src/jsr166y/CountedCompleter.java

/*
 * 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 jsr166y;

/**
 * A {@link ForkJoinTask} with a completion action
 * performed when triggered and there are no remaining pending
 * actions. 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 {@link
 * #onCompletion} action, not just one. Unless initialized otherwise,
 * the {@link #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.
 *
 * <p>A concrete CountedCompleter class must define method {@link
 * #compute}, that should, in almost all use cases, 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. (Method
 * {@link #setRawResult} by default plays no role in CountedCompleters
 * but may be overridden for example to maintain 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.
 *
 * <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 IO) 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:
 *
 * <pre> {@code
 * class MyOperation<E> { void apply(E e) { ... }  }
 *
 * class ForEach<E> extends CountedCompleter<Void> {
 *
 *     public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) {
 *         pool.invoke(new ForEach<E>(null, array, op, 0, array.length));
 *     }
 *
 *     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
 * 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,
 * this version uses {@code helpComplete} to streamline assistance in
 * the execution of forked tasks.
 *
 * <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]);
 *         helpComplete();
 *     }
 * }</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>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(ForkJoinPool pool, E[] array,
 *                                   MyMapper<E> mapper, MyReducer<E> reducer) {
 *         return pool.invoke(new MapReducer<E>(null, array, mapper,
 *                                              reducer, 0, array.length));
 *     }
 * } }</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 tasks 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 tasks 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 there are no pending counts, or when the unconditional
     * method {@link #complete} is invoked.  By default, this method
     * does nothing.
     *
     * @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
     * tasks 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 is successful
     */
    public final boolean compareAndSetPendingCount(int expected, int count) {
        return U.compareAndSwapInt(this, PENDING, expected, count);
    }

    /**
     * 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;
        }
    }

    /**
     * Identical to {@link #tryComplete}, but may additionally execute
     * other tasks within the current computation (i.e., those
     * with the same {@link #getRoot}.
     */
    public final void helpComplete() {
        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)) {
                CountedCompleter<?> root = a.getRoot();
                Thread thread = Thread.currentThread();
                ForkJoinPool.WorkQueue wq =
                    (thread instanceof ForkJoinWorkerThread) ?
                    ((ForkJoinWorkerThread)thread).workQueue : null;
                ForkJoinTask<?> t;
                while ((t = (wq != null) ? wq.popCC(root) :
                        ForkJoinPool.popCCFromCommonPool(root)) != null) {
                    t.doExec();
                    if (root.isDone())
                        break;
                }
                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. This
     * method may be useful when forcing completion as soon as any one
     * (versus all) of several subtask results are obtained.  The
     * given rawResult is used as an argument to {@link #setRawResult}
     * before marking this task as complete; its value is meaningful
     * only for classes overriding {@code setRawResult}.
     *
     * @param rawResult the raw result
     */
    public void complete(T rawResult) {
        CountedCompleter<?> p;
        onCompletion(this);
        setRawResult(rawResult);
        quietlyComplete();
        if ((p = completer) != null)
            p.tryComplete();
    }

    /**
     * Support for FJT 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.
     *
     * @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.
     */
    protected void setRawResult(T t) { }

    // Unsafe mechanics
    private static final sun.misc.Unsafe U;
    private static final long PENDING;
    static {
        try {
            U = getUnsafe();
            PENDING = U.objectFieldOffset
                (CountedCompleter.class.getDeclaredField("pending"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

    /**
     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
     * Replace with a simple call to Unsafe.getUnsafe when integrating
     * into a jdk.
     *
     * @return a sun.misc.Unsafe
     */
    private static sun.misc.Unsafe getUnsafe() {
        try {
            return sun.misc.Unsafe.getUnsafe();
        } catch (SecurityException se) {
            try {
                return java.security.AccessController.doPrivileged
                    (new java.security
                     .PrivilegedExceptionAction<sun.misc.Unsafe>() {
                        public sun.misc.Unsafe run() throws Exception {
                            java.lang.reflect.Field f = sun.misc
                                .Unsafe.class.getDeclaredField("theUnsafe");
                            f.setAccessible(true);
                            return (sun.misc.Unsafe) f.get(null);
                        }});
            } catch (java.security.PrivilegedActionException e) {
                throw new RuntimeException("Could not initialize intrinsics",
                                           e.getCause());
            }
        }
    }

}
Revision:	1.7
Committed:	Tue Oct 30 16:05:35 2012 UTC (11 years, 6 months ago) by jsr166
Branch:	MAIN
Changes since 1.6:	+1 -1 lines
Log Message:	whitespace
#	User	Rev	Content
1	dl	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/publicdomain/zero/1.0/
5			*/
6
7			package jsr166y;
8
9			/**
10	dl	1.3	* A {@link ForkJoinTask} with a completion action
11	dl	1.1	* performed when triggered and there are no remaining pending
12			* actions. Uses of CountedCompleter are similar to those of other
13			* completion based components (such as {@link
14			* java.nio.channels.CompletionHandler}) except that multiple
15			* <em>pending</em> completions may be necessary to trigger the {@link
16			* #onCompletion} action, not just one. Unless initialized otherwise,
17			* the {@link #getPendingCount pending count} starts at zero, but may
18			* be (atomically) changed using methods {@link #setPendingCount},
19			* {@link #addToPendingCount}, and {@link
20			* #compareAndSetPendingCount}. Upon invocation of {@link
21			* #tryComplete}, if the pending action count is nonzero, it is
22			* decremented; otherwise, the completion action is performed, and if
23			* this completer itself has a completer, the process is continued
24	dl	1.2	* with its completer. As is the case with related synchronization
25	jsr166	1.4	* components such as {@link java.util.concurrent.Phaser Phaser} and
26			* {@link java.util.concurrent.Semaphore Semaphore}, these methods
27			* affect only internal counts; they do not establish any further
28			* internal bookkeeping. In particular, the identities of pending
29			* tasks are not maintained. As illustrated below, you can create
30	jsr166	1.5	* subclasses that do record some or all pending tasks or their
31			* results when needed.
32	dl	1.1	*
33			* <p>A concrete CountedCompleter class must define method {@link
34			* #compute}, that should, in almost all use cases, invoke {@code
35	dl	1.2	* tryComplete()} once before returning. The class may also optionally
36	dl	1.1	* override method {@link #onCompletion} to perform an action upon
37	dl	1.2	* normal completion, and method {@link #onExceptionalCompletion} to
38			* perform an action upon any exception.
39	dl	1.1	*
40	dl	1.3	* <p>CountedCompleters most often do not bear results, in which case
41			* they are normally declared as {@code CountedCompleter<Void>}, and
42			* will always return {@code null} as a result value. In other cases,
43			* you should override method {@link #getRawResult} to provide a
44			* result from {@code join(), invoke()}, and related methods. (Method
45			* {@link #setRawResult} by default plays no role in CountedCompleters
46			* but may be overridden for example to maintain fields holding result
47			* data.)
48			*
49	dl	1.1	* <p>A CountedCompleter that does not itself have a completer (i.e.,
50			* one for which {@link #getCompleter} returns {@code null}) can be
51			* used as a regular ForkJoinTask with this added functionality.
52			* However, any completer that in turn has another completer serves
53			* only as an internal helper for other computations, so its own task
54			* status (as reported in methods such as {@link ForkJoinTask#isDone})
55			* is arbitrary; this status changes only upon explicit invocations of
56			* {@link #complete}, {@link ForkJoinTask#cancel}, {@link
57			* ForkJoinTask#completeExceptionally} or upon exceptional completion
58			* of method {@code compute}. Upon any exceptional completion, the
59	dl	1.2	* exception may be relayed to a task's completer (and its completer,
60			* and so on), if one exists and it has not otherwise already
61			* completed.
62	dl	1.1	*
63			* <p><b>Sample Usages.</b>
64			*
65			* <p><b>Parallel recursive decomposition.</b> CountedCompleters may
66			* be arranged in trees similar to those often used with {@link
67			* RecursiveAction}s, although the constructions involved in setting
68	dl	1.3	* them up typically vary. Here, the completer of each task is its
69			* parent in the computation tree. Even though they entail a bit more
70	dl	1.1	* bookkeeping, CountedCompleters may be better choices when applying
71			* a possibly time-consuming operation (that cannot be further
72			* subdivided) to each element of an array or collection; especially
73			* when the operation takes a significantly different amount of time
74			* to complete for some elements than others, either because of
75			* intrinsic variation (for example IO) or auxiliary effects such as
76			* garbage collection. Because CountedCompleters provide their own
77			* continuations, other threads need not block waiting to perform
78			* them.
79			*
80			* <p> For example, here is an initial version of a class that uses
81			* divide-by-two recursive decomposition to divide work into single
82			* pieces (leaf tasks). Even when work is split into individual calls,
83			* tree-based techniques are usually preferable to directly forking
84			* leaf tasks, because they reduce inter-thread communication and
85			* improve load balancing. In the recursive case, the second of each
86			* pair of subtasks to finish triggers completion of its parent
87			* (because no result combination is performed, the default no-op
88			* implementation of method {@code onCompletion} is not overridden). A
89			* static utility method sets up the base task and invokes it:
90			*
91			* <pre> {@code
92			* class MyOperation<E> { void apply(E e) { ... } }
93			*
94	dl	1.3	* class ForEach<E> extends CountedCompleter<Void> {
95	dl	1.1	*
96			* public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) {
97			* pool.invoke(new ForEach<E>(null, array, op, 0, array.length));
98			* }
99			*
100			* final E[] array; final MyOperation<E> op; final int lo, hi;
101	dl	1.3	* ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
102	dl	1.1	* super(p);
103			* this.array = array; this.op = op; this.lo = lo; this.hi = hi;
104			* }
105			*
106			* public void compute() { // version 1
107			* if (hi - lo >= 2) {
108			* int mid = (lo + hi) >>> 1;
109			* setPendingCount(2); // must set pending count before fork
110			* new ForEach(this, array, op, mid, hi).fork(); // right child
111			* new ForEach(this, array, op, lo, mid).fork(); // left child
112			* }
113			* else if (hi > lo)
114			* op.apply(array[lo]);
115			* tryComplete();
116			* }
117			* } }</pre>
118			*
119			* This design can be improved by noticing that in the recursive case,
120			* the task has nothing to do after forking its right task, so can
121			* directly invoke its left task before returning. (This is an analog
122			* of tail recursion removal.) Also, because the task returns upon
123			* executing its left task (rather than falling through to invoke
124			* tryComplete) the pending count is set to one:
125			*
126			* <pre> {@code
127			* class ForEach<E> ...
128			* public void compute() { // version 2
129			* if (hi - lo >= 2) {
130			* int mid = (lo + hi) >>> 1;
131			* setPendingCount(1); // only one pending
132			* new ForEach(this, array, op, mid, hi).fork(); // right child
133			* new ForEach(this, array, op, lo, mid).compute(); // direct invoke
134			* }
135			* else {
136			* if (hi > lo)
137			* op.apply(array[lo]);
138			* tryComplete();
139			* }
140			* }
141			* }</pre>
142			*
143			* As a further improvement, notice that the left task need not even
144			* exist. Instead of creating a new one, we can iterate using the
145	dl	1.6	* original task, and add a pending count for each fork. Additionally,
146			* this version uses {@code helpComplete} to streamline assistance in
147			* the execution of forked tasks.
148	dl	1.1	*
149			* <pre> {@code
150			* class ForEach<E> ...
151			* public void compute() { // version 3
152			* int l = lo, h = hi;
153			* while (h - l >= 2) {
154			* int mid = (l + h) >>> 1;
155			* addToPendingCount(1);
156			* new ForEach(this, array, op, mid, h).fork(); // right child
157			* h = mid;
158			* }
159			* if (h > l)
160			* op.apply(array[l]);
161	dl	1.6	* helpComplete();
162	dl	1.1	* }
163			* }</pre>
164			*
165			* Additional improvements of such classes might entail precomputing
166			* pending counts so that they can be established in constructors,
167			* specializing classes for leaf steps, subdividing by say, four,
168			* instead of two per iteration, and using an adaptive threshold
169			* instead of always subdividing down to single elements.
170			*
171			* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
172			* results of multiple subtasks usually need to access these results
173			* in method {@link #onCompletion}. As illustrated in the following
174			* class (that performs a simplified form of map-reduce where mappings
175			* and reductions are all of type {@code E}), one way to do this in
176			* divide and conquer designs is to have each subtask record its
177			* sibling, so that it can be accessed in method {@code onCompletion}.
178	dl	1.3	* This technique applies to reductions in which the order of
179			* combining left and right results does not matter; ordered
180			* reductions require explicit left/right designations. Variants of
181			* other streamlinings seen in the above examples may also apply.
182	dl	1.1	*
183			* <pre> {@code
184			* class MyMapper<E> { E apply(E v) { ... } }
185			* class MyReducer<E> { E apply(E x, E y) { ... } }
186	dl	1.3	* class MapReducer<E> extends CountedCompleter<E> {
187	dl	1.1	* final E[] array; final MyMapper<E> mapper;
188			* final MyReducer<E> reducer; final int lo, hi;
189	dl	1.3	* MapReducer<E> sibling;
190	dl	1.1	* E result;
191			* MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper,
192			* MyReducer<E> reducer, int lo, int hi) {
193			* super(p);
194			* this.array = array; this.mapper = mapper;
195			* this.reducer = reducer; this.lo = lo; this.hi = hi;
196			* }
197			* public void compute() {
198			* if (hi - lo >= 2) {
199			* int mid = (lo + hi) >>> 1;
200			* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
201			* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
202			* left.sibling = right;
203			* right.sibling = left;
204			* setPendingCount(1); // only right is pending
205			* right.fork();
206			* left.compute(); // directly execute left
207			* }
208			* else {
209			* if (hi > lo)
210			* result = mapper.apply(array[lo]);
211			* tryComplete();
212			* }
213			* }
214			* public void onCompletion(CountedCompleter caller) {
215			* if (caller != this) {
216			* MapReducer<E> child = (MapReducer<E>)caller;
217			* MapReducer<E> sib = child.sibling;
218			* if (sib == null \|\| sib.result == null)
219			* result = child.result;
220			* else
221			* result = reducer.apply(child.result, sib.result);
222			* }
223			* }
224	dl	1.3	* public E getRawResult() { return result; }
225	dl	1.1	*
226			* public static <E> E mapReduce(ForkJoinPool pool, E[] array,
227			* MyMapper<E> mapper, MyReducer<E> reducer) {
228	dl	1.3	* return pool.invoke(new MapReducer<E>(null, array, mapper,
229			* reducer, 0, array.length));
230	dl	1.1	* }
231			* } }</pre>
232			*
233			* <p><b>Triggers.</b> Some CountedCompleters are themselves never
234			* forked, but instead serve as bits of plumbing in other designs;
235			* including those in which the completion of one of more async tasks
236			* triggers another async task. For example:
237			*
238			* <pre> {@code
239	dl	1.3	* class HeaderBuilder extends CountedCompleter<...> { ... }
240			* class BodyBuilder extends CountedCompleter<...> { ... }
241			* class PacketSender extends CountedCompleter<...> {
242	dl	1.1	* PacketSender(...) { super(null, 1); ... } // trigger on second completion
243			* public void compute() { } // never called
244	dl	1.3	* public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
245	dl	1.1	* }
246			* // sample use:
247			* PacketSender p = new PacketSender();
248			* new HeaderBuilder(p, ...).fork();
249			* new BodyBuilder(p, ...).fork();
250			* }</pre>
251			*
252			* @since 1.8
253			* @author Doug Lea
254			*/
255	dl	1.3	public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
256	dl	1.2	private static final long serialVersionUID = 5232453752276485070L;
257
258	dl	1.1	/** This task's completer, or null if none */
259	dl	1.3	final CountedCompleter<?> completer;
260	dl	1.1	/** The number of pending tasks until completion */
261			volatile int pending;
262
263			/**
264			* Creates a new CountedCompleter with the given completer
265			* and initial pending count.
266			*
267			* @param completer this tasks completer, or {@code null} if none
268			* @param initialPendingCount the initial pending count
269			*/
270	dl	1.3	protected CountedCompleter(CountedCompleter<?> completer,
271	dl	1.1	int initialPendingCount) {
272			this.completer = completer;
273			this.pending = initialPendingCount;
274			}
275
276			/**
277			* Creates a new CountedCompleter with the given completer
278			* and an initial pending count of zero.
279			*
280			* @param completer this tasks completer, or {@code null} if none
281			*/
282	dl	1.3	protected CountedCompleter(CountedCompleter<?> completer) {
283	dl	1.1	this.completer = completer;
284			}
285
286			/**
287			* Creates a new CountedCompleter with no completer
288			* and an initial pending count of zero.
289			*/
290			protected CountedCompleter() {
291			this.completer = null;
292			}
293
294			/**
295			* The main computation performed by this task.
296			*/
297			public abstract void compute();
298
299			/**
300	dl	1.2	* Performs an action when method {@link #tryComplete} is invoked
301			* and there are no pending counts, or when the unconditional
302			* method {@link #complete} is invoked. By default, this method
303			* does nothing.
304	dl	1.1	*
305			* @param caller the task invoking this method (which may
306			* be this task itself).
307			*/
308	dl	1.3	public void onCompletion(CountedCompleter<?> caller) {
309	dl	1.1	}
310
311			/**
312	dl	1.2	* Performs an action when method {@link #completeExceptionally}
313			* is invoked or method {@link #compute} throws an exception, and
314			* this task has not otherwise already completed normally. On
315			* entry to this method, this task {@link
316			* ForkJoinTask#isCompletedAbnormally}. The return value of this
317			* method controls further propagation: If {@code true} and this
318			* task has a completer, then this completer is also completed
319			* exceptionally. The default implementation of this method does
320			* nothing except return {@code true}.
321			*
322			* @param ex the exception
323			* @param caller the task invoking this method (which may
324			* be this task itself).
325			* @return true if this exception should be propagated to this
326			* tasks completer, if one exists.
327			*/
328	dl	1.3	public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
329	dl	1.2	return true;
330			}
331
332			/**
333	dl	1.1	* Returns the completer established in this task's constructor,
334			* or {@code null} if none.
335			*
336			* @return the completer
337			*/
338	dl	1.3	public final CountedCompleter<?> getCompleter() {
339	dl	1.1	return completer;
340			}
341
342			/**
343			* Returns the current pending count.
344			*
345			* @return the current pending count
346			*/
347			public final int getPendingCount() {
348			return pending;
349			}
350
351			/**
352			* Sets the pending count to the given value.
353			*
354			* @param count the count
355			*/
356			public final void setPendingCount(int count) {
357			pending = count;
358			}
359
360			/**
361			* Adds (atomically) the given value to the pending count.
362			*
363			* @param delta the value to add
364			*/
365			public final void addToPendingCount(int delta) {
366			int c; // note: can replace with intrinsic in jdk8
367			do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
368			}
369
370			/**
371			* Sets (atomically) the pending count to the given count only if
372			* it currently holds the given expected value.
373			*
374			* @param expected the expected value
375			* @param count the new value
376			* @return true is successful
377			*/
378			public final boolean compareAndSetPendingCount(int expected, int count) {
379			return U.compareAndSwapInt(this, PENDING, expected, count);
380			}
381
382			/**
383	dl	1.6	* Returns the root of the current computation; i.e., this
384			* task if it has no completer, else its completer's root.
385			*
386			* @return the root of the current computation
387			*/
388			public final CountedCompleter<?> getRoot() {
389			CountedCompleter<?> a = this, p;
390			while ((p = a.completer) != null)
391			a = p;
392			return a;
393			}
394
395			/**
396	dl	1.1	* If the pending count is nonzero, decrements the count;
397			* otherwise invokes {@link #onCompletion} and then similarly
398			* tries to complete this task's completer, if one exists,
399			* else marks this task as complete.
400			*/
401			public final void tryComplete() {
402	dl	1.3	CountedCompleter<?> a = this, s = a;
403	dl	1.2	for (int c;;) {
404	dl	1.1	if ((c = a.pending) == 0) {
405			a.onCompletion(s);
406			if ((a = (s = a).completer) == null) {
407			s.quietlyComplete();
408			return;
409			}
410			}
411			else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
412			return;
413			}
414			}
415
416			/**
417	dl	1.6	* Identical to {@link #tryComplete}, but may additionally execute
418			* other tasks within the current computation (i.e., those
419			* with the same {@link #getRoot}.
420			*/
421			public final void helpComplete() {
422			CountedCompleter<?> a = this, s = a;
423			for (int c;;) {
424			if ((c = a.pending) == 0) {
425			a.onCompletion(s);
426			if ((a = (s = a).completer) == null) {
427			s.quietlyComplete();
428			return;
429			}
430			}
431			else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) {
432			CountedCompleter<?> root = a.getRoot();
433			Thread thread = Thread.currentThread();
434			ForkJoinPool.WorkQueue wq =
435	jsr166	1.7	(thread instanceof ForkJoinWorkerThread) ?
436	dl	1.6	((ForkJoinWorkerThread)thread).workQueue : null;
437			ForkJoinTask<?> t;
438			while ((t = (wq != null) ? wq.popCC(root) :
439			ForkJoinPool.popCCFromCommonPool(root)) != null) {
440			t.doExec();
441			if (root.isDone())
442			break;
443			}
444			return;
445			}
446			}
447			}
448
449			/**
450	dl	1.1	* Regardless of pending count, invokes {@link #onCompletion},
451	dl	1.3	* marks this task as complete and further triggers {@link
452			* #tryComplete} on this task's completer, if one exists. This
453			* method may be useful when forcing completion as soon as any one
454			* (versus all) of several subtask results are obtained. The
455			* given rawResult is used as an argument to {@link #setRawResult}
456			* before marking this task as complete; its value is meaningful
457			* only for classes overriding {@code setRawResult}.
458	dl	1.1	*
459	dl	1.3	* @param rawResult the raw result
460	dl	1.1	*/
461	dl	1.3	public void complete(T rawResult) {
462			CountedCompleter<?> p;
463	dl	1.1	onCompletion(this);
464	dl	1.3	setRawResult(rawResult);
465	dl	1.1	quietlyComplete();
466			if ((p = completer) != null)
467			p.tryComplete();
468			}
469
470			/**
471	dl	1.2	* Support for FJT exception propagation
472			*/
473			void internalPropagateException(Throwable ex) {
474	dl	1.3	CountedCompleter<?> a = this, s = a;
475	dl	1.2	while (a.onExceptionalCompletion(ex, s) &&
476			(a = (s = a).completer) != null && a.status >= 0)
477			a.recordExceptionalCompletion(ex);
478			}
479
480			/**
481	dl	1.1	* Implements execution conventions for CountedCompleters
482			*/
483			protected final boolean exec() {
484			compute();
485			return false;
486			}
487
488			/**
489	dl	1.3	* Returns the result of the computation. By default
490			* returns {@code null}, which is appropriate for {@code Void}
491			* actions, but in other cases should be overridden.
492	dl	1.1	*
493	dl	1.3	* @return the result of the computation
494	dl	1.1	*/
495	dl	1.3	public T getRawResult() { return null; }
496	dl	1.1
497			/**
498	dl	1.3	* A method that result-bearing CountedCompleters may optionally
499			* use to help maintain result data. By default, does nothing.
500	dl	1.1	*/
501	dl	1.3	protected void setRawResult(T t) { }
502	dl	1.1
503			// Unsafe mechanics
504			private static final sun.misc.Unsafe U;
505			private static final long PENDING;
506			static {
507			try {
508			U = getUnsafe();
509			PENDING = U.objectFieldOffset
510			(CountedCompleter.class.getDeclaredField("pending"));
511			} catch (Exception e) {
512			throw new Error(e);
513			}
514			}
515
516			/**
517			* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
518			* Replace with a simple call to Unsafe.getUnsafe when integrating
519			* into a jdk.
520			*
521			* @return a sun.misc.Unsafe
522			*/
523			private static sun.misc.Unsafe getUnsafe() {
524			try {
525			return sun.misc.Unsafe.getUnsafe();
526			} catch (SecurityException se) {
527			try {
528			return java.security.AccessController.doPrivileged
529			(new java.security
530			.PrivilegedExceptionAction<sun.misc.Unsafe>() {
531			public sun.misc.Unsafe run() throws Exception {
532			java.lang.reflect.Field f = sun.misc
533			.Unsafe.class.getDeclaredField("theUnsafe");
534			f.setAccessible(true);
535			return (sun.misc.Unsafe) f.get(null);
536			}});
537			} catch (java.security.PrivilegedActionException e) {
538			throw new RuntimeException("Could not initialize intrinsics",
539			e.getCause());
540			}
541			}
542			}
543
544			}