComplete(); // normally complete whether or not found
+ *   }
+ *   boolean matches(E e) { ... } // return true if found
+ *
+ *   public static <E> E search(E[] array) {
+ *       return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke();
+ *   }
+ *}}</pre>
+ *
+ * In this example, as well as others in which tasks have no other
+ * effects except to compareAndSet a common result, the trailing
+ * unconditional invocation of {@code tryComplete} could be made
+ * conditional ({@code if (result.get() == null) tryComplete();})
+ * because no further bookkeeping is required to manage completions
+ * once the root task completes.
+ *
+ * <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
+ * results of multiple subtasks usually need to access these results
+ * in method {@link #onCompletion}. As illustrated in the following
+ * class (that performs a simplified form of map-reduce where mappings
+ * and reductions are all of type {@code E}), one way to do this in
+ * divide and conquer designs is to have each subtask record its
+ * sibling, so that it can be accessed in method {@code onCompletion}.
+ * This technique applies to reductions in which the order of
+ * combining left and right results does not matter; ordered
+ * reductions require explicit left/right designations.  Variants of
+ * other streamlinings seen in the above examples may also apply.
+ *
+ * <pre> {@code
+ * class MyMapper<E> { E apply(E v) {  ...  } }
+ * class MyReducer<E> { E apply(E x, E y) {  ...  } }
+ * class MapReducer<E> extends CountedCompleter<E> {
+ *   final E[] array; final MyMapper<E> mapper;
+ *   final MyReducer<E> reducer; final int lo, hi;
+ *   MapReducer<E> sibling;
+ *   E result;
+ *   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
+ *              MyReducer<E> reducer, int lo, int hi) {
+ *     super(p);
+ *     this.array = array; this.mapper = mapper;
+ *     this.reducer = reducer; this.lo = lo; this.hi = hi;
+ *   }
+ *   public void compute() {
+ *     if (hi - lo >= 2) {
+ *       int mid = (lo + hi) >>> 1;
+ *       MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
+ *       MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
+ *       left.sibling = right;
+ *       right.sibling = left;
+ *       setPendingCount(1); // only right is pending
+ *       right.fork();
+ *       left.compute();     // directly execute left
+ *     }
+ *     else {
+ *       if (hi > lo)
+ *           result = mapper.apply(array[lo]);
+ *       tryComplete();
+ *     }
+ *   }
+ *   public void onCompletion(CountedCompleter<?> caller) {
+ *     if (caller != this) {
+ *       MapReducer<E> child = (MapReducer<E>)caller;
+ *       MapReducer<E> sib = child.sibling;
+ *       if (sib == null || sib.result == null)
+ *         result = child.result;
+ *       else
+ *         result = reducer.apply(child.result, sib.result);
+ *     }
+ *   }
+ *   public E getRawResult() { return result; }
+ *
+ *   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
+ *     return new MapReducer<E>(null, array, mapper, reducer,
+ *                              0, array.length).invoke();
+ *   }
+ * }}</pre>
+ *
+ * Here, method {@code onCompletion} takes a form common to many
+ * completion designs that combine results. This callback-style method
+ * is triggered once per task, in either of the two different contexts
+ * in which the pending count is, or becomes, zero: (1) by a task
+ * itself, if its pending count is zero upon invocation of {@code
+ * tryComplete}, or (2) by any of its subtasks when they complete and
+ * decrement the pending count to zero. The {@code caller} argument
+ * distinguishes cases.  Most often, when the caller is {@code this},
+ * no action is necessary. Otherwise the caller argument can be used
+ * (usually via a cast) to supply a value (and/or links to other
+ * values) to be combined.  Assuming proper use of pending counts, the
+ * actions inside {@code onCompletion} occur (once) upon completion of
+ * a task and its subtasks. No additional synchronization is required
+ * within this method to ensure thread safety of accesses to fields of
+ * this task or other completed tasks.
+ *
+ * <p><b>Completion Traversals</b>. If using {@code onCompletion} to
+ * process completions is inapplicable or inconvenient, you can use
+ * methods {@link #firstComplete} and {@link #nextComplete} to create
+ * custom traversals.  For example, to define a MapReducer that only
+ * splits out right-hand tasks in the form of the third ForEach
+ * example, the completions must cooperatively reduce along
+ * unexhausted subtask links, which can be done as follows:
+ *
+ * <pre> {@code
+ * class MapReducer<E> extends CountedCompleter<E> { // version 2
+ *   final E[] array; final MyMapper<E> mapper;
+ *   final MyReducer<E> reducer; final int lo, hi;
+ *   MapReducer<E> forks, next; // record subtask forks in list
+ *   E result;
+ *   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
+ *              MyReducer<E> reducer, int lo, int hi, MapReducer<E> next) {
+ *     super(p);
+ *     this.array = array; this.mapper = mapper;
+ *     this.reducer = reducer; this.lo = lo; this.hi = hi;
+ *     this.next = next;
+ *   }
+ *   public void compute() {
+ *     int l = lo,  h = hi;
+ *     while (h - l >= 2) {
+ *       int mid = (l + h) >>> 1;
+ *       addToPendingCount(1);
+ *       (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork;
+ *       h = mid;
+ *     }
+ *     if (h > l)
+ *       result = mapper.apply(array[l]);
+ *     // process completions by reducing along and advancing subtask links
+ *     for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
+ *       for (MapReducer t = (MapReducer)c, s = t.forks;  s != null; s = t.forks = s.next)
+ *         t.result = reducer.apply(t.result, s.result);
+ *     }
+ *   }
+ *   public E getRawResult() { return result; }
+ *
+ *   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
+ *     return new MapReducer<E>(null, array, mapper, reducer,
+ *                              0, array.length, null).invoke();
+ *   }
+ * }}</pre>
+ *
+ * <p><b>Triggers.</b> Some CountedCompleters are themselves never
+ * forked, but instead serve as bits of plumbing in other designs;
+ * including those in which the completion of one of more async tasks
+ * triggers another async task. For example:
+ *
+ * <pre> {@code
+ * class HeaderBuilder extends CountedCompleter<...> { ... }
+ * class BodyBuilder extends CountedCompleter<...> { ... }
+ * class PacketSender extends CountedCompleter<...> {
+ *   PacketSender(...) { super(null, 1); ... } // trigger on second completion
+ *   public void compute() { } // never called
+ *   public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
+ * }
+ * // sample use:
+ * PacketSender p = new PacketSender();
+ * new HeaderBuilder(p, ...).fork();
+ * new BodyBuilder(p, ...).fork();
+ * }</pre>
+ *
+ * @since 1.8
+ * @author Doug Lea
+ */
+public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
+    private static final long serialVersionUID = 5232453752276485070L;
+
+    /** This task's completer, or null if none */
+    final CountedCompleter<?> completer;
+    /** The number of pending tasks until completion */
+    volatile int pending;
+
+    /**
+     * Creates a new CountedCompleter with the given completer
+     * and initial pending count.
+     *
+     * @param completer this task's completer, or {@code null} if none
+     * @param initialPendingCount the initial pending count
+     */
+    protected CountedCompleter(CountedCompleter<?> completer,
+                               int initialPendingCount) {
+        this.completer = completer;
+        this.pending = initialPendingCount;
+    }
+
+    /**
+     * Creates a new CountedCompleter with the given completer
+     * and an initial pending count of zero.
+     *
+     * @param completer this task's completer, or {@code null} if none
+     */
+    protected CountedCompleter(CountedCompleter<?> completer) {
+        this.completer = completer;
+    }
+
+    /**
+     * Creates a new CountedCompleter with no completer
+     * and an initial pending count of zero.
+     */
+    protected CountedCompleter() {
+        this.completer = null;
+    }
+
+    /**
+     * The main computation performed by this task.
+     */
+    public abstract void compute();
+
+    /**
+     * Performs an action when method {@link #tryComplete} is invoked
+     * and the pending count is zero, or when the unconditional
+     * method {@link #complete} is invoked.  By default, this method
+     * does nothing. You can distinguish cases by checking the
+     * identity of the given caller argument. If not equal to {@code
+     * this}, then it is typically a subtask that may contain results
+     * (and/or links to other results) to combine.
+     *
+     * @param caller the task invoking this method (which may
+     * be this task itself).
+     */
+    public void onCompletion(CountedCompleter<?> caller) {
+    }
+
+    /**
+     * Performs an action when method {@link #completeExceptionally}
+     * is invoked or method {@link #compute} throws an exception, and
+     * this task has not otherwise already completed normally. On
+     * entry to this method, this task {@link
+     * ForkJoinTask#isCompletedAbnormally}.  The return value of this
+     * method controls further propagation: If {@code true} and this
+     * task has a completer, then this completer is also completed
+     * exceptionally.  The default implementation of this method does
+     * nothing except return {@code true}.
+     *
+     * @param ex the exception
+     * @param caller the task invoking this method (which may
+     * be this task itself).
+     * @return true if this exception should be propagated to this
+     * task's completer, if one exists.
+     */
+    public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
+        return true;
+    }
+
+    /**
+     * Returns the completer established in this task's constructor,
+     * or {@code null} if none.
+     *
+     * @return the completer
+     */
+    public final CountedCompleter<?> getCompleter() {
+        return completer;
+    }
+
+    /**
+     * Returns the current pending count.
+     *
+     * @return the current pending count
+     */
+    public final int getPendingCount() {
+        return pending;
+    }
+
+    /**
+     * Sets the pending count to the given value.
+     *
+     * @param count the count
+     */
+    public final void setPendingCount(int count) {
+        pending = count;
+    }
+
+    /**
+     * Adds (atomically) the given value to the pending count.
+     *
+     * @param delta the value to add
+     */
+    public final void addToPendingCount(int delta) {
+        int c; // note: can replace with intrinsic in jdk8
+        do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
+    }
+
+    /**
+     * Sets (atomically) the pending count to the given count only if
+     * it currently holds the given expected value.
+     *
+     * @param expected the expected value
+     * @param count the new value
+     * @return true if successful
+     */
+    public final boolean compareAndSetPendingCount(int expected, int count) {
+        return U.compareAndSwapInt(this, PENDING, expected, count);
+    }
+
+    /**
+     * If the pending count is nonzero, (atomically) decrements it.
+     *
+     * @return the initial (undecremented) pending count holding on entry
+     * to this method
+     */
+    public final int decrementPendingCountUnlessZero() {
+        int c;
+        do {} while ((c = pending) != 0 &&
+                     !U.compareAndSwapInt(this, PENDING, c, c - 1));
+        return c;
+    }
+
+    /**
+     * Returns the root of the current computation; i.e., this
+     * task if it has no completer, else its completer's root.
+     *
+     * @return the root of the current computation
+     */
+    public final CountedCompleter<?> getRoot() {
+        CountedCompleter<?> a = this, p;
+        while ((p = a.completer) != null)
+            a = p;
+        return a;
+    }
+
+    /**
+     * If the pending count is nonzero, decrements the count;
+     * otherwise invokes {@link #onCompletion} and then similarly
+     * tries to complete this task's completer, if one exists,
+     * else marks this task as complete.
+     */
+    public final void tryComplete() {
+        CountedCompleter<?> a = this, s = a;
+        for (int c;;) {
+            if ((c = a.pending) == 0) {
+                a.onCompletion(s);
+                if ((a = (s = a).completer) == null) {
+                    s.quietlyComplete();
+                    return;
+                }
+            }
+            else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
+                return;
+        }
+    }
+
+    /**
+     * Equivalent to {@link #tryComplete} but does not invoke {@link
+     * #onCompletion} along the completion path: If the pending count
+     * is nonzero, decrements the count; otherwise, similarly tries to
+     * complete this task's completer, if one exists, else marks this
+     * task as complete. This method may be useful in cases where
+     * {@code onCompletion} should not, or need not, be invoked for
+     * each completer in a computation.
+     */
+    public final void propagateCompletion() {
+        CountedCompleter<?> a = this, s = a;
+        for (int c;;) {
+            if ((c = a.pending) == 0) {
+                if ((a = (s = a).completer) == null) {
+                    s.quietlyComplete();
+                    return;
+                }
+            }
+            else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
+                return;
+        }
+    }
+
+    /**
+     * Regardless of pending count, invokes {@link #onCompletion},
+     * marks this task as complete and further triggers {@link
+     * #tryComplete} on this task's completer, if one exists.  The
+     * given rawResult is used as an argument to {@link #setRawResult}
+     * before invoking {@link #onCompletion} or marking this task as
+     * complete; its value is meaningful only for classes overriding
+     * {@code setRawResult}.
+     *
+     * <p>This method may be useful when forcing completion as soon as
+     * any one (versus all) of several subtask results are obtained.
+     * However, in the common (and recommended) case in which {@code
+     * setRawResult} is not overridden, this effect can be obtained
+     * more simply using {@code quietlyCompleteRoot();}.
+     *
+     * @param rawResult the raw result
+     */
+    public void complete(T rawResult) {
+        CountedCompleter<?> p;
+        setRawResult(rawResult);
+        onCompletion(this);
+        quietlyComplete();
+        if ((p = completer) != null)
+            p.tryComplete();
+    }
+
+
+    /**
+     * If this task's pending count is zero, returns this task;
+     * otherwise decrements its pending count and returns {@code
+     * null}. This method is designed to be used with {@link
+     * #nextComplete} in completion traversal loops.
+     *
+     * @return this task, if pending count was zero, else {@code null}
+     */
+    public final CountedCompleter<?> firstComplete() {
+        for (int c;;) {
+            if ((c = pending) == 0)
+                return this;
+            else if (U.compareAndSwapInt(this, PENDING, c, c - 1))
+                return null;
+        }
+    }
+
+    /**
+     * If this task does not have a completer, invokes {@link
+     * ForkJoinTask#quietlyComplete} and returns {@code null}.  Or, if
+     * this task's pending count is non-zero, decrements its pending
+     * count and returns {@code null}.  Otherwise, returns the
+     * completer.  This method can be used as part of a completion
+     * traversal loop for homogeneous task hierarchies:
+     *
+     * <pre> {@code
+     * for (CountedCompleter<?> c = firstComplete();
+     *      c != null;
+     *      c = c.nextComplete()) {
+     *   // ... process c ...
+     * }}</pre>
+     *
+     * @return the completer, or {@code null} if none
+     */
+    public final CountedCompleter<?> nextComplete() {
+        CountedCompleter<?> p;
+        if ((p = completer) != null)
+            return p.firstComplete();
+        else {
+            quietlyComplete();
+            return null;
+        }
+    }
+
+    /**
+     * Equivalent to {@code getRoot().quietlyComplete()}.
+     */
+    public final void quietlyCompleteRoot() {
+        for (CountedCompleter<?> a = this, p;;) {
+            if ((p = a.completer) == null) {
+                a.quietlyComplete();
+                return;
+            }
+            a = p;
+        }
+    }
+
+    /**
+     * Supports ForkJoinTask exception propagation.
+     */
+    void internalPropagateException(Throwable ex) {
+        CountedCompleter<?> a = this, s = a;
+        while (a.onExceptionalCompletion(ex, s) &&
+               (a = (s = a).completer) != null && a.status >= 0)
+            a.recordExceptionalCompletion(ex);
+    }
+
+    /**
+     * Implements execution conventions for CountedCompleters.
+     */
+    protected final boolean exec() {
+        compute();
+        return false;
+    }
+
+    /**
+     * Returns the result of the computation. By default
+     * returns {@code null}, which is appropriate for {@code Void}
+     * actions, but in other cases should be overridden, almost
+     * always to return a field or function of a field that
+     * holds the result upon completion.
+     *
+     * @return the result of the computation
+     */
+    public T getRawResult() { return null; }
+
+    /**
+     * A method that result-bearing CountedCompleters may optionally
+     * use to help maintain result data.  By default, does nothing.
+     * Overrides are not recommended. However, if this method is
+     * overridden to update existing objects or fields, then it must
+     * in general be defined to be thread-safe.
+     */
+    protected void setRawResult(T t) { }
+
+    // Unsafe mechanics
+    private static final sun.misc.Unsafe U;
+    private static final long PENDING;
+    static {
+        try {
+            U = sun.misc.Unsafe.getUnsafe();
+            PENDING = U.objectFieldOffset
+                (CountedCompleter.class.getDeclaredField("pending"));
+        } catch (Exception e) {
+            throw new Error(e);
+        }
+    }
+}

src/share/classes/java/util/concurrent/ForkJoinPool.java

@@ -40,7 +40,6 @@ import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
-import java.util.Random;
 import java.util.concurrent.AbstractExecutorService;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutorService;
@@ -48,11 +47,6 @@ import java.util.concurrent.Future;
 import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.RunnableFuture;
 import java.util.concurrent.TimeUnit;
-import java.util.concurrent.TimeoutException;
-import java.util.concurrent.atomic.AtomicInteger;
-import java.util.concurrent.locks.LockSupport;
-import java.util.concurrent.locks.ReentrantLock;
-import java.util.concurrent.locks.Condition;
 
 /**
  * An {@link ExecutorService} for running {@link ForkJoinTask}s.
@@ -63,21 +57,31 @@ import java.util.concurrent.locks.Condition;
  * <p>A {@code ForkJoinPool} differs from other kinds of {@link
  * ExecutorService} mainly by virtue of employing
  * <em>work-stealing</em>: all threads in the pool attempt to find and
- * execute subtasks created by other active tasks (eventually blocking
- * waiting for work if none exist). This enables efficient processing
- * when most tasks spawn other subtasks (as do most {@code
- * ForkJoinTask}s). When setting <em>asyncMode</em> to true in
- * constructors, {@code ForkJoinPool}s may also be appropriate for use
- * with event-style tasks that are never joined.
+ * execute tasks submitted to the pool and/or created by other active
+ * tasks (eventually blocking waiting for work if none exist). This
+ * enables efficient processing when most tasks spawn other subtasks
+ * (as do most {@code ForkJoinTask}s), as well as when many small
+ * tasks are submitted to the pool from external clients.  Especially
+ * when setting <em>asyncMode</em> to true in constructors, {@code
+ * ForkJoinPool}s may also be appropriate for use with event-style
+ * tasks that are never joined.
  *
- * <p>A {@code ForkJoinPool} is constructed with a given target
- * parallelism level; by default, equal to the number of available
- * processors. The pool attempts to maintain enough active (or
- * available) threads by dynamically adding, suspending, or resuming
- * internal worker threads, even if some tasks are stalled waiting to
- * join others. However, no such adjustments are guaranteed in the
- * face of blocked IO or other unmanaged synchronization. The nested
- * {@link ManagedBlocker} interface enables extension of the kinds of
+ * <p>A static {@link #commonPool()} is available and appropriate for
+ * most applications. The common pool is used by any ForkJoinTask that
+ * is not explicitly submitted to a specified pool. Using the common
+ * pool normally reduces resource usage (its threads are slowly
+ * reclaimed during periods of non-use, and reinstated upon subsequent
+ * use).
+ *
+ * <p>For applications that require separate or custom pools, a {@code
+ * ForkJoinPool} may be constructed with a given target parallelism
+ * level; by default, equal to the number of available processors. The
+ * pool attempts to maintain enough active (or available) threads by
+ * dynamically adding, suspending, or resuming internal worker
+ * threads, even if some tasks are stalled waiting to join
+ * others. However, no such adjustments are guaranteed in the face of
+ * blocked I/O or other unmanaged synchronization. The nested {@link
+ * ManagedBlocker} interface enables extension of the kinds of
  * synchronization accommodated.
  *
  * <p>In addition to execution and lifecycle control methods, this
@@ -87,16 +91,17 @@ import java.util.concurrent.locks.Condition;
  * {@link #toString} returns indications of pool state in a
  * convenient form for informal monitoring.
  *
- * <p> As is the case with other ExecutorServices, there are three
- * main task execution methods summarized in the following
- * table. These are designed to be used by clients not already engaged
- * in fork/join computations in the current pool.  The main forms of
- * these methods accept instances of {@code ForkJoinTask}, but
- * overloaded forms also allow mixed execution of plain {@code
+ * <p>As is the case with other ExecutorServices, there are three
+ * main task execution methods summarized in the following table.
+ * These are designed to be used primarily by clients not already
+ * engaged in fork/join computations in the current pool.  The main
+ * forms of these methods accept instances of {@code ForkJoinTask},
+ * but overloaded forms also allow mixed execution of plain {@code
  * Runnable}- or {@code Callable}- based activities as well.  However,
- * tasks that are already executing in a pool should normally
- * <em>NOT</em> use these pool execution methods, but instead use the
- * within-computation forms listed in the table.
+ * tasks that are already executing in a pool should normally instead
+ * use the within-computation forms listed in the table unless using
+ * async event-style tasks that are not usually joined, in which case
+ * there is little difference among choice of methods.
  *
  * <table BORDER CELLPADDING=3 CELLSPACING=1>
  *  <tr>
@@ -121,23 +126,16 @@ import java.util.concurrent.locks.Condition;
  *  </tr>
  * </table>
  *
- * <p><b>Sample Usage.</b> Normally a single {@code ForkJoinPool} is
- * used for all parallel task execution in a program or subsystem.
- * Otherwise, use would not usually outweigh the construction and
- * bookkeeping overhead of creating a large set of threads. For
- * example, a common pool could be used for the {@code SortTasks}
- * illustrated in {@link RecursiveAction}. Because {@code
- * ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon
- * daemon} mode, there is typically no need to explicitly {@link
- * #shutdown} such a pool upon program exit.
- *
- * <pre>
- * static final ForkJoinPool mainPool = new ForkJoinPool();
- * ...
- * public void sort(long[] array) {
- *   mainPool.invoke(new SortTask(array, 0, array.length));
- * }
- * </pre>
+ * <p>The common pool is by default constructed with default
+ * parameters, but these may be controlled by setting three {@link
+ * System#getProperty system properties} with prefix {@code
+ * java.util.concurrent.ForkJoinPool.common}: {@code parallelism} --
+ * an integer greater than zero, {@code threadFactory} -- the class
+ * name of a {@link ForkJoinWorkerThreadFactory}, and {@code
+ * exceptionHandler} -- the class name of a {@link
+ * java.lang.Thread.UncaughtExceptionHandler
+ * Thread.UncaughtExceptionHandler}. Upon any error in establishing
+ * these settings, default parameters are used.
  *
  * <p><b>Implementation notes</b>: This implementation restricts the
  * maximum number of running threads to 32767. Attempts to create
@@ -156,213 +154,387 @@ public class ForkJoinPool extends AbstractExecutorService {
     /*
      * Implementation Overview
      *
-     * This class provides the central bookkeeping and control for a
-     * set of worker threads: Submissions from non-FJ threads enter
-     * into a submission queue. Workers take these tasks and typically
-     * split them into subtasks that may be stolen by other workers.
-     * Preference rules give first priority to processing tasks from
-     * their own queues (LIFO or FIFO, depending on mode), then to
-     * randomized FIFO steals of tasks in other worker queues, and
-     * lastly to new submissions.
+     * This class and its nested classes provide the main
+     * functionality and control for a set of worker threads:
+     * Submissions from non-FJ threads enter into submission queues.
+     * Workers take these tasks and typically split them into subtasks
+     * that may be stolen by other workers.  Preference rules give
+     * first priority to processing tasks from their own queues (LIFO
+     * or FIFO, depending on mode), then to randomized FIFO steals of
+     * tasks in other queues.
+     *
+     * WorkQueues
+     * ==========
+     *
+     * Most operations occur within work-stealing queues (in nested
+     * class WorkQueue).  These are special forms of Deques that
+     * support only three of the four possible end-operations -- push,
+     * pop, and poll (aka steal), under the further constraints that
+     * push and pop are called only from the owning thread (or, as
+     * extended here, under a lock), while poll may be called from
+     * other threads.  (If you are unfamiliar with them, you probably
+     * want to read Herlihy and Shavit's book "The Art of
+     * Multiprocessor programming", chapter 16 describing these in
+     * more detail before proceeding.)  The main work-stealing queue
+     * design is roughly similar to those in the papers "Dynamic
+     * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
+     * (http://research.sun.com/scalable/pubs/index.html) and
+     * "Idempotent work stealing" by Michael, Saraswat, and Vechev,
+     * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
+     * The main differences ultimately stem from GC requirements that
+     * we null out taken slots as soon as we can, to maintain as small
+     * a footprint as possible even in programs generating huge
+     * numbers of tasks. To accomplish this, we shift the CAS
+     * arbitrating pop vs poll (steal) from being on the indices
+     * ("base" and "top") to the slots themselves.  So, both a
+     * successful pop and poll mainly entail a CAS of a slot from
+     * non-null to null.  Because we rely on CASes of references, we
+     * do not need tag bits on base or top.  They are simple ints as
+     * used in any circular array-based queue (see for example
+     * ArrayDeque).  Updates to the indices must still be ordered in a
+     * way that guarantees that top == base means the queue is empty,
+     * but otherwise may err on the side of possibly making the queue
+     * appear nonempty when a push, pop, or poll have not fully
+     * committed. Note that this means that the poll operation,
+     * considered individually, is not wait-free. One thief cannot
+     * successfully continue until another in-progress one (or, if
+     * previously empty, a push) completes.  However, in the
+     * aggregate, we ensure at least probabilistic non-blockingness.
+     * If an attempted steal fails, a thief always chooses a different
+     * random victim target to try next. So, in order for one thief to
+     * progress, it suffices for any in-progress poll or new push on
+     * any empty queue to complete. (This is why we normally use
+     * method pollAt and its variants that try once at the apparent
+     * base index, else consider alternative actions, rather than
+     * method poll.)
+     *
+     * This approach also enables support of a user mode in which local
+     * task processing is in FIFO, not LIFO order, simply by using
+     * poll rather than pop.  This can be useful in message-passing
+     * frameworks in which tasks are never joined.  However neither
+     * mode considers affinities, loads, cache localities, etc, so
+     * rarely provide the best possible performance on a given
+     * machine, but portably provide good throughput by averaging over
+     * these factors.  (Further, even if we did try to use such
+     * information, we do not usually have a basis for exploiting it.
+     * For example, some sets of tasks profit from cache affinities,
+     * but others are harmed by cache pollution effects.)
+     *
+     * WorkQueues are also used in a similar way for tasks submitted
+     * to the pool. We cannot mix these tasks in the same queues used
+     * for work-stealing (this would contaminate lifo/fifo
+     * processing). Instead, we randomly associate submission queues
+     * with submitting threads, using a form of hashing.  The
+     * ThreadLocal Submitter class contains a value initially used as
+     * a hash code for choosing existing queues, but may be randomly
+     * repositioned upon contention with other submitters.  In
+     * essence, submitters act like workers except that they are
+     * restricted to executing local tasks that they submitted (or in
+     * the case of CountedCompleters, others with the same root task).
+     * However, because most shared/external queue operations are more
+     * expensive than internal, and because, at steady state, external
+     * submitters will compete for CPU with workers, ForkJoinTask.join
+     * and related methods disable them from repeatedly helping to
+     * process tasks if all workers are active.  Insertion of tasks in
+     * shared mode requires a lock (mainly to protect in the case of
+     * resizing) but we use only a simple spinlock (using bits in
+     * field qlock), because submitters encountering a busy queue move
+     * on to try or create other queues -- they block only when
+     * creating and registering new queues.
+     *
+     * Management
+     * ==========
      *
      * The main throughput advantages of work-stealing stem from
      * decentralized control -- workers mostly take tasks from
      * themselves or each other. We cannot negate this in the
      * implementation of other management responsibilities. The main
      * tactic for avoiding bottlenecks is packing nearly all
-     * essentially atomic control state into a single 64bit volatile
-     * variable ("ctl"). This variable is read on the order of 10-100
-     * times as often as it is modified (always via CAS). (There is
-     * some additional control state, for example variable "shutdown"
-     * for which we can cope with uncoordinated updates.)  This
-     * streamlines synchronization and control at the expense of messy
-     * constructions needed to repack status bits upon updates.
-     * Updates tend not to contend with each other except during
-     * bursts while submitted tasks begin or end.  In some cases when
-     * they do contend, threads can instead do something else
-     * (usually, scan for tasks) until contention subsides.
-     *
-     * To enable packing, we restrict maximum parallelism to (1<<15)-1
-     * (which is far in excess of normal operating range) to allow
-     * ids, counts, and their negations (used for thresholding) to fit
-     * into 16bit fields.
-     *
-     * Recording Workers.  Workers are recorded in the "workers" array
-     * that is created upon pool construction and expanded if (rarely)
-     * necessary.  This is an array as opposed to some other data
-     * structure to support index-based random steals by workers.
-     * Updates to the array recording new workers and unrecording
-     * terminated ones are protected from each other by a seqLock
-     * (scanGuard) but the array is otherwise concurrently readable,
-     * and accessed directly by workers. To simplify index-based
-     * operations, the array size is always a power of two, and all
-     * readers must tolerate null slots. To avoid flailing during
-     * start-up, the array is presized to hold twice #parallelism
-     * workers (which is unlikely to need further resizing during
-     * execution). But to avoid dealing with so many null slots,
-     * variable scanGuard includes a mask for the nearest power of two
-     * that contains all current workers.  All worker thread creation
-     * is on-demand, triggered by task submissions, replacement of
-     * terminated workers, and/or compensation for blocked
-     * workers. However, all other support code is set up to work with
-     * other policies.  To ensure that we do not hold on to worker
-     * references that would prevent GC, ALL accesses to workers are
-     * via indices into the workers array (which is one source of some
-     * of the messy code constructions here). In essence, the workers
-     * array serves as a weak reference mechanism. Thus for example
-     * the wait queue field of ctl stores worker indices, not worker
-     * references.  Access to the workers in associated methods (for
-     * example signalWork) must both index-check and null-check the
-     * IDs. All such accesses ignore bad IDs by returning out early
-     * from what they are doing, since this can only be associated
-     * with termination, in which case it is OK to give up.
-     *
-     * All uses of the workers array, as well as queue arrays, check
-     * that the array is non-null (even if previously non-null). This
-     * allows nulling during termination, which is currently not
-     * necessary, but remains an option for resource-revocation-based
-     * shutdown schemes.
-     *
-     * Wait Queuing. Unlike HPC work-stealing frameworks, we cannot
+     * essentially atomic control state into two volatile variables
+     * that are by far most often read (not written) as status and
+     * consistency checks.
+     *
+     * Field "ctl" contains 64 bits holding all the information needed
+     * to atomically decide to add, inactivate, enqueue (on an event
+     * queue), dequeue, and/or re-activate workers.  To enable this
+     * packing, we restrict maximum parallelism to (1<<15)-1 (which is
+     * far in excess of normal operating range) to allow ids, counts,
+     * and their negations (used for thresholding) to fit into 16bit
+     * fields.
+     *
+     * Field "plock" is a form of sequence lock with a saturating
+     * shutdown bit (similarly for per-queue "qlocks"), mainly
+     * protecting updates to the workQueues array, as well as to
+     * enable shutdown.  When used as a lock, it is normally only very
+     * briefly held, so is nearly always available after at most a
+     * brief spin, but we use a monitor-based backup strategy to
+     * block when needed.
+     *
+     * Recording WorkQueues.  WorkQueues are recorded in the
+     * "workQueues" array that is created upon first use and expanded
+     * if necessary.  Updates to the array while recording new workers
+     * and unrecording terminated ones are protected from each other
+     * by a lock but the array is otherwise concurrently readable, and
+     * accessed directly.  To simplify index-based operations, the
+     * array size is always a power of two, and all readers must
+     * tolerate null slots. Worker queues are at odd indices. Shared
+     * (submission) queues are at even indices, up to a maximum of 64
+     * slots, to limit growth even if array needs to expand to add
+     * more workers. Grouping them together in this way simplifies and
+     * speeds up task scanning.
+     *
+     * All worker thread creation is on-demand, triggered by task
+     * submissions, replacement of terminated workers, and/or
+     * compensation for blocked workers. However, all other support
+     * code is set up to work with other policies.  To ensure that we
+     * do not hold on to worker references that would prevent GC, ALL
+     * accesses to workQueues are via indices into the workQueues
+     * array (which is one source of some of the messy code
+     * constructions here). In essence, the workQueues array serves as
+     * a weak reference mechanism. Thus for example the wait queue
+     * field of ctl stores indices, not references.  Access to the
+     * workQueues in associated methods (for example signalWork) must
+     * both index-check and null-check the IDs. All such accesses
+     * ignore bad IDs by returning out early from what they are doing,
+     * since this can only be associated with termination, in which
+     * case it is OK to give up.  All uses of the workQueues array
+     * also check that it is non-null (even if previously
+     * non-null). This allows nulling during termination, which is
+     * currently not necessary, but remains an option for
+     * resource-revocation-based shutdown schemes. It also helps
+     * reduce JIT issuance of uncommon-trap code, which tends to
+     * unnecessarily complicate control flow in some methods.
+     *
+     * Event Queuing. Unlike HPC work-stealing frameworks, we cannot
      * let workers spin indefinitely scanning for tasks when none can
      * be found immediately, and we cannot start/resume workers unless
      * there appear to be tasks available.  On the other hand, we must
      * quickly prod them into action when new tasks are submitted or
-     * generated.  We park/unpark workers after placing in an event
-     * wait queue when they cannot find work. This "queue" is actually
-     * a simple Treiber stack, headed by the "id" field of ctl, plus a
-     * 15bit counter value to both wake up waiters (by advancing their
-     * count) and avoid ABA effects. Successors are held in worker
-     * field "nextWait".  Queuing deals with several intrinsic races,
-     * mainly that a task-producing thread can miss seeing (and
+     * generated. In many usages, ramp-up time to activate workers is
+     * the main limiting factor in overall performance (this is
+     * compounded at program start-up by JIT compilation and
+     * allocation). So we try to streamline this as much as possible.
+     * We park/unpark workers after placing in an event wait queue
+     * when they cannot find work. This "queue" is actually a simple
+     * Treiber stack, headed by the "id" field of ctl, plus a 15bit
+     * counter value (that reflects the number of times a worker has
+     * been inactivated) to avoid ABA effects (we need only as many
+     * version numbers as worker threads). Successors are held in
+     * field WorkQueue.nextWait.  Queuing deals with several intrinsic
+     * races, mainly that a task-producing thread can miss seeing (and
      * signalling) another thread that gave up looking for work but
      * has not yet entered the wait queue. We solve this by requiring
-     * a full sweep of all workers both before (in scan()) and after
-     * (in tryAwaitWork()) a newly waiting worker is added to the wait
-     * queue. During a rescan, the worker might release some other
-     * queued worker rather than itself, which has the same net
-     * effect. Because enqueued workers may actually be rescanning
-     * rather than waiting, we set and clear the "parked" field of
-     * ForkJoinWorkerThread to reduce unnecessary calls to unpark.
-     * (Use of the parked field requires a secondary recheck to avoid
-     * missed signals.)
+     * a full sweep of all workers (via repeated calls to method
+     * scan()) both before and after a newly waiting worker is added
+     * to the wait queue. During a rescan, the worker might release
+     * some other queued worker rather than itself, which has the same
+     * net effect. Because enqueued workers may actually be rescanning
+     * rather than waiting, we set and clear the "parker" field of
+     * WorkQueues to reduce unnecessary calls to unpark.  (This
+     * requires a secondary recheck to avoid missed signals.)  Note
+     * the unusual conventions about Thread.interrupts surrounding
+     * parking and other blocking: Because interrupts are used solely
+     * to alert threads to check termination, which is checked anyway
+     * upon blocking, we clear status (using Thread.interrupted)
+     * before any call to park, so that park does not immediately
+     * return due to status being set via some other unrelated call to
+     * interrupt in user code.
      *
      * Signalling.  We create or wake up workers only when there
      * appears to be at least one task they might be able to find and
-     * execute.  When a submission is added or another worker adds a
-     * task to a queue that previously had two or fewer tasks, they
-     * signal waiting workers (or trigger creation of new ones if
-     * fewer than the given parallelism level -- see signalWork).
-     * These primary signals are buttressed by signals during rescans
-     * as well as those performed when a worker steals a task and
-     * notices that there are more tasks too; together these cover the
-     * signals needed in cases when more than two tasks are pushed
-     * but untaken.
+     * execute. However, many other threads may notice the same task
+     * and each signal to wake up a thread that might take it. So in
+     * general, pools will be over-signalled.  When a submission is
+     * added or another worker adds a task to a queue that has fewer
+     * than two tasks, they signal waiting workers (or trigger
+     * creation of new ones if fewer than the given parallelism level
+     * -- signalWork), and may leave a hint to the unparked worker to
+     * help signal others upon wakeup).  These primary signals are
+     * buttressed by others (see method helpSignal) whenever other
+     * threads scan for work or do not have a task to process.  On
+     * most platforms, signalling (unpark) overhead time is noticeably
+     * long, and the time between signalling a thread and it actually
+     * making progress can be very noticeably long, so it is worth
+     * offloading these delays from critical paths as much as
+     * possible.
      *
      * Trimming workers. To release resources after periods of lack of
      * use, a worker starting to wait when the pool is quiescent will
-     * time out and terminate if the pool has remained quiescent for
-     * SHRINK_RATE nanosecs. This will slowly propagate, eventually
-     * terminating all workers after long periods of non-use.
-     *
-     * Submissions. External submissions are maintained in an
-     * array-based queue that is structured identically to
-     * ForkJoinWorkerThread queues except for the use of
-     * submissionLock in method addSubmission. Unlike the case for
-     * worker queues, multiple external threads can add new
-     * submissions, so adding requires a lock.
-     *
-     * Compensation. Beyond work-stealing support and lifecycle
-     * control, the main responsibility of this framework is to take
-     * actions when one worker is waiting to join a task stolen (or
-     * always held by) another.  Because we are multiplexing many
-     * tasks on to a pool of workers, we can't just let them block (as
-     * in Thread.join).  We also cannot just reassign the joiner's
-     * run-time stack with another and replace it later, which would
-     * be a form of "continuation", that even if possible is not
-     * necessarily a good idea since we sometimes need both an
-     * unblocked task and its continuation to progress. Instead we
-     * combine two tactics:
+     * time out and terminate if the pool has remained quiescent for a
+     * given period -- a short period if there are more threads than
+     * parallelism, longer as the number of threads decreases. This
+     * will slowly propagate, eventually terminating all workers after
+     * periods of non-use.
+     *
+     * Shutdown and Termination. A call to shutdownNow atomically sets
+     * a plock bit and then (non-atomically) sets each worker's
+     * qlock status, cancels all unprocessed tasks, and wakes up
+     * all waiting workers.  Detecting whether termination should
+     * commence after a non-abrupt shutdown() call requires more work
+     * and bookkeeping. We need consensus about quiescence (i.e., that
+     * there is no more work). The active count provides a primary
+     * indication but non-abrupt shutdown still requires a rechecking
+     * scan for any workers that are inactive but not queued.
+     *
+     * Joining Tasks
+     * =============
+     *
+     * Any of several actions may be taken when one worker is waiting
+     * to join a task stolen (or always held) by another.  Because we
+     * are multiplexing many tasks on to a pool of workers, we can't
+     * just let them block (as in Thread.join).  We also cannot just
+     * reassign the joiner's run-time stack with another and replace
+     * it later, which would be a form of "continuation", that even if
+     * possible is not necessarily a good idea since we sometimes need
+     * both an unblocked task and its continuation to progress.
+     * Instead we combine two tactics:
      *
      *   Helping: Arranging for the joiner to execute some task that it
-     *      would be running if the steal had not occurred.  Method
-     *      ForkJoinWorkerThread.joinTask tracks joining->stealing
-     *      links to try to find such a task.
+     *      would be running if the steal had not occurred.
      *
      *   Compensating: Unless there are already enough live threads,
-     *      method tryPreBlock() may create or re-activate a spare
-     *      thread to compensate for blocked joiners until they
-     *      unblock.
+     *      method tryCompensate() may create or re-activate a spare
+     *      thread to compensate for blocked joiners until they unblock.
+     *
+     * A third form (implemented in tryRemoveAndExec) amounts to
+     * helping a hypothetical compensator: If we can readily tell that
+     * a possible action of a compensator is to steal and execute the
+     * task being joined, the joining thread can do so directly,
+     * without the need for a compensation thread (although at the
+     * expense of larger run-time stacks, but the tradeoff is
+     * typically worthwhile).
      *
      * The ManagedBlocker extension API can't use helping so relies
      * only on compensation in method awaitBlocker.
      *
+     * The algorithm in tryHelpStealer entails a form of "linear"
+     * helping: Each worker records (in field currentSteal) the most
+     * recent task it stole from some other worker. Plus, it records
+     * (in field currentJoin) the task it is currently actively
+     * joining. Method tryHelpStealer uses these markers to try to
+     * find a worker to help (i.e., steal back a task from and execute
+     * it) that could hasten completion of the actively joined task.
+     * In essence, the joiner executes a task that would be on its own
+     * local deque had the to-be-joined task not been stolen. This may
+     * be seen as a conservative variant of the approach in Wagner &
+     * Calder "Leapfrogging: a portable technique for implementing
+     * efficient futures" SIGPLAN Notices, 1993
+     * (http://portal.acm.org/citation.cfm?id=155354). It differs in
+     * that: (1) We only maintain dependency links across workers upon
+     * steals, rather than use per-task bookkeeping.  This sometimes
+     * requires a linear scan of workQueues array to locate stealers,
+     * but often doesn't because stealers leave hints (that may become
+     * stale/wrong) of where to locate them.  It is only a hint
+     * because a worker might have had multiple steals and the hint
+     * records only one of them (usually the most current).  Hinting
+     * isolates cost to when it is needed, rather than adding to
+     * per-task overhead.  (2) It is "shallow", ignoring nesting and
+     * potentially cyclic mutual steals.  (3) It is intentionally
+     * racy: field currentJoin is updated only while actively joining,
+     * which means that we miss links in the chain during long-lived
+     * tasks, GC stalls etc (which is OK since blocking in such cases
+     * is usually a good idea).  (4) We bound the number of attempts
+     * to find work (see MAX_HELP) and fall back to suspending the
+     * worker and if necessary replacing it with another.
+     *
+     * Helping actions for CountedCompleters are much simpler: Method
+     * helpComplete can take and execute any task with the same root
+     * as the task being waited on. However, this still entails some
+     * traversal of completer chains, so is less efficient than using
+     * CountedCompleters without explicit joins.
+     *
      * It is impossible to keep exactly the target parallelism number
      * of threads running at any given time.  Determining the
      * existence of conservatively safe helping targets, the
      * availability of already-created spares, and the apparent need
-     * to create new spares are all racy and require heuristic
-     * guidance, so we rely on multiple retries of each.  Currently,
-     * in keeping with on-demand signalling policy, we compensate only
-     * if blocking would leave less than one active (non-waiting,
-     * non-blocked) worker. Additionally, to avoid some false alarms
-     * due to GC, lagging counters, system activity, etc, compensated
-     * blocking for joins is only attempted after rechecks stabilize
-     * (retries are interspersed with Thread.yield, for good
-     * citizenship).  The variable blockedCount, incremented before
-     * blocking and decremented after, is sometimes needed to
-     * distinguish cases of waiting for work vs blocking on joins or
-     * other managed sync. Both cases are equivalent for most pool
-     * control, so we can update non-atomically. (Additionally,
-     * contention on blockedCount alleviates some contention on ctl).
-     *
-     * Shutdown and Termination. A call to shutdownNow atomically sets
-     * the ctl stop bit and then (non-atomically) sets each workers
-     * "terminate" status, cancels all unprocessed tasks, and wakes up
-     * all waiting workers.  Detecting whether termination should
-     * commence after a non-abrupt shutdown() call requires more work
-     * and bookkeeping. We need consensus about quiesence (i.e., that
-     * there is no more work) which is reflected in active counts so
-     * long as there are no current blockers, as well as possible
-     * re-evaluations during independent changes in blocking or
-     * quiescing workers.
-     *
-     * Style notes: There is a lot of representation-level coupling
-     * among classes ForkJoinPool, ForkJoinWorkerThread, and
-     * ForkJoinTask.  Most fields of ForkJoinWorkerThread maintain
-     * data structures managed by ForkJoinPool, so are directly
-     * accessed.  Conversely we allow access to "workers" array by
-     * workers, and direct access to ForkJoinTask.status by both
-     * ForkJoinPool and ForkJoinWorkerThread.  There is little point
+     * to create new spares are all racy, so we rely on multiple
+     * retries of each.  Compensation in the apparent absence of
+     * helping opportunities is challenging to control on JVMs, where
+     * GC and other activities can stall progress of tasks that in
+     * turn stall out many other dependent tasks, without us being
+     * able to determine whether they will ever require compensation.
+     * Even though work-stealing otherwise encounters little
+     * degradation in the presence of more threads than cores,
+     * aggressively adding new threads in such cases entails risk of
+     * unwanted positive feedback control loops in which more threads
+     * cause more dependent stalls (as well as delayed progress of
+     * unblocked threads to the point that we know they are available)
+     * leading to more situations requiring more threads, and so
+     * on. This aspect of control can be seen as an (analytically
+     * intractable) game with an opponent that may choose the worst
+     * (for us) active thread to stall at any time.  We take several
+     * precautions to bound losses (and thus bound gains), mainly in
+     * methods tryCompensate and awaitJoin.
+     *
+     * Common Pool
+     * ===========
+     *
+     * The static commonPool always exists after static
+     * initialization.  Since it (or any other created pool) need
+     * never be used, we minimize initial construction overhead and
+     * footprint to the setup of about a dozen fields, with no nested
+     * allocation. Most bootstrapping occurs within method
+     * fullExternalPush during the first submission to the pool.
+     *
+     * When external threads submit to the common pool, they can
+     * perform some subtask processing (see externalHelpJoin and
+     * related methods).  We do not need to record whether these
+     * submissions are to the common pool -- if not, externalHelpJoin
+     * returns quickly (at the most helping to signal some common pool
+     * workers). These submitters would otherwise be blocked waiting
+     * for completion, so the extra effort (with liberally sprinkled
+     * task status checks) in inapplicable cases amounts to an odd
+     * form of limited spin-wait before blocking in ForkJoinTask.join.
+     *
+     * Style notes
+     * ===========
+     *
+     * There is a lot of representation-level coupling among classes
+     * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask.  The
+     * fields of WorkQueue maintain data structures managed by
+     * ForkJoinPool, so are directly accessed.  There is little point
      * trying to reduce this, since any associated future changes in
      * representations will need to be accompanied by algorithmic
-     * changes anyway. All together, these low-level implementation
-     * choices produce as much as a factor of 4 performance
-     * improvement compared to naive implementations, and enable the
-     * processing of billions of tasks per second, at the expense of
-     * some ugliness.
-     *
-     * Methods signalWork() and scan() are the main bottlenecks so are
-     * especially heavily micro-optimized/mangled.  There are lots of
-     * inline assignments (of form "while ((local = field) != 0)")
-     * which are usually the simplest way to ensure the required read
-     * orderings (which are sometimes critical). This leads to a
-     * "C"-like style of listing declarations of these locals at the
-     * heads of methods or blocks.  There are several occurrences of
-     * the unusual "do {} while (!cas...)"  which is the simplest way
-     * to force an update of a CAS'ed variable. There are also other
-     * coding oddities that help some methods perform reasonably even
-     * when interpreted (not compiled).
-     *
-     * The order of declarations in this file is: (1) declarations of
-     * statics (2) fields (along with constants used when unpacking
-     * some of them), listed in an order that tends to reduce
-     * contention among them a bit under most JVMs.  (3) internal
-     * control methods (4) callbacks and other support for
-     * ForkJoinTask and ForkJoinWorkerThread classes, (5) exported
-     * methods (plus a few little helpers). (6) static block
-     * initializing all statics in a minimally dependent order.
+     * changes anyway. Several methods intrinsically sprawl because
+     * they must accumulate sets of consistent reads of volatiles held
+     * in local variables.  Methods signalWork() and scan() are the
+     * main bottlenecks, so are especially heavily
+     * micro-optimized/mangled.  There are lots of inline assignments
+     * (of form "while ((local = field) != 0)") which are usually the
+     * simplest way to ensure the required read orderings (which are
+     * sometimes critical). This leads to a "C"-like style of listing
+     * declarations of these locals at the heads of methods or blocks.
+     * There are several occurrences of the unusual "do {} while
+     * (!cas...)"  which is the simplest way to force an update of a
+     * CAS'ed variable. There are also other coding oddities (including
+     * several unnecessary-looking hoisted null checks) that help
+     * some methods perform reasonably even when interpreted (not
+     * compiled).
+     *
+     * The order of declarations in this file is:
+     * (1) Static utility functions
+     * (2) Nested (static) classes
+     * (3) Static fields
+     * (4) Fields, along with constants used when unpacking some of them
+     * (5) Internal control methods
+     * (6) Callbacks and other support for ForkJoinTask methods
+     * (7) Exported methods
+     * (8) Static block initializing statics in minimally dependent order
+     */
+
+    // Static utilities
+
+    /**
+     * If there is a security manager, makes sure caller has
+     * permission to modify threads.
      */
+    private static void checkPermission() {
+        SecurityManager security = System.getSecurityManager();
+        if (security != null)
+            security.checkPermission(modifyThreadPermission);
+    }
+
+    // Nested classes
 
     /**
      * Factory for creating new {@link ForkJoinWorkerThread}s.
@@ -384,122 +556,620 @@ public class ForkJoinPool extends AbstractExecutorService {
      * Default ForkJoinWorkerThreadFactory implementation; creates a
      * new ForkJoinWorkerThread.
      */
-    static class DefaultForkJoinWorkerThreadFactory
+    static final class DefaultForkJoinWorkerThreadFactory
         implements ForkJoinWorkerThreadFactory {
-        public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
+        public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
             return new ForkJoinWorkerThread(pool);
         }
     }
 
     /**
-     * Creates a new ForkJoinWorkerThread. This factory is used unless
-     * overridden in ForkJoinPool constructors.
-     */
-    public static final ForkJoinWorkerThreadFactory
-        defaultForkJoinWorkerThreadFactory;
+     * Per-thread records for threads that submit to pools. Currently
+     * holds only pseudo-random seed / index that is used to choose
+     * submission queues in method externalPush. In the future, this may
+     * also incorporate a means to implement different task rejection
+     * and resubmission policies.
+     *
+     * Seeds for submitters and workers/workQueues work in basically
+     * the same way but are initialized and updated using slightly
+     * different mechanics. Both are initialized using the same
+     * approach as in class ThreadLocal, where successive values are
+     * unlikely to collide with previous values. Seeds are then
+     * randomly modified upon collisions using xorshifts, which
+     * requires a non-zero seed.
+     */
+    static final class Submitter {
+        int seed;
+        Submitter(int s) { seed = s; }
+    }
+
+    /**
+     * Class for artificial tasks that are used to replace the target
+     * of local joins if they are removed from an interior queue slot
+     * in WorkQueue.tryRemoveAndExec. We don't need the proxy to
+     * actually do anything beyond having a unique identity.
+     */
+    static final class EmptyTask extends ForkJoinTask<Void> {
+        private static final long serialVersionUID = -7721805057305804111L;
+        EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
+        public final Void getRawResult() { return null; }
+        public final void setRawResult(Void x) {}
+        public final boolean exec() { return true; }
+    }
+
+    /**
+     * Queues supporting work-stealing as well as external task
+     * submission. See above for main rationale and algorithms.
+     * Implementation relies heavily on "Unsafe" intrinsics
+     * and selective use of "volatile":
+     *
+     * Field "base" is the index (mod array.length) of the least valid
+     * queue slot, which is always the next position to steal (poll)
+     * from if nonempty. Reads and writes require volatile orderings
+     * but not CAS, because updates are only performed after slot
+     * CASes.
+     *
+     * Field "top" is the index (mod array.length) of the next queue
+     * slot to push to or pop from. It is written only by owner thread
+     * for push, or under lock for external/shared push, and accessed
+     * by other threads only after reading (volatile) base.  Both top
+     * and base are allowed to wrap around on overflow, but (top -
+     * base) (or more commonly -(base - top) to force volatile read of
+     * base before top) still estimates size. The lock ("qlock") is
+     * forced to -1 on termination, causing all further lock attempts
+     * to fail. (Note: we don't need CAS for termination state because
+     * upon pool shutdown, all shared-queues will stop being used
+     * anyway.)  Nearly all lock bodies are set up so that exceptions
+     * within lock bodies are "impossible" (modulo JVM errors that
+     * would cause failure anyway.)
+     *
+     * The array slots are read and written using the emulation of
+     * volatiles/atomics provided by Unsafe. Insertions must in
+     * general use putOrderedObject as a form of releasing store to
+     * ensure that all writes to the task object are ordered before
+     * its publication in the queue.  All removals entail a CAS to
+     * null.  The array is always a power of two. To ensure safety of
+     * Unsafe array operations, all accesses perform explicit null
+     * checks and implicit bounds checks via power-of-two masking.
+     *
+     * In addition to basic queuing support, this class contains
+     * fields described elsewhere to control execution. It turns out
+     * to work better memory-layout-wise to include them in this class
+     * rather than a separate class.
+     *
+     * Performance on most platforms is very sensitive to placement of
+     * instances of both WorkQueues and their arrays -- we absolutely
+     * do not want multiple WorkQueue instances or multiple queue
+     * arrays sharing cache lines. (It would be best for queue objects
+     * and their arrays to share, but there is nothing available to
+     * help arrange that).  Unfortunately, because they are recorded
+     * in a common array, WorkQueue instances are often moved to be
+     * adjacent by garbage collectors. To reduce impact, we use field
+     * padding that works OK on common platforms; this effectively
+     * trades off slightly slower average field access for the sake of
+     * avoiding really bad worst-case access. (Until better JVM
+     * support is in place, this padding is dependent on transient
+     * properties of JVM field layout rules.) We also take care in
+     * allocating, sizing and resizing the array. Non-shared queue
+     * arrays are initialized by workers before use. Others are
+     * allocated on first use.
+     */
+    static final class WorkQueue {
+        /**
+         * Capacity of work-stealing queue array upon initialization.
+         * Must be a power of two; at least 4, but should be larger to
+         * reduce or eliminate cacheline sharing among queues.
+         * Currently, it is much larger, as a partial workaround for
+         * the fact that JVMs often place arrays in locations that
+         * share GC bookkeeping (especially cardmarks) such that
+         * per-write accesses encounter serious memory contention.
+         */
+        static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
 
-    /**
-     * Permission required for callers of methods that may start or
-     * kill threads.
-     */
-    private static final RuntimePermission modifyThreadPermission;
+        /**
+         * Maximum size for queue arrays. Must be a power of two less
+         * than or equal to 1 << (31 - width of array entry) to ensure
+         * lack of wraparound of index calculations, but defined to a
+         * value a bit less than this to help users trap runaway
+         * programs before saturating systems.
+         */
+        static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M
+
+        // Heuristic padding to ameliorate unfortunate memory placements
+        volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06;
+
+        int seed;                  // for random scanning; initialize nonzero
+        volatile int eventCount;   // encoded inactivation count; < 0 if inactive
+        int nextWait;              // encoded record of next event waiter
+        int hint;                  // steal or signal hint (index)
+        int poolIndex;             // index of this queue in pool (or 0)
+        final int mode;            // 0: lifo, > 0: fifo, < 0: shared
+        int nsteals;               // number of steals
+        volatile int qlock;        // 1: locked, -1: terminate; else 0
+        volatile int base;         // index of next slot for poll
+        int top;                   // index of next slot for push
+        ForkJoinTask<?>[] array;   // the elements (initially unallocated)
+        final ForkJoinPool pool;   // the containing pool (may be null)
+        final ForkJoinWorkerThread owner; // owning thread or null if shared
+        volatile Thread parker;    // == owner during call to park; else null
+        volatile ForkJoinTask<?> currentJoin;  // task being joined in awaitJoin
+        ForkJoinTask<?> currentSteal; // current non-local task being executed
+
+        volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
+        volatile Object pad18, pad19, pad1a, pad1b, pad1c, pad1d;
+
+        WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner, int mode,
+                  int seed) {
+            this.pool = pool;
+            this.owner = owner;
+            this.mode = mode;
+            this.seed = seed;
+            // Place indices in the center of array (that is not yet allocated)
+            base = top = INITIAL_QUEUE_CAPACITY >>> 1;
+        }
 
-    /**
-     * If there is a security manager, makes sure caller has
-     * permission to modify threads.
-     */
-    private static void checkPermission() {
-        SecurityManager security = System.getSecurityManager();
-        if (security != null)
-            security.checkPermission(modifyThreadPermission);
+        /**
+         * Returns the approximate number of tasks in the queue.
+         */
+        final int queueSize() {
+            int n = base - top;       // non-owner callers must read base first
+            return (n >= 0) ? 0 : -n; // ignore transient negative
+        }
+
+       /**
+         * Provides a more accurate estimate of whether this queue has
+         * any tasks than does queueSize, by checking whether a
+         * near-empty queue has at least one unclaimed task.
+         */
+        final boolean isEmpty() {
+            ForkJoinTask<?>[] a; int m, s;
+            int n = base - (s = top);
+            return (n >= 0 ||
+                    (n == -1 &&
+                     ((a = array) == null ||
+                      (m = a.length - 1) < 0 ||
+                      U.getObject
+                      (a, (long)((m & (s - 1)) << ASHIFT) + ABASE) == null)));
+        }
+
+        /**
+         * Pushes a task. Call only by owner in unshared queues.  (The
+         * shared-queue version is embedded in method externalPush.)
+         *
+         * @param task the task. Caller must ensure non-null.
+         * @throw RejectedExecutionException if array cannot be resized
+         */
+        final void push(ForkJoinTask<?> task) {
+            ForkJoinTask<?>[] a; ForkJoinPool p;
+            int s = top, m, n;
+            if ((a = array) != null) {    // ignore if queue removed
+                int j = (((m = a.length - 1) & s) << ASHIFT) + ABASE;
+                U.putOrderedObject(a, j, task);
+                if ((n = (top = s + 1) - base) <= 2) {
+                    if ((p = pool) != null)
+                        p.signalWork(this);
+                }
+                else if (n >= m)
+                    growArray();
+            }
+        }
+
+       /**
+         * Initializes or doubles the capacity of array. Call either
+         * by owner or with lock held -- it is OK for base, but not
+         * top, to move while resizings are in progress.
+         */
+        final ForkJoinTask<?>[] growArray() {
+            ForkJoinTask<?>[] oldA = array;
+            int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
+            if (size > MAXIMUM_QUEUE_CAPACITY)
+                throw new RejectedExecutionException("Queue capacity exceeded");
+            int oldMask, t, b;
+            ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
+            if (oldA != null && (oldMask = oldA.length - 1) >= 0 &&
+                (t = top) - (b = base) > 0) {
+                int mask = size - 1;
+                do {
+                    ForkJoinTask<?> x;
+                    int oldj = ((b & oldMask) << ASHIFT) + ABASE;
+                    int j    = ((b &    mask) << ASHIFT) + ABASE;
+                    x = (ForkJoinTask<?>)U.getObjectVolatile(oldA, oldj);
+                    if (x != null &&
+                        U.compareAndSwapObject(oldA, oldj, x, null))
+                        U.putObjectVolatile(a, j, x);
+                } while (++b != t);
+            }
+            return a;
+        }
+
+        /**
+         * Takes next task, if one exists, in LIFO order.  Call only
+         * by owner in unshared queues.
+         */
+        final ForkJoinTask<?> pop() {
+            ForkJoinTask<?>[] a; ForkJoinTask<?> t; int m;
+            if ((a = array) != null && (m = a.length - 1) >= 0) {
+                for (int s; (s = top - 1) - base >= 0;) {
+                    long j = ((m & s) << ASHIFT) + ABASE;
+                    if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null)
+                        break;
+                    if (U.compareAndSwapObject(a, j, t, null)) {
+                        top = s;
+                        return t;
+                    }
+                }
+            }
+            return null;
+        }
+
+        /**
+         * Takes a task in FIFO order if b is base of queue and a task
+         * can be claimed without contention. Specialized versions
+         * appear in ForkJoinPool methods scan and tryHelpStealer.
+         */
+        final ForkJoinTask<?> pollAt(int b) {
+            ForkJoinTask<?> t; ForkJoinTask<?>[] a;
+            if ((a = array) != null) {
+                int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                if ((t = (ForkJoinTask<?>)U.getObjectVolatile(a, j)) != null &&
+                    base == b &&
+                    U.compareAndSwapObject(a, j, t, null)) {
+                    base = b + 1;
+                    return t;
+                }
+            }
+            return null;
+        }
+
+        /**
+         * Takes next task, if one exists, in FIFO order.
+         */
+        final ForkJoinTask<?> poll() {
+            ForkJoinTask<?>[] a; int b; ForkJoinTask<?> t;
+            while ((b = base) - top < 0 && (a = array) != null) {
+                int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
+                if (t != null) {
+                    if (base == b &&
+                        U.compareAndSwapObject(a, j, t, null)) {
+                        base = b + 1;
+                        return t;
+                    }
+                }
+                else if (base == b) {
+                    if (b + 1 == top)
+                        break;
+                    Thread.yield(); // wait for lagging update (very rare)
+                }
+            }
+            return null;
+        }
+
+        /**
+         * Takes next task, if one exists, in order specified by mode.
+         */
+        final ForkJoinTask<?> nextLocalTask() {
+            return mode == 0 ? pop() : poll();
+        }
+
+        /**
+         * Returns next task, if one exists, in order specified by mode.
+         */
+        final ForkJoinTask<?> peek() {
+            ForkJoinTask<?>[] a = array; int m;
+            if (a == null || (m = a.length - 1) < 0)
+                return null;
+            int i = mode == 0 ? top - 1 : base;
+            int j = ((i & m) << ASHIFT) + ABASE;
+            return (ForkJoinTask<?>)U.getObjectVolatile(a, j);
+        }
+
+        /**
+         * Pops the given task only if it is at the current top.
+         * (A shared version is available only via FJP.tryExternalUnpush)
+         */
+        final boolean tryUnpush(ForkJoinTask<?> t) {
+            ForkJoinTask<?>[] a; int s;
+            if ((a = array) != null && (s = top) != base &&
+                U.compareAndSwapObject
+                (a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
+                top = s;
+                return true;
+            }
+            return false;
+        }
+
+        /**
+         * Removes and cancels all known tasks, ignoring any exceptions.
+         */
+        final void cancelAll() {
+            ForkJoinTask.cancelIgnoringExceptions(currentJoin);
+            ForkJoinTask.cancelIgnoringExceptions(currentSteal);
+            for (ForkJoinTask<?> t; (t = poll()) != null; )
+                ForkJoinTask.cancelIgnoringExceptions(t);
+        }
+
+        /**
+         * Computes next value for random probes.  Scans don't require
+         * a very high quality generator, but also not a crummy one.
+         * Marsaglia xor-shift is cheap and works well enough.  Note:
+         * This is manually inlined in its usages in ForkJoinPool to
+         * avoid writes inside busy scan loops.
+         */
+        final int nextSeed() {
+            int r = seed;
+            r ^= r << 13;
+            r ^= r >>> 17;
+            return seed = r ^= r << 5;
+        }
+
+        // Specialized execution methods
+
+        /**
+         * Pops and runs tasks until empty.
+         */
+        private void popAndExecAll() {
+            // A bit faster than repeated pop calls
+            ForkJoinTask<?>[] a; int m, s; long j; ForkJoinTask<?> t;
+            while ((a = array) != null && (m = a.length - 1) >= 0 &&
+                   (s = top - 1) - base >= 0 &&
+                   (t = ((ForkJoinTask<?>)
+                         U.getObject(a, j = ((m & s) << ASHIFT) + ABASE)))
+                   != null) {
+                if (U.compareAndSwapObject(a, j, t, null)) {
+                    top = s;
+                    t.doExec();
+                }
+            }
+        }
+
+        /**
+         * Polls and runs tasks until empty.
+         */
+        private void pollAndExecAll() {
+            for (ForkJoinTask<?> t; (t = poll()) != null;)
+                t.doExec();
+        }
+
+        /**
+         * If present, removes from queue and executes the given task,
+         * or any other cancelled task. Returns (true) on any CAS
+         * or consistency check failure so caller can retry.
+         *
+         * @return false if no progress can be made, else true;
+         */
+        final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
+            boolean stat = true, removed = false, empty = true;
+            ForkJoinTask<?>[] a; int m, s, b, n;
+            if ((a = array) != null && (m = a.length - 1) >= 0 &&
+                (n = (s = top) - (b = base)) > 0) {
+                for (ForkJoinTask<?> t;;) {           // traverse from s to b
+                    int j = ((--s & m) << ASHIFT) + ABASE;
+                    t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
+                    if (t == null)                    // inconsistent length
+                        break;
+                    else if (t == task) {
+                        if (s + 1 == top) {           // pop
+                            if (!U.compareAndSwapObject(a, j, task, null))
+                                break;
+                            top = s;
+                            removed = true;
+                        }
+                        else if (base == b)           // replace with proxy
+                            removed = U.compareAndSwapObject(a, j, task,
+                                                             new EmptyTask());
+                        break;
+                    }
+                    else if (t.status >= 0)
+                        empty = false;
+                    else if (s + 1 == top) {          // pop and throw away
+                        if (U.compareAndSwapObject(a, j, t, null))
+                            top = s;
+                        break;
+                    }
+                    if (--n == 0) {
+                        if (!empty && base == b)
+                            stat = false;
+                        break;
+                    }
+                }
+            }
+            if (removed)
+                task.doExec();
+            return stat;
+        }
+
+        /**
+         * Polls for and executes the given task or any other task in
+         * its CountedCompleter computation
+         */
+        final boolean pollAndExecCC(ForkJoinTask<?> root) {
+            ForkJoinTask<?>[] a; int b; Object o;
+            outer: while ((b = base) - top < 0 && (a = array) != null) {
+                long j = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                if ((o = U.getObject(a, j)) == null ||
+                    !(o instanceof CountedCompleter))
+                    break;
+                for (CountedCompleter<?> t = (CountedCompleter<?>)o, r = t;;) {
+                    if (r == root) {
+                        if (base == b &&
+                            U.compareAndSwapObject(a, j, t, null)) {
+                            base = b + 1;
+                            t.doExec();
+                            return true;
+                        }
+                        else
+                            break; // restart
+                    }
+                    if ((r = r.completer) == null)
+                        break outer; // not part of root computation
+                }
+            }
+            return false;
+        }
+
+        /**
+         * Executes a top-level task and any local tasks remaining
+         * after execution.
+         */
+        final void runTask(ForkJoinTask<?> t) {
+            if (t != null) {
+                (currentSteal = t).doExec();
+                currentSteal = null;
+                ++nsteals;
+                if (base - top < 0) {       // process remaining local tasks
+                    if (mode == 0)
+                        popAndExecAll();
+                    else
+                        pollAndExecAll();
+                }
+            }
+        }
+
+        /**
+         * Executes a non-top-level (stolen) task.
+         */
+        final void runSubtask(ForkJoinTask<?> t) {
+            if (t != null) {
+                ForkJoinTask<?> ps = currentSteal;
+                (currentSteal = t).doExec();
+                currentSteal = ps;
+            }
+        }
+
+        /**
+         * Returns true if owned and not known to be blocked.
+         */
+        final boolean isApparentlyUnblocked() {
+            Thread wt; Thread.State s;
+            return (eventCount >= 0 &&
+                    (wt = owner) != null &&
+                    (s = wt.getState()) != Thread.State.BLOCKED &&
+                    s != Thread.State.WAITING &&
+                    s != Thread.State.TIMED_WAITING);
+        }
+
+        // Unsafe mechanics
+        private static final sun.misc.Unsafe U;
+        private static final long QLOCK;
+        private static final int ABASE;
+        private static final int ASHIFT;
+        static {
+            int s;
+            try {
+                U = sun.misc.Unsafe.getUnsafe();
+                Class<?> k = WorkQueue.class;
+                Class<?> ak = ForkJoinTask[].class;
+                QLOCK = U.objectFieldOffset
+                    (k.getDeclaredField("qlock"));
+                ABASE = U.arrayBaseOffset(ak);
+                s = U.arrayIndexScale(ak);
+            } catch (Exception e) {
+                throw new Error(e);
+            }
+            if ((s & (s-1)) != 0)
+                throw new Error("data type scale not a power of two");
+            ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
+        }
     }
 
+    // static fields (initialized in static initializer below)
+
     /**
-     * Generator for assigning sequence numbers as pool names.
+     * Creates a new ForkJoinWorkerThread. This factory is used unless
+     * overridden in ForkJoinPool constructors.
      */
-    private static final AtomicInteger poolNumberGenerator;
+    public static final ForkJoinWorkerThreadFactory
+        defaultForkJoinWorkerThreadFactory;
 
     /**
-     * Generator for initial random seeds for worker victim
-     * selection. This is used only to create initial seeds. Random
-     * steals use a cheaper xorshift generator per steal attempt. We
-     * don't expect much contention on seedGenerator, so just use a
-     * plain Random.
+     * Per-thread submission bookkeeping. Shared across all pools
+     * to reduce ThreadLocal pollution and because random motion
+     * to avoid contention in one pool is likely to hold for others.
+     * Lazily initialized on first submission (but null-checked
+     * in other contexts to avoid unnecessary initialization).
      */
-    static final Random workerSeedGenerator;
+    static final ThreadLocal<Submitter> submitters;
 
     /**
-     * Array holding all worker threads in the pool.  Initialized upon
-     * construction. Array size must be a power of two.  Updates and
-     * replacements are protected by scanGuard, but the array is
-     * always kept in a consistent enough state to be randomly
-     * accessed without locking by workers performing work-stealing,
-     * as well as other traversal-based methods in this class, so long
-     * as reads memory-acquire by first reading ctl. All readers must
-     * tolerate that some array slots may be null.
+     * Permission required for callers of methods that may start or
+     * kill threads.
      */
-    ForkJoinWorkerThread[] workers;
+    private static final RuntimePermission modifyThreadPermission;
 
     /**
-     * Initial size for submission queue array. Must be a power of
-     * two.  In many applications, these always stay small so we use a
-     * small initial cap.
+     * Common (static) pool. Non-null for public use unless a static
+     * construction exception, but internal usages null-check on use
+     * to paranoically avoid potential initialization circularities
+     * as well as to simplify generated code.
      */
-    private static final int INITIAL_QUEUE_CAPACITY = 8;
+    static final ForkJoinPool commonPool;
 
     /**
-     * Maximum size for submission queue array. Must be a power of two
-     * less than or equal to 1 << (31 - width of array entry) to
-     * ensure lack of index wraparound, but is capped at a lower
-     * value to help users trap runaway computations.
+     * Common pool parallelism. Must equal commonPool.parallelism.
      */
-    private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M
+    static final int commonPoolParallelism;
 
     /**
-     * Array serving as submission queue. Initialized upon construction.
+     * Sequence number for creating workerNamePrefix.
      */
-    private ForkJoinTask<?>[] submissionQueue;
+    private static int poolNumberSequence;
 
     /**
-     * Lock protecting submissions array for addSubmission
+     * Return the next sequence number. We don't expect this to
+     * ever contend so use simple builtin sync.
      */
-    private final ReentrantLock submissionLock;
+    private static final synchronized int nextPoolId() {
+        return ++poolNumberSequence;
+    }
+
+    // static constants
 
     /**
-     * Condition for awaitTermination, using submissionLock for
-     * convenience.
+     * Initial timeout value (in nanoseconds) for the thread
+     * triggering quiescence to park waiting for new work. On timeout,
+     * the thread will instead try to shrink the number of
+     * workers. The value should be large enough to avoid overly
+     * aggressive shrinkage during most transient stalls (long GCs
+     * etc).
      */
-    private final Condition termination;
+    private static final long IDLE_TIMEOUT      = 2000L * 1000L * 1000L; // 2sec
 
     /**
-     * Creation factory for worker threads.
+     * Timeout value when there are more threads than parallelism level
      */
-    private final ForkJoinWorkerThreadFactory factory;
+    private static final long FAST_IDLE_TIMEOUT =  200L * 1000L * 1000L;
 
     /**
-     * The uncaught exception handler used when any worker abruptly
-     * terminates.
+     * Tolerance for idle timeouts, to cope with timer undershoots
      */
-    final Thread.UncaughtExceptionHandler ueh;
+    private static final long TIMEOUT_SLOP = 2000000L;
 
     /**
-     * Prefix for assigning names to worker threads
+     * The maximum stolen->joining link depth allowed in method
+     * tryHelpStealer.  Must be a power of two.  Depths for legitimate
+     * chains are unbounded, but we use a fixed constant to avoid
+     * (otherwise unchecked) cycles and to bound staleness of
+     * traversal parameters at the expense of sometimes blocking when
+     * we could be helping.
      */
-    private final String workerNamePrefix;
+    private static final int MAX_HELP = 64;
 
     /**
-     * Sum of per-thread steal counts, updated only when threads are
-     * idle or terminating.
+     * Increment for seed generators. See class ThreadLocal for
+     * explanation.
      */
-    private volatile long stealCount;
+    private static final int SEED_INCREMENT = 0x61c88647;
 
     /**
-     * Main pool control -- a long packed with:
+     * Bits and masks for control variables
+     *
+     * Field ctl is a long packed with:
      * AC: Number of active running workers minus target parallelism (16 bits)
-     * TC: Number of total workers minus target parallelism (16bits)
+     * TC: Number of total workers minus target parallelism (16 bits)
      * ST: true if pool is terminating (1 bit)
      * EC: the wait count of top waiting thread (15 bits)
-     * ID: ~poolIndex of top of Treiber stack of waiting threads (16 bits)
+     * ID: poolIndex of top of Treiber stack of waiters (16 bits)
      *
      * When convenient, we can extract the upper 32 bits of counts and
      * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e =
@@ -508,13 +1178,26 @@ public class ForkJoinPool extends AbstractExecutorService {
      * parallelism and the positionings of fields makes it possible to
      * perform the most common checks via sign tests of fields: When
      * ac is negative, there are not enough active workers, when tc is
-     * negative, there are not enough total workers, when id is
-     * negative, there is at least one waiting worker, and when e is
+     * negative, there are not enough total workers, and when e is
      * negative, the pool is terminating.  To deal with these possibly
      * negative fields, we use casts in and out of "short" and/or
      * signed shifts to maintain signedness.
+     *
+     * When a thread is queued (inactivated), its eventCount field is
+     * set negative, which is the only way to tell if a worker is
+     * prevented from executing tasks, even though it must continue to
+     * scan for them to avoid queuing races. Note however that
+     * eventCount updates lag releases so usage requires care.
+     *
+     * Field plock is an int packed with:
+     * SHUTDOWN: true if shutdown is enabled (1 bit)
+     * SEQ:  a sequence lock, with PL_LOCK bit set if locked (30 bits)
+     * SIGNAL: set when threads may be waiting on the lock (1 bit)
+     *
+     * The sequence number enables simple consistency checks:
+     * Staleness of read-only operations on the workQueues array can
+     * be checked by comparing plock before vs after the reads.
      */
-    volatile long ctl;
 
     // bit positions/shifts for fields
     private static final int  AC_SHIFT   = 48;
@@ -523,8 +1206,10 @@ public class ForkJoinPool extends AbstractExecutorService {
     private static final int  EC_SHIFT   = 16;
 
     // bounds
-    private static final int  MAX_ID     = 0x7fff;  // max poolIndex
-    private static final int  SMASK      = 0xffff;  // mask short bits
+    private static final int  SMASK      = 0xffff;  // short bits
+    private static final int  MAX_CAP    = 0x7fff;  // max #workers - 1
+    private static final int  EVENMASK   = 0xfffe;  // even short bits
+    private static final int  SQMASK     = 0x007e;  // max 64 (even) slots
     private static final int  SHORT_SIGN = 1 << 15;
     private static final int  INT_SIGN   = 1 << 31;
 
@@ -546,853 +1231,1281 @@ public class ForkJoinPool extends AbstractExecutorService {
     private static final int  UTC_UNIT   = 1 << UTC_SHIFT;
 
     // masks and units for dealing with e = (int)ctl
-    private static final int  E_MASK     = 0x7fffffff; // no STOP_BIT
-    private static final int  EC_UNIT    = 1 << EC_SHIFT;
+    private static final int E_MASK      = 0x7fffffff; // no STOP_BIT
+    private static final int E_SEQ       = 1 << EC_SHIFT;
 
-    /**
-     * The target parallelism level.
-     */
-    final int parallelism;
-
-    /**
-     * Index (mod submission queue length) of next element to take
-     * from submission queue. Usage is identical to that for
-     * per-worker queues -- see ForkJoinWorkerThread internal
-     * documentation.
-     */
-    volatile int queueBase;
+    // plock bits
+    private static final int SHUTDOWN    = 1 << 31;
+    private static final int PL_LOCK     = 2;
+    private static final int PL_SIGNAL   = 1;
+    private static final int PL_SPINS    = 1 << 8;
 
-    /**
-     * Index (mod submission queue length) of next element to add
-     * in submission queue. Usage is identical to that for
-     * per-worker queues -- see ForkJoinWorkerThread internal
-     * documentation.
-     */
-    int queueTop;
-
-    /**
-     * True when shutdown() has been called.
-     */
-    volatile boolean shutdown;
+    // access mode for WorkQueue
+    static final int LIFO_QUEUE          =  0;
+    static final int FIFO_QUEUE          =  1;
+    static final int SHARED_QUEUE        = -1;
 
-    /**
-     * True if use local fifo, not default lifo, for local polling
-     * Read by, and replicated by ForkJoinWorkerThreads
-     */
-    final boolean locallyFifo;
+    // bounds for #steps in scan loop -- must be power 2 minus 1
+    private static final int MIN_SCAN    = 0x1ff;   // cover estimation slop
+    private static final int MAX_SCAN    = 0x1ffff; // 4 * max workers
 
-    /**
-     * The number of threads in ForkJoinWorkerThreads.helpQuiescePool.
-     * When non-zero, suppresses automatic shutdown when active
-     * counts become zero.
-     */
-    volatile int quiescerCount;
+    // Instance fields
 
-    /**
-     * The number of threads blocked in join.
+    /*
+     * Field layout of this class tends to matter more than one would
+     * like. Runtime layout order is only loosely related to
+     * declaration order and may differ across JVMs, but the following
+     * empirically works OK on current JVMs.
      */
-    volatile int blockedCount;
 
-    /**
-     * Counter for worker Thread names (unrelated to their poolIndex)
-     */
-    private volatile int nextWorkerNumber;
+    // Heuristic padding to ameliorate unfortunate memory placements
+    volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06;
 
-    /**
-     * The index for the next created worker. Accessed under scanGuard.
-     */
-    private int nextWorkerIndex;
+    volatile long stealCount;                  // collects worker counts
+    volatile long ctl;                         // main pool control
+    volatile int plock;                        // shutdown status and seqLock
+    volatile int indexSeed;                    // worker/submitter index seed
+    final int config;                          // mode and parallelism level
+    WorkQueue[] workQueues;                    // main registry
+    final ForkJoinWorkerThreadFactory factory;
+    final Thread.UncaughtExceptionHandler ueh; // per-worker UEH
+    final String workerNamePrefix;             // to create worker name string
 
-    /**
-     * SeqLock and index masking for updates to workers array.  Locked
-     * when SG_UNIT is set. Unlocking clears bit by adding
-     * SG_UNIT. Staleness of read-only operations can be checked by
-     * comparing scanGuard to value before the reads. The low 16 bits
-     * (i.e, anding with SMASK) hold (the smallest power of two
-     * covering all worker indices, minus one, and is used to avoid
-     * dealing with large numbers of null slots when the workers array
-     * is overallocated.
-     */
-    volatile int scanGuard;
+    volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
+    volatile Object pad18, pad19, pad1a, pad1b;
 
-    private static final int SG_UNIT = 1 << 16;
+    /*
+     * Acquires the plock lock to protect worker array and related
+     * updates. This method is called only if an initial CAS on plock
+     * fails. This acts as a spinLock for normal cases, but falls back
+     * to builtin monitor to block when (rarely) needed. This would be
+     * a terrible idea for a highly contended lock, but works fine as
+     * a more conservative alternative to a pure spinlock.
+     */
+    private int acquirePlock() {
+        int spins = PL_SPINS, r = 0, ps, nps;
+        for (;;) {
+            if (((ps = plock) & PL_LOCK) == 0 &&
+                U.compareAndSwapInt(this, PLOCK, ps, nps = ps + PL_LOCK))
+                return nps;
+            else if (r == 0) { // randomize spins if possible
+                Thread t = Thread.currentThread(); WorkQueue w; Submitter z;
+                if ((t instanceof ForkJoinWorkerThread) &&
+                    (w = ((ForkJoinWorkerThread)t).workQueue) != null)
+                    r = w.seed;
+                else if ((z = submitters.get()) != null)
+                    r = z.seed;
+                else
+                    r = 1;
+            }
+            else if (spins >= 0) {
+                r ^= r << 1; r ^= r >>> 3; r ^= r << 10; // xorshift
+                if (r >= 0)
+                    --spins;
+            }
+            else if (U.compareAndSwapInt(this, PLOCK, ps, ps | PL_SIGNAL)) {
+                synchronized (this) {
+                    if ((plock & PL_SIGNAL) != 0) {
+                        try {
+                            wait();
+                        } catch (InterruptedException ie) {
+                            try {
+                                Thread.currentThread().interrupt();
+                            } catch (SecurityException ignore) {
+                            }
+                        }
+                    }
+                    else
+                        notifyAll();
+                }
+            }
+        }
+    }
 
     /**
-     * The wakeup interval (in nanoseconds) for a worker waiting for a
-     * task when the pool is quiescent to instead try to shrink the
-     * number of workers.  The exact value does not matter too
-     * much. It must be short enough to release resources during
-     * sustained periods of idleness, but not so short that threads
-     * are continually re-created.
+     * Unlocks and signals any thread waiting for plock. Called only
+     * when CAS of seq value for unlock fails.
      */
-    private static final long SHRINK_RATE =
-        4L * 1000L * 1000L * 1000L; // 4 seconds
+    private void releasePlock(int ps) {
+        plock = ps;
+        synchronized (this) { notifyAll(); }
+    }
 
     /**
-     * Top-level loop for worker threads: On each step: if the
-     * previous step swept through all queues and found no tasks, or
-     * there are excess threads, then possibly blocks. Otherwise,
-     * scans for and, if found, executes a task. Returns when pool
-     * and/or worker terminate.
+     * Performs secondary initialization, called when plock is zero.
+     * Creates workQueue array and sets plock to a valid value.  The
+     * lock body must be exception-free (so no try/finally) so we
+     * optimistically allocate new array outside the lock and throw
+     * away if (very rarely) not needed. (A similar tactic is used in
+     * fullExternalPush.)  Because the plock seq value can eventually
+     * wrap around zero, this method harmlessly fails to reinitialize
+     * if workQueues exists, while still advancing plock.
      *
-     * @param w the worker
+     * Additionally tries to create the first worker.
      */
-    final void work(ForkJoinWorkerThread w) {
-        boolean swept = false;                // true on empty scans
-        long c;
-        while (!w.terminate && (int)(c = ctl) >= 0) {
-            int a;                            // active count
-            if (!swept && (a = (int)(c >> AC_SHIFT)) <= 0)
-                swept = scan(w, a);
-            else if (tryAwaitWork(w, c))
-                swept = false;
+    private void initWorkers() {
+        WorkQueue[] ws, nws; int ps;
+        int p = config & SMASK;        // find power of two table size
+        int n = (p > 1) ? p - 1 : 1;   // ensure at least 2 slots
+        n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16;
+        n = (n + 1) << 1;
+        if ((ws = workQueues) == null || ws.length == 0)
+            nws = new WorkQueue[n];
+        else
+            nws = null;
+        if (((ps = plock) & PL_LOCK) != 0 ||
+            !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+            ps = acquirePlock();
+        if (((ws = workQueues) == null || ws.length == 0) && nws != null)
+            workQueues = nws;
+        int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+        if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+            releasePlock(nps);
+        tryAddWorker();
+    }
+
+    /**
+     * Tries to create and start one worker if fewer than target
+     * parallelism level exist. Adjusts counts etc on failure.
+     */
+    private void tryAddWorker() {
+        long c; int u;
+        while ((u = (int)((c = ctl) >>> 32)) < 0 &&
+               (u & SHORT_SIGN) != 0 && (int)c == 0) {
+            long nc = (long)(((u + UTC_UNIT) & UTC_MASK) |
+                             ((u + UAC_UNIT) & UAC_MASK)) << 32;
+            if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                ForkJoinWorkerThreadFactory fac;
+                Throwable ex = null;
+                ForkJoinWorkerThread wt = null;
+                try {
+                    if ((fac = factory) != null &&
+                        (wt = fac.newThread(this)) != null) {
+                        wt.start();
+                        break;
+                    }
+                } catch (Throwable e) {
+                    ex = e;
+                }
+                deregisterWorker(wt, ex);
+                break;
+            }
         }
     }
 
-    // Signalling
+    //  Registering and deregistering workers
 
     /**
-     * Wakes up or creates a worker.
+     * Callback from ForkJoinWorkerThread to establish and record its
+     * WorkQueue. To avoid scanning bias due to packing entries in
+     * front of the workQueues array, we treat the array as a simple
+     * power-of-two hash table using per-thread seed as hash,
+     * expanding as needed.
+     *
+     * @param wt the worker thread
+     * @return the worker's queue
      */
-    final void signalWork() {
-        /*
-         * The while condition is true if: (there is are too few total
-         * workers OR there is at least one waiter) AND (there are too
-         * few active workers OR the pool is terminating).  The value
-         * of e distinguishes the remaining cases: zero (no waiters)
-         * for create, negative if terminating (in which case do
-         * nothing), else release a waiter. The secondary checks for
-         * release (non-null array etc) can fail if the pool begins
-         * terminating after the test, and don't impose any added cost
-         * because JVMs must perform null and bounds checks anyway.
-         */
-        long c; int e, u;
-        while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) &
-                (INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN) && e >= 0) {
-            if (e > 0) {                         // release a waiting worker
-                int i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
-                if ((ws = workers) == null ||
-                    (i = ~e & SMASK) >= ws.length ||
-                    (w = ws[i]) == null)
-                    break;
-                long nc = (((long)(w.nextWait & E_MASK)) |
-                           ((long)(u + UAC_UNIT) << 32));
-                if (w.eventCount == e &&
-                    UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
-                    w.eventCount = (e + EC_UNIT) & E_MASK;
-                    if (w.parked)
-                        UNSAFE.unpark(w);
-                    break;
+    final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
+        Thread.UncaughtExceptionHandler handler; WorkQueue[] ws; int s, ps;
+        wt.setDaemon(true);
+        if ((handler = ueh) != null)
+            wt.setUncaughtExceptionHandler(handler);
+        do {} while (!U.compareAndSwapInt(this, INDEXSEED, s = indexSeed,
+                                          s += SEED_INCREMENT) ||
+                     s == 0); // skip 0
+        WorkQueue w = new WorkQueue(this, wt, config >>> 16, s);
+        if (((ps = plock) & PL_LOCK) != 0 ||
+            !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+            ps = acquirePlock();
+        int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+        try {
+            if ((ws = workQueues) != null) {    // skip if shutting down
+                int n = ws.length, m = n - 1;
+                int r = (s << 1) | 1;           // use odd-numbered indices
+                if (ws[r &= m] != null) {       // collision
+                    int probes = 0;             // step by approx half size
+                    int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
+                    while (ws[r = (r + step) & m] != null) {
+                        if (++probes >= n) {
+                            workQueues = ws = Arrays.copyOf(ws, n <<= 1);
+                            m = n - 1;
+                            probes = 0;
+                        }
+                    }
                 }
+                w.eventCount = w.poolIndex = r; // volatile write orders
+                ws[r] = w;
             }
-            else if (UNSAFE.compareAndSwapLong
-                     (this, ctlOffset, c,
-                      (long)(((u + UTC_UNIT) & UTC_MASK) |
-                             ((u + UAC_UNIT) & UAC_MASK)) << 32)) {
-                addWorker();
-                break;
+        } finally {
+            if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+                releasePlock(nps);
+        }
+        wt.setName(workerNamePrefix.concat(Integer.toString(w.poolIndex)));
+        return w;
+    }
+
+    /**
+     * Final callback from terminating worker, as well as upon failure
+     * to construct or start a worker.  Removes record of worker from
+     * array, and adjusts counts. If pool is shutting down, tries to
+     * complete termination.
+     *
+     * @param wt the worker thread or null if construction failed
+     * @param ex the exception causing failure, or null if none
+     */
+    final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
+        WorkQueue w = null;
+        if (wt != null && (w = wt.workQueue) != null) {
+            int ps;
+            w.qlock = -1;                // ensure set
+            long ns = w.nsteals, sc;     // collect steal count
+            do {} while (!U.compareAndSwapLong(this, STEALCOUNT,
+                                               sc = stealCount, sc + ns));
+            if (((ps = plock) & PL_LOCK) != 0 ||
+                !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+                ps = acquirePlock();
+            int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+            try {
+                int idx = w.poolIndex;
+                WorkQueue[] ws = workQueues;
+                if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w)
+                    ws[idx] = null;
+            } finally {
+                if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+                    releasePlock(nps);
             }
         }
-    }
 
-    /**
-     * Variant of signalWork to help release waiters on rescans.
-     * Tries once to release a waiter if active count < 0.
-     *
-     * @return false if failed due to contention, else true
-     */
-    private boolean tryReleaseWaiter() {
-        long c; int e, i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
-        if ((e = (int)(c = ctl)) > 0 &&
-            (int)(c >> AC_SHIFT) < 0 &&
-            (ws = workers) != null &&
-            (i = ~e & SMASK) < ws.length &&
-            (w = ws[i]) != null) {
-            long nc = ((long)(w.nextWait & E_MASK) |
-                       ((c + AC_UNIT) & (AC_MASK|TC_MASK)));
-            if (w.eventCount != e ||
-                !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
-                return false;
-            w.eventCount = (e + EC_UNIT) & E_MASK;
-            if (w.parked)
-                UNSAFE.unpark(w);
+        long c;                          // adjust ctl counts
+        do {} while (!U.compareAndSwapLong
+                     (this, CTL, c = ctl, (((c - AC_UNIT) & AC_MASK) |
+                                           ((c - TC_UNIT) & TC_MASK) |
+                                           (c & ~(AC_MASK|TC_MASK)))));
+
+        if (!tryTerminate(false, false) && w != null && w.array != null) {
+            w.cancelAll();               // cancel remaining tasks
+            WorkQueue[] ws; WorkQueue v; Thread p; int u, i, e;
+            while ((u = (int)((c = ctl) >>> 32)) < 0 && (e = (int)c) >= 0) {
+                if (e > 0) {             // activate or create replacement
+                    if ((ws = workQueues) == null ||
+                        (i = e & SMASK) >= ws.length ||
+                        (v = ws[i]) != null)
+                        break;
+                    long nc = (((long)(v.nextWait & E_MASK)) |
+                               ((long)(u + UAC_UNIT) << 32));
+                    if (v.eventCount != (e | INT_SIGN))
+                        break;
+                    if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                        v.eventCount = (e + E_SEQ) & E_MASK;
+                        if ((p = v.parker) != null)
+                            U.unpark(p);
+                        break;
+                    }
+                }
+                else {
+                    if ((short)u < 0)
+                        tryAddWorker();
+                    break;
+                }
+            }
         }
-        return true;
+        if (ex == null)                     // help clean refs on way out
+            ForkJoinTask.helpExpungeStaleExceptions();
+        else                                // rethrow
+            ForkJoinTask.rethrow(ex);
     }
 
-    // Scanning for tasks
+    // Submissions
 
     /**
-     * Scans for and, if found, executes one task. Scans start at a
-     * random index of workers array, and randomly select the first
-     * (2*#workers)-1 probes, and then, if all empty, resort to 2
-     * circular sweeps, which is necessary to check quiescence. and
-     * taking a submission only if no stealable tasks were found.  The
-     * steal code inside the loop is a specialized form of
-     * ForkJoinWorkerThread.deqTask, followed bookkeeping to support
-     * helpJoinTask and signal propagation. The code for submission
-     * queues is almost identical. On each steal, the worker completes
-     * not only the task, but also all local tasks that this task may
-     * have generated. On detecting staleness or contention when
-     * trying to take a task, this method returns without finishing
-     * sweep, which allows global state rechecks before retry.
-     *
-     * @param w the worker
-     * @param a the number of active workers
-     * @return true if swept all queues without finding a task
-     */
-    private boolean scan(ForkJoinWorkerThread w, int a) {
-        int g = scanGuard; // mask 0 avoids useless scans if only one active
-        int m = (parallelism == 1 - a && blockedCount == 0) ? 0 : g & SMASK;
-        ForkJoinWorkerThread[] ws = workers;
-        if (ws == null || ws.length <= m)         // staleness check
-            return false;
-        for (int r = w.seed, k = r, j = -(m + m); j <= m + m; ++j) {
-            ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
-            ForkJoinWorkerThread v = ws[k & m];
-            if (v != null && (b = v.queueBase) != v.queueTop &&
-                (q = v.queue) != null && (i = (q.length - 1) & b) >= 0) {
-                long u = (i << ASHIFT) + ABASE;
-                if ((t = q[i]) != null && v.queueBase == b &&
-                    UNSAFE.compareAndSwapObject(q, u, t, null)) {
-                    int d = (v.queueBase = b + 1) - v.queueTop;
-                    v.stealHint = w.poolIndex;
-                    if (d != 0)
-                        signalWork();             // propagate if nonempty
-                    w.execTask(t);
+     * Unless shutting down, adds the given task to a submission queue
+     * at submitter's current queue index (modulo submission
+     * range). Only the most common path is directly handled in this
+     * method. All others are relayed to fullExternalPush.
+     *
+     * @param task the task. Caller must ensure non-null.
+     */
+    final void externalPush(ForkJoinTask<?> task) {
+        WorkQueue[] ws; WorkQueue q; Submitter z; int m; ForkJoinTask<?>[] a;
+        if ((z = submitters.get()) != null && plock > 0 &&
+            (ws = workQueues) != null && (m = (ws.length - 1)) >= 0 &&
+            (q = ws[m & z.seed & SQMASK]) != null &&
+            U.compareAndSwapInt(q, QLOCK, 0, 1)) { // lock
+            int b = q.base, s = q.top, n, an;
+            if ((a = q.array) != null && (an = a.length) > (n = s + 1 - b)) {
+                int j = (((an - 1) & s) << ASHIFT) + ABASE;
+                U.putOrderedObject(a, j, task);
+                q.top = s + 1;                     // push on to deque
+                q.qlock = 0;
+                if (n <= 2)
+                    signalWork(q);
+                return;
+            }
+            q.qlock = 0;
+        }
+        fullExternalPush(task);
+    }
+
+    /**
+     * Full version of externalPush. This method is called, among
+     * other times, upon the first submission of the first task to the
+     * pool, so must perform secondary initialization (via
+     * initWorkers). It also detects first submission by an external
+     * thread by looking up its ThreadLocal, and creates a new shared
+     * queue if the one at index if empty or contended. The plock lock
+     * body must be exception-free (so no try/finally) so we
+     * optimistically allocate new queues outside the lock and throw
+     * them away if (very rarely) not needed.
+     */
+    private void fullExternalPush(ForkJoinTask<?> task) {
+        int r = 0; // random index seed
+        for (Submitter z = submitters.get();;) {
+            WorkQueue[] ws; WorkQueue q; int ps, m, k;
+            if (z == null) {
+                if (U.compareAndSwapInt(this, INDEXSEED, r = indexSeed,
+                                        r += SEED_INCREMENT) && r != 0)
+                    submitters.set(z = new Submitter(r));
+            }
+            else if (r == 0) {               // move to a different index
+                r = z.seed;
+                r ^= r << 13;                // same xorshift as WorkQueues
+                r ^= r >>> 17;
+                z.seed = r ^ (r << 5);
+            }
+            else if ((ps = plock) < 0)
+                throw new RejectedExecutionException();
+            else if (ps == 0 || (ws = workQueues) == null ||
+                     (m = ws.length - 1) < 0)
+                initWorkers();
+            else if ((q = ws[k = r & m & SQMASK]) != null) {
+                if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+                    ForkJoinTask<?>[] a = q.array;
+                    int s = q.top;
+                    boolean submitted = false;
+                    try {                      // locked version of push
+                        if ((a != null && a.length > s + 1 - q.base) ||
+                            (a = q.growArray()) != null) {   // must presize
+                            int j = (((a.length - 1) & s) << ASHIFT) + ABASE;
+                            U.putOrderedObject(a, j, task);
+                            q.top = s + 1;
+                            submitted = true;
+                        }
+                    } finally {
+                        q.qlock = 0;  // unlock
+                    }
+                    if (submitted) {
+                        signalWork(q);
+                        return;
+                    }
                 }
-                r ^= r << 13; r ^= r >>> 17; w.seed = r ^ (r << 5);
-                return false;                     // store next seed
+                r = 0; // move on failure
             }
-            else if (j < 0) {                     // xorshift
-                r ^= r << 13; r ^= r >>> 17; k = r ^= r << 5;
+            else if (((ps = plock) & PL_LOCK) == 0) { // create new queue
+                q = new WorkQueue(this, null, SHARED_QUEUE, r);
+                if (((ps = plock) & PL_LOCK) != 0 ||
+                    !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+                    ps = acquirePlock();
+                if ((ws = workQueues) != null && k < ws.length && ws[k] == null)
+                    ws[k] = q;
+                int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+                if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+                    releasePlock(nps);
             }
             else
-                ++k;
+                r = 0; // try elsewhere while lock held
         }
-        if (scanGuard != g)                       // staleness check
-            return false;
-        else {                                    // try to take submission
-            ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
-            if ((b = queueBase) != queueTop &&
-                (q = submissionQueue) != null &&
-                (i = (q.length - 1) & b) >= 0) {
-                long u = (i << ASHIFT) + ABASE;
-                if ((t = q[i]) != null && queueBase == b &&
-                    UNSAFE.compareAndSwapObject(q, u, t, null)) {
-                    queueBase = b + 1;
-                    w.execTask(t);
+    }
+
+    // Maintaining ctl counts
+
+    /**
+     * Increments active count; mainly called upon return from blocking.
+     */
+    final void incrementActiveCount() {
+        long c;
+        do {} while (!U.compareAndSwapLong(this, CTL, c = ctl, c + AC_UNIT));
+    }
+
+    /**
+     * Tries to create or activate a worker if too few are active.
+     *
+     * @param q the (non-null) queue holding tasks to be signalled
+     */
+    final void signalWork(WorkQueue q) {
+        int hint = q.poolIndex;
+        long c; int e, u, i, n; WorkQueue[] ws; WorkQueue w; Thread p;
+        while ((u = (int)((c = ctl) >>> 32)) < 0) {
+            if ((e = (int)c) > 0) {
+                if ((ws = workQueues) != null && ws.length > (i = e & SMASK) &&
+                    (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) {
+                    long nc = (((long)(w.nextWait & E_MASK)) |
+                               ((long)(u + UAC_UNIT) << 32));
+                    if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                        w.hint = hint;
+                        w.eventCount = (e + E_SEQ) & E_MASK;
+                        if ((p = w.parker) != null)
+                            U.unpark(p);
+                        break;
+                    }
+                    if (q.top - q.base <= 0)
+                        break;
                 }
-                return false;
+                else
+                    break;
+            }
+            else {
+                if ((short)u < 0)
+                    tryAddWorker();
+                break;
             }
-            return true;                         // all queues empty
         }
     }
 
+    // Scanning for tasks
+
+    /**
+     * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
+     */
+    final void runWorker(WorkQueue w) {
+        w.growArray(); // allocate queue
+        do { w.runTask(scan(w)); } while (w.qlock >= 0);
+    }
+
     /**
-     * Tries to enqueue worker w in wait queue and await change in
-     * worker's eventCount.  If the pool is quiescent and there is
-     * more than one worker, possibly terminates worker upon exit.
-     * Otherwise, before blocking, rescans queues to avoid missed
-     * signals.  Upon finding work, releases at least one worker
-     * (which may be the current worker). Rescans restart upon
-     * detected staleness or failure to release due to
-     * contention. Note the unusual conventions about Thread.interrupt
-     * here and elsewhere: Because interrupts are used solely to alert
-     * threads to check termination, which is checked here anyway, we
-     * clear status (using Thread.interrupted) before any call to
-     * park, so that park does not immediately return due to status
-     * being set via some other unrelated call to interrupt in user
-     * code.
+     * Scans for and, if found, returns one task, else possibly
+     * inactivates the worker. This method operates on single reads of
+     * volatile state and is designed to be re-invoked continuously,
+     * in part because it returns upon detecting inconsistencies,
+     * contention, or state changes that indicate possible success on
+     * re-invocation.
+     *
+     * The scan searches for tasks across queues (starting at a random
+     * index, and relying on registerWorker to irregularly scatter
+     * them within array to avoid bias), checking each at least twice.
+     * The scan terminates upon either finding a non-empty queue, or
+     * completing the sweep. If the worker is not inactivated, it
+     * takes and returns a task from this queue. Otherwise, if not
+     * activated, it signals workers (that may include itself) and
+     * returns so caller can retry. Also returns for true if the
+     * worker array may have changed during an empty scan.  On failure
+     * to find a task, we take one of the following actions, after
+     * which the caller will retry calling this method unless
+     * terminated.
+     *
+     * * If pool is terminating, terminate the worker.
+     *
+     * * If not already enqueued, try to inactivate and enqueue the
+     * worker on wait queue. Or, if inactivating has caused the pool
+     * to be quiescent, relay to idleAwaitWork to possibly shrink
+     * pool.
      *
-     * @param w the calling worker
-     * @param c the ctl value on entry
-     * @return true if waited or another thread was released upon enq
-     */
-    private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) {
-        int v = w.eventCount;
-        w.nextWait = (int)c;                      // w's successor record
-        long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK));
-        if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
-            long d = ctl; // return true if lost to a deq, to force scan
-            return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L;
-        }
-        for (int sc = w.stealCount; sc != 0;) {   // accumulate stealCount
-            long s = stealCount;
-            if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc))
-                sc = w.stealCount = 0;
-            else if (w.eventCount != v)
-                return true;                      // update next time
-        }
-        if ((!shutdown || !tryTerminate(false)) &&
-            (int)c != 0 && parallelism + (int)(nc >> AC_SHIFT) == 0 &&
-            blockedCount == 0 && quiescerCount == 0)
-            idleAwaitWork(w, nc, c, v);           // quiescent
-        for (boolean rescanned = false;;) {
-            if (w.eventCount != v)
-                return true;
-            if (!rescanned) {
-                int g = scanGuard, m = g & SMASK;
-                ForkJoinWorkerThread[] ws = workers;
-                if (ws != null && m < ws.length) {
-                    rescanned = true;
-                    for (int i = 0; i <= m; ++i) {
-                        ForkJoinWorkerThread u = ws[i];
-                        if (u != null) {
-                            if (u.queueBase != u.queueTop &&
-                                !tryReleaseWaiter())
-                                rescanned = false; // contended
-                            if (w.eventCount != v)
-                                return true;
-                        }
+     * * If already enqueued and none of the above apply, possibly
+     * park awaiting signal, else lingering to help scan and signal.
+     *
+     * * If a non-empty queue discovered or left as a hint,
+     * help wake up other workers before return
+     *
+     * @param w the worker (via its WorkQueue)
+     * @return a task or null if none found
+     */
+    private final ForkJoinTask<?> scan(WorkQueue w) {
+        WorkQueue[] ws; int m;
+        int ps = plock;                          // read plock before ws
+        if (w != null && (ws = workQueues) != null && (m = ws.length - 1) >= 0) {
+            int ec = w.eventCount;               // ec is negative if inactive
+            int r = w.seed; r ^= r << 13; r ^= r >>> 17; w.seed = r ^= r << 5;
+            w.hint = -1;                         // update seed and clear hint
+            int j = ((m + m + 1) | MIN_SCAN) & MAX_SCAN;
+            do {
+                WorkQueue q; ForkJoinTask<?>[] a; int b;
+                if ((q = ws[(r + j) & m]) != null && (b = q.base) - q.top < 0 &&
+                    (a = q.array) != null) {     // probably nonempty
+                    int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                    ForkJoinTask<?> t = (ForkJoinTask<?>)
+                        U.getObjectVolatile(a, i);
+                    if (q.base == b && ec >= 0 && t != null &&
+                        U.compareAndSwapObject(a, i, t, null)) {
+                        if ((q.base = b + 1) - q.top < 0)
+                            signalWork(q);
+                        return t;                // taken
+                    }
+                    else if ((ec < 0 || j < m) && (int)(ctl >> AC_SHIFT) <= 0) {
+                        w.hint = (r + j) & m;    // help signal below
+                        break;                   // cannot take
                     }
                 }
-                if (scanGuard != g ||              // stale
-                    (queueBase != queueTop && !tryReleaseWaiter()))
-                    rescanned = false;
-                if (!rescanned)
-                    Thread.yield();                // reduce contention
-                else
-                    Thread.interrupted();          // clear before park
+            } while (--j >= 0);
+
+            int h, e, ns; long c, sc; WorkQueue q;
+            if ((ns = w.nsteals) != 0) {
+                if (U.compareAndSwapLong(this, STEALCOUNT,
+                                         sc = stealCount, sc + ns))
+                    w.nsteals = 0;               // collect steals and rescan
             }
+            else if (plock != ps)                // consistency check
+                ;                                // skip
+            else if ((e = (int)(c = ctl)) < 0)
+                w.qlock = -1;                    // pool is terminating
             else {
-                w.parked = true;                   // must recheck
-                if (w.eventCount != v) {
-                    w.parked = false;
-                    return true;
+                if ((h = w.hint) < 0) {
+                    if (ec >= 0) {               // try to enqueue/inactivate
+                        long nc = (((long)ec |
+                                    ((c - AC_UNIT) & (AC_MASK|TC_MASK))));
+                        w.nextWait = e;          // link and mark inactive
+                        w.eventCount = ec | INT_SIGN;
+                        if (ctl != c || !U.compareAndSwapLong(this, CTL, c, nc))
+                            w.eventCount = ec;   // unmark on CAS failure
+                        else if ((int)(c >> AC_SHIFT) == 1 - (config & SMASK))
+                            idleAwaitWork(w, nc, c);
+                    }
+                    else if (w.eventCount < 0 && !tryTerminate(false, false) &&
+                             ctl == c) {         // block
+                        Thread wt = Thread.currentThread();
+                        Thread.interrupted();    // clear status
+                        U.putObject(wt, PARKBLOCKER, this);
+                        w.parker = wt;           // emulate LockSupport.park
+                        if (w.eventCount < 0)    // recheck
+                            U.park(false, 0L);
+                        w.parker = null;
+                        U.putObject(wt, PARKBLOCKER, null);
+                    }
+                }
+                if ((h >= 0 || (h = w.hint) >= 0) &&
+                    (ws = workQueues) != null && h < ws.length &&
+                    (q = ws[h]) != null) {      // signal others before retry
+                    WorkQueue v; Thread p; int u, i, s;
+                    for (int n = (config & SMASK) >>> 1;;) {
+                        int idleCount = (w.eventCount < 0) ? 0 : -1;
+                        if (((s = idleCount - q.base + q.top) <= n &&
+                             (n = s) <= 0) ||
+                            (u = (int)((c = ctl) >>> 32)) >= 0 ||
+                            (e = (int)c) <= 0 || m < (i = e & SMASK) ||
+                            (v = ws[i]) == null)
+                            break;
+                        long nc = (((long)(v.nextWait & E_MASK)) |
+                                   ((long)(u + UAC_UNIT) << 32));
+                        if (v.eventCount != (e | INT_SIGN) ||
+                            !U.compareAndSwapLong(this, CTL, c, nc))
+                            break;
+                        v.hint = h;
+                        v.eventCount = (e + E_SEQ) & E_MASK;
+                        if ((p = v.parker) != null)
+                            U.unpark(p);
+                        if (--n <= 0)
+                            break;
+                    }
                 }
-                LockSupport.park(this);
-                rescanned = w.parked = false;
             }
         }
+        return null;
     }
 
     /**
-     * If inactivating worker w has caused pool to become
-     * quiescent, check for pool termination, and wait for event
-     * for up to SHRINK_RATE nanosecs (rescans are unnecessary in
-     * this case because quiescence reflects consensus about lack
-     * of work). On timeout, if ctl has not changed, terminate the
-     * worker. Upon its termination (see deregisterWorker), it may
-     * wake up another worker to possibly repeat this process.
+     * If inactivating worker w has caused the pool to become
+     * quiescent, checks for pool termination, and, so long as this is
+     * not the only worker, waits for event for up to a given
+     * duration.  On timeout, if ctl has not changed, terminates the
+     * worker, which will in turn wake up another worker to possibly
+     * repeat this process.
      *
      * @param w the calling worker
-     * @param currentCtl the ctl value after enqueuing w
-     * @param prevCtl the ctl value if w terminated
-     * @param v the eventCount w awaits change
-     */
-    private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl,
-                               long prevCtl, int v) {
-        if (w.eventCount == v) {
-            if (shutdown)
-                tryTerminate(false);
-            ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs
+     * @param currentCtl the ctl value triggering possible quiescence
+     * @param prevCtl the ctl value to restore if thread is terminated
+     */
+    private void idleAwaitWork(WorkQueue w, long currentCtl, long prevCtl) {
+        if (w != null && w.eventCount < 0 &&
+            !tryTerminate(false, false) && (int)prevCtl != 0) {
+            int dc = -(short)(currentCtl >>> TC_SHIFT);
+            long parkTime = dc < 0 ? FAST_IDLE_TIMEOUT: (dc + 1) * IDLE_TIMEOUT;
+            long deadline = System.nanoTime() + parkTime - TIMEOUT_SLOP;
+            Thread wt = Thread.currentThread();
             while (ctl == currentCtl) {
-                long startTime = System.nanoTime();
-                w.parked = true;
-                if (w.eventCount == v)             // must recheck
-                    LockSupport.parkNanos(this, SHRINK_RATE);
-                w.parked = false;
-                if (w.eventCount != v)
+                Thread.interrupted();  // timed variant of version in scan()
+                U.putObject(wt, PARKBLOCKER, this);
+                w.parker = wt;
+                if (ctl == currentCtl)
+                    U.park(false, parkTime);
+                w.parker = null;
+                U.putObject(wt, PARKBLOCKER, null);
+                if (ctl != currentCtl)
                     break;
-                else if (System.nanoTime() - startTime <
-                         SHRINK_RATE - (SHRINK_RATE / 10)) // timing slop
-                    Thread.interrupted();          // spurious wakeup
-                else if (UNSAFE.compareAndSwapLong(this, ctlOffset,
-                                                   currentCtl, prevCtl)) {
-                    w.terminate = true;            // restore previous
-                    w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK;
+                if (deadline - System.nanoTime() <= 0L &&
+                    U.compareAndSwapLong(this, CTL, currentCtl, prevCtl)) {
+                    w.eventCount = (w.eventCount + E_SEQ) | E_MASK;
+                    w.qlock = -1;   // shrink
                     break;
                 }
             }
         }
     }
 
-    // Submissions
-
     /**
-     * Enqueues the given task in the submissionQueue.  Same idea as
-     * ForkJoinWorkerThread.pushTask except for use of submissionLock.
+     * Scans through queues looking for work while joining a task; if
+     * any present, signals. May return early if more signalling is
+     * detectably unneeded.
      *
-     * @param t the task
+     * @param task return early if done
+     * @param origin an index to start scan
      */
-    private void addSubmission(ForkJoinTask<?> t) {
-        final ReentrantLock lock = this.submissionLock;
-        lock.lock();
-        try {
-            ForkJoinTask<?>[] q; int s, m;
-            if ((q = submissionQueue) != null) {    // ignore if queue removed
-                long u = (((s = queueTop) & (m = q.length-1)) << ASHIFT)+ABASE;
-                UNSAFE.putOrderedObject(q, u, t);
-                queueTop = s + 1;
-                if (s - queueBase == m)
-                    growSubmissionQueue();
-            }
-        } finally {
-            lock.unlock();
-        }
-        signalWork();
-    }
-
-    //  (pollSubmission is defined below with exported methods)
-
-    /**
-     * Creates or doubles submissionQueue array.
-     * Basically identical to ForkJoinWorkerThread version.
-     */
-    private void growSubmissionQueue() {
-        ForkJoinTask<?>[] oldQ = submissionQueue;
-        int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
-        if (size > MAXIMUM_QUEUE_CAPACITY)
-            throw new RejectedExecutionException("Queue capacity exceeded");
-        if (size < INITIAL_QUEUE_CAPACITY)
-            size = INITIAL_QUEUE_CAPACITY;
-        ForkJoinTask<?>[] q = submissionQueue = new ForkJoinTask<?>[size];
-        int mask = size - 1;
-        int top = queueTop;
-        int oldMask;
-        if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
-            for (int b = queueBase; b != top; ++b) {
-                long u = ((b & oldMask) << ASHIFT) + ABASE;
-                Object x = UNSAFE.getObjectVolatile(oldQ, u);
-                if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
-                    UNSAFE.putObjectVolatile
-                        (q, ((b & mask) << ASHIFT) + ABASE, x);
+    private void helpSignal(ForkJoinTask<?> task, int origin) {
+        WorkQueue[] ws; WorkQueue w; Thread p; long c; int m, u, e, i, s;
+        if (task != null && task.status >= 0 &&
+            (u = (int)(ctl >>> 32)) < 0 && (u >> UAC_SHIFT) < 0 &&
+            (ws = workQueues) != null && (m = ws.length - 1) >= 0) {
+            outer: for (int k = origin, j = m; j >= 0; --j) {
+                WorkQueue q = ws[k++ & m];
+                for (int n = m;;) { // limit to at most m signals
+                    if (task.status < 0)
+                        break outer;
+                    if (q == null ||
+                        ((s = -q.base + q.top) <= n && (n = s) <= 0))
+                        break;
+                    if ((u = (int)((c = ctl) >>> 32)) >= 0 ||
+                        (e = (int)c) <= 0 || m < (i = e & SMASK) ||
+                        (w = ws[i]) == null)
+                        break outer;
+                    long nc = (((long)(w.nextWait & E_MASK)) |
+                               ((long)(u + UAC_UNIT) << 32));
+                    if (w.eventCount != (e | INT_SIGN))
+                        break outer;
+                    if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                        w.eventCount = (e + E_SEQ) & E_MASK;
+                        if ((p = w.parker) != null)
+                            U.unpark(p);
+                        if (--n <= 0)
+                            break;
+                    }
+                }
             }
         }
     }
 
-    // Blocking support
-
     /**
-     * Tries to increment blockedCount, decrement active count
-     * (sometimes implicitly) and possibly release or create a
-     * compensating worker in preparation for blocking. Fails
-     * on contention or termination.
-     *
-     * @return true if the caller can block, else should recheck and retry
-     */
-    private boolean tryPreBlock() {
-        int b = blockedCount;
-        if (UNSAFE.compareAndSwapInt(this, blockedCountOffset, b, b + 1)) {
-            int pc = parallelism;
-            do {
-                ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w;
-                int e, ac, tc, rc, i;
-                long c = ctl;
-                int u = (int)(c >>> 32);
-                if ((e = (int)c) < 0) {
-                                                 // skip -- terminating
-                }
-                else if ((ac = (u >> UAC_SHIFT)) <= 0 && e != 0 &&
-                         (ws = workers) != null &&
-                         (i = ~e & SMASK) < ws.length &&
-                         (w = ws[i]) != null) {
-                    long nc = ((long)(w.nextWait & E_MASK) |
-                               (c & (AC_MASK|TC_MASK)));
-                    if (w.eventCount == e &&
-                        UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
-                        w.eventCount = (e + EC_UNIT) & E_MASK;
-                        if (w.parked)
-                            UNSAFE.unpark(w);
-                        return true;             // release an idle worker
+     * Tries to locate and execute tasks for a stealer of the given
+     * task, or in turn one of its stealers, Traces currentSteal ->
+     * currentJoin links looking for a thread working on a descendant
+     * of the given task and with a non-empty queue to steal back and
+     * execute tasks from. The first call to this method upon a
+     * waiting join will often entail scanning/search, (which is OK
+     * because the joiner has nothing better to do), but this method
+     * leaves hints in workers to speed up subsequent calls. The
+     * implementation is very branchy to cope with potential
+     * inconsistencies or loops encountering chains that are stale,
+     * unknown, or so long that they are likely cyclic.
+     *
+     * @param joiner the joining worker
+     * @param task the task to join
+     * @return 0 if no progress can be made, negative if task
+     * known complete, else positive
+     */
+    private int tryHelpStealer(WorkQueue joiner, ForkJoinTask<?> task) {
+        int stat = 0, steps = 0;                    // bound to avoid cycles
+        if (joiner != null && task != null) {       // hoist null checks
+            restart: for (;;) {
+                ForkJoinTask<?> subtask = task;     // current target
+                for (WorkQueue j = joiner, v;;) {   // v is stealer of subtask
+                    WorkQueue[] ws; int m, s, h;
+                    if ((s = task.status) < 0) {
+                        stat = s;
+                        break restart;
                     }
-                }
-                else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) {
-                    long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
-                    if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
-                        return true;             // no compensation needed
-                }
-                else if (tc + pc < MAX_ID) {
-                    long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
-                    if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
-                        addWorker();
-                        return true;            // create a replacement
+                    if ((ws = workQueues) == null || (m = ws.length - 1) <= 0)
+                        break restart;              // shutting down
+                    if ((v = ws[h = (j.hint | 1) & m]) == null ||
+                        v.currentSteal != subtask) {
+                        for (int origin = h;;) {    // find stealer
+                            if (((h = (h + 2) & m) & 15) == 1 &&
+                                (subtask.status < 0 || j.currentJoin != subtask))
+                                continue restart;   // occasional staleness check
+                            if ((v = ws[h]) != null &&
+                                v.currentSteal == subtask) {
+                                j.hint = h;        // save hint
+                                break;
+                            }
+                            if (h == origin)
+                                break restart;      // cannot find stealer
+                        }
+                    }
+                    for (;;) { // help stealer or descend to its stealer
+                        ForkJoinTask[] a;  int b;
+                        if (subtask.status < 0)     // surround probes with
+                            continue restart;       //   consistency checks
+                        if ((b = v.base) - v.top < 0 && (a = v.array) != null) {
+                            int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                            ForkJoinTask<?> t =
+                                (ForkJoinTask<?>)U.getObjectVolatile(a, i);
+                            if (subtask.status < 0 || j.currentJoin != subtask ||
+                                v.currentSteal != subtask)
+                                continue restart;   // stale
+                            stat = 1;               // apparent progress
+                            if (t != null && v.base == b &&
+                                U.compareAndSwapObject(a, i, t, null)) {
+                                v.base = b + 1;     // help stealer
+                                joiner.runSubtask(t);
+                            }
+                            else if (v.base == b && ++steps == MAX_HELP)
+                                break restart;      // v apparently stalled
+                        }
+                        else {                      // empty -- try to descend
+                            ForkJoinTask<?> next = v.currentJoin;
+                            if (subtask.status < 0 || j.currentJoin != subtask ||
+                                v.currentSteal != subtask)
+                                continue restart;   // stale
+                            else if (next == null || ++steps == MAX_HELP)
+                                break restart;      // dead-end or maybe cyclic
+                            else {
+                                subtask = next;
+                                j = v;
+                                break;
+                            }
+                        }
                     }
                 }
-                // try to back out on any failure and let caller retry
-            } while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
-                                               b = blockedCount, b - 1));
+            }
         }
-        return false;
+        return stat;
     }
 
     /**
-     * Decrements blockedCount and increments active count
-     */
-    private void postBlock() {
-        long c;
-        do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset,  // no mask
-                                                c = ctl, c + AC_UNIT));
-        int b;
-        do {} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
-                                               b = blockedCount, b - 1));
-    }
-
-    /**
-     * Possibly blocks waiting for the given task to complete, or
-     * cancels the task if terminating.  Fails to wait if contended.
+     * Analog of tryHelpStealer for CountedCompleters. Tries to steal
+     * and run tasks within the target's computation.
+     *
+     * @param task the task to join
+     * @param mode if shared, exit upon completing any task
+     * if all workers are active
      *
-     * @param joinMe the task
      */
-    final void tryAwaitJoin(ForkJoinTask<?> joinMe) {
-        int s;
-        Thread.interrupted(); // clear interrupts before checking termination
-        if (joinMe.status >= 0) {
-            if (tryPreBlock()) {
-                joinMe.tryAwaitDone(0L);
-                postBlock();
+    private int helpComplete(ForkJoinTask<?> task, int mode) {
+        WorkQueue[] ws; WorkQueue q; int m, n, s, u;
+        if (task != null && (ws = workQueues) != null &&
+            (m = ws.length - 1) >= 0) {
+            for (int j = 1, origin = j;;) {
+                if ((s = task.status) < 0)
+                    return s;
+                if ((q = ws[j & m]) != null && q.pollAndExecCC(task)) {
+                    origin = j;
+                    if (mode == SHARED_QUEUE &&
+                        ((u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0))
+                        break;
+                }
+                else if ((j = (j + 2) & m) == origin)
+                    break;
+            }
+        }
+        return 0;
+    }
+
+    /**
+     * Tries to decrement active count (sometimes implicitly) and
+     * possibly release or create a compensating worker in preparation
+     * for blocking. Fails on contention or termination. Otherwise,
+     * adds a new thread if no idle workers are available and pool
+     * may become starved.
+     */
+    final boolean tryCompensate() {
+        int pc = config & SMASK, e, i, tc; long c;
+        WorkQueue[] ws; WorkQueue w; Thread p;
+        if ((ws = workQueues) != null && (e = (int)(c = ctl)) >= 0) {
+            if (e != 0 && (i = e & SMASK) < ws.length &&
+                (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) {
+                long nc = ((long)(w.nextWait & E_MASK) |
+                           (c & (AC_MASK|TC_MASK)));
+                if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                    w.eventCount = (e + E_SEQ) & E_MASK;
+                    if ((p = w.parker) != null)
+                        U.unpark(p);
+                    return true;   // replace with idle worker
+                }
+            }
+            else if ((tc = (short)(c >>> TC_SHIFT)) >= 0 &&
+                     (int)(c >> AC_SHIFT) + pc > 1) {
+                long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
+                if (U.compareAndSwapLong(this, CTL, c, nc))
+                    return true;   // no compensation
+            }
+            else if (tc + pc < MAX_CAP) {
+                long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
+                if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                    ForkJoinWorkerThreadFactory fac;
+                    Throwable ex = null;
+                    ForkJoinWorkerThread wt = null;
+                    try {
+                        if ((fac = factory) != null &&
+                            (wt = fac.newThread(this)) != null) {
+                            wt.start();
+                            return true;
+                        }
+                    } catch (Throwable rex) {
+                        ex = rex;
+                    }
+                    deregisterWorker(wt, ex); // clean up and return false
+                }
             }
-            else if ((ctl & STOP_BIT) != 0L)
-                joinMe.cancelIgnoringExceptions();
         }
+        return false;
     }
 
     /**
-     * Possibly blocks the given worker waiting for joinMe to
-     * complete or timeout
+     * Helps and/or blocks until the given task is done.
      *
-     * @param joinMe the task
-     * @param millis the wait time for underlying Object.wait
-     */
-    final void timedAwaitJoin(ForkJoinTask<?> joinMe, long nanos) {
-        while (joinMe.status >= 0) {
-            Thread.interrupted();
-            if ((ctl & STOP_BIT) != 0L) {
-                joinMe.cancelIgnoringExceptions();
-                break;
+     * @param joiner the joining worker
+     * @param task the task
+     * @return task status on exit
+     */
+    final int awaitJoin(WorkQueue joiner, ForkJoinTask<?> task) {
+        int s = 0;
+        if (joiner != null && task != null && (s = task.status) >= 0) {
+            ForkJoinTask<?> prevJoin = joiner.currentJoin;
+            joiner.currentJoin = task;
+            do {} while ((s = task.status) >= 0 && !joiner.isEmpty() &&
+                         joiner.tryRemoveAndExec(task)); // process local tasks
+            if (s >= 0 && (s = task.status) >= 0) {
+                helpSignal(task, joiner.poolIndex);
+                if ((s = task.status) >= 0 &&
+                    (task instanceof CountedCompleter))
+                    s = helpComplete(task, LIFO_QUEUE);
             }
-            if (tryPreBlock()) {
-                long last = System.nanoTime();
-                while (joinMe.status >= 0) {
-                    long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
-                    if (millis <= 0)
-                        break;
-                    joinMe.tryAwaitDone(millis);
-                    if (joinMe.status < 0)
-                        break;
-                    if ((ctl & STOP_BIT) != 0L) {
-                        joinMe.cancelIgnoringExceptions();
-                        break;
+            while (s >= 0 && (s = task.status) >= 0) {
+                if ((!joiner.isEmpty() ||           // try helping
+                     (s = tryHelpStealer(joiner, task)) == 0) &&
+                    (s = task.status) >= 0) {
+                    helpSignal(task, joiner.poolIndex);
+                    if ((s = task.status) >= 0 && tryCompensate()) {
+                        if (task.trySetSignal() && (s = task.status) >= 0) {
+                            synchronized (task) {
+                                if (task.status >= 0) {
+                                    try {                // see ForkJoinTask
+                                        task.wait();     //  for explanation
+                                    } catch (InterruptedException ie) {
+                                    }
+                                }
+                                else
+                                    task.notifyAll();
+                            }
+                        }
+                        long c;                          // re-activate
+                        do {} while (!U.compareAndSwapLong
+                                     (this, CTL, c = ctl, c + AC_UNIT));
                     }
-                    long now = System.nanoTime();
-                    nanos -= now - last;
-                    last = now;
                 }
-                postBlock();
-                break;
             }
+            joiner.currentJoin = prevJoin;
         }
+        return s;
     }
 
     /**
-     * If necessary, compensates for blocker, and blocks
+     * Stripped-down variant of awaitJoin used by timed joins. Tries
+     * to help join only while there is continuous progress. (Caller
+     * will then enter a timed wait.)
+     *
+     * @param joiner the joining worker
+     * @param task the task
      */
-    private void awaitBlocker(ManagedBlocker blocker)
-        throws InterruptedException {
-        while (!blocker.isReleasable()) {
-            if (tryPreBlock()) {
-                try {
-                    do {} while (!blocker.isReleasable() && !blocker.block());
-                } finally {
-                    postBlock();
-                }
-                break;
+    final void helpJoinOnce(WorkQueue joiner, ForkJoinTask<?> task) {
+        int s;
+        if (joiner != null && task != null && (s = task.status) >= 0) {
+            ForkJoinTask<?> prevJoin = joiner.currentJoin;
+            joiner.currentJoin = task;
+            do {} while ((s = task.status) >= 0 && !joiner.isEmpty() &&
+                         joiner.tryRemoveAndExec(task));
+            if (s >= 0 && (s = task.status) >= 0) {
+                helpSignal(task, joiner.poolIndex);
+                if ((s = task.status) >= 0 &&
+                    (task instanceof CountedCompleter))
+                    s = helpComplete(task, LIFO_QUEUE);
             }
+            if (s >= 0 && joiner.isEmpty()) {
+                do {} while (task.status >= 0 &&
+                             tryHelpStealer(joiner, task) > 0);
+            }
+            joiner.currentJoin = prevJoin;
         }
     }
 
-    // Creating, registering and deregistring workers
-
     /**
-     * Tries to create and start a worker; minimally rolls back counts
-     * on failure.
-     */
-    private void addWorker() {
-        Throwable ex = null;
-        ForkJoinWorkerThread t = null;
-        try {
-            t = factory.newThread(this);
-        } catch (Throwable e) {
-            ex = e;
-        }
-        if (t == null) {  // null or exceptional factory return
-            long c;       // adjust counts
-            do {} while (!UNSAFE.compareAndSwapLong
-                         (this, ctlOffset, c = ctl,
-                          (((c - AC_UNIT) & AC_MASK) |
-                           ((c - TC_UNIT) & TC_MASK) |
-                           (c & ~(AC_MASK|TC_MASK)))));
-            // Propagate exception if originating from an external caller
-            if (!tryTerminate(false) && ex != null &&
-                !(Thread.currentThread() instanceof ForkJoinWorkerThread))
-                UNSAFE.throwException(ex);
+     * Returns a (probably) non-empty steal queue, if one is found
+     * during a random, then cyclic scan, else null.  This method must
+     * be retried by caller if, by the time it tries to use the queue,
+     * it is empty.
+     * @param r a (random) seed for scanning
+     */
+    private WorkQueue findNonEmptyStealQueue(int r) {
+        for (WorkQueue[] ws;;) {
+            int ps = plock, m, n;
+            if ((ws = workQueues) == null || (m = ws.length - 1) < 1)
+                return null;
+            for (int j = (m + 1) << 2; ;) {
+                WorkQueue q = ws[(((r + j) << 1) | 1) & m];
+                if (q != null && (n = q.base - q.top) < 0) {
+                    if (n < -1)
+                        signalWork(q);
+                    return q;
+                }
+                else if (--j < 0) {
+                    if (plock == ps)
+                        return null;
+                    break;
+                }
+            }
         }
-        else
-            t.start();
     }
 
     /**
-     * Callback from ForkJoinWorkerThread constructor to assign a
-     * public name
-     */
-    final String nextWorkerName() {
-        for (int n;;) {
-            if (UNSAFE.compareAndSwapInt(this, nextWorkerNumberOffset,
-                                         n = nextWorkerNumber, ++n))
-                return workerNamePrefix + n;
+     * Runs tasks until {@code isQuiescent()}. We piggyback on
+     * active count ctl maintenance, but rather than blocking
+     * when tasks cannot be found, we rescan until all others cannot
+     * find tasks either.
+     */
+    final void helpQuiescePool(WorkQueue w) {
+        for (boolean active = true;;) {
+            ForkJoinTask<?> localTask; // exhaust local queue
+            while ((localTask = w.nextLocalTask()) != null)
+                localTask.doExec();
+            // Similar to loop in scan(), but ignoring submissions
+            WorkQueue q = findNonEmptyStealQueue(w.nextSeed());
+            if (q != null) {
+                ForkJoinTask<?> t; int b;
+                if (!active) {      // re-establish active count
+                    long c;
+                    active = true;
+                    do {} while (!U.compareAndSwapLong
+                                 (this, CTL, c = ctl, c + AC_UNIT));
+                }
+                if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null)
+                    w.runSubtask(t);
+            }
+            else {
+                long c;
+                if (active) {       // decrement active count without queuing
+                    active = false;
+                    do {} while (!U.compareAndSwapLong
+                                 (this, CTL, c = ctl, c -= AC_UNIT));
+                }
+                else
+                    c = ctl;        // re-increment on exit
+                if ((int)(c >> AC_SHIFT) + (config & SMASK) == 0) {
+                    do {} while (!U.compareAndSwapLong
+                                 (this, CTL, c = ctl, c + AC_UNIT));
+                    break;
+                }
+            }
         }
     }
 
     /**
-     * Callback from ForkJoinWorkerThread constructor to
-     * determine its poolIndex and record in workers array.
+     * Gets and removes a local or stolen task for the given worker.
      *
-     * @param w the worker
-     * @return the worker's pool index
+     * @return a task, if available
      */
-    final int registerWorker(ForkJoinWorkerThread w) {
-        /*
-         * In the typical case, a new worker acquires the lock, uses
-         * next available index and returns quickly.  Since we should
-         * not block callers (ultimately from signalWork or
-         * tryPreBlock) waiting for the lock needed to do this, we
-         * instead help release other workers while waiting for the
-         * lock.
-         */
-        for (int g;;) {
-            ForkJoinWorkerThread[] ws;
-            if (((g = scanGuard) & SG_UNIT) == 0 &&
-                UNSAFE.compareAndSwapInt(this, scanGuardOffset,
-                                         g, g | SG_UNIT)) {
-                int k = nextWorkerIndex;
-                try {
-                    if ((ws = workers) != null) { // ignore on shutdown
-                        int n = ws.length;
-                        if (k < 0 || k >= n || ws[k] != null) {
-                            for (k = 0; k < n && ws[k] != null; ++k)
-                                ;
-                            if (k == n)
-                                ws = workers = Arrays.copyOf(ws, n << 1);
-                        }
-                        ws[k] = w;
-                        nextWorkerIndex = k + 1;
-                        int m = g & SMASK;
-                        g = (k > m) ? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1);
-                    }
-                } finally {
-                    scanGuard = g;
-                }
-                return k;
-            }
-            else if ((ws = workers) != null) { // help release others
-                for (ForkJoinWorkerThread u : ws) {
-                    if (u != null && u.queueBase != u.queueTop) {
-                        if (tryReleaseWaiter())
-                            break;
-                    }
-                }
-            }
+    final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
+        for (ForkJoinTask<?> t;;) {
+            WorkQueue q; int b;
+            if ((t = w.nextLocalTask()) != null)
+                return t;
+            if ((q = findNonEmptyStealQueue(w.nextSeed())) == null)
+                return null;
+            if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null)
+                return t;
         }
     }
 
     /**
-     * Final callback from terminating worker.  Removes record of
-     * worker from array, and adjusts counts. If pool is shutting
-     * down, tries to complete termination.
-     *
-     * @param w the worker
-     */
-    final void deregisterWorker(ForkJoinWorkerThread w, Throwable ex) {
-        int idx = w.poolIndex;
-        int sc = w.stealCount;
-        int steps = 0;
-        // Remove from array, adjust worker counts and collect steal count.
-        // We can intermix failed removes or adjusts with steal updates
-        do {
-            long s, c;
-            int g;
-            if (steps == 0 && ((g = scanGuard) & SG_UNIT) == 0 &&
-                UNSAFE.compareAndSwapInt(this, scanGuardOffset,
-                                         g, g |= SG_UNIT)) {
-                ForkJoinWorkerThread[] ws = workers;
-                if (ws != null && idx >= 0 &&
-                    idx < ws.length && ws[idx] == w)
-                    ws[idx] = null;    // verify
-                nextWorkerIndex = idx;
-                scanGuard = g + SG_UNIT;
-                steps = 1;
-            }
-            if (steps == 1 &&
-                UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
-                                          (((c - AC_UNIT) & AC_MASK) |
-                                           ((c - TC_UNIT) & TC_MASK) |
-                                           (c & ~(AC_MASK|TC_MASK)))))
-                steps = 2;
-            if (sc != 0 &&
-                UNSAFE.compareAndSwapLong(this, stealCountOffset,
-                                          s = stealCount, s + sc))
-                sc = 0;
-        } while (steps != 2 || sc != 0);
-        if (!tryTerminate(false)) {
-            if (ex != null)   // possibly replace if died abnormally
-                signalWork();
-            else
-                tryReleaseWaiter();
+     * Returns a cheap heuristic guide for task partitioning when
+     * programmers, frameworks, tools, or languages have little or no
+     * idea about task granularity.  In essence by offering this
+     * method, we ask users only about tradeoffs in overhead vs
+     * expected throughput and its variance, rather than how finely to
+     * partition tasks.
+     *
+     * In a steady state strict (tree-structured) computation, each
+     * thread makes available for stealing enough tasks for other
+     * threads to remain active. Inductively, if all threads play by
+     * the same rules, each thread should make available only a
+     * constant number of tasks.
+     *
+     * The minimum useful constant is just 1. But using a value of 1
+     * would require immediate replenishment upon each steal to
+     * maintain enough tasks, which is infeasible.  Further,
+     * partitionings/granularities of offered tasks should minimize
+     * steal rates, which in general means that threads nearer the top
+     * of computation tree should generate more than those nearer the
+     * bottom. In perfect steady state, each thread is at
+     * approximately the same level of computation tree. However,
+     * producing extra tasks amortizes the uncertainty of progress and
+     * diffusion assumptions.
+     *
+     * So, users will want to use values larger, but not much larger
+     * than 1 to both smooth over transient shortages and hedge
+     * against uneven progress; as traded off against the cost of
+     * extra task overhead. We leave the user to pick a threshold
+     * value to compare with the results of this call to guide
+     * decisions, but recommend values such as 3.
+     *
+     * When all threads are active, it is on average OK to estimate
+     * surplus strictly locally. In steady-state, if one thread is
+     * maintaining say 2 surplus tasks, then so are others. So we can
+     * just use estimated queue length.  However, this strategy alone
+     * leads to serious mis-estimates in some non-steady-state
+     * conditions (ramp-up, ramp-down, other stalls). We can detect
+     * many of these by further considering the number of "idle"
+     * threads, that are known to have zero queued tasks, so
+     * compensate by a factor of (#idle/#active) threads.
+     *
+     * Note: The approximation of #busy workers as #active workers is
+     * not very good under current signalling scheme, and should be
+     * improved.
+     */
+    static int getSurplusQueuedTaskCount() {
+        Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
+        if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
+            int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).config & SMASK;
+            int n = (q = wt.workQueue).top - q.base;
+            int a = (int)(pool.ctl >> AC_SHIFT) + p;
+            return n - (a > (p >>>= 1) ? 0 :
+                        a > (p >>>= 1) ? 1 :
+                        a > (p >>>= 1) ? 2 :
+                        a > (p >>>= 1) ? 4 :
+                        8);
         }
+        return 0;
     }
 
-    // Shutdown and termination
+    //  Termination
 
     /**
-     * Possibly initiates and/or completes termination.
+     * Possibly initiates and/or completes termination.  The caller
+     * triggering termination runs three passes through workQueues:
+     * (0) Setting termination status, followed by wakeups of queued
+     * workers; (1) cancelling all tasks; (2) interrupting lagging
+     * threads (likely in external tasks, but possibly also blocked in
+     * joins).  Each pass repeats previous steps because of potential
+     * lagging thread creation.
      *
      * @param now if true, unconditionally terminate, else only
-     * if shutdown and empty queue and no active workers
+     * if no work and no active workers
+     * @param enable if true, enable shutdown when next possible
      * @return true if now terminating or terminated
      */
-    private boolean tryTerminate(boolean now) {
-        long c;
-        while (((c = ctl) & STOP_BIT) == 0) {
-            if (!now) {
-                if ((int)(c >> AC_SHIFT) != -parallelism)
-                    return false;
-                if (!shutdown || blockedCount != 0 || quiescerCount != 0 ||
-                    queueBase != queueTop) {
-                    if (ctl == c) // staleness check
-                        return false;
-                    continue;
+    private boolean tryTerminate(boolean now, boolean enable) {
+        if (this == commonPool)                     // cannot shut down
+            return false;
+        for (long c;;) {
+            if (((c = ctl) & STOP_BIT) != 0) {      // already terminating
+                if ((short)(c >>> TC_SHIFT) == -(config & SMASK)) {
+                    synchronized (this) {
+                        notifyAll();                // signal when 0 workers
+                    }
                 }
+                return true;
             }
-            if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT))
-                startTerminating();
-        }
-        if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers
-            final ReentrantLock lock = this.submissionLock;
-            lock.lock();
-            try {
-                termination.signalAll();
-            } finally {
-                lock.unlock();
+            if (plock >= 0) {                       // not yet enabled
+                int ps;
+                if (!enable)
+                    return false;
+                if (((ps = plock) & PL_LOCK) != 0 ||
+                    !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+                    ps = acquirePlock();
+                if (!U.compareAndSwapInt(this, PLOCK, ps, SHUTDOWN))
+                    releasePlock(SHUTDOWN);
             }
-        }
-        return true;
-    }
-
-    /**
-     * Runs up to three passes through workers: (0) Setting
-     * termination status for each worker, followed by wakeups up to
-     * queued workers; (1) helping cancel tasks; (2) interrupting
-     * lagging threads (likely in external tasks, but possibly also
-     * blocked in joins).  Each pass repeats previous steps because of
-     * potential lagging thread creation.
-     */
-    private void startTerminating() {
-        cancelSubmissions();
-        for (int pass = 0; pass < 3; ++pass) {
-            ForkJoinWorkerThread[] ws = workers;
-            if (ws != null) {
-                for (ForkJoinWorkerThread w : ws) {
-                    if (w != null) {
-                        w.terminate = true;
-                        if (pass > 0) {
-                            w.cancelTasks();
-                            if (pass > 1 && !w.isInterrupted()) {
-                                try {
-                                    w.interrupt();
-                                } catch (SecurityException ignore) {
+            if (!now) {                             // check if idle & no tasks
+                if ((int)(c >> AC_SHIFT) != -(config & SMASK) ||
+                    hasQueuedSubmissions())
+                    return false;
+                // Check for unqueued inactive workers. One pass suffices.
+                WorkQueue[] ws = workQueues; WorkQueue w;
+                if (ws != null) {
+                    for (int i = 1; i < ws.length; i += 2) {
+                        if ((w = ws[i]) != null && w.eventCount >= 0)
+                            return false;
+                    }
+                }
+            }
+            if (U.compareAndSwapLong(this, CTL, c, c | STOP_BIT)) {
+                for (int pass = 0; pass < 3; ++pass) {
+                    WorkQueue[] ws = workQueues;
+                    if (ws != null) {
+                        WorkQueue w; Thread wt;
+                        int n = ws.length;
+                        for (int i = 0; i < n; ++i) {
+                            if ((w = ws[i]) != null) {
+                                w.qlock = -1;
+                                if (pass > 0) {
+                                    w.cancelAll();
+                                    if (pass > 1 && (wt = w.owner) != null) {
+                                        if (!wt.isInterrupted()) {
+                                            try {
+                                                wt.interrupt();
+                                            } catch (SecurityException ignore) {
+                                            }
+                                        }
+                                        U.unpark(wt);
+                                    }
                                 }
                             }
                         }
+                        // Wake up workers parked on event queue
+                        int i, e; long cc; Thread p;
+                        while ((e = (int)(cc = ctl) & E_MASK) != 0 &&
+                               (i = e & SMASK) < n &&
+                               (w = ws[i]) != null) {
+                            long nc = ((long)(w.nextWait & E_MASK) |
+                                       ((cc + AC_UNIT) & AC_MASK) |
+                                       (cc & (TC_MASK|STOP_BIT)));
+                            if (w.eventCount == (e | INT_SIGN) &&
+                                U.compareAndSwapLong(this, CTL, cc, nc)) {
+                                w.eventCount = (e + E_SEQ) & E_MASK;
+                                w.qlock = -1;
+                                if ((p = w.parker) != null)
+                                    U.unpark(p);
+                            }
+                        }
                     }
                 }
-                terminateWaiters();
             }
         }
     }
 
+    // external operations on common pool
+
     /**
-     * Polls and cancels all submissions. Called only during termination.
+     * Returns common pool queue for a thread that has submitted at
+     * least one task.
      */
-    private void cancelSubmissions() {
-        while (queueBase != queueTop) {
-            ForkJoinTask<?> task = pollSubmission();
-            if (task != null) {
-                try {
-                    task.cancel(false);
-                } catch (Throwable ignore) {
-                }
-            }
-        }
+    static WorkQueue commonSubmitterQueue() {
+        ForkJoinPool p; WorkQueue[] ws; int m; Submitter z;
+        return ((z = submitters.get()) != null &&
+                (p = commonPool) != null &&
+                (ws = p.workQueues) != null &&
+                (m = ws.length - 1) >= 0) ?
+            ws[m & z.seed & SQMASK] : null;
     }
 
     /**
-     * Tries to set the termination status of waiting workers, and
-     * then wakes them up (after which they will terminate).
-     */
-    private void terminateWaiters() {
-        ForkJoinWorkerThread[] ws = workers;
-        if (ws != null) {
-            ForkJoinWorkerThread w; long c; int i, e;
-            int n = ws.length;
-            while ((i = ~(e = (int)(c = ctl)) & SMASK) < n &&
-                   (w = ws[i]) != null && w.eventCount == (e & E_MASK)) {
-                if (UNSAFE.compareAndSwapLong(this, ctlOffset, c,
-                                              (long)(w.nextWait & E_MASK) |
-                                              ((c + AC_UNIT) & AC_MASK) |
-                                              (c & (TC_MASK|STOP_BIT)))) {
-                    w.terminate = true;
-                    w.eventCount = e + EC_UNIT;
-                    if (w.parked)
-                        UNSAFE.unpark(w);
+     * Tries to pop the given task from submitter's queue in common pool.
+     */
+    static boolean tryExternalUnpush(ForkJoinTask<?> t) {
+        ForkJoinPool p; WorkQueue[] ws; WorkQueue q; Submitter z;
+        ForkJoinTask<?>[] a;  int m, s;
+        if (t != null &&
+            (z = submitters.get()) != null &&
+            (p = commonPool) != null &&
+            (ws = p.workQueues) != null &&
+            (m = ws.length - 1) >= 0 &&
+            (q = ws[m & z.seed & SQMASK]) != null &&
+            (s = q.top) != q.base &&
+            (a = q.array) != null) {
+            long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
+            if (U.getObject(a, j) == t &&
+                U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+                if (q.array == a && q.top == s && // recheck
+                    U.compareAndSwapObject(a, j, t, null)) {
+                    q.top = s - 1;
+                    q.qlock = 0;
+                    return true;
                 }
+                q.qlock = 0;
             }
         }
+        return false;
     }
 
-    // misc ForkJoinWorkerThread support
-
     /**
-     * Increment or decrement quiescerCount. Needed only to prevent
-     * triggering shutdown if a worker is transiently inactive while
-     * checking quiescence.
-     *
-     * @param delta 1 for increment, -1 for decrement
+     * Tries to pop and run local tasks within the same computation
+     * as the given root. On failure, tries to help complete from
+     * other queues via helpComplete.
      */
-    final void addQuiescerCount(int delta) {
-        int c;
-        do {} while (!UNSAFE.compareAndSwapInt(this, quiescerCountOffset,
-                                               c = quiescerCount, c + delta));
+    private void externalHelpComplete(WorkQueue q, ForkJoinTask<?> root) {
+        ForkJoinTask<?>[] a; int m;
+        if (q != null && (a = q.array) != null && (m = (a.length - 1)) >= 0 &&
+            root != null && root.status >= 0) {
+            for (;;) {
+                int s, u; Object o; CountedCompleter<?> task = null;
+                if ((s = q.top) - q.base > 0) {
+                    long j = ((m & (s - 1)) << ASHIFT) + ABASE;
+                    if ((o = U.getObject(a, j)) != null &&
+                        (o instanceof CountedCompleter)) {
+                        CountedCompleter<?> t = (CountedCompleter<?>)o, r = t;
+                        do {
+                            if (r == root) {
+                                if (U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+                                    if (q.array == a && q.top == s &&
+                                        U.compareAndSwapObject(a, j, t, null)) {
+                                        q.top = s - 1;
+                                        task = t;
+                                    }
+                                    q.qlock = 0;
+                                }
+                                break;
+                            }
+                        } while ((r = r.completer) != null);
+                    }
+                }
+                if (task != null)
+                    task.doExec();
+                if (root.status < 0 ||
+                    (u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0)
+                    break;
+                if (task == null) {
+                    helpSignal(root, q.poolIndex);
+                    if (root.status >= 0)
+                        helpComplete(root, SHARED_QUEUE);
+                    break;
+                }
+            }
+        }
     }
 
     /**
-     * Directly increment or decrement active count without
-     * queuing. This method is used to transiently assert inactivation
-     * while checking quiescence.
-     *
-     * @param delta 1 for increment, -1 for decrement
-     */
-    final void addActiveCount(int delta) {
-        long d = delta < 0 ? -AC_UNIT : AC_UNIT;
-        long c;
-        do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
-                                                ((c + d) & AC_MASK) |
-                                                (c & ~AC_MASK)));
+     * Tries to help execute or signal availability of the given task
+     * from submitter's queue in common pool.
+     */
+    static void externalHelpJoin(ForkJoinTask<?> t) {
+        // Some hard-to-avoid overlap with tryExternalUnpush
+        ForkJoinPool p; WorkQueue[] ws; WorkQueue q, w; Submitter z;
+        ForkJoinTask<?>[] a;  int m, s, n;
+        if (t != null &&
+            (z = submitters.get()) != null &&
+            (p = commonPool) != null &&
+            (ws = p.workQueues) != null &&
+            (m = ws.length - 1) >= 0 &&
+            (q = ws[m & z.seed & SQMASK]) != null &&
+            (a = q.array) != null) {
+            int am = a.length - 1;
+            if ((s = q.top) != q.base) {
+                long j = ((am & (s - 1)) << ASHIFT) + ABASE;
+                if (U.getObject(a, j) == t &&
+                    U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+                    if (q.array == a && q.top == s &&
+                        U.compareAndSwapObject(a, j, t, null)) {
+                        q.top = s - 1;
+                        q.qlock = 0;
+                        t.doExec();
+                    }
+                    else
+                        q.qlock = 0;
+                }
+            }
+            if (t.status >= 0) {
+                if (t instanceof CountedCompleter)
+                    p.externalHelpComplete(q, t);
+                else
+                    p.helpSignal(t, q.poolIndex);
+            }
+        }
     }
 
     /**
-     * Returns the approximate (non-atomic) number of idle threads per
-     * active thread.
+     * Restricted version of helpQuiescePool for external callers
      */
-    final int idlePerActive() {
-        // Approximate at powers of two for small values, saturate past 4
-        int p = parallelism;
-        int a = p + (int)(ctl >> AC_SHIFT);
-        return (a > (p >>>= 1) ? 0 :
-                a > (p >>>= 1) ? 1 :
-                a > (p >>>= 1) ? 2 :
-                a > (p >>>= 1) ? 4 :
-                8);
+    static void externalHelpQuiescePool() {
+        ForkJoinPool p; ForkJoinTask<?> t; WorkQueue q; int b;
+        if ((p = commonPool) != null &&
+            (q = p.findNonEmptyStealQueue(1)) != null &&
+            (b = q.base) - q.top < 0 &&
+            (t = q.pollAt(b)) != null)
+            t.doExec();
     }
 
     // Exported methods
@@ -1464,31 +2577,46 @@ public class ForkJoinPool extends AbstractExecutorService {
         checkPermission();
         if (factory == null)
             throw new NullPointerException();
-        if (parallelism <= 0 || parallelism > MAX_ID)
+        if (parallelism <= 0 || parallelism > MAX_CAP)
             throw new IllegalArgumentException();
-        this.parallelism = parallelism;
         this.factory = factory;
         this.ueh = handler;
-        this.locallyFifo = asyncMode;
+        this.config = parallelism | (asyncMode ? (FIFO_QUEUE << 16) : 0);
         long np = (long)(-parallelism); // offset ctl counts
         this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
-        this.submissionQueue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
-        // initialize workers array with room for 2*parallelism if possible
-        int n = parallelism << 1;
-        if (n >= MAX_ID)
-            n = MAX_ID;
-        else { // See Hackers Delight, sec 3.2, where n < (1 << 16)
-            n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8;
-        }
-        workers = new ForkJoinWorkerThread[n + 1];
-        this.submissionLock = new ReentrantLock();
-        this.termination = submissionLock.newCondition();
+        int pn = nextPoolId();
         StringBuilder sb = new StringBuilder("ForkJoinPool-");
-        sb.append(poolNumberGenerator.incrementAndGet());
+        sb.append(Integer.toString(pn));
         sb.append("-worker-");
         this.workerNamePrefix = sb.toString();
     }
 
+    /**
+     * Constructor for common pool, suitable only for static initialization.
+     * Basically the same as above, but uses smallest possible initial footprint.
+     */
+    ForkJoinPool(int parallelism, long ctl,
+                 ForkJoinWorkerThreadFactory factory,
+                 Thread.UncaughtExceptionHandler handler) {
+        this.config = parallelism;
+        this.ctl = ctl;
+        this.factory = factory;
+        this.ueh = handler;
+        this.workerNamePrefix = "ForkJoinPool.commonPool-worker-";
+    }
+
+    /**
+     * Returns the common pool instance. This pool is statically
+     * constructed; its run state is unaffected by attempts to
+     * {@link #shutdown} or {@link #shutdownNow}.
+     *
+     * @return the common pool instance
+     */
+    public static ForkJoinPool commonPool() {
+        // assert commonPool != null : "static init error";
+        return commonPool;
+    }
+
     // Execution methods
 
     /**
@@ -1508,34 +2636,10 @@ public class ForkJoinPool extends AbstractExecutorService {
      *         scheduled for execution
      */
     public <T> T invoke(ForkJoinTask<T> task) {
-        Thread t = Thread.currentThread();
         if (task == null)
             throw new NullPointerException();
-        if (shutdown)
-            throw new RejectedExecutionException();
-        if ((t instanceof ForkJoinWorkerThread) &&
-            ((ForkJoinWorkerThread)t).pool == this)
-            return task.invoke();  // bypass submit if in same pool
-        else {
-            addSubmission(task);
-            return task.join();
-        }
-    }
-
-    /**
-     * Unless terminating, forks task if within an ongoing FJ
-     * computation in the current pool, else submits as external task.
-     */
-    private <T> void forkOrSubmit(ForkJoinTask<T> task) {
-        ForkJoinWorkerThread w;
-        Thread t = Thread.currentThread();
-        if (shutdown)
-            throw new RejectedExecutionException();
-        if ((t instanceof ForkJoinWorkerThread) &&
-            (w = (ForkJoinWorkerThread)t).pool == this)
-            w.pushTask(task);
-        else
-            addSubmission(task);
+        externalPush(task);
+        return task.join();
     }
 
     /**
@@ -1549,7 +2653,7 @@ public class ForkJoinPool extends AbstractExecutorService {
     public void execute(ForkJoinTask<?> task) {
         if (task == null)
             throw new NullPointerException();
-        forkOrSubmit(task);
+        externalPush(task);
     }
 
     // AbstractExecutorService methods
@@ -1566,8 +2670,8 @@ public class ForkJoinPool extends AbstractExecutorService {
         if (task instanceof ForkJoinTask<?>) // avoid re-wrap
             job = (ForkJoinTask<?>) task;
         else
-            job = ForkJoinTask.adapt(task, null);
-        forkOrSubmit(job);
+            job = new ForkJoinTask.AdaptedRunnableAction(task);
+        externalPush(job);
     }
 
     /**
@@ -1582,7 +2686,7 @@ public class ForkJoinPool extends AbstractExecutorService {
     public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
         if (task == null)
             throw new NullPointerException();
-        forkOrSubmit(task);
+        externalPush(task);
         return task;
     }
 
@@ -1592,10 +2696,8 @@ public class ForkJoinPool extends AbstractExecutorService {
      *         scheduled for execution
      */
     public <T> ForkJoinTask<T> submit(Callable<T> task) {
-        if (task == null)
-            throw new NullPointerException();
-        ForkJoinTask<T> job = ForkJoinTask.adapt(task);
-        forkOrSubmit(job);
+        ForkJoinTask<T> job = new ForkJoinTask.AdaptedCallable<T>(task);
+        externalPush(job);
         return job;
     }
 
@@ -1605,10 +2707,8 @@ public class ForkJoinPool extends AbstractExecutorService {
      *         scheduled for execution
      */
     public <T> ForkJoinTask<T> submit(Runnable task, T result) {
-        if (task == null)
-            throw new NullPointerException();
-        ForkJoinTask<T> job = ForkJoinTask.adapt(task, result);
-        forkOrSubmit(job);
+        ForkJoinTask<T> job = new ForkJoinTask.AdaptedRunnable<T>(task, result);
+        externalPush(job);
         return job;
     }
 
@@ -1624,8 +2724,8 @@ public class ForkJoinPool extends AbstractExecutorService {
         if (task instanceof ForkJoinTask<?>) // avoid re-wrap
             job = (ForkJoinTask<?>) task;
         else
-            job = ForkJoinTask.adapt(task, null);
-        forkOrSubmit(job);
+            job = new ForkJoinTask.AdaptedRunnableAction(task);
+        externalPush(job);
         return job;
     }
 
@@ -1634,25 +2734,31 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @throws RejectedExecutionException {@inheritDoc}
      */
     public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
-        ArrayList<ForkJoinTask<T>> forkJoinTasks =
-            new ArrayList<ForkJoinTask<T>>(tasks.size());
-        for (Callable<T> task : tasks)
-            forkJoinTasks.add(ForkJoinTask.adapt(task));
-        invoke(new InvokeAll<T>(forkJoinTasks));
-
+        // In previous versions of this class, this method constructed
+        // a task to run ForkJoinTask.invokeAll, but now external
+        // invocation of multiple tasks is at least as efficient.
+        List<ForkJoinTask<T>> fs = new ArrayList<ForkJoinTask<T>>(tasks.size());
+        // Workaround needed because method wasn't declared with
+        // wildcards in return type but should have been.
         @SuppressWarnings({"unchecked", "rawtypes"})
-            List<Future<T>> futures = (List<Future<T>>) (List) forkJoinTasks;
-        return futures;
-    }
+            List<Future<T>> futures = (List<Future<T>>) (List) fs;
 
-    static final class InvokeAll<T> extends RecursiveAction {
-        final ArrayList<ForkJoinTask<T>> tasks;
-        InvokeAll(ArrayList<ForkJoinTask<T>> tasks) { this.tasks = tasks; }
-        public void compute() {
-            try { invokeAll(tasks); }
-            catch (Exception ignore) {}
+        boolean done = false;
+        try {
+            for (Callable<T> t : tasks) {
+                ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
+                externalPush(f);
+                fs.add(f);
+            }
+            for (ForkJoinTask<T> f : fs)
+                f.quietlyJoin();
+            done = true;
+            return futures;
+        } finally {
+            if (!done)
+                for (ForkJoinTask<T> f : fs)
+                    f.cancel(false);
         }
-        private static final long serialVersionUID = -7914297376763021607L;
     }
 
     /**
@@ -1680,7 +2786,16 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return the targeted parallelism level of this pool
      */
     public int getParallelism() {
-        return parallelism;
+        return config & SMASK;
+    }
+
+    /**
+     * Returns the targeted parallelism level of the common pool.
+     *
+     * @return the targeted parallelism level of the common pool
+     */
+    public static int getCommonPoolParallelism() {
+        return commonPoolParallelism;
     }
 
     /**
@@ -1692,7 +2807,7 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return the number of worker threads
      */
     public int getPoolSize() {
-        return parallelism + (short)(ctl >>> TC_SHIFT);
+        return (config & SMASK) + (short)(ctl >>> TC_SHIFT);
     }
 
     /**
@@ -1702,7 +2817,7 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return {@code true} if this pool uses async mode
      */
     public boolean getAsyncMode() {
-        return locallyFifo;
+        return (config >>> 16) == FIFO_QUEUE;
     }
 
     /**
@@ -1714,8 +2829,15 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return the number of worker threads
      */
     public int getRunningThreadCount() {
-        int r = parallelism + (int)(ctl >> AC_SHIFT);
-        return (r <= 0) ? 0 : r; // suppress momentarily negative values
+        int rc = 0;
+        WorkQueue[] ws; WorkQueue w;
+        if ((ws = workQueues) != null) {
+            for (int i = 1; i < ws.length; i += 2) {
+                if ((w = ws[i]) != null && w.isApparentlyUnblocked())
+                    ++rc;
+            }
+        }
+        return rc;
     }
 
     /**
@@ -1726,7 +2848,7 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return the number of active threads
      */
     public int getActiveThreadCount() {
-        int r = parallelism + (int)(ctl >> AC_SHIFT) + blockedCount;
+        int r = (config & SMASK) + (int)(ctl >> AC_SHIFT);
         return (r <= 0) ? 0 : r; // suppress momentarily negative values
     }
 
@@ -1742,7 +2864,7 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return {@code true} if all threads are currently idle
      */
     public boolean isQuiescent() {
-        return parallelism + (int)(ctl >> AC_SHIFT) + blockedCount == 0;
+        return (int)(ctl >> AC_SHIFT) + (config & SMASK) == 0;
     }
 
     /**
@@ -1757,7 +2879,15 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return the number of steals
      */
     public long getStealCount() {
-        return stealCount;
+        long count = stealCount;
+        WorkQueue[] ws; WorkQueue w;
+        if ((ws = workQueues) != null) {
+            for (int i = 1; i < ws.length; i += 2) {
+                if ((w = ws[i]) != null)
+                    count += w.nsteals;
+            }
+        }
+        return count;
     }
 
     /**
@@ -1772,12 +2902,12 @@ public class ForkJoinPool extends AbstractExecutorService {
      */
     public long getQueuedTaskCount() {
         long count = 0;
-        ForkJoinWorkerThread[] ws;
-        if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
-            (ws = workers) != null) {
-            for (ForkJoinWorkerThread w : ws)
-                if (w != null)
-                    count -= w.queueBase - w.queueTop; // must read base first
+        WorkQueue[] ws; WorkQueue w;
+        if ((ws = workQueues) != null) {
+            for (int i = 1; i < ws.length; i += 2) {
+                if ((w = ws[i]) != null)
+                    count += w.queueSize();
+            }
         }
         return count;
     }
@@ -1790,7 +2920,15 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return the number of queued submissions
      */
     public int getQueuedSubmissionCount() {
-        return -queueBase + queueTop;
+        int count = 0;
+        WorkQueue[] ws; WorkQueue w;
+        if ((ws = workQueues) != null) {
+            for (int i = 0; i < ws.length; i += 2) {
+                if ((w = ws[i]) != null)
+                    count += w.queueSize();
+            }
+        }
+        return count;
     }
 
     /**
@@ -1800,7 +2938,14 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return {@code true} if there are any queued submissions
      */
     public boolean hasQueuedSubmissions() {
-        return queueBase != queueTop;
+        WorkQueue[] ws; WorkQueue w;
+        if ((ws = workQueues) != null) {
+            for (int i = 0; i < ws.length; i += 2) {
+                if ((w = ws[i]) != null && !w.isEmpty())
+                    return true;
+            }
+        }
+        return false;
     }
 
     /**
@@ -1811,16 +2956,11 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return the next submission, or {@code null} if none
      */
     protected ForkJoinTask<?> pollSubmission() {
-        ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
-        while ((b = queueBase) != queueTop &&
-               (q = submissionQueue) != null &&
-               (i = (q.length - 1) & b) >= 0) {
-            long u = (i << ASHIFT) + ABASE;
-            if ((t = q[i]) != null &&
-                queueBase == b &&
-                UNSAFE.compareAndSwapObject(q, u, t, null)) {
-                queueBase = b + 1;
-                return t;
+        WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
+        if ((ws = workQueues) != null) {
+            for (int i = 0; i < ws.length; i += 2) {
+                if ((w = ws[i]) != null && (t = w.poll()) != null)
+                    return t;
             }
         }
         return null;
@@ -1845,20 +2985,17 @@ public class ForkJoinPool extends AbstractExecutorService {
      */
     protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
         int count = 0;
-        while (queueBase != queueTop) {
-            ForkJoinTask<?> t = pollSubmission();
-            if (t != null) {
-                c.add(t);
-                ++count;
+        WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
+        if ((ws = workQueues) != null) {
+            for (int i = 0; i < ws.length; ++i) {
+                if ((w = ws[i]) != null) {
+                    while ((t = w.poll()) != null) {
+                        c.add(t);
+                        ++count;
+                    }
+                }
             }
         }
-        ForkJoinWorkerThread[] ws;
-        if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
-            (ws = workers) != null) {
-            for (ForkJoinWorkerThread w : ws)
-                if (w != null)
-                    count += w.drainTasksTo(c);
-        }
         return count;
     }
 
@@ -1870,21 +3007,36 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return a string identifying this pool, as well as its state
      */
     public String toString() {
-        long st = getStealCount();
-        long qt = getQueuedTaskCount();
-        long qs = getQueuedSubmissionCount();
-        int pc = parallelism;
+        // Use a single pass through workQueues to collect counts
+        long qt = 0L, qs = 0L; int rc = 0;
+        long st = stealCount;
         long c = ctl;
+        WorkQueue[] ws; WorkQueue w;
+        if ((ws = workQueues) != null) {
+            for (int i = 0; i < ws.length; ++i) {
+                if ((w = ws[i]) != null) {
+                    int size = w.queueSize();
+                    if ((i & 1) == 0)
+                        qs += size;
+                    else {
+                        qt += size;
+                        st += w.nsteals;
+                        if (w.isApparentlyUnblocked())
+                            ++rc;
+                    }
+                }
+            }
+        }
+        int pc = (config & SMASK);
         int tc = pc + (short)(c >>> TC_SHIFT);
-        int rc = pc + (int)(c >> AC_SHIFT);
-        if (rc < 0) // ignore transient negative
-            rc = 0;
-        int ac = rc + blockedCount;
+        int ac = pc + (int)(c >> AC_SHIFT);
+        if (ac < 0) // ignore transient negative
+            ac = 0;
         String level;
         if ((c & STOP_BIT) != 0)
             level = (tc == 0) ? "Terminated" : "Terminating";
         else
-            level = shutdown ? "Shutting down" : "Running";
+            level = plock < 0 ? "Shutting down" : "Running";
         return super.toString() +
             "[" + level +
             ", parallelism = " + pc +
@@ -1898,11 +3050,13 @@ public class ForkJoinPool extends AbstractExecutorService {
     }
 
     /**
-     * Initiates an orderly shutdown in which previously submitted
-     * tasks are executed, but no new tasks will be accepted.
-     * Invocation has no additional effect if already shut down.
-     * Tasks that are in the process of being submitted concurrently
-     * during the course of this method may or may not be rejected.
+     * Possibly initiates an orderly shutdown in which previously
+     * submitted tasks are executed, but no new tasks will be
+     * accepted. Invocation has no effect on execution state if this
+     * is the {@link #commonPool}, and no additional effect if
+     * already shut down.  Tasks that are in the process of being
+     * submitted concurrently during the course of this method may or
+     * may not be rejected.
      *
      * @throws SecurityException if a security manager exists and
      *         the caller is not permitted to modify threads
@@ -1911,19 +3065,20 @@ public class ForkJoinPool extends AbstractExecutorService {
      */
     public void shutdown() {
         checkPermission();
-        shutdown = true;
-        tryTerminate(false);
+        tryTerminate(false, true);
     }
 
     /**
-     * Attempts to cancel and/or stop all tasks, and reject all
-     * subsequently submitted tasks.  Tasks that are in the process of
-     * being submitted or executed concurrently during the course of
-     * this method may or may not be rejected. This method cancels
-     * both existing and unexecuted tasks, in order to permit
-     * termination in the presence of task dependencies. So the method
-     * always returns an empty list (unlike the case for some other
-     * Executors).
+     * Possibly attempts to cancel and/or stop all tasks, and reject
+     * all subsequently submitted tasks.  Invocation has no effect on
+     * execution state if this is the {@link #commonPool}, and no
+     * additional effect if already shut down. Otherwise, tasks that
+     * are in the process of being submitted or executed concurrently
+     * during the course of this method may or may not be
+     * rejected. This method cancels both existing and unexecuted
+     * tasks, in order to permit termination in the presence of task
+     * dependencies. So the method always returns an empty list
+     * (unlike the case for some other Executors).
      *
      * @return an empty list
      * @throws SecurityException if a security manager exists and
@@ -1933,8 +3088,7 @@ public class ForkJoinPool extends AbstractExecutorService {
      */
     public List<Runnable> shutdownNow() {
         checkPermission();
-        shutdown = true;
-        tryTerminate(true);
+        tryTerminate(true, true);
         return Collections.emptyList();
     }
 
@@ -1946,7 +3100,7 @@ public class ForkJoinPool extends AbstractExecutorService {
     public boolean isTerminated() {
         long c = ctl;
         return ((c & STOP_BIT) != 0L &&
-                (short)(c >>> TC_SHIFT) == -parallelism);
+                (short)(c >>> TC_SHIFT) == -(config & SMASK));
     }
 
     /**
@@ -1954,7 +3108,7 @@ public class ForkJoinPool extends AbstractExecutorService {
      * commenced but not yet completed.  This method may be useful for
      * debugging. A return of {@code true} reported a sufficient
      * period after shutdown may indicate that submitted tasks have
-     * ignored or suppressed interruption, or are waiting for IO,
+     * ignored or suppressed interruption, or are waiting for I/O,
      * causing this executor not to properly terminate. (See the
      * advisory notes for class {@link ForkJoinTask} stating that
      * tasks should not normally entail blocking operations.  But if
@@ -1965,14 +3119,7 @@ public class ForkJoinPool extends AbstractExecutorService {
     public boolean isTerminating() {
         long c = ctl;
         return ((c & STOP_BIT) != 0L &&
-                (short)(c >>> TC_SHIFT) != -parallelism);
-    }
-
-    /**
-     * Returns true if terminating or terminated. Used by ForkJoinWorkerThread.
-     */
-    final boolean isAtLeastTerminating() {
-        return (ctl & STOP_BIT) != 0L;
+                (short)(c >>> TC_SHIFT) != -(config & SMASK));
     }
 
     /**
@@ -1981,13 +3128,15 @@ public class ForkJoinPool extends AbstractExecutorService {
      * @return {@code true} if this pool has been shut down
      */
     public boolean isShutdown() {
-        return shutdown;
+        return plock < 0;
     }
 
     /**
-     * Blocks until all tasks have completed execution after a shutdown
-     * request, or the timeout occurs, or the current thread is
-     * interrupted, whichever happens first.
+     * Blocks until all tasks have completed execution after a
+     * shutdown request, or the timeout occurs, or the current thread
+     * is interrupted, whichever happens first. Note that the {@link
+     * #commonPool()} never terminates until program shutdown so
+     * this method will always time out.
      *
      * @param timeout the maximum time to wait
      * @param unit the time unit of the timeout argument
@@ -1998,19 +3147,21 @@ public class ForkJoinPool extends AbstractExecutorService {
     public boolean awaitTermination(long timeout, TimeUnit unit)
         throws InterruptedException {
         long nanos = unit.toNanos(timeout);
-        final ReentrantLock lock = this.submissionLock;
-        lock.lock();
-        try {
-            for (;;) {
-                if (isTerminated())
-                    return true;
-                if (nanos <= 0)
-                    return false;
-                nanos = termination.awaitNanos(nanos);
+        if (isTerminated())
+            return true;
+        long startTime = System.nanoTime();
+        boolean terminated = false;
+        synchronized (this) {
+            for (long waitTime = nanos, millis = 0L;;) {
+                if (terminated = isTerminated() ||
+                    waitTime <= 0L ||
+                    (millis = unit.toMillis(waitTime)) <= 0L)
+                    break;
+                wait(millis);
+                waitTime = nanos - (System.nanoTime() - startTime);
             }
-        } finally {
-            lock.unlock();
         }
+        return terminated;
     }
 
     /**
@@ -2110,11 +3261,35 @@ public class ForkJoinPool extends AbstractExecutorService {
         throws InterruptedException {
         Thread t = Thread.currentThread();
         if (t instanceof ForkJoinWorkerThread) {
-            ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
-            w.pool.awaitBlocker(blocker);
+            ForkJoinPool p = ((ForkJoinWorkerThread)t).pool;
+            while (!blocker.isReleasable()) { // variant of helpSignal
+                WorkQueue[] ws; WorkQueue q; int m, u;
+                if ((ws = p.workQueues) != null && (m = ws.length - 1) >= 0) {
+                    for (int i = 0; i <= m; ++i) {
+                        if (blocker.isReleasable())
+                            return;
+                        if ((q = ws[i]) != null && q.base - q.top < 0) {
+                            p.signalWork(q);
+                            if ((u = (int)(p.ctl >>> 32)) >= 0 ||
+                                (u >> UAC_SHIFT) >= 0)
+                                break;
+                        }
+                    }
+                }
+                if (p.tryCompensate()) {
+                    try {
+                        do {} while (!blocker.isReleasable() &&
+                                     !blocker.block());
+                    } finally {
+                        p.incrementActiveCount();
+                    }
+                    break;
+                }
+            }
         }
         else {
-            do {} while (!blocker.isReleasable() && !blocker.block());
+            do {} while (!blocker.isReleasable() &&
+                         !blocker.block());
         }
     }
 
@@ -2123,55 +3298,93 @@ public class ForkJoinPool extends AbstractExecutorService {
     // implement RunnableFuture.
 
     protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
-        return (RunnableFuture<T>) ForkJoinTask.adapt(runnable, value);
+        return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
     }
 
     protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
-        return (RunnableFuture<T>) ForkJoinTask.adapt(callable);
+        return new ForkJoinTask.AdaptedCallable<T>(callable);
     }
 
     // Unsafe mechanics
-    private static final sun.misc.Unsafe UNSAFE;
-    private static final long ctlOffset;
-    private static final long stealCountOffset;
-    private static final long blockedCountOffset;
-    private static final long quiescerCountOffset;
-    private static final long scanGuardOffset;
-    private static final long nextWorkerNumberOffset;
-    private static final long ABASE;
+    private static final sun.misc.Unsafe U;
+    private static final long CTL;
+    private static final long PARKBLOCKER;
+    private static final int ABASE;
     private static final int ASHIFT;
+    private static final long STEALCOUNT;
+    private static final long PLOCK;
+    private static final long INDEXSEED;
+    private static final long QLOCK;
 
     static {
-        poolNumberGenerator = new AtomicInteger();
-        workerSeedGenerator = new Random();
-        modifyThreadPermission = new RuntimePermission("modifyThread");
-        defaultForkJoinWorkerThreadFactory =
-            new DefaultForkJoinWorkerThreadFactory();
-        int s;
+        int s; // initialize field offsets for CAS etc
         try {
-            UNSAFE = sun.misc.Unsafe.getUnsafe();
+            U = sun.misc.Unsafe.getUnsafe();
             Class<?> k = ForkJoinPool.class;
-            ctlOffset = UNSAFE.objectFieldOffset
+            CTL = U.objectFieldOffset
                 (k.getDeclaredField("ctl"));
-            stealCountOffset = UNSAFE.objectFieldOffset
+            STEALCOUNT = U.objectFieldOffset
                 (k.getDeclaredField("stealCount"));
-            blockedCountOffset = UNSAFE.objectFieldOffset
-                (k.getDeclaredField("blockedCount"));
-            quiescerCountOffset = UNSAFE.objectFieldOffset
-                (k.getDeclaredField("quiescerCount"));
-            scanGuardOffset = UNSAFE.objectFieldOffset
-                (k.getDeclaredField("scanGuard"));
-            nextWorkerNumberOffset = UNSAFE.objectFieldOffset
-                (k.getDeclaredField("nextWorkerNumber"));
-            Class<?> a = ForkJoinTask[].class;
-            ABASE = UNSAFE.arrayBaseOffset(a);
-            s = UNSAFE.arrayIndexScale(a);
+            PLOCK = U.objectFieldOffset
+                (k.getDeclaredField("plock"));
+            INDEXSEED = U.objectFieldOffset
+                (k.getDeclaredField("indexSeed"));
+            Class<?> tk = Thread.class;
+            PARKBLOCKER = U.objectFieldOffset
+                (tk.getDeclaredField("parkBlocker"));
+            Class<?> wk = WorkQueue.class;
+            QLOCK = U.objectFieldOffset
+                (wk.getDeclaredField("qlock"));
+            Class<?> ak = ForkJoinTask[].class;
+            ABASE = U.arrayBaseOffset(ak);
+            s = U.arrayIndexScale(ak);
+            ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
         } catch (Exception e) {
             throw new Error(e);
         }
         if ((s & (s-1)) != 0)
             throw new Error("data type scale not a power of two");
-        ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
+
+        submitters = new ThreadLocal<Submitter>();
+        ForkJoinWorkerThreadFactory fac = defaultForkJoinWorkerThreadFactory =
+            new DefaultForkJoinWorkerThreadFactory();
+        modifyThreadPermission = new RuntimePermission("modifyThread");
+
+        /*
+         * Establish common pool parameters.  For extra caution,
+         * computations to set up common pool state are here; the
+         * constructor just assigns these values to fields.
+         */
+
+        int par = 0;
+        Thread.UncaughtExceptionHandler handler = null;
+        try {  // TBD: limit or report ignored exceptions?
+            String pp = System.getProperty
+                ("java.util.concurrent.ForkJoinPool.common.parallelism");
+            String hp = System.getProperty
+                ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
+            String fp = System.getProperty
+                ("java.util.concurrent.ForkJoinPool.common.threadFactory");
+            if (fp != null)
+                fac = ((ForkJoinWorkerThreadFactory)ClassLoader.
+                       getSystemClassLoader().loadClass(fp).newInstance());
+            if (hp != null)
+                handler = ((Thread.UncaughtExceptionHandler)ClassLoader.
+                           getSystemClassLoader().loadClass(hp).newInstance());
+            if (pp != null)
+                par = Integer.parseInt(pp);
+        } catch (Exception ignore) {
+        }
+
+        if (par <= 0)
+            par = Runtime.getRuntime().availableProcessors();
+        if (par > MAX_CAP)
+            par = MAX_CAP;
+        commonPoolParallelism = par;
+        long np = (long)(-par); // precompute initial ctl value
+        long ct = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
+
+        commonPool = new ForkJoinPool(par, ct, fac, handler);
     }
 
 }

src/share/classes/java/util/concurrent/ForkJoinTask.java

@@ -37,17 +37,13 @@ package java.util.concurrent;
 
 import java.io.Serializable;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 import java.util.RandomAccess;
-import java.util.Map;
 import java.lang.ref.WeakReference;
 import java.lang.ref.ReferenceQueue;
 import java.util.concurrent.Callable;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.ExecutionException;
-import java.util.concurrent.Executor;
-import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Future;
 import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.RunnableFuture;
@@ -63,46 +59,59 @@ import java.lang.reflect.Constructor;
  * subtasks may be hosted by a small number of actual threads in a
  * ForkJoinPool, at the price of some usage limitations.
  *
- * <p>A "main" {@code ForkJoinTask} begins execution when submitted
- * to a {@link ForkJoinPool}.  Once started, it will usually in turn
- * start other subtasks.  As indicated by the name of this class,
- * many programs using {@code ForkJoinTask} employ only methods
- * {@link #fork} and {@link #join}, or derivatives such as {@link
+ * <p>A "main" {@code ForkJoinTask} begins execution when it is
+ * explicitly submitted to a {@link ForkJoinPool}, or, if not already
+ * engaged in a ForkJoin computation, commenced in the {@link
+ * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or
+ * related methods.  Once started, it will usually in turn start other
+ * subtasks.  As indicated by the name of this class, many programs
+ * using {@code ForkJoinTask} employ only methods {@link #fork} and
+ * {@link #join}, or derivatives such as {@link
  * #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also
  * provides a number of other methods that can come into play in
- * advanced usages, as well as extension mechanics that allow
- * support of new forms of fork/join processing.
+ * advanced usages, as well as extension mechanics that allow support
+ * of new forms of fork/join processing.
  *
  * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.
  * The efficiency of {@code ForkJoinTask}s stems from a set of
  * restrictions (that are only partially statically enforceable)
- * reflecting their intended use as computational tasks calculating
- * pure functions or operating on purely isolated objects.  The
- * primary coordination mechanisms are {@link #fork}, that arranges
+ * reflecting their main use as computational tasks calculating pure
+ * functions or operating on purely isolated objects.  The primary
+ * coordination mechanisms are {@link #fork}, that arranges
  * asynchronous execution, and {@link #join}, that doesn't proceed
  * until the task's result has been computed.  Computations should
- * avoid {@code synchronized} methods or blocks, and should minimize
- * other blocking synchronization apart from joining other tasks or
- * using synchronizers such as Phasers that are advertised to
- * cooperate with fork/join scheduling. Tasks should also not perform
- * blocking IO, and should ideally access variables that are
- * completely independent of those accessed by other running
- * tasks. Minor breaches of these restrictions, for example using
- * shared output streams, may be tolerable in practice, but frequent
- * use may result in poor performance, and the potential to
- * indefinitely stall if the number of threads not waiting for IO or
- * other external synchronization becomes exhausted. This usage
- * restriction is in part enforced by not permitting checked
- * exceptions such as {@code IOExceptions} to be thrown. However,
- * computations may still encounter unchecked exceptions, that are
- * rethrown to callers attempting to join them. These exceptions may
- * additionally include {@link RejectedExecutionException} stemming
- * from internal resource exhaustion, such as failure to allocate
- * internal task queues. Rethrown exceptions behave in the same way as
- * regular exceptions, but, when possible, contain stack traces (as
- * displayed for example using {@code ex.printStackTrace()}) of both
- * the thread that initiated the computation as well as the thread
- * actually encountering the exception; minimally only the latter.
+ * ideally avoid {@code synchronized} methods or blocks, and should
+ * minimize other blocking synchronization apart from joining other
+ * tasks or using synchronizers such as Phasers that are advertised to
+ * cooperate with fork/join scheduling. Subdividable tasks should also
+ * not perform blocking I/O, and should ideally access variables that
+ * are completely independent of those accessed by other running
+ * tasks. These guidelines are loosely enforced by not permitting
+ * checked exceptions such as {@code IOExceptions} to be
+ * thrown. However, computations may still encounter unchecked
+ * exceptions, that are rethrown to callers attempting to join
+ * them. These exceptions may additionally include {@link
+ * RejectedExecutionException} stemming from internal resource
+ * exhaustion, such as failure to allocate internal task
+ * queues. Rethrown exceptions behave in the same way as regular
+ * exceptions, but, when possible, contain stack traces (as displayed
+ * for example using {@code ex.printStackTrace()}) of both the thread
+ * that initiated the computation as well as the thread actually
+ * encountering the exception; minimally only the latter.
+ *
+ * <p>It is possible to define and use ForkJoinTasks that may block,
+ * but doing do requires three further considerations: (1) Completion
+ * of few if any <em>other</em> tasks should be dependent on a task
+ * that blocks on external synchronization or I/O. Event-style async
+ * tasks that are never joined (for example, those subclassing {@link
+ * CountedCompleter}) often fall into this category.  (2) To minimize
+ * resource impact, tasks should be small; ideally performing only the
+ * (possibly) blocking action. (3) Unless the {@link
+ * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly
+ * blocked tasks is known to be less than the pool's {@link
+ * ForkJoinPool#getParallelism} level, the pool cannot guarantee that
+ * enough threads will be available to ensure progress or good
+ * performance.
  *
  * <p>The primary method for awaiting completion and extracting
  * results of a task is {@link #join}, but there are several variants:
@@ -118,6 +127,13 @@ import java.lang.reflect.Constructor;
  * performs the most common form of parallel invocation: forking a set
  * of tasks and joining them all.
  *
+ * <p>In the most typical usages, a fork-join pair act like a call
+ * (fork) and return (join) from a parallel recursive function. As is
+ * the case with other forms of recursive calls, returns (joins)
+ * should be performed innermost-first. For example, {@code a.fork();
+ * b.fork(); b.join(); a.join();} is likely to be substantially more
+ * efficient than joining {@code a} before {@code b}.
+ *
  * <p>The execution status of tasks may be queried at several levels
  * of detail: {@link #isDone} is true if a task completed in any way
  * (including the case where a task was cancelled without executing);
@@ -133,18 +149,13 @@ import java.lang.reflect.Constructor;
  * <p>The ForkJoinTask class is not usually directly subclassed.
  * Instead, you subclass one of the abstract classes that support a
  * particular style of fork/join processing, typically {@link
- * RecursiveAction} for computations that do not return results, or
- * {@link RecursiveTask} for those that do.  Normally, a concrete
- * ForkJoinTask subclass declares fields comprising its parameters,
- * established in a constructor, and then defines a {@code compute}
- * method that somehow uses the control methods supplied by this base
- * class. While these methods have {@code public} access (to allow
- * instances of different task subclasses to call each other's
- * methods), some of them may only be called from within other
- * ForkJoinTasks (as may be determined using method {@link
- * #inForkJoinPool}).  Attempts to invoke them in other contexts
- * result in exceptions or errors, possibly including
- * {@code ClassCastException}.
+ * RecursiveAction} for most computations that do not return results,
+ * {@link RecursiveTask} for those that do, and {@link
+ * CountedCompleter} for those in which completed actions trigger
+ * other actions.  Normally, a concrete ForkJoinTask subclass declares
+ * fields comprising its parameters, established in a constructor, and
+ * then defines a {@code compute} method that somehow uses the control
+ * methods supplied by this base class.
  *
  * <p>Method {@link #join} and its variants are appropriate for use
  * only when completion dependencies are acyclic; that is, the
@@ -154,7 +165,17 @@ import java.lang.reflect.Constructor;
  * supports other methods and techniques (for example the use of
  * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
  * may be of use in constructing custom subclasses for problems that
- * are not statically structured as DAGs.
+ * are not statically structured as DAGs. To support such usages a
+ * ForkJoinTask may be atomically <em>tagged</em> with a {@code short}
+ * value using {@link #setForkJoinTaskTag} or {@link
+ * #compareAndSetForkJoinTaskTag} and checked using {@link
+ * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use
+ * these {@code protected} methods or tags for any purpose, but they
+ * may be of use in the construction of specialized subclasses.  For
+ * example, parallel graph traversals can use the supplied methods to
+ * avoid revisiting nodes/tasks that have already been processed.
+ * (Method names for tagging are bulky in part to encourage definition
+ * of methods that reflect their usage patterns.)
  *
  * <p>Most base support methods are {@code final}, to prevent
  * overriding of implementations that are intrinsically tied to the
@@ -194,41 +215,50 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * See the internal documentation of class ForkJoinPool for a
      * general implementation overview.  ForkJoinTasks are mainly
      * responsible for maintaining their "status" field amidst relays
-     * to methods in ForkJoinWorkerThread and ForkJoinPool. The
-     * methods of this class are more-or-less layered into (1) basic
-     * status maintenance (2) execution and awaiting completion (3)
-     * user-level methods that additionally report results. This is
-     * sometimes hard to see because this file orders exported methods
-     * in a way that flows well in javadocs.
+     * to methods in ForkJoinWorkerThread and ForkJoinPool.
+     *
+     * The methods of this class are more-or-less layered into
+     * (1) basic status maintenance
+     * (2) execution and awaiting completion
+     * (3) user-level methods that additionally report results.
+     * This is sometimes hard to see because this file orders exported
+     * methods in a way that flows well in javadocs.
      */
 
     /*
      * The status field holds run control status bits packed into a
      * single int to minimize footprint and to ensure atomicity (via
      * CAS).  Status is initially zero, and takes on nonnegative
-     * values until completed, upon which status holds value
-     * NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking
-     * waits by other threads have the SIGNAL bit set.  Completion of
-     * a stolen task with SIGNAL set awakens any waiters via
-     * notifyAll. Even though suboptimal for some purposes, we use
-     * basic builtin wait/notify to take advantage of "monitor
-     * inflation" in JVMs that we would otherwise need to emulate to
-     * avoid adding further per-task bookkeeping overhead.  We want
-     * these monitors to be "fat", i.e., not use biasing or thin-lock
-     * techniques, so use some odd coding idioms that tend to avoid
-     * them.
+     * values until completed, upon which status (anded with
+     * DONE_MASK) holds value NORMAL, CANCELLED, or EXCEPTIONAL. Tasks
+     * undergoing blocking waits by other threads have the SIGNAL bit
+     * set.  Completion of a stolen task with SIGNAL set awakens any
+     * waiters via notifyAll. Even though suboptimal for some
+     * purposes, we use basic builtin wait/notify to take advantage of
+     * "monitor inflation" in JVMs that we would otherwise need to
+     * emulate to avoid adding further per-task bookkeeping overhead.
+     * We want these monitors to be "fat", i.e., not use biasing or
+     * thin-lock techniques, so use some odd coding idioms that tend
+     * to avoid them, mainly by arranging that every synchronized
+     * block performs a wait, notifyAll or both.
+     *
+     * These control bits occupy only (some of) the upper half (16
+     * bits) of status field. The lower bits are used for user-defined
+     * tags.
      */
 
     /** The run status of this task */
     volatile int status; // accessed directly by pool and workers
-    private static final int NORMAL      = -1;
-    private static final int CANCELLED   = -2;
-    private static final int EXCEPTIONAL = -3;
-    private static final int SIGNAL      =  1;
+    static final int DONE_MASK   = 0xf0000000;  // mask out non-completion bits
+    static final int NORMAL      = 0xf0000000;  // must be negative
+    static final int CANCELLED   = 0xc0000000;  // must be < NORMAL
+    static final int EXCEPTIONAL = 0x80000000;  // must be < CANCELLED
+    static final int SIGNAL      = 0x00010000;  // must be >= 1 << 16
+    static final int SMASK       = 0x0000ffff;  // short bits for tags
 
     /**
-     * Marks completion and wakes up threads waiting to join this task,
-     * also clearing signal request bits.
+     * Marks completion and wakes up threads waiting to join this
+     * task.
      *
      * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
      * @return completion status on exit
@@ -237,8 +267,8 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
         for (int s;;) {
             if ((s = status) < 0)
                 return s;
-            if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {
-                if (s != 0)
+            if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
+                if ((s >>> 16) != 0)
                     synchronized (this) { notifyAll(); }
                 return completion;
             }
@@ -246,27 +276,36 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
     }
 
     /**
-     * Tries to block a worker thread until completed or timed out.
-     * Uses Object.wait time argument conventions.
-     * May fail on contention or interrupt.
+     * Primary execution method for stolen tasks. Unless done, calls
+     * exec and records status if completed, but doesn't wait for
+     * completion otherwise.
      *
-     * @param millis if > 0, wait time.
+     * @return status on exit from this method
      */
-    final void tryAwaitDone(long millis) {
-        int s;
-        try {
-            if (((s = status) > 0 ||
-                 (s == 0 &&
-                  UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL))) &&
-                status > 0) {
-                synchronized (this) {
-                    if (status > 0)
-                        wait(millis);
-                }
+    final int doExec() {
+        int s; boolean completed;
+        if ((s = status) >= 0) {
+            try {
+                completed = exec();
+            } catch (Throwable rex) {
+                return setExceptionalCompletion(rex);
             }
-        } catch (InterruptedException ie) {
-            // caller must check termination
+            if (completed)
+                s = setCompletion(NORMAL);
         }
+        return s;
+    }
+
+    /**
+     * Tries to set SIGNAL status unless already completed. Used by
+     * ForkJoinPool. Other variants are directly incorporated into
+     * externalAwaitDone etc.
+     *
+     * @return true if successful
+     */
+    final boolean trySetSignal() {
+        int s = status;
+        return s >= 0 && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL);
     }
 
     /**
@@ -275,113 +314,78 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      */
     private int externalAwaitDone() {
         int s;
-        if ((s = status) >= 0) {
-            boolean interrupted = false;
-            synchronized (this) {
-                while ((s = status) >= 0) {
-                    if (s == 0)
-                        UNSAFE.compareAndSwapInt(this, statusOffset,
-                                                 0, SIGNAL);
-                    else {
+        ForkJoinPool.externalHelpJoin(this);
+        boolean interrupted = false;
+        while ((s = status) >= 0) {
+            if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
+                synchronized (this) {
+                    if (status >= 0) {
                         try {
                             wait();
                         } catch (InterruptedException ie) {
                             interrupted = true;
                         }
                     }
+                    else
+                        notifyAll();
                 }
             }
-            if (interrupted)
-                Thread.currentThread().interrupt();
         }
+        if (interrupted)
+            Thread.currentThread().interrupt();
         return s;
     }
 
     /**
-     * Blocks a non-worker-thread until completion or interruption or timeout.
+     * Blocks a non-worker-thread until completion or interruption.
      */
-    private int externalInterruptibleAwaitDone(long millis)
-        throws InterruptedException {
+    private int externalInterruptibleAwaitDone() throws InterruptedException {
         int s;
         if (Thread.interrupted())
             throw new InterruptedException();
-        if ((s = status) >= 0) {
-            synchronized (this) {
-                while ((s = status) >= 0) {
-                    if (s == 0)
-                        UNSAFE.compareAndSwapInt(this, statusOffset,
-                                                 0, SIGNAL);
-                    else {
-                        wait(millis);
-                        if (millis > 0L)
-                            break;
-                    }
+        ForkJoinPool.externalHelpJoin(this);
+        while ((s = status) >= 0) {
+            if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
+                synchronized (this) {
+                    if (status >= 0)
+                        wait();
+                    else
+                        notifyAll();
                 }
             }
         }
         return s;
     }
 
-    /**
-     * Primary execution method for stolen tasks. Unless done, calls
-     * exec and records status if completed, but doesn't wait for
-     * completion otherwise.
-     */
-    final void doExec() {
-        if (status >= 0) {
-            boolean completed;
-            try {
-                completed = exec();
-            } catch (Throwable rex) {
-                setExceptionalCompletion(rex);
-                return;
-            }
-            if (completed)
-                setCompletion(NORMAL); // must be outside try block
-        }
-    }
 
     /**
-     * Primary mechanics for join, get, quietlyJoin.
+     * Implementation for join, get, quietlyJoin. Directly handles
+     * only cases of already-completed, external wait, and
+     * unfork+exec.  Others are relayed to ForkJoinPool.awaitJoin.
+     *
      * @return status upon completion
      */
     private int doJoin() {
-        Thread t; ForkJoinWorkerThread w; int s; boolean completed;
-        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
-            if ((s = status) < 0)
-                return s;
-            if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {
-                try {
-                    completed = exec();
-                } catch (Throwable rex) {
-                    return setExceptionalCompletion(rex);
-                }
-                if (completed)
-                    return setCompletion(NORMAL);
-            }
-            return w.joinTask(this);
-        }
-        else
-            return externalAwaitDone();
+        int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
+        return (s = status) < 0 ? s :
+            ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+            (w = (wt = (ForkJoinWorkerThread)t).workQueue).
+            tryUnpush(this) && (s = doExec()) < 0 ? s :
+            wt.pool.awaitJoin(w, this) :
+            externalAwaitDone();
     }
 
     /**
-     * Primary mechanics for invoke, quietlyInvoke.
+     * Implementation for invoke, quietlyInvoke.
+     *
      * @return status upon completion
      */
     private int doInvoke() {
-        int s; boolean completed;
-        if ((s = status) < 0)
-            return s;
-        try {
-            completed = exec();
-        } catch (Throwable rex) {
-            return setExceptionalCompletion(rex);
-        }
-        if (completed)
-            return setCompletion(NORMAL);
-        else
-            return doJoin();
+        int s; Thread t; ForkJoinWorkerThread wt;
+        return (s = doExec()) < 0 ? s :
+            ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+            (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this) :
+            externalAwaitDone();
     }
 
     // Exception table support
@@ -416,7 +420,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * any ForkJoinPool will call helpExpungeStaleExceptions when its
      * pool becomes isQuiescent.
      */
-    static final class ExceptionNode extends WeakReference<ForkJoinTask<?>>{
+    static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> {
         final Throwable ex;
         ExceptionNode next;
         final long thrower;  // use id not ref to avoid weak cycles
@@ -429,30 +433,67 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
     }
 
     /**
-     * Records exception and sets exceptional completion.
+     * Records exception and sets status.
      *
      * @return status on exit
      */
-    private int setExceptionalCompletion(Throwable ex) {
-        int h = System.identityHashCode(this);
-        final ReentrantLock lock = exceptionTableLock;
-        lock.lock();
-        try {
-            expungeStaleExceptions();
-            ExceptionNode[] t = exceptionTable;
-            int i = h & (t.length - 1);
-            for (ExceptionNode e = t[i]; ; e = e.next) {
-                if (e == null) {
-                    t[i] = new ExceptionNode(this, ex, t[i]);
-                    break;
+    final int recordExceptionalCompletion(Throwable ex) {
+        int s;
+        if ((s = status) >= 0) {
+            int h = System.identityHashCode(this);
+            final ReentrantLock lock = exceptionTableLock;
+            lock.lock();
+            try {
+                expungeStaleExceptions();
+                ExceptionNode[] t = exceptionTable;
+                int i = h & (t.length - 1);
+                for (ExceptionNode e = t[i]; ; e = e.next) {
+                    if (e == null) {
+                        t[i] = new ExceptionNode(this, ex, t[i]);
+                        break;
+                    }
+                    if (e.get() == this) // already present
+                        break;
                 }
-                if (e.get() == this) // already present
-                    break;
+            } finally {
+                lock.unlock();
+            }
+            s = setCompletion(EXCEPTIONAL);
+        }
+        return s;
+    }
+
+    /**
+     * Records exception and possibly propagates
+     *
+     * @return status on exit
+     */
+    private int setExceptionalCompletion(Throwable ex) {
+        int s = recordExceptionalCompletion(ex);
+        if ((s & DONE_MASK) == EXCEPTIONAL)
+            internalPropagateException(ex);
+        return s;
+    }
+
+    /**
+     * Hook for exception propagation support for tasks with completers.
+     */
+    void internalPropagateException(Throwable ex) {
+    }
+
+    /**
+     * Cancels, ignoring any exceptions thrown by cancel. Used during
+     * worker and pool shutdown. Cancel is spec'ed not to throw any
+     * exceptions, but if it does anyway, we have no recourse during
+     * shutdown, so guard against this case.
+     */
+    static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
+        if (t != null && t.status >= 0) {
+            try {
+                t.cancel(false);
+            } catch (Throwable ignore) {
             }
-        } finally {
-            lock.unlock();
         }
-        return setCompletion(EXCEPTIONAL);
     }
 
     /**
@@ -501,7 +542,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * @return the exception, or null if none
      */
     private Throwable getThrowableException() {
-        if (status != EXCEPTIONAL)
+        if ((status & DONE_MASK) != EXCEPTIONAL)
             return null;
         int h = System.identityHashCode(this);
         ExceptionNode e;
@@ -519,7 +560,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
         Throwable ex;
         if (e == null || (ex = e.ex) == null)
             return null;
-        if (e.thrower != Thread.currentThread().getId()) {
+        if (false && e.thrower != Thread.currentThread().getId()) {
             Class<? extends Throwable> ec = ex.getClass();
             try {
                 Constructor<?> noArgCtor = null;
@@ -586,41 +627,61 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
     }
 
     /**
-     * Report the result of invoke or join; called only upon
-     * non-normal return of internal versions.
+     * A version of "sneaky throw" to relay exceptions
+     */
+    static void rethrow(final Throwable ex) {
+        if (ex != null) {
+            if (ex instanceof Error)
+                throw (Error)ex;
+            if (ex instanceof RuntimeException)
+                throw (RuntimeException)ex;
+            throw uncheckedThrowable(ex, RuntimeException.class);
+        }
+    }
+
+    /**
+     * The sneaky part of sneaky throw, relying on generics
+     * limitations to evade compiler complaints about rethrowing
+     * unchecked exceptions
      */
-    private V reportResult() {
-        int s; Throwable ex;
-        if ((s = status) == CANCELLED)
+    @SuppressWarnings("unchecked") static <T extends Throwable>
+        T uncheckedThrowable(final Throwable t, final Class<T> c) {
+        return (T)t; // rely on vacuous cast
+    }
+
+    /**
+     * Throws exception, if any, associated with the given status.
+     */
+    private void reportException(int s) {
+        if (s == CANCELLED)
             throw new CancellationException();
-        if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
-            UNSAFE.throwException(ex);
-        return getRawResult();
+        if (s == EXCEPTIONAL)
+            rethrow(getThrowableException());
     }
 
     // public methods
 
     /**
-     * Arranges to asynchronously execute this task.  While it is not
-     * necessarily enforced, it is a usage error to fork a task more
-     * than once unless it has completed and been reinitialized.
-     * Subsequent modifications to the state of this task or any data
-     * it operates on are not necessarily consistently observable by
-     * any thread other than the one executing it unless preceded by a
-     * call to {@link #join} or related methods, or a call to {@link
-     * #isDone} returning {@code true}.
-     *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
+     * Arranges to asynchronously execute this task in the pool the
+     * current task is running in, if applicable, or using the {@link
+     * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.  While
+     * it is not necessarily enforced, it is a usage error to fork a
+     * task more than once unless it has completed and been
+     * reinitialized.  Subsequent modifications to the state of this
+     * task or any data it operates on are not necessarily
+     * consistently observable by any thread other than the one
+     * executing it unless preceded by a call to {@link #join} or
+     * related methods, or a call to {@link #isDone} returning {@code
+     * true}.
      *
      * @return {@code this}, to simplify usage
      */
     public final ForkJoinTask<V> fork() {
-        ((ForkJoinWorkerThread) Thread.currentThread())
-            .pushTask(this);
+        Thread t;
+        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
+            ((ForkJoinWorkerThread)t).workQueue.push(this);
+        else
+            ForkJoinPool.commonPool.externalPush(this);
         return this;
     }
 
@@ -636,10 +697,10 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * @return the computed result
      */
     public final V join() {
-        if (doJoin() != NORMAL)
-            return reportResult();
-        else
-            return getRawResult();
+        int s;
+        if ((s = doJoin() & DONE_MASK) != NORMAL)
+            reportException(s);
+        return getRawResult();
     }
 
     /**
@@ -651,10 +712,10 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * @return the computed result
      */
     public final V invoke() {
-        if (doInvoke() != NORMAL)
-            return reportResult();
-        else
-            return getRawResult();
+        int s;
+        if ((s = doInvoke() & DONE_MASK) != NORMAL)
+            reportException(s);
+        return getRawResult();
     }
 
     /**
@@ -670,20 +731,17 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * cancelled, completed normally or exceptionally, or left
      * unprocessed.
      *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
-     *
      * @param t1 the first task
      * @param t2 the second task
      * @throws NullPointerException if any task is null
      */
     public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
+        int s1, s2;
         t2.fork();
-        t1.invoke();
-        t2.join();
+        if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL)
+            t1.reportException(s1);
+        if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL)
+            t2.reportException(s2);
     }
 
     /**
@@ -698,12 +756,6 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * related methods to check if they have been cancelled, completed
      * normally or exceptionally, or left unprocessed.
      *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
-     *
      * @param tasks the tasks
      * @throws NullPointerException if any task is null
      */
@@ -726,12 +778,12 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
             if (t != null) {
                 if (ex != null)
                     t.cancel(false);
-                else if (t.doJoin() < NORMAL && ex == null)
+                else if (t.doJoin() < NORMAL)
                     ex = t.getException();
             }
         }
         if (ex != null)
-            UNSAFE.throwException(ex);
+            rethrow(ex);
     }
 
     /**
@@ -747,12 +799,6 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * cancelled, completed normally or exceptionally, or left
      * unprocessed.
      *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
-     *
      * @param tasks the collection of tasks
      * @return the tasks argument, to simplify usage
      * @throws NullPointerException if tasks or any element are null
@@ -783,12 +829,12 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
             if (t != null) {
                 if (ex != null)
                     t.cancel(false);
-                else if (t.doJoin() < NORMAL && ex == null)
+                else if (t.doJoin() < NORMAL)
                     ex = t.getException();
             }
         }
         if (ex != null)
-            UNSAFE.throwException(ex);
+            rethrow(ex);
         return tasks;
     }
 
@@ -820,20 +866,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * @return {@code true} if this task is now cancelled
      */
     public boolean cancel(boolean mayInterruptIfRunning) {
-        return setCompletion(CANCELLED) == CANCELLED;
-    }
-
-    /**
-     * Cancels, ignoring any exceptions thrown by cancel. Used during
-     * worker and pool shutdown. Cancel is spec'ed not to throw any
-     * exceptions, but if it does anyway, we have no recourse during
-     * shutdown, so guard against this case.
-     */
-    final void cancelIgnoringExceptions() {
-        try {
-            cancel(false);
-        } catch (Throwable ignore) {
-        }
+        return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
     }
 
     public final boolean isDone() {
@@ -841,7 +874,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
     }
 
     public final boolean isCancelled() {
-        return status == CANCELLED;
+        return (status & DONE_MASK) == CANCELLED;
     }
 
     /**
@@ -861,7 +894,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * exception and was not cancelled
      */
     public final boolean isCompletedNormally() {
-        return status == NORMAL;
+        return (status & DONE_MASK) == NORMAL;
     }
 
     /**
@@ -872,7 +905,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * @return the exception, or {@code null} if none
      */
     public final Throwable getException() {
-        int s = status;
+        int s = status & DONE_MASK;
         return ((s >= NORMAL)    ? null :
                 (s == CANCELLED) ? new CancellationException() :
                 getThrowableException());
@@ -921,6 +954,18 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
         setCompletion(NORMAL);
     }
 
+    /**
+     * Completes this task normally without setting a value. The most
+     * recent value established by {@link #setRawResult} (or {@code
+     * null} by default) will be returned as the result of subsequent
+     * invocations of {@code join} and related operations.
+     *
+     * @since 1.8
+     */
+    public final void quietlyComplete() {
+        setCompletion(NORMAL);
+    }
+
     /**
      * Waits if necessary for the computation to complete, and then
      * retrieves its result.
@@ -934,9 +979,9 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      */
     public final V get() throws InterruptedException, ExecutionException {
         int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
-            doJoin() : externalInterruptibleAwaitDone(0L);
+            doJoin() : externalInterruptibleAwaitDone();
         Throwable ex;
-        if (s == CANCELLED)
+        if ((s &= DONE_MASK) == CANCELLED)
             throw new CancellationException();
         if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
             throw new ExecutionException(ex);
@@ -959,32 +1004,62 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      */
     public final V get(long timeout, TimeUnit unit)
         throws InterruptedException, ExecutionException, TimeoutException {
-        Thread t = Thread.currentThread();
-        if (t instanceof ForkJoinWorkerThread) {
-            ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
-            long nanos = unit.toNanos(timeout);
-            if (status >= 0) {
-                boolean completed = false;
-                if (w.unpushTask(this)) {
-                    try {
-                        completed = exec();
-                    } catch (Throwable rex) {
-                        setExceptionalCompletion(rex);
+        if (Thread.interrupted())
+            throw new InterruptedException();
+        // Messy in part because we measure in nanosecs, but wait in millisecs
+        int s; long ns, ms;
+        if ((s = status) >= 0 && (ns = unit.toNanos(timeout)) > 0L) {
+            long deadline = System.nanoTime() + ns;
+            ForkJoinPool p = null;
+            ForkJoinPool.WorkQueue w = null;
+            Thread t = Thread.currentThread();
+            if (t instanceof ForkJoinWorkerThread) {
+                ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
+                p = wt.pool;
+                w = wt.workQueue;
+                p.helpJoinOnce(w, this); // no retries on failure
+            }
+            else
+                ForkJoinPool.externalHelpJoin(this);
+            boolean canBlock = false;
+            boolean interrupted = false;
+            try {
+                while ((s = status) >= 0) {
+                    if (w != null && w.qlock < 0)
+                        cancelIgnoringExceptions(this);
+                    else if (!canBlock) {
+                        if (p == null || p.tryCompensate())
+                            canBlock = true;
+                    }
+                    else {
+                        if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L &&
+                            U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
+                            synchronized (this) {
+                                if (status >= 0) {
+                                    try {
+                                        wait(ms);
+                                    } catch (InterruptedException ie) {
+                                        if (p == null)
+                                            interrupted = true;
+                                    }
+                                }
+                                else
+                                    notifyAll();
+                            }
+                        }
+                        if ((s = status) < 0 || interrupted ||
+                            (ns = deadline - System.nanoTime()) <= 0L)
+                            break;
                     }
                 }
-                if (completed)
-                    setCompletion(NORMAL);
-                else if (status >= 0 && nanos > 0)
-                    w.pool.timedAwaitJoin(this, nanos);
+            } finally {
+                if (p != null && canBlock)
+                    p.incrementActiveCount();
             }
+            if (interrupted)
+                throw new InterruptedException();
         }
-        else {
-            long millis = unit.toMillis(timeout);
-            if (millis > 0)
-                externalInterruptibleAwaitDone(millis);
-        }
-        int s = status;
-        if (s != NORMAL) {
+        if ((s &= DONE_MASK) != NORMAL) {
             Throwable ex;
             if (s == CANCELLED)
                 throw new CancellationException();
@@ -1021,16 +1096,15 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * be of use in designs in which many tasks are forked, but none
      * are explicitly joined, instead executing them until all are
      * processed.
-     *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
      */
     public static void helpQuiesce() {
-        ((ForkJoinWorkerThread) Thread.currentThread())
-            .helpQuiescePool();
+        Thread t;
+        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
+            ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
+            wt.pool.helpQuiescePool(wt.workQueue);
+        }
+        else
+            ForkJoinPool.externalHelpQuiescePool();
     }
 
     /**
@@ -1050,7 +1124,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * setRawResult(null)}.
      */
     public void reinitialize() {
-        if (status == EXCEPTIONAL)
+        if ((status & DONE_MASK) == EXCEPTIONAL)
             clearExceptionalCompletion();
         else
             status = 0;
@@ -1083,23 +1157,19 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
 
     /**
      * Tries to unschedule this task for execution. This method will
-     * typically succeed if this task is the most recently forked task
-     * by the current thread, and has not commenced executing in
-     * another thread.  This method may be useful when arranging
-     * alternative local processing of tasks that could have been, but
-     * were not, stolen.
-     *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
+     * typically (but is not guaranteed to) succeed if this task is
+     * the most recently forked task by the current thread, and has
+     * not commenced executing in another thread.  This method may be
+     * useful when arranging alternative local processing of tasks
+     * that could have been, but were not, stolen.
      *
      * @return {@code true} if unforked
      */
     public boolean tryUnfork() {
-        return ((ForkJoinWorkerThread) Thread.currentThread())
-            .unpushTask(this);
+        Thread t;
+        return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+                ((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) :
+                ForkJoinPool.tryExternalUnpush(this));
     }
 
     /**
@@ -1108,40 +1178,32 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * value may be useful for heuristic decisions about whether to
      * fork other tasks.
      *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
-     *
      * @return the number of tasks
      */
     public static int getQueuedTaskCount() {
-        return ((ForkJoinWorkerThread) Thread.currentThread())
-            .getQueueSize();
+        Thread t; ForkJoinPool.WorkQueue q;
+        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
+            q = ((ForkJoinWorkerThread)t).workQueue;
+        else
+            q = ForkJoinPool.commonSubmitterQueue();
+        return (q == null) ? 0 : q.queueSize();
     }
 
     /**
      * Returns an estimate of how many more locally queued tasks are
      * held by the current worker thread than there are other worker
-     * threads that might steal them.  This value may be useful for
+     * threads that might steal them, or zero if this thread is not
+     * operating in a ForkJoinPool. This value may be useful for
      * heuristic decisions about whether to fork other tasks. In many
      * usages of ForkJoinTasks, at steady state, each worker should
      * aim to maintain a small constant surplus (for example, 3) of
      * tasks, and to process computations locally if this threshold is
      * exceeded.
      *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
-     *
      * @return the surplus number of tasks, which may be negative
      */
     public static int getSurplusQueuedTaskCount() {
-        return ((ForkJoinWorkerThread) Thread.currentThread())
-            .getEstimatedSurplusTaskCount();
+        return ForkJoinPool.getSurplusQueuedTaskCount();
     }
 
     // Extension methods
@@ -1167,15 +1229,18 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
     protected abstract void setRawResult(V value);
 
     /**
-     * Immediately performs the base action of this task.  This method
-     * is designed to support extensions, and should not in general be
-     * called otherwise. The return value controls whether this task
-     * is considered to be done normally. It may return false in
+     * Immediately performs the base action of this task and returns
+     * true if, upon return from this method, this task is guaranteed
+     * to have completed normally. This method may return false
+     * otherwise, to indicate that this task is not necessarily
+     * complete (or is not known to be complete), for example in
      * asynchronous actions that require explicit invocations of
-     * {@link #complete} to become joinable. It may also throw an
-     * (unchecked) exception to indicate abnormal exit.
+     * completion methods. This method may also throw an (unchecked)
+     * exception to indicate abnormal exit. This method is designed to
+     * support extensions, and should not in general be called
+     * otherwise.
      *
-     * @return {@code true} if completed normally
+     * @return {@code true} if this task is known to have completed normally
      */
     protected abstract boolean exec();
 
@@ -1189,59 +1254,102 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * primarily to support extensions, and is unlikely to be useful
      * otherwise.
      *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
-     *
      * @return the next task, or {@code null} if none are available
      */
     protected static ForkJoinTask<?> peekNextLocalTask() {
-        return ((ForkJoinWorkerThread) Thread.currentThread())
-            .peekTask();
+        Thread t; ForkJoinPool.WorkQueue q;
+        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
+            q = ((ForkJoinWorkerThread)t).workQueue;
+        else
+            q = ForkJoinPool.commonSubmitterQueue();
+        return (q == null) ? null : q.peek();
     }
 
     /**
      * Unschedules and returns, without executing, the next task
-     * queued by the current thread but not yet executed.  This method
-     * is designed primarily to support extensions, and is unlikely to
-     * be useful otherwise.
-     *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
+     * queued by the current thread but not yet executed, if the
+     * current thread is operating in a ForkJoinPool.  This method is
+     * designed primarily to support extensions, and is unlikely to be
+     * useful otherwise.
      *
      * @return the next task, or {@code null} if none are available
      */
     protected static ForkJoinTask<?> pollNextLocalTask() {
-        return ((ForkJoinWorkerThread) Thread.currentThread())
-            .pollLocalTask();
+        Thread t;
+        return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+            ((ForkJoinWorkerThread)t).workQueue.nextLocalTask() :
+            null;
     }
 
     /**
-     * Unschedules and returns, without executing, the next task
+     * If the current thread is operating in a ForkJoinPool,
+     * unschedules and returns, without executing, the next task
      * queued by the current thread but not yet executed, if one is
      * available, or if not available, a task that was forked by some
      * other thread, if available. Availability may be transient, so a
-     * {@code null} result does not necessarily imply quiescence
-     * of the pool this task is operating in.  This method is designed
+     * {@code null} result does not necessarily imply quiescence of
+     * the pool this task is operating in.  This method is designed
      * primarily to support extensions, and is unlikely to be useful
      * otherwise.
      *
-     * <p>This method may be invoked only from within {@code
-     * ForkJoinPool} computations (as may be determined using method
-     * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
-     * result in exceptions or errors, possibly including {@code
-     * ClassCastException}.
-     *
      * @return a task, or {@code null} if none are available
      */
     protected static ForkJoinTask<?> pollTask() {
-        return ((ForkJoinWorkerThread) Thread.currentThread())
-            .pollTask();
+        Thread t; ForkJoinWorkerThread wt;
+        return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+            (wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) :
+            null;
+    }
+
+    // tag operations
+
+    /**
+     * Returns the tag for this task.
+     *
+     * @return the tag for this task
+     * @since 1.8
+     */
+    public final short getForkJoinTaskTag() {
+        return (short)status;
+    }
+
+    /**
+     * Atomically sets the tag value for this task.
+     *
+     * @param tag the tag value
+     * @return the previous value of the tag
+     * @since 1.8
+     */
+    public final short setForkJoinTaskTag(short tag) {
+        for (int s;;) {
+            if (U.compareAndSwapInt(this, STATUS, s = status,
+                                    (s & ~SMASK) | (tag & SMASK)))
+                return (short)s;
+        }
+    }
+
+    /**
+     * Atomically conditionally sets the tag value for this task.
+     * Among other applications, tags can be used as visit markers
+     * in tasks operating on graphs, as in methods that check: {@code
+     * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))}
+     * before processing, otherwise exiting because the node has
+     * already been visited.
+     *
+     * @param e the expected tag value
+     * @param tag the new tag value
+     * @return true if successful; i.e., the current value was
+     * equal to e and is now tag.
+     * @since 1.8
+     */
+    public final boolean compareAndSetForkJoinTaskTag(short e, short tag) {
+        for (int s;;) {
+            if ((short)(s = status) != e)
+                return false;
+            if (U.compareAndSwapInt(this, STATUS, s,
+                                    (s & ~SMASK) | (tag & SMASK)))
+                return true;
+        }
     }
 
     /**
@@ -1252,21 +1360,33 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
     static final class AdaptedRunnable<T> extends ForkJoinTask<T>
         implements RunnableFuture<T> {
         final Runnable runnable;
-        final T resultOnCompletion;
         T result;
         AdaptedRunnable(Runnable runnable, T result) {
             if (runnable == null) throw new NullPointerException();
             this.runnable = runnable;
-            this.resultOnCompletion = result;
+            this.result = result; // OK to set this even before completion
         }
-        public T getRawResult() { return result; }
-        public void setRawResult(T v) { result = v; }
-        public boolean exec() {
-            runnable.run();
-            result = resultOnCompletion;
-            return true;
+        public final T getRawResult() { return result; }
+        public final void setRawResult(T v) { result = v; }
+        public final boolean exec() { runnable.run(); return true; }
+        public final void run() { invoke(); }
+        private static final long serialVersionUID = 5232453952276885070L;
+    }
+
+    /**
+     * Adaptor for Runnables without results
+     */
+    static final class AdaptedRunnableAction extends ForkJoinTask<Void>
+        implements RunnableFuture<Void> {
+        final Runnable runnable;
+        AdaptedRunnableAction(Runnable runnable) {
+            if (runnable == null) throw new NullPointerException();
+            this.runnable = runnable;
         }
-        public void run() { invoke(); }
+        public final Void getRawResult() { return null; }
+        public final void setRawResult(Void v) { }
+        public final boolean exec() { runnable.run(); return true; }
+        public final void run() { invoke(); }
         private static final long serialVersionUID = 5232453952276885070L;
     }
 
@@ -1281,9 +1401,9 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
             if (callable == null) throw new NullPointerException();
             this.callable = callable;
         }
-        public T getRawResult() { return result; }
-        public void setRawResult(T v) { result = v; }
-        public boolean exec() {
+        public final T getRawResult() { return result; }
+        public final void setRawResult(T v) { result = v; }
+        public final boolean exec() {
             try {
                 result = callable.call();
                 return true;
@@ -1295,7 +1415,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
                 throw new RuntimeException(ex);
             }
         }
-        public void run() { invoke(); }
+        public final void run() { invoke(); }
         private static final long serialVersionUID = 2838392045355241008L;
     }
 
@@ -1308,7 +1428,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
      * @return the task
      */
     public static ForkJoinTask<?> adapt(Runnable runnable) {
-        return new AdaptedRunnable<Void>(runnable, null);
+        return new AdaptedRunnableAction(runnable);
     }
 
     /**
@@ -1342,11 +1462,10 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
     private static final long serialVersionUID = -7721805057305804111L;
 
     /**
-     * Saves the state to a stream (that is, serializes it).
+     * Saves this task to a stream (that is, serializes it).
      *
      * @serialData the current run status and the exception thrown
      * during execution, or {@code null} if none
-     * @param s the stream
      */
     private void writeObject(java.io.ObjectOutputStream s)
         throws java.io.IOException {
@@ -1355,9 +1474,7 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
     }
 
     /**
-     * Reconstitutes the instance from a stream (that is, deserializes it).
-     *
-     * @param s the stream
+     * Reconstitutes this task from a stream (that is, deserializes it).
      */
     private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOException, ClassNotFoundException {
@@ -1368,16 +1485,18 @@ public abstract class ForkJoinTask<V> implements Future<V>, Serializable {
     }
 
     // Unsafe mechanics
-    private static final sun.misc.Unsafe UNSAFE;
-    private static final long statusOffset;
+    private static final sun.misc.Unsafe U;
+    private static final long STATUS;
+
     static {
         exceptionTableLock = new ReentrantLock();
         exceptionTableRefQueue = new ReferenceQueue<Object>();
         exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
         try {
-            UNSAFE = sun.misc.Unsafe.getUnsafe();
-            statusOffset = UNSAFE.objectFieldOffset
-                (ForkJoinTask.class.getDeclaredField("status"));
+            U = sun.misc.Unsafe.getUnsafe();
+            Class<?> k = ForkJoinTask.class;
+            STATUS = U.objectFieldOffset
+                (k.getDeclaredField("status"));
         } catch (Exception e) {
             throw new Error(e);
         }

src/share/classes/java/util/concurrent/ForkJoinWorkerThread.java

@@ -35,9 +35,6 @@
 
 package java.util.concurrent;
 
-import java.util.Collection;
-import java.util.concurrent.RejectedExecutionException;
-
 /**
  * A thread managed by a {@link ForkJoinPool}, which executes
  * {@link ForkJoinTask}s.
@@ -54,238 +51,20 @@ import java.util.concurrent.RejectedExecutionException;
  */
 public class ForkJoinWorkerThread extends Thread {
     /*
-     * Overview:
-     *
      * ForkJoinWorkerThreads are managed by ForkJoinPools and perform
-     * ForkJoinTasks. This class includes bookkeeping in support of
-     * worker activation, suspension, and lifecycle control described
-     * in more detail in the internal documentation of class
-     * ForkJoinPool. And as described further below, this class also
-     * includes special-cased support for some ForkJoinTask
-     * methods. But the main mechanics involve work-stealing:
-     *
-     * Work-stealing queues are special forms of Deques that support
-     * only three of the four possible end-operations -- push, pop,
-     * and deq (aka steal), under the further constraints that push
-     * and pop are called only from the owning thread, while deq may
-     * be called from other threads.  (If you are unfamiliar with
-     * them, you probably want to read Herlihy and Shavit's book "The
-     * Art of Multiprocessor programming", chapter 16 describing these
-     * in more detail before proceeding.)  The main work-stealing
-     * queue design is roughly similar to those in the papers "Dynamic
-     * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
-     * (http://research.sun.com/scalable/pubs/index.html) and
-     * "Idempotent work stealing" by Michael, Saraswat, and Vechev,
-     * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
-     * The main differences ultimately stem from gc requirements that
-     * we null out taken slots as soon as we can, to maintain as small
-     * a footprint as possible even in programs generating huge
-     * numbers of tasks. To accomplish this, we shift the CAS
-     * arbitrating pop vs deq (steal) from being on the indices
-     * ("queueBase" and "queueTop") to the slots themselves (mainly
-     * via method "casSlotNull()"). So, both a successful pop and deq
-     * mainly entail a CAS of a slot from non-null to null.  Because
-     * we rely on CASes of references, we do not need tag bits on
-     * queueBase or queueTop.  They are simple ints as used in any
-     * circular array-based queue (see for example ArrayDeque).
-     * Updates to the indices must still be ordered in a way that
-     * guarantees that queueTop == queueBase means the queue is empty,
-     * but otherwise may err on the side of possibly making the queue
-     * appear nonempty when a push, pop, or deq have not fully
-     * committed. Note that this means that the deq operation,
-     * considered individually, is not wait-free. One thief cannot
-     * successfully continue until another in-progress one (or, if
-     * previously empty, a push) completes.  However, in the
-     * aggregate, we ensure at least probabilistic non-blockingness.
-     * If an attempted steal fails, a thief always chooses a different
-     * random victim target to try next. So, in order for one thief to
-     * progress, it suffices for any in-progress deq or new push on
-     * any empty queue to complete.
-     *
-     * This approach also enables support for "async mode" where local
-     * task processing is in FIFO, not LIFO order; simply by using a
-     * version of deq rather than pop when locallyFifo is true (as set
-     * by the ForkJoinPool).  This allows use in message-passing
-     * frameworks in which tasks are never joined.  However neither
-     * mode considers affinities, loads, cache localities, etc, so
-     * rarely provide the best possible performance on a given
-     * machine, but portably provide good throughput by averaging over
-     * these factors.  (Further, even if we did try to use such
-     * information, we do not usually have a basis for exploiting
-     * it. For example, some sets of tasks profit from cache
-     * affinities, but others are harmed by cache pollution effects.)
+     * ForkJoinTasks. For explanation, see the internal documentation
+     * of class ForkJoinPool.
      *
-     * When a worker would otherwise be blocked waiting to join a
-     * task, it first tries a form of linear helping: Each worker
-     * records (in field currentSteal) the most recent task it stole
-     * from some other worker. Plus, it records (in field currentJoin)
-     * the task it is currently actively joining. Method joinTask uses
-     * these markers to try to find a worker to help (i.e., steal back
-     * a task from and execute it) that could hasten completion of the
-     * actively joined task. In essence, the joiner executes a task
-     * that would be on its own local deque had the to-be-joined task
-     * not been stolen. This may be seen as a conservative variant of
-     * the approach in Wagner & Calder "Leapfrogging: a portable
-     * technique for implementing efficient futures" SIGPLAN Notices,
-     * 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs
-     * in that: (1) We only maintain dependency links across workers
-     * upon steals, rather than use per-task bookkeeping.  This may
-     * require a linear scan of workers array to locate stealers, but
-     * usually doesn't because stealers leave hints (that may become
-     * stale/wrong) of where to locate them. This isolates cost to
-     * when it is needed, rather than adding to per-task overhead.
-     * (2) It is "shallow", ignoring nesting and potentially cyclic
-     * mutual steals.  (3) It is intentionally racy: field currentJoin
-     * is updated only while actively joining, which means that we
-     * miss links in the chain during long-lived tasks, GC stalls etc
-     * (which is OK since blocking in such cases is usually a good
-     * idea).  (4) We bound the number of attempts to find work (see
-     * MAX_HELP) and fall back to suspending the worker and if
-     * necessary replacing it with another.
-     *
-     * Efficient implementation of these algorithms currently relies
-     * on an uncomfortable amount of "Unsafe" mechanics. To maintain
-     * correct orderings, reads and writes of variable queueBase
-     * require volatile ordering.  Variable queueTop need not be
-     * volatile because non-local reads always follow those of
-     * queueBase.  Similarly, because they are protected by volatile
-     * queueBase reads, reads of the queue array and its slots by
-     * other threads do not need volatile load semantics, but writes
-     * (in push) require store order and CASes (in pop and deq)
-     * require (volatile) CAS semantics.  (Michael, Saraswat, and
-     * Vechev's algorithm has similar properties, but without support
-     * for nulling slots.)  Since these combinations aren't supported
-     * using ordinary volatiles, the only way to accomplish these
-     * efficiently is to use direct Unsafe calls. (Using external
-     * AtomicIntegers and AtomicReferenceArrays for the indices and
-     * array is significantly slower because of memory locality and
-     * indirection effects.)
-     *
-     * Further, performance on most platforms is very sensitive to
-     * placement and sizing of the (resizable) queue array.  Even
-     * though these queues don't usually become all that big, the
-     * initial size must be large enough to counteract cache
-     * contention effects across multiple queues (especially in the
-     * presence of GC cardmarking). Also, to improve thread-locality,
-     * queues are initialized after starting.
-     */
-
-    /**
-     * Mask for pool indices encoded as shorts
-     */
-    private static final int  SMASK  = 0xffff;
-
-    /**
-     * Capacity of work-stealing queue array upon initialization.
-     * Must be a power of two. Initial size must be at least 4, but is
-     * padded to minimize cache effects.
-     */
-    private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
-
-    /**
-     * Maximum size for queue array. Must be a power of two
-     * less than or equal to 1 << (31 - width of array entry) to
-     * ensure lack of index wraparound, but is capped at a lower
-     * value to help users trap runaway computations.
-     */
-    private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M
-
-    /**
-     * The work-stealing queue array. Size must be a power of two.
-     * Initialized when started (as opposed to when constructed), to
-     * improve memory locality.
-     */
-    ForkJoinTask<?>[] queue;
-
-    /**
-     * The pool this thread works in. Accessed directly by ForkJoinTask.
-     */
-    final ForkJoinPool pool;
-
-    /**
-     * Index (mod queue.length) of next queue slot to push to or pop
-     * from. It is written only by owner thread, and accessed by other
-     * threads only after reading (volatile) queueBase.  Both queueTop
-     * and queueBase are allowed to wrap around on overflow, but
-     * (queueTop - queueBase) still estimates size.
-     */
-    int queueTop;
-
-    /**
-     * Index (mod queue.length) of least valid queue slot, which is
-     * always the next position to steal from if nonempty.
+     * This class just maintains links to its pool and WorkQueue.  The
+     * pool field is set immediately upon construction, but the
+     * workQueue field is not set until a call to registerWorker
+     * completes. This leads to a visibility race, that is tolerated
+     * by requiring that the workQueue field is only accessed by the
+     * owning thread.
      */
-    volatile int queueBase;
 
-    /**
-     * The index of most recent stealer, used as a hint to avoid
-     * traversal in method helpJoinTask. This is only a hint because a
-     * worker might have had multiple steals and this only holds one
-     * of them (usually the most current). Declared non-volatile,
-     * relying on other prevailing sync to keep reasonably current.
-     */
-    int stealHint;
-
-    /**
-     * Index of this worker in pool array. Set once by pool before
-     * running, and accessed directly by pool to locate this worker in
-     * its workers array.
-     */
-    final int poolIndex;
-
-    /**
-     * Encoded record for pool task waits. Usages are always
-     * surrounded by volatile reads/writes
-     */
-    int nextWait;
-
-    /**
-     * Complement of poolIndex, offset by count of entries of task
-     * waits. Accessed by ForkJoinPool to manage event waiters.
-     */
-    volatile int eventCount;
-
-    /**
-     * Seed for random number generator for choosing steal victims.
-     * Uses Marsaglia xorshift. Must be initialized as nonzero.
-     */
-    int seed;
-
-    /**
-     * Number of steals. Directly accessed (and reset) by pool when
-     * idle.
-     */
-    int stealCount;
-
-    /**
-     * True if this worker should or did terminate
-     */
-    volatile boolean terminate;
-
-    /**
-     * Set to true before LockSupport.park; false on return
-     */
-    volatile boolean parked;
-
-    /**
-     * True if use local fifo, not default lifo, for local polling.
-     * Shadows value from ForkJoinPool.
-     */
-    final boolean locallyFifo;
-
-    /**
-     * The task most recently stolen from another worker (or
-     * submission queue).  All uses are surrounded by enough volatile
-     * reads/writes to maintain as non-volatile.
-     */
-    ForkJoinTask<?> currentSteal;
-
-    /**
-     * The task currently being joined, set only when actively trying
-     * to help other stealers in helpJoinTask. All uses are surrounded
-     * by enough volatile reads/writes to maintain as non-volatile.
-     */
-    ForkJoinTask<?> currentJoin;
+    final ForkJoinPool pool;                // the pool this thread works in
+    final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics
 
     /**
      * Creates a ForkJoinWorkerThread operating in the given pool.
@@ -294,20 +73,12 @@ public class ForkJoinWorkerThread extends Thread {
      * @throws NullPointerException if pool is null
      */
     protected ForkJoinWorkerThread(ForkJoinPool pool) {
-        super(pool.nextWorkerName());
+        // Use a placeholder until a useful name can be set in registerWorker
+        super("aForkJoinWorkerThread");
         this.pool = pool;
-        int k = pool.registerWorker(this);
-        poolIndex = k;
-        eventCount = ~k & SMASK; // clear wait count
-        locallyFifo = pool.locallyFifo;
-        Thread.UncaughtExceptionHandler ueh = pool.ueh;
-        if (ueh != null)
-            setUncaughtExceptionHandler(ueh);
-        setDaemon(true);
+        this.workQueue = pool.registerWorker(this);
     }
 
-    // Public methods
-
     /**
      * Returns the pool hosting this thread.
      *
@@ -327,28 +98,9 @@ public class ForkJoinWorkerThread extends Thread {
      * @return the index number
      */
     public int getPoolIndex() {
-        return poolIndex;
-    }
-
-    // Randomization
-
-    /**
-     * Computes next value for random victim probes and backoffs.
-     * Scans don't require a very high quality generator, but also not
-     * a crummy one.  Marsaglia xor-shift is cheap and works well
-     * enough.  Note: This is manually inlined in FJP.scan() to avoid
-     * writes inside busy loops.
-     */
-    private int nextSeed() {
-        int r = seed;
-        r ^= r << 13;
-        r ^= r >>> 17;
-        r ^= r << 5;
-        return seed = r;
+        return workQueue.poolIndex;
     }
 
-    // Run State management
-
     /**
      * Initializes internal state after construction but before
      * processing any tasks. If you override this method, you must
@@ -359,9 +111,6 @@ public class ForkJoinWorkerThread extends Thread {
      * processing tasks.
      */
     protected void onStart() {
-        queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
-        int r = ForkJoinPool.workerSeedGenerator.nextInt();
-        seed = (r == 0) ? 1 : r; //  must be nonzero
     }
 
     /**
@@ -373,17 +122,6 @@ public class ForkJoinWorkerThread extends Thread {
      * to an unrecoverable error, or {@code null} if completed normally
      */
     protected void onTermination(Throwable exception) {
-        try {
-            terminate = true;
-            cancelTasks();
-            pool.deregisterWorker(this, exception);
-        } catch (Throwable ex) {        // Shouldn't ever happen
-            if (exception == null)      // but if so, at least rethrown
-                exception = ex;
-        } finally {
-            if (exception != null)
-                UNSAFE.throwException(exception);
-        }
     }
 
     /**
@@ -395,604 +133,18 @@ public class ForkJoinWorkerThread extends Thread {
         Throwable exception = null;
         try {
             onStart();
-            pool.work(this);
+            pool.runWorker(workQueue);
         } catch (Throwable ex) {
             exception = ex;
         } finally {
-            onTermination(exception);
-        }
-    }
-
-    /*
-     * Intrinsics-based atomic writes for queue slots. These are
-     * basically the same as methods in AtomicReferenceArray, but
-     * specialized for (1) ForkJoinTask elements (2) requirement that
-     * nullness and bounds checks have already been performed by
-     * callers and (3) effective offsets are known not to overflow
-     * from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't
-     * need corresponding version for reads: plain array reads are OK
-     * because they are protected by other volatile reads and are
-     * confirmed by CASes.
-     *
-     * Most uses don't actually call these methods, but instead
-     * contain inlined forms that enable more predictable
-     * optimization.  We don't define the version of write used in
-     * pushTask at all, but instead inline there a store-fenced array
-     * slot write.
-     *
-     * Also in most methods, as a performance (not correctness) issue,
-     * we'd like to encourage compilers not to arbitrarily postpone
-     * setting queueTop after writing slot.  Currently there is no
-     * intrinsic for arranging this, but using Unsafe putOrderedInt
-     * may be a preferable strategy on some compilers even though its
-     * main effect is a pre-, not post- fence. To simplify possible
-     * changes, the option is left in comments next to the associated
-     * assignments.
-     */
-
-    /**
-     * CASes slot i of array q from t to null. Caller must ensure q is
-     * non-null and index is in range.
-     */
-    private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i,
-                                             ForkJoinTask<?> t) {
-        return UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null);
-    }
-
-    /**
-     * Performs a volatile write of the given task at given slot of
-     * array q.  Caller must ensure q is non-null and index is in
-     * range. This method is used only during resets and backouts.
-     */
-    private static final void writeSlot(ForkJoinTask<?>[] q, int i,
-                                        ForkJoinTask<?> t) {
-        UNSAFE.putObjectVolatile(q, (i << ASHIFT) + ABASE, t);
-    }
-
-    // queue methods
-
-    /**
-     * Pushes a task. Call only from this thread.
-     *
-     * @param t the task. Caller must ensure non-null.
-     */
-    final void pushTask(ForkJoinTask<?> t) {
-        ForkJoinTask<?>[] q; int s, m;
-        if ((q = queue) != null) {    // ignore if queue removed
-            long u = (((s = queueTop) & (m = q.length - 1)) << ASHIFT) + ABASE;
-            UNSAFE.putOrderedObject(q, u, t);
-            queueTop = s + 1;         // or use putOrderedInt
-            if ((s -= queueBase) <= 2)
-                pool.signalWork();
-            else if (s == m)
-                growQueue();
-        }
-    }
-
-    /**
-     * Creates or doubles queue array.  Transfers elements by
-     * emulating steals (deqs) from old array and placing, oldest
-     * first, into new array.
-     */
-    private void growQueue() {
-        ForkJoinTask<?>[] oldQ = queue;
-        int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
-        if (size > MAXIMUM_QUEUE_CAPACITY)
-            throw new RejectedExecutionException("Queue capacity exceeded");
-        if (size < INITIAL_QUEUE_CAPACITY)
-            size = INITIAL_QUEUE_CAPACITY;
-        ForkJoinTask<?>[] q = queue = new ForkJoinTask<?>[size];
-        int mask = size - 1;
-        int top = queueTop;
-        int oldMask;
-        if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
-            for (int b = queueBase; b != top; ++b) {
-                long u = ((b & oldMask) << ASHIFT) + ABASE;
-                Object x = UNSAFE.getObjectVolatile(oldQ, u);
-                if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
-                    UNSAFE.putObjectVolatile
-                        (q, ((b & mask) << ASHIFT) + ABASE, x);
-            }
-        }
-    }
-
-    /**
-     * Tries to take a task from the base of the queue, failing if
-     * empty or contended. Note: Specializations of this code appear
-     * in locallyDeqTask and elsewhere.
-     *
-     * @return a task, or null if none or contended
-     */
-    final ForkJoinTask<?> deqTask() {
-        ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
-        if (queueTop != (b = queueBase) &&
-            (q = queue) != null && // must read q after b
-            (i = (q.length - 1) & b) >= 0 &&
-            (t = q[i]) != null && queueBase == b &&
-            UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null)) {
-            queueBase = b + 1;
-            return t;
-        }
-        return null;
-    }
-
-    /**
-     * Tries to take a task from the base of own queue.  Called only
-     * by this thread.
-     *
-     * @return a task, or null if none
-     */
-    final ForkJoinTask<?> locallyDeqTask() {
-        ForkJoinTask<?> t; int m, b, i;
-        ForkJoinTask<?>[] q = queue;
-        if (q != null && (m = q.length - 1) >= 0) {
-            while (queueTop != (b = queueBase)) {
-                if ((t = q[i = m & b]) != null &&
-                    queueBase == b &&
-                    UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE,
-                                                t, null)) {
-                    queueBase = b + 1;
-                    return t;
-                }
+            try {
+                onTermination(exception);
+            } catch (Throwable ex) {
+                if (exception == null)
+                    exception = ex;
+            } finally {
+                pool.deregisterWorker(this, exception);
             }
         }
-        return null;
     }
-
-    /**
-     * Returns a popped task, or null if empty.
-     * Called only by this thread.
-     */
-    private ForkJoinTask<?> popTask() {
-        int m;
-        ForkJoinTask<?>[] q = queue;
-        if (q != null && (m = q.length - 1) >= 0) {
-            for (int s; (s = queueTop) != queueBase;) {
-                int i = m & --s;
-                long u = (i << ASHIFT) + ABASE; // raw offset
-                ForkJoinTask<?> t = q[i];
-                if (t == null)   // lost to stealer
-                    break;
-                if (UNSAFE.compareAndSwapObject(q, u, t, null)) {
-                    queueTop = s; // or putOrderedInt
-                    return t;
-                }
-            }
-        }
-        return null;
-    }
-
-    /**
-     * Specialized version of popTask to pop only if topmost element
-     * is the given task. Called only by this thread.
-     *
-     * @param t the task. Caller must ensure non-null.
-     */
-    final boolean unpushTask(ForkJoinTask<?> t) {
-        ForkJoinTask<?>[] q;
-        int s;
-        if ((q = queue) != null && (s = queueTop) != queueBase &&
-            UNSAFE.compareAndSwapObject
-            (q, (((q.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
-            queueTop = s; // or putOrderedInt
-            return true;
-        }
-        return false;
-    }
-
-    /**
-     * Returns next task, or null if empty or contended.
-     */
-    final ForkJoinTask<?> peekTask() {
-        int m;
-        ForkJoinTask<?>[] q = queue;
-        if (q == null || (m = q.length - 1) < 0)
-            return null;
-        int i = locallyFifo ? queueBase : (queueTop - 1);
-        return q[i & m];
-    }
-
-    // Support methods for ForkJoinPool
-
-    /**
-     * Runs the given task, plus any local tasks until queue is empty
-     */
-    final void execTask(ForkJoinTask<?> t) {
-        currentSteal = t;
-        for (;;) {
-            if (t != null)
-                t.doExec();
-            if (queueTop == queueBase)
-                break;
-            t = locallyFifo ? locallyDeqTask() : popTask();
-        }
-        ++stealCount;
-        currentSteal = null;
-    }
-
-    /**
-     * Removes and cancels all tasks in queue.  Can be called from any
-     * thread.
-     */
-    final void cancelTasks() {
-        ForkJoinTask<?> cj = currentJoin; // try to cancel ongoing tasks
-        if (cj != null && cj.status >= 0)
-            cj.cancelIgnoringExceptions();
-        ForkJoinTask<?> cs = currentSteal;
-        if (cs != null && cs.status >= 0)
-            cs.cancelIgnoringExceptions();
-        while (queueBase != queueTop) {
-            ForkJoinTask<?> t = deqTask();
-            if (t != null)
-                t.cancelIgnoringExceptions();
-        }
-    }
-
-    /**
-     * Drains tasks to given collection c.
-     *
-     * @return the number of tasks drained
-     */
-    final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
-        int n = 0;
-        while (queueBase != queueTop) {
-            ForkJoinTask<?> t = deqTask();
-            if (t != null) {
-                c.add(t);
-                ++n;
-            }
-        }
-        return n;
-    }
-
-    // Support methods for ForkJoinTask
-
-    /**
-     * Returns an estimate of the number of tasks in the queue.
-     */
-    final int getQueueSize() {
-        return queueTop - queueBase;
-    }
-
-    /**
-     * Gets and removes a local task.
-     *
-     * @return a task, if available
-     */
-    final ForkJoinTask<?> pollLocalTask() {
-        return locallyFifo ? locallyDeqTask() : popTask();
-    }
-
-    /**
-     * Gets and removes a local or stolen task.
-     *
-     * @return a task, if available
-     */
-    final ForkJoinTask<?> pollTask() {
-        ForkJoinWorkerThread[] ws;
-        ForkJoinTask<?> t = pollLocalTask();
-        if (t != null || (ws = pool.workers) == null)
-            return t;
-        int n = ws.length; // cheap version of FJP.scan
-        int steps = n << 1;
-        int r = nextSeed();
-        int i = 0;
-        while (i < steps) {
-            ForkJoinWorkerThread w = ws[(i++ + r) & (n - 1)];
-            if (w != null && w.queueBase != w.queueTop && w.queue != null) {
-                if ((t = w.deqTask()) != null)
-                    return t;
-                i = 0;
-            }
-        }
-        return null;
-    }
-
-    /**
-     * The maximum stolen->joining link depth allowed in helpJoinTask,
-     * as well as the maximum number of retries (allowing on average
-     * one staleness retry per level) per attempt to instead try
-     * compensation.  Depths for legitimate chains are unbounded, but
-     * we use a fixed constant to avoid (otherwise unchecked) cycles
-     * and bound staleness of traversal parameters at the expense of
-     * sometimes blocking when we could be helping.
-     */
-    private static final int MAX_HELP = 16;
-
-    /**
-     * Possibly runs some tasks and/or blocks, until joinMe is done.
-     *
-     * @param joinMe the task to join
-     * @return completion status on exit
-     */
-    final int joinTask(ForkJoinTask<?> joinMe) {
-        ForkJoinTask<?> prevJoin = currentJoin;
-        currentJoin = joinMe;
-        for (int s, retries = MAX_HELP;;) {
-            if ((s = joinMe.status) < 0) {
-                currentJoin = prevJoin;
-                return s;
-            }
-            if (retries > 0) {
-                if (queueTop != queueBase) {
-                    if (!localHelpJoinTask(joinMe))
-                        retries = 0;           // cannot help
-                }
-                else if (retries == MAX_HELP >>> 1) {
-                    --retries;                 // check uncommon case
-                    if (tryDeqAndExec(joinMe) >= 0)
-                        Thread.yield();        // for politeness
-                }
-                else
-                    retries = helpJoinTask(joinMe) ? MAX_HELP : retries - 1;
-            }
-            else {
-                retries = MAX_HELP;           // restart if not done
-                pool.tryAwaitJoin(joinMe);
-            }
-        }
-    }
-
-    /**
-     * If present, pops and executes the given task, or any other
-     * cancelled task
-     *
-     * @return false if any other non-cancelled task exists in local queue
-     */
-    private boolean localHelpJoinTask(ForkJoinTask<?> joinMe) {
-        int s, i; ForkJoinTask<?>[] q; ForkJoinTask<?> t;
-        if ((s = queueTop) != queueBase && (q = queue) != null &&
-            (i = (q.length - 1) & --s) >= 0 &&
-            (t = q[i]) != null) {
-            if (t != joinMe && t.status >= 0)
-                return false;
-            if (UNSAFE.compareAndSwapObject
-                (q, (i << ASHIFT) + ABASE, t, null)) {
-                queueTop = s;           // or putOrderedInt
-                t.doExec();
-            }
-        }
-        return true;
-    }
-
-    /**
-     * Tries to locate and execute tasks for a stealer of the given
-     * task, or in turn one of its stealers, Traces
-     * currentSteal->currentJoin links looking for a thread working on
-     * a descendant of the given task and with a non-empty queue to
-     * steal back and execute tasks from.  The implementation is very
-     * branchy to cope with potential inconsistencies or loops
-     * encountering chains that are stale, unknown, or of length
-     * greater than MAX_HELP links.  All of these cases are dealt with
-     * by just retrying by caller.
-     *
-     * @param joinMe the task to join
-     * @param canSteal true if local queue is empty
-     * @return true if ran a task
-     */
-    private boolean helpJoinTask(ForkJoinTask<?> joinMe) {
-        boolean helped = false;
-        int m = pool.scanGuard & SMASK;
-        ForkJoinWorkerThread[] ws = pool.workers;
-        if (ws != null && ws.length > m && joinMe.status >= 0) {
-            int levels = MAX_HELP;              // remaining chain length
-            ForkJoinTask<?> task = joinMe;      // base of chain
-            outer:for (ForkJoinWorkerThread thread = this;;) {
-                // Try to find v, the stealer of task, by first using hint
-                ForkJoinWorkerThread v = ws[thread.stealHint & m];
-                if (v == null || v.currentSteal != task) {
-                    for (int j = 0; ;) {        // search array
-                        if ((v = ws[j]) != null && v.currentSteal == task) {
-                            thread.stealHint = j;
-                            break;              // save hint for next time
-                        }
-                        if (++j > m)
-                            break outer;        // can't find stealer
-                    }
-                }
-                // Try to help v, using specialized form of deqTask
-                for (;;) {
-                    ForkJoinTask<?>[] q; int b, i;
-                    if (joinMe.status < 0)
-                        break outer;
-                    if ((b = v.queueBase) == v.queueTop ||
-                        (q = v.queue) == null ||
-                        (i = (q.length-1) & b) < 0)
-                        break;                  // empty
-                    long u = (i << ASHIFT) + ABASE;
-                    ForkJoinTask<?> t = q[i];
-                    if (task.status < 0)
-                        break outer;            // stale
-                    if (t != null && v.queueBase == b &&
-                        UNSAFE.compareAndSwapObject(q, u, t, null)) {
-                        v.queueBase = b + 1;
-                        v.stealHint = poolIndex;
-                        ForkJoinTask<?> ps = currentSteal;
-                        currentSteal = t;
-                        t.doExec();
-                        currentSteal = ps;
-                        helped = true;
-                    }
-                }
-                // Try to descend to find v's stealer
-                ForkJoinTask<?> next = v.currentJoin;
-                if (--levels > 0 && task.status >= 0 &&
-                    next != null && next != task) {
-                    task = next;
-                    thread = v;
-                }
-                else
-                    break;  // max levels, stale, dead-end, or cyclic
-            }
-        }
-        return helped;
-    }
-
-    /**
-     * Performs an uncommon case for joinTask: If task t is at base of
-     * some workers queue, steals and executes it.
-     *
-     * @param t the task
-     * @return t's status
-     */
-    private int tryDeqAndExec(ForkJoinTask<?> t) {
-        int m = pool.scanGuard & SMASK;
-        ForkJoinWorkerThread[] ws = pool.workers;
-        if (ws != null && ws.length > m && t.status >= 0) {
-            for (int j = 0; j <= m; ++j) {
-                ForkJoinTask<?>[] q; int b, i;
-                ForkJoinWorkerThread v = ws[j];
-                if (v != null &&
-                    (b = v.queueBase) != v.queueTop &&
-                    (q = v.queue) != null &&
-                    (i = (q.length - 1) & b) >= 0 &&
-                    q[i] == t) {
-                    long u = (i << ASHIFT) + ABASE;
-                    if (v.queueBase == b &&
-                        UNSAFE.compareAndSwapObject(q, u, t, null)) {
-                        v.queueBase = b + 1;
-                        v.stealHint = poolIndex;
-                        ForkJoinTask<?> ps = currentSteal;
-                        currentSteal = t;
-                        t.doExec();
-                        currentSteal = ps;
-                    }
-                    break;
-                }
-            }
-        }
-        return t.status;
-    }
-
-    /**
-     * Implements ForkJoinTask.getSurplusQueuedTaskCount().  Returns
-     * an estimate of the number of tasks, offset by a function of
-     * number of idle workers.
-     *
-     * This method provides a cheap heuristic guide for task
-     * partitioning when programmers, frameworks, tools, or languages
-     * have little or no idea about task granularity.  In essence by
-     * offering this method, we ask users only about tradeoffs in
-     * overhead vs expected throughput and its variance, rather than
-     * how finely to partition tasks.
-     *
-     * In a steady state strict (tree-structured) computation, each
-     * thread makes available for stealing enough tasks for other
-     * threads to remain active. Inductively, if all threads play by
-     * the same rules, each thread should make available only a
-     * constant number of tasks.
-     *
-     * The minimum useful constant is just 1. But using a value of 1
-     * would require immediate replenishment upon each steal to
-     * maintain enough tasks, which is infeasible.  Further,
-     * partitionings/granularities of offered tasks should minimize
-     * steal rates, which in general means that threads nearer the top
-     * of computation tree should generate more than those nearer the
-     * bottom. In perfect steady state, each thread is at
-     * approximately the same level of computation tree. However,
-     * producing extra tasks amortizes the uncertainty of progress and
-     * diffusion assumptions.
-     *
-     * So, users will want to use values larger, but not much larger
-     * than 1 to both smooth over transient shortages and hedge
-     * against uneven progress; as traded off against the cost of
-     * extra task overhead. We leave the user to pick a threshold
-     * value to compare with the results of this call to guide
-     * decisions, but recommend values such as 3.
-     *
-     * When all threads are active, it is on average OK to estimate
-     * surplus strictly locally. In steady-state, if one thread is
-     * maintaining say 2 surplus tasks, then so are others. So we can
-     * just use estimated queue length (although note that (queueTop -
-     * queueBase) can be an overestimate because of stealers lagging
-     * increments of queueBase).  However, this strategy alone leads
-     * to serious mis-estimates in some non-steady-state conditions
-     * (ramp-up, ramp-down, other stalls). We can detect many of these
-     * by further considering the number of "idle" threads, that are
-     * known to have zero queued tasks, so compensate by a factor of
-     * (#idle/#active) threads.
-     */
-    final int getEstimatedSurplusTaskCount() {
-        return queueTop - queueBase - pool.idlePerActive();
-    }
-
-    /**
-     * Runs tasks until {@code pool.isQuiescent()}. We piggyback on
-     * pool's active count ctl maintenance, but rather than blocking
-     * when tasks cannot be found, we rescan until all others cannot
-     * find tasks either. The bracketing by pool quiescerCounts
-     * updates suppresses pool auto-shutdown mechanics that could
-     * otherwise prematurely terminate the pool because all threads
-     * appear to be inactive.
-     */
-    final void helpQuiescePool() {
-        boolean active = true;
-        ForkJoinTask<?> ps = currentSteal; // to restore below
-        ForkJoinPool p = pool;
-        p.addQuiescerCount(1);
-        for (;;) {
-            ForkJoinWorkerThread[] ws = p.workers;
-            ForkJoinWorkerThread v = null;
-            int n;
-            if (queueTop != queueBase)
-                v = this;
-            else if (ws != null && (n = ws.length) > 1) {
-                ForkJoinWorkerThread w;
-                int r = nextSeed(); // cheap version of FJP.scan
-                int steps = n << 1;
-                for (int i = 0; i < steps; ++i) {
-                    if ((w = ws[(i + r) & (n - 1)]) != null &&
-                        w.queueBase != w.queueTop) {
-                        v = w;
-                        break;
-                    }
-                }
-            }
-            if (v != null) {
-                ForkJoinTask<?> t;
-                if (!active) {
-                    active = true;
-                    p.addActiveCount(1);
-                }
-                if ((t = (v != this) ? v.deqTask() :
-                     locallyFifo ? locallyDeqTask() : popTask()) != null) {
-                    currentSteal = t;
-                    t.doExec();
-                    currentSteal = ps;
-                }
-            }
-            else {
-                if (active) {
-                    active = false;
-                    p.addActiveCount(-1);
-                }
-                if (p.isQuiescent()) {
-                    p.addActiveCount(1);
-                    p.addQuiescerCount(-1);
-                    break;
-                }
-            }
-        }
-    }
-
-    // Unsafe mechanics
-    private static final sun.misc.Unsafe UNSAFE;
-    private static final long ABASE;
-    private static final int ASHIFT;
-
-    static {
-        int s;
-        try {
-            UNSAFE = sun.misc.Unsafe.getUnsafe();
-            Class<?> a = ForkJoinTask[].class;
-            ABASE = UNSAFE.arrayBaseOffset(a);
-            s = UNSAFE.arrayIndexScale(a);
-        } catch (Exception e) {
-            throw new Error(e);
-        }
-        if ((s & (s-1)) != 0)
-            throw new Error("data type scale not a power of two");
-        ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
-    }
-
 }