package org.infinispan.util.concurrent.jdk8backported;

/*
 * 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/
 */

import java.util.ArrayList;
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;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;

/**
 * An {@link ExecutorService} for running {@link ForkJoinTask}s.
 * A {@code ForkJoinPool} provides the entry point for submissions
 * from non-{@code ForkJoinTask} clients, as well as management and
 * monitoring operations.
 *
 * <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 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
 * synchronization accommodated.
 *
 * <p>In addition to execution and lifecycle control methods, this
 * class provides status check methods (for example
 * {@link #getStealCount}) that are intended to aid in developing,
 * tuning, and monitoring fork/join applications. Also, method
 * {@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 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 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>
 *    <td></td>
 *    <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td>
 *    <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td>
 *  </tr>
 *  <tr>
 *    <td> <b>Arrange async execution</td>
 *    <td> {@link #execute(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#fork}</td>
 *  </tr>
 *  <tr>
 *    <td> <b>Await and obtain result</td>
 *    <td> {@link #invoke(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#invoke}</td>
 *  </tr>
 *  <tr>
 *    <td> <b>Arrange exec and obtain Future</td>
 *    <td> {@link #submit(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
 *  </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> {@code
 * static final ForkJoinPool mainPool = new ForkJoinPool();
 * ...
 * public void sort(long[] array) {
 *   mainPool.invoke(new SortTask(array, 0, array.length));
 * }}</pre>
 *
 * <p><b>Implementation notes</b>: This implementation restricts the
 * maximum number of running threads to 32767. Attempts to create
 * pools with greater than the maximum number result in
 * {@code IllegalArgumentException}.
 *
 * <p>This implementation rejects submitted tasks (that is, by throwing
 * {@link RejectedExecutionException}) only when the pool is shut down
 * or internal resources have been exhausted.
 *
 * @since 1.7
 * @author Doug Lea
 */
public class ForkJoinPool extends AbstractExecutorService {

   /*
   * Implementation Overview
   *
   * 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 loosely 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 never
   * take tasks, and they are multiplexed on to a finite number of
   * shared work queues. However, classes are set up so that future
   * extensions could allow submitters to optionally help perform
   * tasks as well. 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 runState), 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 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 "runState" contains 32 bits needed to register and
   * deregister WorkQueues, as well as to enable shutdown. It is
   * only modified under a lock (normally briefly held, but
   * occasionally protecting allocations and resizings) but even
   * when locked remains available to check consistency.
   *
   * Recording WorkQueues.  WorkQueues are recorded in the
   * "workQueues" array that is created upon pool construction 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. Shared (submission) queues
   * are at even indices, worker queues at odd indices. 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. 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 (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 fewer than two 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;
   * together these cover the signals needed in cases when more
   * tasks are pushed but untaken, and improve performance compared
   * to having one thread wake up all workers.
   *
   * 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.
   *
   * Shutdown and Termination. A call to shutdownNow atomically sets
   * a runState bit and then (non-atomically) sets each worker's
   * runState 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.
   *
   *   Compensating: Unless there are already enough live threads,
   *      method tryCompensate() may create or re-activate a spare
   *      thread to compensate for blocked joiners until they unblock.
   *
   * A third form (implemented in tryRemoveAndExec and
   * tryPollForAndExec) 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.  A stealHint 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.
   *
   * 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, 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: (1) We only try
   * compensation after attempting enough helping steps (measured
   * via counting and timing) that we have already consumed the
   * estimated cost of creating and activating a new thread.  (2) We
   * allow up to 50% of threads to be blocked before initially
   * adding any others, and unless completely saturated, check that
   * some work is available for a new worker before adding. Also, we
   * create up to only 50% more threads until entering a mode that
   * only adds a thread if all others are possibly blocked.  All
   * together, this means that we might be half as fast to react,
   * and create half as many threads as possible in the ideal case,
   * but present vastly fewer anomalies in all other cases compared
   * to both more aggressive and more conservative alternatives.
   *
   * 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. 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 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.
    * A {@code ForkJoinWorkerThreadFactory} must be defined and used
    * for {@code ForkJoinWorkerThread} subclasses that extend base
    * functionality or initialize threads with different contexts.
    */
   public static interface ForkJoinWorkerThreadFactory {
      /**
       * Returns a new worker thread operating in the given pool.
       *
       * @param pool the pool this thread works in
       * @throws NullPointerException if the pool is null
       */
      public ForkJoinWorkerThread newThread(ForkJoinPool pool);
   }

   /**
    * Default ForkJoinWorkerThreadFactory implementation; creates a
    * new ForkJoinWorkerThread.
    */
   static class DefaultForkJoinWorkerThreadFactory
         implements ForkJoinWorkerThreadFactory {
      public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
         return new ForkJoinWorkerThread(pool);
      }
   }

   /**
    * A simple non-reentrant lock used for exclusion when managing
    * queues and workers. We use a custom lock so that we can readily
    * probe lock state in constructions that check among alternative
    * actions. The lock is normally only very briefly held, and
    * sometimes treated as a spinlock, but other usages block to
    * reduce overall contention in those cases where locked code
    * bodies perform allocation/resizing.
    */
   static final class Mutex extends AbstractQueuedSynchronizer {
      public final boolean tryAcquire(int ignore) {
         return compareAndSetState(0, 1);
      }
      public final boolean tryRelease(int ignore) {
         setState(0);
         return true;
      }
      public final void lock() { acquire(0); }
      public final void unlock() { release(0); }
      public final boolean isHeldExclusively() { return getState() == 1; }
      public final Condition newCondition() { return new ConditionObject(); }
   }

   /**
    * 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> {
      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 trySharedPush, 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 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. (Although we can avoid one case
    * of this when locked in trySharedPush.) 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 (via method growArray) 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;

      /**
       * 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

      volatile long totalSteals; // cumulative number of steals
      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 rescans;               // remaining scans until block
      int nsteals;               // top-level task executions since last idle
      final int mode;            // lifo, fifo, or shared
      int poolIndex;             // index of this queue in pool (or 0)
      int stealHint;             // index of most recent known stealer
      volatile int runState;     // 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
      // Heuristic padding to ameliorate unfortunate memory placements
      Object p00, p01, p02, p03, p04, p05, p06, p07;
      Object p08, p09, p0a, p0b, p0c, p0d, p0e;

      WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner, int mode) {
         this.mode = mode;
         this.pool = pool;
         this.owner = owner;
         // Place indices in the center of array (that is not yet allocated)
         base = top = INITIAL_QUEUE_CAPACITY >>> 1;
      }

      /**
       * 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.getObjectVolatile
                                           (a, ((m & (s - 1)) << ASHIFT) + ABASE) == null)));
      }

      /**
       * Pushes a task. Call only by owner in unshared queues.
       *
       * @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
            U.putOrderedObject
                  (a, (((m = a.length - 1) & s) << ASHIFT) + ABASE, task);
            if ((n = (top = s + 1) - base) <= 2) {
               if ((p = pool) != null)
                  p.signalWork();
            }
            else if (n >= m)
               growArray(true);
         }
      }

      /**
       * Pushes a task if lock is free and array is either big
       * enough or can be resized to be big enough.
       *
       * @param task the task. Caller must ensure non-null.
       * @return true if submitted
       */
      final boolean trySharedPush(ForkJoinTask<?> task) {
         boolean submitted = false;
         if (runState == 0 && U.compareAndSwapInt(this, RUNSTATE, 0, 1)) {
            ForkJoinTask<?>[] a = array;
            int s = top;
            try {
               if ((a != null && a.length > s + 1 - base) ||
                     (a = growArray(false)) != null) { // must presize
                  int j = (((a.length - 1) & s) << ASHIFT) + ABASE;
                  U.putObject(a, (long)j, task);    // don't need "ordered"
                  top = s + 1;
                  submitted = true;
               }
            } finally {
               runState = 0;                         // unlock
            }
         }
         return submitted;
      }

      /**
       * Takes next task, if one exists, in LIFO order.  Call only
       * by owner in unshared queues. (We do not have a shared
       * version of this method because it is never needed.)
       */
      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
            }
         }
         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.
       */
      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;
      }

      /**
       * Polls the given task only if it is at the current base.
       */
      final boolean pollFor(ForkJoinTask<?> task) {
         ForkJoinTask<?>[] a; int b;
         if ((b = base) - top < 0 && (a = array) != null) {
            int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
            if (U.getObjectVolatile(a, j) == task && base == b &&
                  U.compareAndSwapObject(a, j, task, null)) {
               base = b + 1;
               return true;
            }
         }
         return false;
      }

      /**
       * 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.
       *
       * @param rejectOnFailure if true, throw exception if capacity
       * exceeded (relayed ultimately to user); else return null.
       */
      final ForkJoinTask<?>[] growArray(boolean rejectOnFailure) {
         ForkJoinTask<?>[] oldA = array;
         int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
         if (size <= MAXIMUM_QUEUE_CAPACITY) {
            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;
         }
         else if (!rejectOnFailure)
            return null;
         else
            throw new RejectedExecutionException("Queue capacity exceeded");
      }

      /**
       * 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;
      }

      // 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) immediately on any CAS
       * or consistency check failure so caller can retry.
       *
       * @return 0 if no progress can be made, else positive
       * (this unusual convention simplifies use with tryHelpStealer.)
       */
      final int tryRemoveAndExec(ForkJoinTask<?> task) {
         int stat = 1;
         boolean 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 = 0;
                  break;
               }
            }
         }
         if (removed)
            task.doExec();
         return stat;
      }

      /**
       * Executes a top-level task and any local tasks remaining
       * after execution.
       */
      final void runTask(ForkJoinTask<?> t) {
         if (t != null) {
            currentSteal = t;
            t.doExec();
            if (top != base) {       // process remaining local tasks
               if (mode == 0)
                  popAndExecAll();
               else
                  pollAndExecAll();
            }
            ++nsteals;
            currentSteal = null;
         }
      }

      /**
       * Executes a non-top-level (stolen) task.
       */
      final void runSubtask(ForkJoinTask<?> t) {
         if (t != null) {
            ForkJoinTask<?> ps = currentSteal;
            currentSteal = t;
            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);
      }

      /**
       * If this owned and is not already interrupted, try to
       * interrupt and/or unpark, ignoring exceptions.
       */
      final void interruptOwner() {
         Thread wt, p;
         if ((wt = owner) != null && !wt.isInterrupted()) {
            try {
               wt.interrupt();
            } catch (SecurityException ignore) {
            }
         }
         if ((p = parker) != null)
            U.unpark(p);
      }

      // Unsafe mechanics
      private static final sun.misc.Unsafe U;
      private static final long RUNSTATE;
      private static final int ABASE;
      private static final int ASHIFT;
      static {
         int s;
         try {
            U = getUnsafe();
            Class<?> k = WorkQueue.class;
            Class<?> ak = ForkJoinTask[].class;
            RUNSTATE = U.objectFieldOffset
                  (k.getDeclaredField("runState"));
            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);
      }
   }

   /**
    * 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 doSubmit. 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. This is done during
    * registration for workers, but requires a separate AtomicInteger
    * for submitters. Seeds are then randomly modified upon
    * collisions using xorshifts, which requires a non-zero seed.
    */
   static final class Submitter {
      int seed;
      Submitter() {
         int s = nextSubmitterSeed.getAndAdd(SEED_INCREMENT);
         seed = (s == 0) ? 1 : s; // ensure non-zero
      }
   }

   /** ThreadLocal class for Submitters */
   static final class ThreadSubmitter extends ThreadLocal<Submitter> {
      public Submitter initialValue() { return new Submitter(); }
   }

   // static fields (initialized in static initializer below)

   /**
    * Creates a new ForkJoinWorkerThread. This factory is used unless
    * overridden in ForkJoinPool constructors.
    */
   public static final ForkJoinWorkerThreadFactory
         defaultForkJoinWorkerThreadFactory;

   /**
    * Generator for assigning sequence numbers as pool names.
    */
   private static final AtomicInteger poolNumberGenerator;

   /**
    * Generator for initial hashes/seeds for submitters. Accessed by
    * Submitter class constructor.
    */
   static final AtomicInteger nextSubmitterSeed;

   /**
    * Permission required for callers of methods that may start or
    * kill threads.
    */
   private static final RuntimePermission modifyThreadPermission;

   /**
    * 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.
    */
   private static final ThreadSubmitter submitters;

   // static constants

   /**
    * 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.
    */
   private static final long SHRINK_RATE =
         4L * 1000L * 1000L * 1000L; // 4 seconds

   /**
    * The timeout value for attempted shrinkage, includes
    * some slop to cope with system timer imprecision.
    */
   private static final long SHRINK_TIMEOUT = SHRINK_RATE - (SHRINK_RATE / 10);

   /**
    * The maximum stolen->joining link depth allowed in method
    * tryHelpStealer.  Must be a power of two. This value also
    * controls the maximum number of times to try to help join a task
    * without any apparent progress or change in pool state before
    * giving up and blocking (see awaitJoin).  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 static final int MAX_HELP = 64;

   /**
    * Secondary time-based bound (in nanosecs) for helping attempts
    * before trying compensated blocking in awaitJoin. Used in
    * conjunction with MAX_HELP to reduce variance due to different
    * polling rates associated with different helping options. The
    * value should roughly approximate the time required to create
    * and/or activate a worker thread.
    */
   private static final long COMPENSATION_DELAY = 1L << 18; // ~0.25 millisec

   /**
    * Increment for seed generators. See class ThreadLocal for
    * explanation.
    */
   private static final int SEED_INCREMENT = 0x61c88647;

   /**
    * 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 (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 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 =
    * (int)ctl.  The ec field is never accessed alone, but always
    * together with id and st. The offsets of counts by the target
    * 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, 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 runState is an int packed with:
    * SHUTDOWN: true if shutdown is enabled (1 bit)
    * SEQ:  a sequence number updated upon (de)registering workers (30 bits)
    * INIT: set true after workQueues array construction (1 bit)
    *
    * The sequence number enables simple consistency checks:
    * Staleness of read-only operations on the workQueues array can
    * be checked by comparing runState before vs after the reads.
    */

   // bit positions/shifts for fields
   private static final int  AC_SHIFT   = 48;
   private static final int  TC_SHIFT   = 32;
   private static final int  ST_SHIFT   = 31;
   private static final int  EC_SHIFT   = 16;

   // bounds
   private static final int  SMASK      = 0xffff;  // short bits
   private static final int  MAX_CAP    = 0x7fff;  // max #workers - 1
   private static final int  SQMASK     = 0xfffe;  // even short bits
   private static final int  SHORT_SIGN = 1 << 15;
   private static final int  INT_SIGN   = 1 << 31;

   // masks
   private static final long STOP_BIT   = 0x0001L << ST_SHIFT;
   private static final long AC_MASK    = ((long)SMASK) << AC_SHIFT;
   private static final long TC_MASK    = ((long)SMASK) << TC_SHIFT;

   // units for incrementing and decrementing
   private static final long TC_UNIT    = 1L << TC_SHIFT;
   private static final long AC_UNIT    = 1L << AC_SHIFT;

   // masks and units for dealing with u = (int)(ctl >>> 32)
   private static final int  UAC_SHIFT  = AC_SHIFT - 32;
   private static final int  UTC_SHIFT  = TC_SHIFT - 32;
   private static final int  UAC_MASK   = SMASK << UAC_SHIFT;
   private static final int  UTC_MASK   = SMASK << UTC_SHIFT;
   private static final int  UAC_UNIT   = 1 << UAC_SHIFT;
   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 E_SEQ       = 1 << EC_SHIFT;

   // runState bits
   private static final int SHUTDOWN    = 1 << 31;

   // access mode for WorkQueue
   static final int LIFO_QUEUE          =  0;
   static final int FIFO_QUEUE          =  1;
   static final int SHARED_QUEUE        = -1;

   // Instance fields

   /*
   * Field layout order in 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 long ctl;                         // main pool control
   final int parallelism;                     // parallelism level
   final int localMode;                       // per-worker scheduling mode
   final int submitMask;                      // submit queue index bound
   int nextSeed;                              // for initializing worker seeds
   volatile int runState;                     // shutdown status and seq
   WorkQueue[] workQueues;                    // main registry
   final Mutex lock;                          // for registration
   final Condition termination;               // for awaitTermination
   final ForkJoinWorkerThreadFactory factory; // factory for new workers
   final Thread.UncaughtExceptionHandler ueh; // per-worker UEH
   final AtomicLong stealCount;               // collect counts when terminated
   final AtomicInteger nextWorkerNumber;      // to create worker name string
   final String workerNamePrefix;             // to create worker name string

   //  Creating, registering, and deregistering workers

   /**
    * Tries to create and start a worker
    */
   private void addWorker() {
      Throwable ex = null;
      ForkJoinWorkerThread wt = null;
      try {
         if ((wt = factory.newThread(this)) != null) {
            wt.start();
            return;
         }
      } catch (Throwable e) {
         ex = e;
      }
      deregisterWorker(wt, ex); // adjust counts etc on failure
   }

   /**
    * Callback from ForkJoinWorkerThread constructor to assign a
    * public name. This must be separate from registerWorker because
    * it is called during the "super" constructor call in
    * ForkJoinWorkerThread.
    */
   final String nextWorkerName() {
      return workerNamePrefix.concat
            (Integer.toString(nextWorkerNumber.addAndGet(1)));
   }

   /**
    * Callback from ForkJoinWorkerThread constructor to establish its
    * poolIndex 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 w the worker's queue
    */

   final void registerWorker(WorkQueue w) {
      Mutex lock = this.lock;
      lock.lock();
      try {
         WorkQueue[] ws = workQueues;
         if (w != null && ws != null) {          // skip on shutdown/failure
            int rs, n = ws.length, m = n - 1;
            int s = nextSeed += SEED_INCREMENT; // rarely-colliding sequence
            w.seed = (s == 0) ? 1 : s;          // ensure non-zero seed
            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) & SQMASK) + 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;     // establish before recording
            ws[r] = w;                          // also update seq
            runState = ((rs = runState) & SHUTDOWN) | ((rs + 2) & ~SHUTDOWN);
         }
      } finally {
         lock.unlock();
      }
   }

   /**
    * Final callback from terminating worker, as well as upon failure
    * to construct or start a worker in addWorker.  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 addWorker failed
    * @param ex the exception causing failure, or null if none
    */
   final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
      Mutex lock = this.lock;
      WorkQueue w = null;
      if (wt != null && (w = wt.workQueue) != null) {
         w.runState = -1;                // ensure runState is set
         stealCount.getAndAdd(w.totalSteals + w.nsteals);
         int idx = w.poolIndex;
         lock.lock();
         try {                           // remove record from array
            WorkQueue[] ws = workQueues;
            if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w)
               ws[idx] = null;
         } finally {
            lock.unlock();
         }
      }

      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.cancelAll();                  // cancel remaining tasks
         if (w.array != null)            // suppress signal if never ran
            signalWork();               // wake up or create replacement
         if (ex == null)                 // help clean refs on way out
            ForkJoinTask.helpExpungeStaleExceptions();
      }

      if (ex != null)                     // rethrow
         U.throwException(ex);
   }


   // Submissions

   /**
    * Unless shutting down, adds the given task to a submission queue
    * at submitter's current queue index (modulo submission
    * range). If no queue exists at the index, one is created.  If
    * the queue is busy, another index is randomly chosen. The
    * submitMask bounds the effective number of queues to the
    * (nearest power of two for) parallelism level.
    *
    * @param task the task. Caller must ensure non-null.
    */
   private void doSubmit(ForkJoinTask<?> task) {
      Submitter s = submitters.get();
      for (int r = s.seed, m = submitMask;;) {
         WorkQueue[] ws; WorkQueue q;
         int k = r & m & SQMASK;          // use only even indices
         if (runState < 0 || (ws = workQueues) == null || ws.length <= k)
            throw new RejectedExecutionException(); // shutting down
         else if ((q = ws[k]) == null) {  // create new queue
            WorkQueue nq = new WorkQueue(this, null, SHARED_QUEUE);
            Mutex lock = this.lock;      // construct outside lock
            lock.lock();
            try {                        // recheck under lock
               int rs = runState;       // to update seq
               if (ws == workQueues && ws[k] == null) {
                  ws[k] = nq;
                  runState = ((rs & SHUTDOWN) | ((rs + 2) & ~SHUTDOWN));
               }
            } finally {
               lock.unlock();
            }
         }
         else if (q.trySharedPush(task)) {
            signalWork();
            return;
         }
         else if (m > 1) {                // move to a different index
            r ^= r << 13;                // same xorshift as WorkQueues
            r ^= r >>> 17;
            s.seed = r ^= r << 5;
         }
         else
            Thread.yield();              // yield if no alternatives
      }
   }

   // 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 activate or create a worker if too few are active.
    */
   final void signalWork() {
      long c; int u;
      while ((u = (int)((c = ctl) >>> 32)) < 0) {     // too few active
         WorkQueue[] ws = workQueues; int e, i; WorkQueue w; Thread p;
         if ((e = (int)c) > 0) {                     // at least one waiting
            if (ws != null && (i = e & SMASK) < ws.length &&
                  (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.eventCount = (e + E_SEQ) & E_MASK;
                  if ((p = w.parker) != null)
                     U.unpark(p);                // activate and release
                  break;
               }
            }
            else
               break;
         }
         else if (e == 0 && (u & SHORT_SIGN) != 0) { // too few total
            long nc = (long)(((u + UTC_UNIT) & UTC_MASK) |
                                   ((u + UAC_UNIT) & UAC_MASK)) << 32;
            if (U.compareAndSwapLong(this, CTL, c, nc)) {
               addWorker();
               break;
            }
         }
         else
            break;
      }
   }

   // Scanning for tasks

   /**
    * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
    */
   final void runWorker(WorkQueue w) {
      w.growArray(false);         // initialize queue array in this thread
      do { w.runTask(scan(w)); } while (w.runState >= 0);
   }

   /**
    * 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 a random permutation of
    * queues (starting at a random index and stepping by a random
    * relative prime, checking each at least once).  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.  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 a complete sweep, try to release a waiting worker.  If
    * the scan terminated because the worker is inactivated, then the
    * released worker will often be the calling worker, and it can
    * succeed obtaining a task on the next call. Or maybe it is
    * another worker, but with same net effect. Releasing in other
    * cases as well ensures that we have enough workers running.
    *
    * * 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 check for
    * termination and possibly shrink pool.
    *
    * * If already inactive, and the caller has run a task since the
    * last empty scan, return (to allow rescan) unless others are
    * also inactivated.  Field WorkQueue.rescans counts down on each
    * scan to ensure eventual inactivation and blocking.
    *
    * * If already enqueued and none of the above apply, park
    * awaiting signal,
    *
    * @param w the worker (via its WorkQueue)
    * @return a task or null of none found
    */
   private final ForkJoinTask<?> scan(WorkQueue w) {
      WorkQueue[] ws;                       // first update random seed
      int r = w.seed; r ^= r << 13; r ^= r >>> 17; w.seed = r ^= r << 5;
      int rs = runState, m;                 // volatile read order matters
      if ((ws = workQueues) != null && (m = ws.length - 1) > 0) {
         int ec = w.eventCount;            // ec is negative if inactive
         int step = (r >>> 16) | 1;        // relative prime
         for (int j = (m + 1) << 2; ; r += step) {
            WorkQueue q; ForkJoinTask<?> t; ForkJoinTask<?>[] a; int b;
            if ((q = ws[r & m]) != null && (b = q.base) - q.top < 0 &&
                  (a = q.array) != null) {  // probably nonempty
               int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
               t = (ForkJoinTask<?>)U.getObjectVolatile(a, i);
               if (q.base == b && ec >= 0 && t != null &&
                     U.compareAndSwapObject(a, i, t, null)) {
                  if (q.top - (q.base = b + 1) > 1)
                     signalWork();    // help pushes signal
                  return t;
               }
               else if (ec < 0 || j <= m) {
                  rs = 0;               // mark scan as imcomplete
                  break;                // caller can retry after release
               }
            }
            if (--j < 0)
               break;
         }

         long c = ctl; int e = (int)c, a = (int)(c >> AC_SHIFT), nr, ns;
         if (e < 0)                        // decode ctl on empty scan
            w.runState = -1;              // pool is terminating
         else if (rs == 0 || rs != runState) { // incomplete scan
            WorkQueue v; Thread p;        // try to release a waiter
            if (e > 0 && a < 0 && w.eventCount == ec &&
                  (v = ws[e & m]) != null && v.eventCount == (e | INT_SIGN)) {
               long nc = ((long)(v.nextWait & E_MASK) |
                                ((c + AC_UNIT) & (AC_MASK|TC_MASK)));
               if (ctl == c && U.compareAndSwapLong(this, CTL, c, nc)) {
                  v.eventCount = (e + E_SEQ) & E_MASK;
                  if ((p = v.parker) != null)
                     U.unpark(p);
               }
            }
         }
         else if (ec >= 0) {               // try to enqueue/inactivate
            long nc = (long)ec | ((c - AC_UNIT) & (AC_MASK|TC_MASK));
            w.nextWait = e;
            w.eventCount = ec | INT_SIGN; // mark as inactive
            if (ctl != c || !U.compareAndSwapLong(this, CTL, c, nc))
               w.eventCount = ec;        // unmark on CAS failure
            else {
               if ((ns = w.nsteals) != 0) {
                  w.nsteals = 0;        // set rescans if ran task
                  w.rescans = (a > 0) ? 0 : a + parallelism;
                  w.totalSteals += ns;
               }
               if (a == 1 - parallelism) // quiescent
                  idleAwaitWork(w, nc, c);
            }
         }
         else if (w.eventCount < 0) {      // already queued
            if ((nr = w.rescans) > 0) {   // continue rescanning
               int ac = a + parallelism;
               if (((w.rescans = (ac < nr) ? ac : nr - 1) & 3) == 0)
                  Thread.yield();       // yield before block
            }
            else {
               Thread.interrupted();     // clear status
               Thread wt = Thread.currentThread();
               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);
            }
         }
      }
      return null;
   }

   /**
    * 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 SHRINK_RATE
    * nanosecs.  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 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.eventCount < 0 && !tryTerminate(false, false) &&
            (int)prevCtl != 0 && !hasQueuedSubmissions() && ctl == currentCtl) {
         Thread wt = Thread.currentThread();
         Thread.yield();            // yield before block
         while (ctl == currentCtl) {
            long startTime = System.nanoTime();
            Thread.interrupted();  // timed variant of version in scan()
            U.putObject(wt, PARKBLOCKER, this);
            w.parker = wt;
            if (ctl == currentCtl)
               U.park(false, SHRINK_RATE);
            w.parker = null;
            U.putObject(wt, PARKBLOCKER, null);
            if (ctl != currentCtl)
               break;
            if (System.nanoTime() - startTime >= SHRINK_TIMEOUT &&
                  U.compareAndSwapLong(this, CTL, currentCtl, prevCtl)) {
               w.eventCount = (w.eventCount + E_SEQ) | E_MASK;
               w.runState = -1;   // shrink
               break;
            }
         }
      }
   }

   /**
    * 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;
               }
               if ((ws = workQueues) == null || (m = ws.length - 1) <= 0)
                  break restart;              // shutting down
               if ((v = ws[h = (j.stealHint | 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.stealHint = 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;
                     }
                  }
               }
            }
         }
      }
      return stat;
   }

   /**
    * If task is at base of some steal queue, steals and executes it.
    *
    * @param joiner the joining worker
    * @param task the task
    */
   private void tryPollForAndExec(WorkQueue joiner, ForkJoinTask<?> task) {
      WorkQueue[] ws;
      if ((ws = workQueues) != null) {
         for (int j = 1; j < ws.length && task.status >= 0; j += 2) {
            WorkQueue q = ws[j];
            if (q != null && q.pollFor(task)) {
               joiner.runSubtask(task);
               break;
            }
         }
      }
   }

   /**
    * 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 either
    * pool would become completely starved or: (at least half
    * starved, and fewer than 50% spares exist, and there is at least
    * one task apparently available). Even though the availability
    * check requires a full scan, it is worthwhile in reducing false
    * alarms.
    *
    * @param task if non-null, a task being waited for
    * @param blocker if non-null, a blocker being waited for
    * @return true if the caller can block, else should recheck and retry
    */
   final boolean tryCompensate(ForkJoinTask<?> task, ManagedBlocker blocker) {
      int pc = parallelism, e;
      long c = ctl;
      WorkQueue[] ws = workQueues;
      if ((e = (int)c) >= 0 && ws != null) {
         int u, a, ac, hc;
         int tc = (short)((u = (int)(c >>> 32)) >>> UTC_SHIFT) + pc;
         boolean replace = false;
         if ((a = u >> UAC_SHIFT) <= 0) {
            if ((ac = a + pc) <= 1)
               replace = true;
            else if ((e > 0 || (task != null &&
                                      ac <= (hc = pc >>> 1) && tc < pc + hc))) {
               WorkQueue w;
               for (int j = 0; j < ws.length; ++j) {
                  if ((w = ws[j]) != null && !w.isEmpty()) {
                     replace = true;
                     break;   // in compensation range and tasks available
                  }
               }
            }
         }
         if ((task == null || task.status >= 0) && // recheck need to block
               (blocker == null || !blocker.isReleasable()) && ctl == c) {
            if (!replace) {          // no compensation
               long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
               if (U.compareAndSwapLong(this, CTL, c, nc))
                  return true;
            }
            else if (e != 0) {       // release an idle worker
               WorkQueue w; Thread p; int i;
               if ((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 | INT_SIGN) &&
                        U.compareAndSwapLong(this, CTL, c, nc)) {
                     w.eventCount = (e + E_SEQ) & E_MASK;
                     if ((p = w.parker) != null)
                        U.unpark(p);
                     return true;
                  }
               }
            }
            else if (tc < MAX_CAP) { // create replacement
               long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
               if (U.compareAndSwapLong(this, CTL, c, nc)) {
                  addWorker();
                  return true;
               }
            }
         }
      }
      return false;
   }

   /**
    * Helps and/or blocks until the given task is done.
    *
    * @param joiner the joining worker
    * @param task the task
    * @return task status on exit
    */
   final int awaitJoin(WorkQueue joiner, ForkJoinTask<?> task) {
      int s;
      if ((s = task.status) >= 0) {
         ForkJoinTask<?> prevJoin = joiner.currentJoin;
         joiner.currentJoin = task;
         long startTime = 0L;
         for (int k = 0;;) {
            if ((s = (joiner.isEmpty() ?           // try to help
                            tryHelpStealer(joiner, task) :
                            joiner.tryRemoveAndExec(task))) == 0 &&
                  (s = task.status) >= 0) {
               if (k == 0) {
                  startTime = System.nanoTime();
                  tryPollForAndExec(joiner, task); // check uncommon case
               }
               else if ((k & (MAX_HELP - 1)) == 0 &&
                     System.nanoTime() - startTime >=
                           COMPENSATION_DELAY &&
                     tryCompensate(task, null)) {
                  if (task.trySetSignal()) {
                     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));
               }
            }
            if (s < 0 || (s = task.status) < 0) {
               joiner.currentJoin = prevJoin;
               break;
            }
            else if ((k++ & (MAX_HELP - 1)) == MAX_HELP >>> 1)
               Thread.yield();                     // for politeness
         }
      }
      return s;
   }

   /**
    * 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
    * @return task status on exit
    */
   final int helpJoinOnce(WorkQueue joiner, ForkJoinTask<?> task) {
      int s;
      while ((s = task.status) >= 0 &&
            (joiner.isEmpty() ?
                   tryHelpStealer(joiner, task) :
                   joiner.tryRemoveAndExec(task)) != 0)
         ;
      return s;
   }

   /**
    * 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.
    */
   private WorkQueue findNonEmptyStealQueue(WorkQueue w) {
      // Similar to loop in scan(), but ignoring submissions
      int r = w.seed; r ^= r << 13; r ^= r >>> 17; w.seed = r ^= r << 5;
      int step = (r >>> 16) | 1;
      for (WorkQueue[] ws;;) {
         int rs = runState, m;
         if ((ws = workQueues) == null || (m = ws.length - 1) < 1)
            return null;
         for (int j = (m + 1) << 2; ; r += step) {
            WorkQueue q = ws[((r << 1) | 1) & m];
            if (q != null && !q.isEmpty())
               return q;
            else if (--j < 0) {
               if (runState == rs)
                  return null;
               break;
            }
         }
      }
   }


   /**
    * 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();
         WorkQueue q = findNonEmptyStealQueue(w);
         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) + parallelism == 0) {
               do {} while (!U.compareAndSwapLong
                     (this, CTL, c = ctl, c + AC_UNIT));
               break;
            }
         }
      }
   }

   /**
    * Gets and removes a local or stolen task for the given worker.
    *
    * @return a task, if available
    */
   final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
      for (ForkJoinTask<?> t;;) {
         WorkQueue q; int b;
         if ((t = w.nextLocalTask()) != null)
            return t;
         if ((q = findNonEmptyStealQueue(w)) == null)
            return null;
         if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null)
            return t;
      }
   }

   /**
    * Returns the approximate (non-atomic) number of idle threads per
    * active thread to offset steal queue size for method
    * ForkJoinTask.getSurplusQueuedTaskCount().
    */
   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);
   }

   //  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 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, boolean enable) {
      Mutex lock = this.lock;
      for (long c;;) {
         if (((c = ctl) & STOP_BIT) != 0) {      // already terminating
            if ((short)(c >>> TC_SHIFT) == -parallelism) {
               lock.lock();                    // don't need try/finally
               termination.signalAll();        // signal when 0 workers
               lock.unlock();
            }
            return true;
         }
         if (runState >= 0) {                    // not yet enabled
            if (!enable)
               return false;
            lock.lock();
            runState |= SHUTDOWN;
            lock.unlock();
         }
         if (!now) {                             // check if idle & no tasks
            if ((int)(c >> AC_SHIFT) != -parallelism ||
                  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;
                  int n = ws.length;
                  for (int i = 0; i < n; ++i) {
                     if ((w = ws[i]) != null) {
                        w.runState = -1;
                        if (pass > 0) {
                           w.cancelAll();
                           if (pass > 1)
                              w.interruptOwner();
                        }
                     }
                  }
                  // 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.runState = -1;
                        if ((p = w.parker) != null)
                           U.unpark(p);
                     }
                  }
               }
            }
         }
      }
   }

   // Exported methods

   // Constructors

   /**
    * Creates a {@code ForkJoinPool} with parallelism equal to {@link
    * java.lang.Runtime#availableProcessors}, using the {@linkplain
    * #defaultForkJoinWorkerThreadFactory default thread factory},
    * no UncaughtExceptionHandler, and non-async LIFO processing mode.
    *
    * @throws SecurityException if a security manager exists and
    *         the caller is not permitted to modify threads
    *         because it does not hold {@link
    *         java.lang.RuntimePermission}{@code ("modifyThread")}
    */
   public ForkJoinPool() {
      this(Runtime.getRuntime().availableProcessors(),
           defaultForkJoinWorkerThreadFactory, null, false);
   }

   /**
    * Creates a {@code ForkJoinPool} with the indicated parallelism
    * level, the {@linkplain
    * #defaultForkJoinWorkerThreadFactory default thread factory},
    * no UncaughtExceptionHandler, and non-async LIFO processing mode.
    *
    * @param parallelism the parallelism level
    * @throws IllegalArgumentException if parallelism less than or
    *         equal to zero, or greater than implementation limit
    * @throws SecurityException if a security manager exists and
    *         the caller is not permitted to modify threads
    *         because it does not hold {@link
    *         java.lang.RuntimePermission}{@code ("modifyThread")}
    */
   public ForkJoinPool(int parallelism) {
      this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
   }

   /**
    * Creates a {@code ForkJoinPool} with the given parameters.
    *
    * @param parallelism the parallelism level. For default value,
    * use {@link java.lang.Runtime#availableProcessors}.
    * @param factory the factory for creating new threads. For default value,
    * use {@link #defaultForkJoinWorkerThreadFactory}.
    * @param handler the handler for internal worker threads that
    * terminate due to unrecoverable errors encountered while executing
    * tasks. For default value, use {@code null}.
    * @param asyncMode if true,
    * establishes local first-in-first-out scheduling mode for forked
    * tasks that are never joined. This mode may be more appropriate
    * than default locally stack-based mode in applications in which
    * worker threads only process event-style asynchronous tasks.
    * For default value, use {@code false}.
    * @throws IllegalArgumentException if parallelism less than or
    *         equal to zero, or greater than implementation limit
    * @throws NullPointerException if the factory is null
    * @throws SecurityException if a security manager exists and
    *         the caller is not permitted to modify threads
    *         because it does not hold {@link
    *         java.lang.RuntimePermission}{@code ("modifyThread")}
    */
   public ForkJoinPool(int parallelism,
                       ForkJoinWorkerThreadFactory factory,
                       Thread.UncaughtExceptionHandler handler,
                       boolean asyncMode) {
      checkPermission();
      if (factory == null)
         throw new NullPointerException();
      if (parallelism <= 0 || parallelism > MAX_CAP)
         throw new IllegalArgumentException();
      this.parallelism = parallelism;
      this.factory = factory;
      this.ueh = handler;
      this.localMode = asyncMode ? FIFO_QUEUE : LIFO_QUEUE;
      long np = (long)(-parallelism); // offset ctl counts
      this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
      // Use nearest power 2 for workQueues size. See Hackers Delight sec 3.2.
      int n = parallelism - 1;
      n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16;
      int size = (n + 1) << 1;        // #slots = 2*#workers
      this.submitMask = size - 1;     // room for max # of submit queues
      this.workQueues = new WorkQueue[size];
      this.termination = (this.lock = new Mutex()).newCondition();
      this.stealCount = new AtomicLong();
      this.nextWorkerNumber = new AtomicInteger();
      int pn = poolNumberGenerator.incrementAndGet();
      StringBuilder sb = new StringBuilder("ForkJoinPool-");
      sb.append(Integer.toString(pn));
      sb.append("-worker-");
      this.workerNamePrefix = sb.toString();
      lock.lock();
      this.runState = 1;              // set init flag
      lock.unlock();
   }

   // Execution methods

   /**
    * Performs the given task, returning its result upon completion.
    * If the computation encounters an unchecked Exception or Error,
    * it is rethrown as the outcome of this invocation.  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 current thread
    * as well as the thread actually encountering the exception;
    * minimally only the latter.
    *
    * @param task the task
    * @return the task's result
    * @throws NullPointerException if the task is null
    * @throws RejectedExecutionException if the task cannot be
    *         scheduled for execution
    */
   public <T> T invoke(ForkJoinTask<T> task) {
      if (task == null)
         throw new NullPointerException();
      doSubmit(task);
      return task.join();
   }

   /**
    * Arranges for (asynchronous) execution of the given task.
    *
    * @param task the task
    * @throws NullPointerException if the task is null
    * @throws RejectedExecutionException if the task cannot be
    *         scheduled for execution
    */
   public void execute(ForkJoinTask<?> task) {
      if (task == null)
         throw new NullPointerException();
      doSubmit(task);
   }

   // AbstractExecutorService methods

   /**
    * @throws NullPointerException if the task is null
    * @throws RejectedExecutionException if the task cannot be
    *         scheduled for execution
    */
   public void execute(Runnable task) {
      if (task == null)
         throw new NullPointerException();
      ForkJoinTask<?> job;
      if (task instanceof ForkJoinTask<?>) // avoid re-wrap
         job = (ForkJoinTask<?>) task;
      else
         job = new ForkJoinTask.AdaptedRunnableAction(task);
      doSubmit(job);
   }

   /**
    * Submits a ForkJoinTask for execution.
    *
    * @param task the task to submit
    * @return the task
    * @throws NullPointerException if the task is null
    * @throws RejectedExecutionException if the task cannot be
    *         scheduled for execution
    */
   public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
      if (task == null)
         throw new NullPointerException();
      doSubmit(task);
      return task;
   }

   /**
    * @throws NullPointerException if the task is null
    * @throws RejectedExecutionException if the task cannot be
    *         scheduled for execution
    */
   public <T> ForkJoinTask<T> submit(Callable<T> task) {
      ForkJoinTask<T> job = new ForkJoinTask.AdaptedCallable<T>(task);
      doSubmit(job);
      return job;
   }

   /**
    * @throws NullPointerException if the task is null
    * @throws RejectedExecutionException if the task cannot be
    *         scheduled for execution
    */
   public <T> ForkJoinTask<T> submit(Runnable task, T result) {
      ForkJoinTask<T> job = new ForkJoinTask.AdaptedRunnable<T>(task, result);
      doSubmit(job);
      return job;
   }

   /**
    * @throws NullPointerException if the task is null
    * @throws RejectedExecutionException if the task cannot be
    *         scheduled for execution
    */
   public ForkJoinTask<?> submit(Runnable task) {
      if (task == null)
         throw new NullPointerException();
      ForkJoinTask<?> job;
      if (task instanceof ForkJoinTask<?>) // avoid re-wrap
         job = (ForkJoinTask<?>) task;
      else
         job = new ForkJoinTask.AdaptedRunnableAction(task);
      doSubmit(job);
      return job;
   }

   /**
    * @throws NullPointerException       {@inheritDoc}
    * @throws RejectedExecutionException {@inheritDoc}
    */
   public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
      // 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) fs;

      boolean done = false;
      try {
         for (Callable<T> t : tasks) {
            ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
            doSubmit(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);
      }
   }

   /**
    * Returns the factory used for constructing new workers.
    *
    * @return the factory used for constructing new workers
    */
   public ForkJoinWorkerThreadFactory getFactory() {
      return factory;
   }

   /**
    * Returns the handler for internal worker threads that terminate
    * due to unrecoverable errors encountered while executing tasks.
    *
    * @return the handler, or {@code null} if none
    */
   public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() {
      return ueh;
   }

   /**
    * Returns the targeted parallelism level of this pool.
    *
    * @return the targeted parallelism level of this pool
    */
   public int getParallelism() {
      return parallelism;
   }

   /**
    * Returns the number of worker threads that have started but not
    * yet terminated.  The result returned by this method may differ
    * from {@link #getParallelism} when threads are created to
    * maintain parallelism when others are cooperatively blocked.
    *
    * @return the number of worker threads
    */
   public int getPoolSize() {
      return parallelism + (short)(ctl >>> TC_SHIFT);
   }

   /**
    * Returns {@code true} if this pool uses local first-in-first-out
    * scheduling mode for forked tasks that are never joined.
    *
    * @return {@code true} if this pool uses async mode
    */
   public boolean getAsyncMode() {
      return localMode != 0;
   }

   /**
    * Returns an estimate of the number of worker threads that are
    * not blocked waiting to join tasks or for other managed
    * synchronization. This method may overestimate the
    * number of running threads.
    *
    * @return the number of worker threads
    */
   public int getRunningThreadCount() {
      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;
   }

   /**
    * Returns an estimate of the number of threads that are currently
    * stealing or executing tasks. This method may overestimate the
    * number of active threads.
    *
    * @return the number of active threads
    */
   public int getActiveThreadCount() {
      int r = parallelism + (int)(ctl >> AC_SHIFT);
      return (r <= 0) ? 0 : r; // suppress momentarily negative values
   }

   /**
    * Returns {@code true} if all worker threads are currently idle.
    * An idle worker is one that cannot obtain a task to execute
    * because none are available to steal from other threads, and
    * there are no pending submissions to the pool. This method is
    * conservative; it might not return {@code true} immediately upon
    * idleness of all threads, but will eventually become true if
    * threads remain inactive.
    *
    * @return {@code true} if all threads are currently idle
    */
   public boolean isQuiescent() {
      return (int)(ctl >> AC_SHIFT) + parallelism == 0;
   }

   /**
    * Returns an estimate of the total number of tasks stolen from
    * one thread's work queue by another. The reported value
    * underestimates the actual total number of steals when the pool
    * is not quiescent. This value may be useful for monitoring and
    * tuning fork/join programs: in general, steal counts should be
    * high enough to keep threads busy, but low enough to avoid
    * overhead and contention across threads.
    *
    * @return the number of steals
    */
   public long getStealCount() {
      long count = stealCount.get();
      WorkQueue[] ws; WorkQueue w;
      if ((ws = workQueues) != null) {
         for (int i = 1; i < ws.length; i += 2) {
            if ((w = ws[i]) != null)
               count += w.totalSteals;
         }
      }
      return count;
   }

   /**
    * Returns an estimate of the total number of tasks currently held
    * in queues by worker threads (but not including tasks submitted
    * to the pool that have not begun executing). This value is only
    * an approximation, obtained by iterating across all threads in
    * the pool. This method may be useful for tuning task
    * granularities.
    *
    * @return the number of queued tasks
    */
   public long getQueuedTaskCount() {
      long count = 0;
      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;
   }

   /**
    * Returns an estimate of the number of tasks submitted to this
    * pool that have not yet begun executing.  This method may take
    * time proportional to the number of submissions.
    *
    * @return the number of queued submissions
    */
   public int getQueuedSubmissionCount() {
      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;
   }

   /**
    * Returns {@code true} if there are any tasks submitted to this
    * pool that have not yet begun executing.
    *
    * @return {@code true} if there are any queued submissions
    */
   public boolean hasQueuedSubmissions() {
      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;
   }

   /**
    * Removes and returns the next unexecuted submission if one is
    * available.  This method may be useful in extensions to this
    * class that re-assign work in systems with multiple pools.
    *
    * @return the next submission, or {@code null} if none
    */
   protected ForkJoinTask<?> pollSubmission() {
      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;
   }

   /**
    * Removes all available unexecuted submitted and forked tasks
    * from scheduling queues and adds them to the given collection,
    * without altering their execution status. These may include
    * artificially generated or wrapped tasks. This method is
    * designed to be invoked only when the pool is known to be
    * quiescent. Invocations at other times may not remove all
    * tasks. A failure encountered while attempting to add elements
    * to collection {@code c} may result in elements being in
    * neither, either or both collections when the associated
    * exception is thrown.  The behavior of this operation is
    * undefined if the specified collection is modified while the
    * operation is in progress.
    *
    * @param c the collection to transfer elements into
    * @return the number of elements transferred
    */
   protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
      int count = 0;
      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;
               }
            }
         }
      }
      return count;
   }

   /**
    * Returns a string identifying this pool, as well as its state,
    * including indications of run state, parallelism level, and
    * worker and task counts.
    *
    * @return a string identifying this pool, as well as its state
    */
   public String toString() {
      // Use a single pass through workQueues to collect counts
      long qt = 0L, qs = 0L; int rc = 0;
      long st = stealCount.get();
      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.totalSteals;
                  if (w.isApparentlyUnblocked())
                     ++rc;
               }
            }
         }
      }
      int pc = parallelism;
      int tc = pc + (short)(c >>> TC_SHIFT);
      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 = runState < 0 ? "Shutting down" : "Running";
      return super.toString() +
            "[" + level +
            ", parallelism = " + pc +
            ", size = " + tc +
            ", active = " + ac +
            ", running = " + rc +
            ", steals = " + st +
            ", tasks = " + qt +
            ", submissions = " + qs +
            "]";
   }

   /**
    * 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.
    *
    * @throws SecurityException if a security manager exists and
    *         the caller is not permitted to modify threads
    *         because it does not hold {@link
    *         java.lang.RuntimePermission}{@code ("modifyThread")}
    */
   public void shutdown() {
      checkPermission();
      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).
    *
    * @return an empty list
    * @throws SecurityException if a security manager exists and
    *         the caller is not permitted to modify threads
    *         because it does not hold {@link
    *         java.lang.RuntimePermission}{@code ("modifyThread")}
    */
   public List<Runnable> shutdownNow() {
      checkPermission();
      tryTerminate(true, true);
      return Collections.emptyList();
   }

   /**
    * Returns {@code true} if all tasks have completed following shut down.
    *
    * @return {@code true} if all tasks have completed following shut down
    */
   public boolean isTerminated() {
      long c = ctl;
      return ((c & STOP_BIT) != 0L &&
                    (short)(c >>> TC_SHIFT) == -parallelism);
   }

   /**
    * Returns {@code true} if the process of termination has
    * 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,
    * 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
    * they do, they must abort them on interrupt.)
    *
    * @return {@code true} if terminating but not yet terminated
    */
   public boolean isTerminating() {
      long c = ctl;
      return ((c & STOP_BIT) != 0L &&
                    (short)(c >>> TC_SHIFT) != -parallelism);
   }

   /**
    * Returns {@code true} if this pool has been shut down.
    *
    * @return {@code true} if this pool has been shut down
    */
   public boolean isShutdown() {
      return runState < 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.
    *
    * @param timeout the maximum time to wait
    * @param unit the time unit of the timeout argument
    * @return {@code true} if this executor terminated and
    *         {@code false} if the timeout elapsed before termination
    * @throws InterruptedException if interrupted while waiting
    */
   public boolean awaitTermination(long timeout, TimeUnit unit)
         throws InterruptedException {
      long nanos = unit.toNanos(timeout);
      final Mutex lock = this.lock;
      lock.lock();
      try {
         for (;;) {
            if (isTerminated())
               return true;
            if (nanos <= 0)
               return false;
            nanos = termination.awaitNanos(nanos);
         }
      } finally {
         lock.unlock();
      }
   }

   /**
    * Interface for extending managed parallelism for tasks running
    * in {@link ForkJoinPool}s.
    *
    * <p>A {@code ManagedBlocker} provides two methods.  Method
    * {@code isReleasable} must return {@code true} if blocking is
    * not necessary. Method {@code block} blocks the current thread
    * if necessary (perhaps internally invoking {@code isReleasable}
    * before actually blocking). These actions are performed by any
    * thread invoking {@link ForkJoinPool#managedBlock}.  The
    * unusual methods in this API accommodate synchronizers that may,
    * but don't usually, block for long periods. Similarly, they
    * allow more efficient internal handling of cases in which
    * additional workers may be, but usually are not, needed to
    * ensure sufficient parallelism.  Toward this end,
    * implementations of method {@code isReleasable} must be amenable
    * to repeated invocation.
    *
    * <p>For example, here is a ManagedBlocker based on a
    * ReentrantLock:
    *  <pre> {@code
    * class ManagedLocker implements ManagedBlocker {
    *   final ReentrantLock lock;
    *   boolean hasLock = false;
    *   ManagedLocker(ReentrantLock lock) { this.lock = lock; }
    *   public boolean block() {
    *     if (!hasLock)
    *       lock.lock();
    *     return true;
    *   }
    *   public boolean isReleasable() {
    *     return hasLock || (hasLock = lock.tryLock());
    *   }
    * }}</pre>
    *
    * <p>Here is a class that possibly blocks waiting for an
    * item on a given queue:
    *  <pre> {@code
    * class QueueTaker<E> implements ManagedBlocker {
    *   final BlockingQueue<E> queue;
    *   volatile E item = null;
    *   QueueTaker(BlockingQueue<E> q) { this.queue = q; }
    *   public boolean block() throws InterruptedException {
    *     if (item == null)
    *       item = queue.take();
    *     return true;
    *   }
    *   public boolean isReleasable() {
    *     return item != null || (item = queue.poll()) != null;
    *   }
    *   public E getItem() { // call after pool.managedBlock completes
    *     return item;
    *   }
    * }}</pre>
    */
   public static interface ManagedBlocker {
      /**
       * Possibly blocks the current thread, for example waiting for
       * a lock or condition.
       *
       * @return {@code true} if no additional blocking is necessary
       * (i.e., if isReleasable would return true)
       * @throws InterruptedException if interrupted while waiting
       * (the method is not required to do so, but is allowed to)
       */
      boolean block() throws InterruptedException;

      /**
       * Returns {@code true} if blocking is unnecessary.
       */
      boolean isReleasable();
   }

   /**
    * Blocks in accord with the given blocker.  If the current thread
    * is a {@link ForkJoinWorkerThread}, this method possibly
    * arranges for a spare thread to be activated if necessary to
    * ensure sufficient parallelism while the current thread is blocked.
    *
    * <p>If the caller is not a {@link ForkJoinTask}, this method is
    * behaviorally equivalent to
    *  <pre> {@code
    * while (!blocker.isReleasable())
    *   if (blocker.block())
    *     return;
    * }</pre>
    *
    * If the caller is a {@code ForkJoinTask}, then the pool may
    * first be expanded to ensure parallelism, and later adjusted.
    *
    * @param blocker the blocker
    * @throws InterruptedException if blocker.block did so
    */
   public static void managedBlock(ManagedBlocker blocker)
         throws InterruptedException {
      Thread t = Thread.currentThread();
      ForkJoinPool p = ((t instanceof ForkJoinWorkerThread) ?
                              ((ForkJoinWorkerThread)t).pool : null);
      while (!blocker.isReleasable()) {
         if (p == null || p.tryCompensate(null, blocker)) {
            try {
               do {} while (!blocker.isReleasable() && !blocker.block());
            } finally {
               if (p != null)
                  p.incrementActiveCount();
            }
            break;
         }
      }
   }

   // AbstractExecutorService overrides.  These rely on undocumented
   // fact that ForkJoinTask.adapt returns ForkJoinTasks that also
   // implement RunnableFuture.

   protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
      return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
   }

   protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
      return new ForkJoinTask.AdaptedCallable<T>(callable);
   }

   // Unsafe mechanics
   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;

   static {
      poolNumberGenerator = new AtomicInteger();
      nextSubmitterSeed = new AtomicInteger(0x55555555);
      modifyThreadPermission = new RuntimePermission("modifyThread");
      defaultForkJoinWorkerThreadFactory =
            new DefaultForkJoinWorkerThreadFactory();
      submitters = new ThreadSubmitter();
      int s;
      try {
         U = getUnsafe();
         Class<?> k = ForkJoinPool.class;
         Class<?> ak = ForkJoinTask[].class;
         CTL = U.objectFieldOffset
               (k.getDeclaredField("ctl"));
         Class<?> tk = Thread.class;
         PARKBLOCKER = U.objectFieldOffset
               (tk.getDeclaredField("parkBlocker"));
         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);
   }

   /**
    * 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());
         }
      }
   }

}