content: /*
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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  /*
   * This file is available under and governed by the GNU General Public
   * License version 2 only, as published by the Free Software Foundation.
   * However, the following notice accompanied the original version of this
   * file:
   *
   * Written by Doug Lea with assistance from members of JCP JSR-166
   * Expert Group and released to the public domain, as explained at
   * http://creativecommons.org/publicdomain/zero/1.0/
   */
  
  package java.util.concurrent;
  
  import java.lang.Thread.UncaughtExceptionHandler;
  import java.security.AccessControlContext;
  import java.security.Permissions;
  import java.security.ProtectionDomain;
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.List;
  import java.util.concurrent.locks.ReentrantLock;
  import java.util.concurrent.locks.LockSupport;
  
  /**
   * 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 static {@link #commonPool()} is available and appropriate for
   * most applications. The common pool is used by any ForkJoinTask that
   * is not explicitly submitted to a specified pool. Using the common
   * pool normally reduces resource usage (its threads are slowly
   * reclaimed during periods of non-use, and reinstated upon subsequent
   * use).
   *
   * <p>For applications that require separate or custom pools, a {@code
   * ForkJoinPool} may be constructed with a given target parallelism
   * level; by default, equal to the number of available processors.
   * The pool attempts to maintain enough active (or available) threads
   * by dynamically adding, suspending, or resuming internal worker
   * threads, even if some tasks are stalled waiting to join others.
   * However, no such adjustments are guaranteed in the face of blocked
   * I/O or other unmanaged synchronization. The nested {@link
   * ManagedBlocker} interface enables extension of the kinds of
   * synchronization accommodated.
   *
   * <p>In addition to execution and lifecycle control methods, this
   * 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>
   * <caption>Summary of task execution methods</caption>
   *  <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</b></td>
   *    <td> {@link #execute(ForkJoinTask)}</td>
   *    <td> {@link ForkJoinTask#fork}</td>
   *  </tr>
   *  <tr>
   *    <td> <b>Await and obtain result</b></td>
   *    <td> {@link #invoke(ForkJoinTask)}</td>
   *    <td> {@link ForkJoinTask#invoke}</td>
   *  </tr>
   *  <tr>
   *    <td> <b>Arrange exec and obtain Future</b></td>
   *    <td> {@link #submit(ForkJoinTask)}</td>
   *    <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
   *  </tr>
   * </table>
   *
   * <p>The common pool is by default constructed with default
   * parameters, but these may be controlled by setting three
   * {@linkplain System#getProperty system properties}:
   * <ul>
   * <li>{@code java.util.concurrent.ForkJoinPool.common.parallelism}
   * - the parallelism level, a non-negative integer
   * <li>{@code java.util.concurrent.ForkJoinPool.common.threadFactory}
   * - the class name of a {@link ForkJoinWorkerThreadFactory}
   * <li>{@code java.util.concurrent.ForkJoinPool.common.exceptionHandler}
   * - the class name of a {@link UncaughtExceptionHandler}
   * <li>{@code java.util.concurrent.ForkJoinPool.common.maximumSpares}
   * - the maximum number of allowed extra threads to maintain target
   * parallelism (default 256).
   * </ul>
   * If a {@link SecurityManager} is present and no factory is
   * specified, then the default pool uses a factory supplying
   * threads that have no {@link Permissions} enabled.
   * The system class loader is used to load these classes.
   * Upon any error in establishing these settings, default parameters
   * are used. It is possible to disable or limit the use of threads in
   * the common pool by setting the parallelism property to zero, and/or
   * using a factory that may return {@code null}. However doing so may
   * cause unjoined tasks to never be executed.
   *
   * <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
   */
  // Android-removed: @Contended, this hint is not used by the Android runtime.
  //@jdk.internal.vm.annotation.Contended
  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.  This framework began as vehicle for
       * supporting tree-structured parallelism using work-stealing.
       * Over time, its scalability advantages led to extensions and
       * changes to better support more diverse usage contexts.  Because
       * most internal methods and nested classes are interrelated,
       * their main rationale and descriptions are presented here;
       * individual methods and nested classes contain only brief
       * comments about details.
       *
       * 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.
       *
       * Adding tasks then takes the form of a classic array push(task)
       * in a circular buffer:
       *    q.array[q.top++ % length] = task;
       *
       * (The actual code needs to null-check and size-check the array,
       * uses masking, not mod, for indexing a power-of-two-sized array,
       * properly fences accesses, and possibly signals waiting workers
       * to start scanning -- see below.)  Both a successful pop and
       * poll mainly entail a CAS of a slot from non-null to null.
       *
       * The pop operation (always performed by owner) is:
       *   if ((the task at top slot is not null) and
       *        (CAS slot to null))
       *           decrement top and return task;
       *
       * And the poll operation (usually by a stealer) is
       *    if ((the task at base slot is not null) and
       *        (CAS slot to null))
       *           increment base and return task;
       *
       * There are several variants of each of these; for example most
       * versions of poll pre-screen the CAS by rechecking that the base
       * has not changed since reading the slot, and most methods only
       * attempt the CAS if base appears not to be equal to top.
       *
       * Memory ordering.  See "Correct and Efficient Work-Stealing for
       * Weak Memory Models" by Le, Pop, Cohen, and Nardelli, PPoPP 2013
       * (http://www.di.ens.fr/~zappa/readings/ppopp13.pdf) for an
       * analysis of memory ordering requirements in work-stealing
       * algorithms similar to (but different than) the one used here.
       * Extracting tasks in array slots via (fully fenced) CAS provides
       * primary synchronization. The base and top indices imprecisely
       * guide where to extract from. We do not always require strict
       * orderings of array and index updates, so sometimes let them be
       * subject to compiler and processor reorderings. However, the
       * volatile "base" index also serves as a basis for memory
       * ordering: Slot accesses are preceded by a read of base,
       * ensuring happens-before ordering with respect to stealers (so
       * the slots themselves can be read via plain array reads.)  The
       * only other memory orderings relied on are maintained in the
       * course of signalling and activation (see below).  A check that
       * base == top indicates (momentary) emptiness, but otherwise may
       * err on the side of possibly making the queue appear nonempty
       * when a push, pop, or poll have not fully committed, or making
       * it appear empty when an update of top has not yet been visibly
       * written.  (Method isEmpty() checks the case of a partially
       * completed removal of the last element.)  Because of this, 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) visibly completes.
       * However, in the aggregate, we ensure at least probabilistic
       * non-blockingness.  If an attempted steal fails, a scanning
       * thief 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, which
       * retries.)
       *
       * 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.
       *
       * WorkQueues are also used in a similar way for tasks submitted
       * to the pool. We cannot mix these tasks in the same queues used
       * by workers. Instead, we randomly associate submission queues
       * with submitting threads, using a form of hashing.  The
       * ThreadLocalRandom probe value serves as a hash code for
       * choosing existing queues, and may be randomly repositioned upon
       * contention with other submitters.  In essence, submitters act
       * like workers except that they are restricted to executing local
       * tasks that they submitted (or in the case of CountedCompleters,
       * others with the same root task).  Insertion of tasks in shared
       * mode requires a lock but we use only a simple spinlock (using
       * field qlock), because submitters encountering a busy queue move
       * on to try or create other queues -- they block only when
       * creating and registering new queues. Because it is used only as
       * a spinlock, unlocking requires only a "releasing" store (using
       * putOrderedInt).  The qlock is also used during termination
       * detection, in which case it is forced to a negative
       * non-lockable value.
       *
       * Management
       * ==========
       *
       * The main throughput advantages of work-stealing stem from
       * decentralized control -- workers mostly take tasks from
       * themselves or each other, at rates that can exceed a billion
       * per second.  The pool itself creates, activates (enables
       * scanning for and running tasks), deactivates, blocks, and
       * terminates threads, all with minimal central information.
       * There are only a few properties that we can globally track or
       * maintain, so we pack them into a small number of variables,
       * often maintaining atomicity without blocking or locking.
       * Nearly all essentially atomic control state is held in two
       * volatile variables that are by far most often read (not
       * written) as status and consistency checks. (Also, field
       * "config" holds unchanging configuration state.)
       *
       * Field "ctl" contains 64 bits holding 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
       * subfields.
       *
       * Field "runState" holds lifetime status, atomically and
       * monotonically setting STARTED, SHUTDOWN, STOP, and finally
       * TERMINATED bits.
       *
       * Field "auxState" is a ReentrantLock subclass that also
       * opportunistically holds some other bookkeeping fields accessed
       * only when locked.  It is mainly used to lock (infrequent)
       * updates to workQueues.  The auxState instance is itself lazily
       * constructed (see tryInitialize), requiring a double-check-style
       * bootstrapping use of field runState, and locking a private
       * static.
       *
       * Field "workQueues" holds references to WorkQueues.  It is
       * updated (only during worker creation and termination) under the
       * lock, but is otherwise concurrently readable, and accessed
       * directly. We also ensure that reads of the array reference
       * itself never become too stale (for example, re-reading before
       * each scan). To simplify index-based operations, the array size
       * is always a power of two, and all readers must tolerate null
       * slots. Worker queues are at odd indices. Shared (submission)
       * queues are at even indices, up to a maximum of 64 slots, to
       * limit growth even if array needs to expand to add more
       * workers. Grouping them together in this way simplifies and
       * speeds up task scanning.
       *
       * All worker thread creation is on-demand, triggered by task
       * submissions, replacement of terminated workers, and/or
       * compensation for blocked workers. However, all other support
       * code is set up to work with other policies.  To ensure that we
       * do not hold on to worker references that would prevent GC, all
       * accesses to workQueues are via indices into the workQueues
       * array (which is one source of some of the messy code
       * constructions here). In essence, the workQueues array serves as
       * a weak reference mechanism. Thus for example the stack top
       * subfield of ctl stores indices, not references.
       *
       * Queuing Idle Workers. 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, which is compounded at program start-up by JIT
       * compilation and allocation. So we streamline this as much as
       * possible.
       *
       * The "ctl" field atomically maintains active and total worker
       * counts as well as a queue to place waiting threads so they can
       * be located for signalling. Active counts also play the role of
       * quiescence indicators, so are decremented when workers believe
       * that there are no more tasks to execute. The "queue" is
       * actually a form of Treiber stack.  A stack is ideal for
       * activating threads in most-recently used order. This improves
       * performance and locality, outweighing the disadvantages of
       * being prone to contention and inability to release a worker
       * unless it is topmost on stack.  We block/unblock workers after
       * pushing on the idle worker stack (represented by the lower
       * 32bit subfield of ctl) when they cannot find work.  The top
       * stack state holds the value of the "scanState" field of the
       * worker: its index and status, plus a version counter that, in
       * addition to the count subfields (also serving as version
       * stamps) provide protection against Treiber stack ABA effects.
       *
       * Creating workers. To create a worker, we pre-increment total
       * count (serving as a reservation), and attempt to construct a
       * ForkJoinWorkerThread via its factory. Upon construction, the
       * new thread invokes registerWorker, where it constructs a
       * WorkQueue and is assigned an index in the workQueues array
       * (expanding the array if necessary). The thread is then started.
       * Upon any exception across these steps, or null return from
       * factory, deregisterWorker adjusts counts and records
       * accordingly.  If a null return, the pool continues running with
       * fewer than the target number workers. If exceptional, the
       * exception is propagated, generally to some external caller.
       * Worker index assignment avoids the bias in scanning that would
       * occur if entries were sequentially packed starting at the front
       * of the workQueues array. We treat the array as a simple
       * power-of-two hash table, expanding as needed. The seedIndex
       * increment ensures no collisions until a resize is needed or a
       * worker is deregistered and replaced, and thereafter keeps
       * probability of collision low. We cannot use
       * ThreadLocalRandom.getProbe() for similar purposes here because
       * the thread has not started yet, but do so for creating
       * submission queues for existing external threads (see
       * externalPush).
       *
       * WorkQueue field scanState is used by both workers and the pool
       * to manage and track whether a worker is UNSIGNALLED (possibly
       * blocked waiting for a signal).  When a worker is inactivated,
       * its scanState field is set, and is prevented from executing
       * tasks, even though it must scan once for them to avoid queuing
       * races. Note that scanState updates lag queue CAS releases so
       * usage requires care. When queued, the lower 16 bits of
       * scanState must hold its pool index. So we place the index there
       * upon initialization (see registerWorker) and otherwise keep it
       * there or restore it when necessary.
       *
       * The ctl field also serves as the basis for memory
       * synchronization surrounding activation. This uses a more
       * efficient version of a Dekker-like rule that task producers and
       * consumers sync with each other by both writing/CASing ctl (even
       * if to its current value).  This would be extremely costly. So
       * we relax it in several ways: (1) Producers only signal when
       * their queue is empty. Other workers propagate this signal (in
       * method scan) when they find tasks. (2) Workers only enqueue
       * after scanning (see below) and not finding any tasks.  (3)
       * Rather than CASing ctl to its current value in the common case
       * where no action is required, we reduce write contention by
       * equivalently prefacing signalWork when called by an external
       * task producer using a memory access with full-volatile
       * semantics or a "fullFence". (4) For internal task producers we
       * rely on the fact that even if no other workers awaken, the
       * producer itself will eventually see the task and execute it.
       *
       * Almost always, too many signals are issued. A task producer
       * cannot in general tell if some existing worker is in the midst
       * of finishing one task (or already scanning) and ready to take
       * another without being signalled. So the producer might instead
       * activate a different worker that does not find any work, and
       * then inactivates. This scarcely matters in steady-state
       * computations involving all workers, but can create contention
       * and bookkeeping bottlenecks during ramp-up, ramp-down, and small
       * computations involving only a few workers.
       *
       * Scanning. Method scan() performs top-level scanning for tasks.
       * Each scan traverses (and tries to poll from) each queue in
       * pseudorandom permutation order by randomly selecting an origin
       * index and a step value.  (The pseudorandom generator need not
       * have high-quality statistical properties in the long term, but
       * just within computations; We use 64bit and 32bit Marsaglia
       * XorShifts, which are cheap and suffice here.)  Scanning also
       * employs contention reduction: When scanning workers fail a CAS
       * polling for work, they soon restart with a different
       * pseudorandom scan order (thus likely retrying at different
       * intervals). This improves throughput when many threads are
       * trying to take tasks from few queues.  Scans do not otherwise
       * explicitly take into account core affinities, loads, cache
       * localities, etc, However, they do exploit temporal locality
       * (which usually approximates these) by preferring to re-poll (up
       * to POLL_LIMIT times) from the same queue after a successful
       * poll before trying others.  Restricted forms of scanning occur
       * in methods helpComplete and findNonEmptyStealQueue, and take
       * similar but simpler forms.
       *
       * Deactivation and waiting. Queuing encounters several intrinsic
       * races; most notably that an inactivating scanning worker can
       * miss seeing a task produced during a scan.  So when a worker
       * cannot find a task to steal, it inactivates and enqueues, and
       * then rescans to ensure that it didn't miss one, reactivating
       * upon seeing one with probability approximately proportional to
       * probability of a miss.  (In most cases, the worker will be
       * signalled before self-signalling, avoiding cascades of multiple
       * signals for the same task).
       *
       * Workers block (in method awaitWork) using park/unpark;
       * advertising the need for signallers to unpark by setting their
       * "parker" fields.
       *
       * 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 (see awaitWork) if the pool has remained
       * quiescent for period given by IDLE_TIMEOUT_MS, increasing the
       * period as the number of threads decreases, eventually removing
       * all workers.
       *
       * Shutdown and Termination. A call to shutdownNow invokes
       * tryTerminate to atomically set a runState bit. The calling
       * thread, as well as every other worker thereafter terminating,
       * helps terminate others by setting their (qlock) status,
       * cancelling their unprocessed tasks, and waking them up, doing
       * so repeatedly until stable. Calls to non-abrupt shutdown()
       * preface this by checking whether termination should commence.
       * This relies primarily on the active count bits of "ctl"
       * maintaining consensus -- tryTerminate is called from awaitWork
       * whenever quiescent. However, external submitters do not take
       * part in this consensus.  So, tryTerminate sweeps through queues
       * (until stable) to ensure lack of in-flight submissions and
       * workers about to process them before triggering the "STOP"
       * phase of termination. (Note: there is an intrinsic conflict if
       * helpQuiescePool is called when shutdown is enabled. Both wait
       * for quiescence, but tryTerminate is biased to not trigger until
       * helpQuiescePool completes.)
       *
       * 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 may 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) 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 helpStealer entails a form of "linear
       * helping".  Each worker records (in field currentSteal) the most
       * recent task it stole from some other worker (or a submission).
       * It also records (in field currentJoin) the task it is currently
       * actively joining. Method helpStealer 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.  Thus, the joiner executes a task that would be on its
       * own local deque had the to-be-joined task not been stolen. This
       * is a conservative variant of the approach described in Wagner &
       * Calder "Leapfrogging: a portable technique for implementing
       * efficient futures" SIGPLAN Notices, 1993
       * (http://portal.acm.org/citation.cfm?id=155354). It differs in
       * that: (1) We only maintain dependency links across workers upon
       * steals, rather than use per-task bookkeeping.  This sometimes
       * requires a linear scan of workQueues array to locate stealers,
       * but often doesn't because stealers leave hints (that may become
       * stale/wrong) of where to locate them.  It is only a hint
       * because a worker might have had multiple steals and the hint
       * records only one of them (usually the most current).  Hinting
       * isolates cost to when it is needed, rather than adding to
       * per-task overhead.  (2) It is "shallow", ignoring nesting and
       * potentially cyclic mutual steals.  (3) It is intentionally
       * racy: field currentJoin is updated only while actively joining,
       * which means that we miss links in the chain during long-lived
       * tasks, GC stalls etc (which is OK since blocking in such cases
       * is usually a good idea).  (4) We bound the number of attempts
       * to find work using checksums and fall back to suspending the
       * worker and if necessary replacing it with another.
       *
       * Helping actions for CountedCompleters do not require tracking
       * currentJoins: Method helpComplete takes and executes any task
       * with the same root as the task being waited on (preferring
       * local pops to non-local polls). However, this still entails
       * some traversal of completer chains, so is less efficient than
       * using CountedCompleters without explicit joins.
       *
       * Compensation does not aim to keep exactly the target
       * parallelism number of unblocked threads running at any given
       * time. Some previous versions of this class employed immediate
       * compensations for any blocked join. However, in practice, the
       * vast majority of blockages are transient byproducts of GC and
       * other JVM or OS activities that are made worse by replacement.
       * Currently, compensation is attempted only after validating that
       * all purportedly active threads are processing tasks by checking
       * field WorkQueue.scanState, which eliminates most false
       * positives.  Also, compensation is bypassed (tolerating fewer
       * threads) in the most common case in which it is rarely
       * beneficial: when a worker with an empty queue (thus no
       * continuation tasks) blocks on a join and there still remain
       * enough threads to ensure liveness.
       *
       * Spare threads are removed as soon as they notice that the
       * target parallelism level has been exceeded, in method
       * tryDropSpare. (Method scan arranges returns for rechecks upon
       * each probe via the "bound" parameter.)
       *
       * The compensation mechanism may be bounded.  Bounds for the
       * commonPool (see COMMON_MAX_SPARES) better enable JVMs to cope
       * with programming errors and abuse before running out of
       * resources to do so. In other cases, users may supply factories
       * that limit thread construction. The effects of bounding in this
       * pool (like all others) is imprecise.  Total worker counts are
       * decremented when threads deregister, not when they exit and
       * resources are reclaimed by the JVM and OS. So the number of
       * simultaneously live threads may transiently exceed bounds.
       *
       * Common Pool
       * ===========
       *
       * The static common pool always exists after static
       * initialization.  Since it (or any other created pool) need
       * never be used, we minimize initial construction overhead and
       * footprint to the setup of about a dozen fields, with no nested
       * allocation. Most bootstrapping occurs within method
       * externalSubmit during the first submission to the pool.
       *
       * When external threads submit to the common pool, they can
       * perform subtask processing (see externalHelpComplete and
       * related methods) upon joins.  This caller-helps policy makes it
       * sensible to set common pool parallelism level to one (or more)
       * less than the total number of available cores, or even zero for
       * pure caller-runs.  We do not need to record whether external
       * submissions are to the common pool -- if not, external help
       * methods return quickly. These submitters would otherwise be
       * blocked waiting for completion, so the extra effort (with
       * liberally sprinkled task status checks) in inapplicable cases
       * amounts to an odd form of limited spin-wait before blocking in
       * ForkJoinTask.join.
       *
       * As a more appropriate default in managed environments, unless
       * overridden by system properties, we use workers of subclass
       * InnocuousForkJoinWorkerThread when there is a SecurityManager
       * present. These workers have no permissions set, do not belong
       * to any user-defined ThreadGroup, and erase all ThreadLocals
       * after executing any top-level task (see WorkQueue.runTask).
       * The associated mechanics (mainly in ForkJoinWorkerThread) may
       * be JVM-dependent and must access particular Thread class fields
       * to achieve this effect.
       *
       * Style notes
       * ===========
       *
       * Memory ordering relies mainly on Unsafe intrinsics that carry
       * the further responsibility of explicitly performing null- and
       * bounds- checks otherwise carried out implicitly by JVMs.  This
       * can be awkward and ugly, but also reflects the need to control
       * outcomes across the unusual cases that arise in very racy code
       * with very few invariants. So these explicit checks would exist
       * in some form anyway.  All fields are read into locals before
       * use, and null-checked if they are references.  This is usually
       * done in a "C"-like style of listing declarations at the heads
       * of methods or blocks, and using inline assignments on first
       * encounter.  Array bounds-checks are usually performed by
       * masking with array.length-1, which relies on the invariant that
       * these arrays are created with positive lengths, which is itself
       * paranoically checked. Nearly all explicit checks lead to
       * bypass/return, not exception throws, because they may
       * legitimately arise due to cancellation/revocation during
       * shutdown.
       *
       * 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 fields held in
       * local variables.  There are also other coding oddities
       * (including several unnecessary-looking hoisted null checks)
       * that help some methods perform reasonably even when interpreted
       * (not compiled).
       *
       * The order of declarations in this file is (with a few exceptions):
       * (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
           * @return the new worker thread, or {@code null} if the request
           *         to create a thread is rejected
           * @throws NullPointerException if the pool is null
           */
          public ForkJoinWorkerThread newThread(ForkJoinPool pool);
      }
  
      /**
       * Default ForkJoinWorkerThreadFactory implementation; creates a
       * new ForkJoinWorkerThread.
       */
      private static final class DefaultForkJoinWorkerThreadFactory
          implements ForkJoinWorkerThreadFactory {
          public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
              return new ForkJoinWorkerThread(pool);
          }
      }
  
      /**
       * 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.
       */
      private static final class EmptyTask extends ForkJoinTask<Void> {
          private static final long serialVersionUID = -7721805057305804111L;
          EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
          public final Void getRawResult() { return null; }
          public final void setRawResult(Void x) {}
          public final boolean exec() { return true; }
      }
  
      /**
       * Additional fields and lock created upon initialization.
       */
      private static final class AuxState extends ReentrantLock {
          private static final long serialVersionUID = -6001602636862214147L;
          volatile long stealCount;     // cumulative steal count
          long indexSeed;               // index bits for registerWorker
          AuxState() {}
      }
  
      // Constants shared across ForkJoinPool and WorkQueue
  
      // Bounds
      static final int SMASK        = 0xffff;        // short bits == max index
      static final int MAX_CAP      = 0x7fff;        // max #workers - 1
      static final int EVENMASK     = 0xfffe;        // even short bits
      static final int SQMASK       = 0x007e;        // max 64 (even) slots
  
      // Masks and units for WorkQueue.scanState and ctl sp subfield
      static final int UNSIGNALLED  = 1 << 31;       // must be negative
      static final int SS_SEQ       = 1 << 16;       // version count
  
      // Mode bits for ForkJoinPool.config and WorkQueue.config
      static final int MODE_MASK    = 0xffff << 16;  // top half of int
      static final int SPARE_WORKER = 1 << 17;       // set if tc > 0 on creation
      static final int UNREGISTERED = 1 << 18;       // to skip some of deregister
      static final int FIFO_QUEUE   = 1 << 31;       // must be negative
      static final int LIFO_QUEUE   = 0;             // for clarity
      static final int IS_OWNED     = 1;             // low bit 0 if shared
  
      /**
       * The maximum number of task executions from the same queue
       * before checking other queues, bounding unfairness and impact of
       * infinite user task recursion.  Must be a power of two minus 1.
       */
      static final int POLL_LIMIT = (1 << 10) - 1;
  
      /**
       * Queues supporting work-stealing as well as external task
       * submission. See above for descriptions and algorithms.
       * 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. The @Contended annotation alerts
       * JVMs to try to keep instances apart.
       */
      // Android-removed: @Contended, this hint is not used by the Android runtime.
      //@jdk.internal.vm.annotation.Contended
      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
  
          // Instance fields
  
          volatile int scanState;    // versioned, negative if inactive
          int stackPred;             // pool stack (ctl) predecessor
          int nsteals;               // number of steals
          int hint;                  // randomization and stealer index hint
          int config;                // pool index and mode
          volatile int qlock;        // 1: locked, < 0: 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
  
        // Android-removed: @Contended, this hint is not used by the Android runtime.
        // @jdk.internal.vm.annotation.Contended("group2") // segregate
          volatile ForkJoinTask<?> currentSteal; // nonnull when running some task
  
          WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner) {
              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 an exportable index (used by ForkJoinWorkerThread).
           */
          final int getPoolIndex() {
              return (config & 0xffff) >>> 1; // ignore odd/even tag bit
          }
  
          /**
           * Returns the approximate number of tasks in the queue.
           */
          final int queueSize() {
              int n = base - top;       // 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 n, al, s;
              return ((n = base - (s = top)) >= 0 || // possibly one task
                      (n == -1 && ((a = array) == null ||
                                   (al = a.length) == 0 ||
                                   a[(al - 1) & (s - 1)] == null)));
          }
  
          /**
           * Pushes a task. Call only by owner in unshared queues.
           *
           * @param task the task. Caller must ensure non-null.
           * @throws RejectedExecutionException if array cannot be resized
           */
          final void push(ForkJoinTask<?> task) {
              U.storeFence();              // ensure safe publication
              int s = top, al, d; ForkJoinTask<?>[] a;
              if ((a = array) != null && (al = a.length) > 0) {
                  a[(al - 1) & s] = task;  // relaxed writes OK
                  top = s + 1;
                  ForkJoinPool p = pool;
                  if ((d = base - s) == 0 && p != null) {
                      U.fullFence();
                      p.signalWork();
                  }
                  else if (al + d == 1)
                      growArray();
              }
          }
  
          /**
           * Initializes or doubles the capacity of array. Call either
           * by owner or with lock held -- it is OK for base, but not
           * top, to move while resizings are in progress.
           */
          final ForkJoinTask<?>[] growArray() {
              ForkJoinTask<?>[] oldA = array;
              int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
              if (size < INITIAL_QUEUE_CAPACITY || size > MAXIMUM_QUEUE_CAPACITY)
                  throw new RejectedExecutionException("Queue capacity exceeded");
              int oldMask, t, b;
              ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
              if (oldA != null && (oldMask = oldA.length - 1) > 0 &&
                  (t = top) - (b = base) > 0) {
                  int mask = size - 1;
                  do { // emulate poll from old array, push to new array
                      int index = b & oldMask;
                      long offset = ((long)index << ASHIFT) + ABASE;
                      ForkJoinTask<?> x = (ForkJoinTask<?>)
                          U.getObjectVolatile(oldA, offset);
                      if (x != null &&
                          U.compareAndSwapObject(oldA, offset, x, null))
                          a[b & mask] = x;
                  } while (++b != t);
                  U.storeFence();
              }
              return a;
          }
  
          /**
           * Takes next task, if one exists, in LIFO order.  Call only
           * by owner in unshared queues.
           */
          final ForkJoinTask<?> pop() {
              int b = base, s = top, al, i; ForkJoinTask<?>[] a;
              if ((a = array) != null && b != s && (al = a.length) > 0) {
                  int index = (al - 1) & --s;
                  long offset = ((long)index << ASHIFT) + ABASE;
                  ForkJoinTask<?> t = (ForkJoinTask<?>)
                      U.getObject(a, offset);
                  if (t != null &&
                      U.compareAndSwapObject(a, offset, 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 helpStealer.
           */
          final ForkJoinTask<?> pollAt(int b) {
              ForkJoinTask<?>[] a; int al;
              if ((a = array) != null && (al = a.length) > 0) {
                  int index = (al - 1) & b;
                  long offset = ((long)index << ASHIFT) + ABASE;
                  ForkJoinTask<?> t = (ForkJoinTask<?>)
                      U.getObjectVolatile(a, offset);
                  if (t != null && b++ == base &&
                      U.compareAndSwapObject(a, offset, t, null)) {
                      base = b;
                      return t;
                  }
              }
              return null;
          }
  
          /**
           * Takes next task, if one exists, in FIFO order.
           */
          final ForkJoinTask<?> poll() {
              for (;;) {
                  int b = base, s = top, d, al; ForkJoinTask<?>[] a;
                  if ((a = array) != null && (d = b - s) < 0 &&
                      (al = a.length) > 0) {
                      int index = (al - 1) & b;
                      long offset = ((long)index << ASHIFT) + ABASE;
                      ForkJoinTask<?> t = (ForkJoinTask<?>)
                          U.getObjectVolatile(a, offset);
                      if (b++ == base) {
                          if (t != null) {
                              if (U.compareAndSwapObject(a, offset, t, null)) {
                                  base = b;
                                  return t;
                              }
                          }
                          else if (d == -1)
                              break; // now empty
                      }
                  }
                  else
                      break;
              }
              return null;
          }
  
          /**
           * Takes next task, if one exists, in order specified by mode.
           */
          final ForkJoinTask<?> nextLocalTask() {
              return (config < 0) ? poll() : pop();
          }
  
          /**
           * Returns next task, if one exists, in order specified by mode.
           */
          final ForkJoinTask<?> peek() {
              int al; ForkJoinTask<?>[] a;
              return ((a = array) != null && (al = a.length) > 0) ?
                  a[(al - 1) & (config < 0 ? base : top - 1)] : null;
          }
  
          /**
           * Pops the given task only if it is at the current top.
           */
          final boolean tryUnpush(ForkJoinTask<?> task) {
              int b = base, s = top, al; ForkJoinTask<?>[] a;
              if ((a = array) != null && b != s && (al = a.length) > 0) {
                  int index = (al - 1) & --s;
                  long offset = ((long)index << ASHIFT) + ABASE;
                  if (U.compareAndSwapObject(a, offset, task, null)) {
                      top = s;
                      return true;
                  }
              }
              return false;
          }
  
          /**
           * Shared version of push. Fails if already locked.
           *
           * @return status: > 0 locked, 0 possibly was empty, < 0 was nonempty
           */
          final int sharedPush(ForkJoinTask<?> task) {
              int stat;
              if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
                  int b = base, s = top, al, d; ForkJoinTask<?>[] a;
                  if ((a = array) != null && (al = a.length) > 0 &&
                      al - 1 + (d = b - s) > 0) {
                      a[(al - 1) & s] = task;
                      top = s + 1;                 // relaxed writes OK here
                      qlock = 0;
                      stat = (d < 0 && b == base) ? d : 0;
                  }
                  else {
                      growAndSharedPush(task);
                      stat = 0;
                  }
              }
              else
                  stat = 1;
              return stat;
          }
  
          /**
           * Helper for sharedPush; called only when locked and resize
           * needed.
           */
          private void growAndSharedPush(ForkJoinTask<?> task) {
              try {
                  growArray();
                  int s = top, al; ForkJoinTask<?>[] a;
                  if ((a = array) != null && (al = a.length) > 0) {
                      a[(al - 1) & s] = task;
                      top = s + 1;
                  }
              } finally {
                  qlock = 0;
              }
          }
  
          /**
           * Shared version of tryUnpush.
           */
          final boolean trySharedUnpush(ForkJoinTask<?> task) {
              boolean popped = false;
              int s = top - 1, al; ForkJoinTask<?>[] a;
              if ((a = array) != null && (al = a.length) > 0) {
                  int index = (al - 1) & s;
                  long offset = ((long)index << ASHIFT) + ABASE;
                  ForkJoinTask<?> t = (ForkJoinTask<?>) U.getObject(a, offset);
                  if (t == task &&
                      U.compareAndSwapInt(this, QLOCK, 0, 1)) {
                      if (top == s + 1 && array == a &&
                          U.compareAndSwapObject(a, offset, task, null)) {
                          popped = true;
                          top = s;
                      }
                      U.putOrderedInt(this, QLOCK, 0);
                  }
              }
              return popped;
          }
  
          /**
           * Removes and cancels all known tasks, ignoring any exceptions.
           */
          final void cancelAll() {
              ForkJoinTask<?> t;
              if ((t = currentJoin) != null) {
                  currentJoin = null;
                  ForkJoinTask.cancelIgnoringExceptions(t);
              }
              if ((t = currentSteal) != null) {
                  currentSteal = null;
                  ForkJoinTask.cancelIgnoringExceptions(t);
              }
              while ((t = poll()) != null)
                  ForkJoinTask.cancelIgnoringExceptions(t);
          }
  
          // Specialized execution methods
  
          /**
           * Pops and executes up to POLL_LIMIT tasks or until empty.
           */
          final void localPopAndExec() {
              for (int nexec = 0;;) {
                  int b = base, s = top, al; ForkJoinTask<?>[] a;
                  if ((a = array) != null && b != s && (al = a.length) > 0) {
                      int index = (al - 1) & --s;
                      long offset = ((long)index << ASHIFT) + ABASE;
                      ForkJoinTask<?> t = (ForkJoinTask<?>)
                          U.getAndSetObject(a, offset, null);
                      if (t != null) {
                          top = s;
                          (currentSteal = t).doExec();
                          if (++nexec > POLL_LIMIT)
                              break;
                      }
                      else
                          break;
                  }
                  else
                      break;
              }
          }
  
          /**
           * Polls and executes up to POLL_LIMIT tasks or until empty.
           */
          final void localPollAndExec() {
              for (int nexec = 0;;) {
                  int b = base, s = top, al; ForkJoinTask<?>[] a;
                  if ((a = array) != null && b != s && (al = a.length) > 0) {
                      int index = (al - 1) & b++;
                      long offset = ((long)index << ASHIFT) + ABASE;
                      ForkJoinTask<?> t = (ForkJoinTask<?>)
                          U.getAndSetObject(a, offset, null);
                      if (t != null) {
                          base = b;
                          t.doExec();
                          if (++nexec > POLL_LIMIT)
                              break;
                      }
                  }
                  else
                      break;
              }
          }
  
          /**
           * Executes the given task and (some) remaining local tasks.
           */
          final void runTask(ForkJoinTask<?> task) {
              if (task != null) {
                  task.doExec();
                  if (config < 0)
                      localPollAndExec();
                  else
                      localPopAndExec();
                  int ns = ++nsteals;
                  ForkJoinWorkerThread thread = owner;
                  currentSteal = null;
                  if (ns < 0)           // collect on overflow
                      transferStealCount(pool);
                  if (thread != null)
                      thread.afterTopLevelExec();
              }
          }
  
          /**
           * Adds steal count to pool steal count if it exists, and resets.
           */
          final void transferStealCount(ForkJoinPool p) {
              AuxState aux;
              if (p != null && (aux = p.auxState) != null) {
                  long s = nsteals;
                  nsteals = 0;            // if negative, correct for overflow
                  if (s < 0) s = Integer.MAX_VALUE;
                  aux.lock();
                  try {
                      aux.stealCount += s;
                  } finally {
                      aux.unlock();
                  }
              }
          }
  
          /**
           * If present, removes from queue and executes the given task,
           * or any other cancelled task. Used only by awaitJoin.
           *
           * @return true if queue empty and task not known to be done
           */
          final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
              if (task != null && task.status >= 0) {
                  int b, s, d, al; ForkJoinTask<?>[] a;
                  while ((d = (b = base) - (s = top)) < 0 &&
                         (a = array) != null && (al = a.length) > 0) {
                      for (;;) {      // traverse from s to b
                          int index = --s & (al - 1);
                          long offset = (index << ASHIFT) + ABASE;
                          ForkJoinTask<?> t = (ForkJoinTask<?>)
                              U.getObjectVolatile(a, offset);
                          if (t == null)
                              break;                   // restart
                          else if (t == task) {
                              boolean removed = false;
                              if (s + 1 == top) {      // pop
                                  if (U.compareAndSwapObject(a, offset, t, null)) {
                                      top = s;
                                      removed = true;
                                  }
                              }
                              else if (base == b)      // replace with proxy
                                  removed = U.compareAndSwapObject(a, offset, t,
                                                                   new EmptyTask());
                              if (removed) {
                                  ForkJoinTask<?> ps = currentSteal;
                                  (currentSteal = task).doExec();
                                  currentSteal = ps;
                              }
                              break;
                          }
                          else if (t.status < 0 && s + 1 == top) {
                              if (U.compareAndSwapObject(a, offset, t, null)) {
                                  top = s;
                              }
                              break;                  // was cancelled
                          }
                          else if (++d == 0) {
                              if (base != b)          // rescan
                                  break;
                              return false;
                          }
                      }
                      if (task.status < 0)
                          return false;
                  }
              }
              return true;
          }
  
          /**
           * Pops task if in the same CC computation as the given task,
           * in either shared or owned mode. Used only by helpComplete.
           */
          final CountedCompleter<?> popCC(CountedCompleter<?> task, int mode) {
              int b = base, s = top, al; ForkJoinTask<?>[] a;
              if ((a = array) != null && b != s && (al = a.length) > 0) {
                  int index = (al - 1) & (s - 1);
                  long offset = ((long)index << ASHIFT) + ABASE;
                  ForkJoinTask<?> o = (ForkJoinTask<?>)
                      U.getObjectVolatile(a, offset);
                  if (o instanceof CountedCompleter) {
                      CountedCompleter<?> t = (CountedCompleter<?>)o;
                      for (CountedCompleter<?> r = t;;) {
                          if (r == task) {
                              if ((mode & IS_OWNED) == 0) {
                                  boolean popped = false;
                                  if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
                                      if (top == s && array == a &&
                                          U.compareAndSwapObject(a, offset,
                                                                 t, null)) {
                                          popped = true;
                                          top = s - 1;
                                      }
                                      U.putOrderedInt(this, QLOCK, 0);
                                      if (popped)
                                          return t;
                                  }
                              }
                              else if (U.compareAndSwapObject(a, offset,
                                                              t, null)) {
                                  top = s - 1;
                                  return t;
                              }
                              break;
                          }
                          else if ((r = r.completer) == null) // try parent
                              break;
                      }
                  }
              }
              return null;
          }
  
          /**
           * Steals and runs a task in the same CC computation as the
           * given task if one exists and can be taken without
           * contention. Otherwise returns a checksum/control value for
           * use by method helpComplete.
           *
           * @return 1 if successful, 2 if retryable (lost to another
           * stealer), -1 if non-empty but no matching task found, else
           * the base index, forced negative.
           */
          final int pollAndExecCC(CountedCompleter<?> task) {
              ForkJoinTask<?>[] a;
              int b = base, s = top, al, h;
              if ((a = array) != null && b != s && (al = a.length) > 0) {
                  int index = (al - 1) & b;
                  long offset = ((long)index << ASHIFT) + ABASE;
                  ForkJoinTask<?> o = (ForkJoinTask<?>)
                      U.getObjectVolatile(a, offset);
                  if (o == null)
                      h = 2;                      // retryable
                  else if (!(o instanceof CountedCompleter))
                      h = -1;                     // unmatchable
                  else {
                      CountedCompleter<?> t = (CountedCompleter<?>)o;
                      for (CountedCompleter<?> r = t;;) {
                          if (r == task) {
                              if (b++ == base &&
                                  U.compareAndSwapObject(a, offset, t, null)) {
                                  base = b;
                                  t.doExec();
                                  h = 1;          // success
                              }
                              else
                                  h = 2;          // lost CAS
                              break;
                          }
                          else if ((r = r.completer) == null) {
                              h = -1;             // unmatched
                              break;
                          }
                      }
                  }
              }
              else
                  h = b | Integer.MIN_VALUE;      // to sense movement on re-poll
              return h;
          }
  
          /**
           * Returns true if owned and not known to be blocked.
           */
          final boolean isApparentlyUnblocked() {
              Thread wt; Thread.State s;
              return (scanState >= 0 &&
                      (wt = owner) != null &&
                      (s = wt.getState()) != Thread.State.BLOCKED &&
                      s != Thread.State.WAITING &&
                      s != Thread.State.TIMED_WAITING);
          }
  
          // Unsafe mechanics. Note that some are (and must be) the same as in FJP
          private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
          private static final long QLOCK;
          private static final int ABASE;
          private static final int ASHIFT;
          static {
              try {
                  QLOCK = U.objectFieldOffset
                      (WorkQueue.class.getDeclaredField("qlock"));
                  ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
                  int scale = U.arrayIndexScale(ForkJoinTask[].class);
                  if ((scale & (scale - 1)) != 0)
                      throw new Error("array index scale not a power of two");
                  ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
              } catch (ReflectiveOperationException e) {
                  throw new Error(e);
              }
          }
      }
  
      // 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;
  
      /**
       * Permission required for callers of methods that may start or
       * kill threads.  Also used as a static lock in tryInitialize.
       */
      static final RuntimePermission modifyThreadPermission;
  
      /**
       * Common (static) pool. Non-null for public use unless a static
       * construction exception, but internal usages null-check on use
       * to paranoically avoid potential initialization circularities
       * as well as to simplify generated code.
       */
      static final ForkJoinPool common;
  
      /**
       * Common pool parallelism. To allow simpler use and management
       * when common pool threads are disabled, we allow the underlying
       * common.parallelism field to be zero, but in that case still report
       * parallelism as 1 to reflect resulting caller-runs mechanics.
       */
      static final int COMMON_PARALLELISM;
  
      /**
       * Limit on spare thread construction in tryCompensate.
       */
      private static final int COMMON_MAX_SPARES;
  
      /**
       * Sequence number for creating workerNamePrefix.
       */
      private static int poolNumberSequence;
  
      /**
       * Returns the next sequence number. We don't expect this to
       * ever contend, so use simple builtin sync.
       */
      private static final synchronized int nextPoolId() {
          return ++poolNumberSequence;
      }
  
      // static configuration constants
  
      /**
       * Initial timeout value (in milliseconds) for the thread
       * triggering quiescence to park waiting for new work. On timeout,
       * the thread will instead try to shrink the number of workers.
       * The value should be large enough to avoid overly aggressive
       * shrinkage during most transient stalls (long GCs etc).
       */
      private static final long IDLE_TIMEOUT_MS = 2000L; // 2sec
  
      /**
       * Tolerance for idle timeouts, to cope with timer undershoots.
       */
      private static final long TIMEOUT_SLOP_MS =   20L; // 20ms
  
      /**
       * The default value for COMMON_MAX_SPARES.  Overridable using the
       * "java.util.concurrent.ForkJoinPool.common.maximumSpares" system
       * property.  The default value is far in excess of normal
       * requirements, but also far short of MAX_CAP and typical OS
       * thread limits, so allows JVMs to catch misuse/abuse before
       * running out of resources needed to do so.
       */
      private static final int DEFAULT_COMMON_MAX_SPARES = 256;
  
      /**
       * Increment for seed generators. See class ThreadLocal for
       * explanation.
       */
      private static final int SEED_INCREMENT = 0x9e3779b9;
  
      /*
       * Bits and masks for field ctl, packed with 4 16 bit subfields:
       * AC: Number of active running workers minus target parallelism
       * TC: Number of total workers minus target parallelism
       * SS: version count and status of top waiting thread
       * ID: poolIndex of top of Treiber stack of waiters
       *
       * When convenient, we can extract the lower 32 stack top bits
       * (including version bits) as sp=(int)ctl.  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.  When sp is non-zero, there are waiting workers.  To
       * deal with possibly negative fields, we use casts in and out of
       * "short" and/or signed shifts to maintain signedness.
       *
       * Because it occupies uppermost bits, we can add one active count
       * using getAndAddLong of AC_UNIT, rather than CAS, when returning
       * from a blocked join.  Other updates entail multiple subfields
       * and masking, requiring CAS.
       */
  
      // Lower and upper word masks
      private static final long SP_MASK    = 0xffffffffL;
      private static final long UC_MASK    = ~SP_MASK;
  
      // Active counts
      private static final int  AC_SHIFT   = 48;
      private static final long AC_UNIT    = 0x0001L << AC_SHIFT;
      private static final long AC_MASK    = 0xffffL << AC_SHIFT;
  
      // Total counts
      private static final int  TC_SHIFT   = 32;
      private static final long TC_UNIT    = 0x0001L << TC_SHIFT;
      private static final long TC_MASK    = 0xffffL << TC_SHIFT;
      private static final long ADD_WORKER = 0x0001L << (TC_SHIFT + 15); // sign
  
      // runState bits: SHUTDOWN must be negative, others arbitrary powers of two
      private static final int  STARTED    = 1;
      private static final int  STOP       = 1 << 1;
      private static final int  TERMINATED = 1 << 2;
      private static final int  SHUTDOWN   = 1 << 31;
  
      // Instance fields
      volatile long ctl;                   // main pool control
      volatile int runState;
      final int config;                    // parallelism, mode
      AuxState auxState;                   // lock, steal counts
      volatile WorkQueue[] workQueues;     // main registry
      final String workerNamePrefix;       // to create worker name string
      final ForkJoinWorkerThreadFactory factory;
      final UncaughtExceptionHandler ueh;  // per-worker UEH
  
      /**
       * Instantiates fields upon first submission, or upon shutdown if
       * no submissions. If checkTermination true, also responds to
       * termination by external calls submitting tasks.
       */
      private void tryInitialize(boolean checkTermination) {
          if (runState == 0) { // bootstrap by locking static field
              int p = config & SMASK;
              int n = (p > 1) ? p - 1 : 1; // ensure at least 2 slots
              n |= n >>> 1;    // create workQueues array with size a power of two
              n |= n >>> 2;
              n |= n >>> 4;
              n |= n >>> 8;
              n |= n >>> 16;
              n = ((n + 1) << 1) & SMASK;
              AuxState aux = new AuxState();
              WorkQueue[] ws = new WorkQueue[n];
              synchronized (modifyThreadPermission) { // double-check
                  if (runState == 0) {
                      workQueues = ws;
                      auxState = aux;
                      runState = STARTED;
                  }
              }
          }
          if (checkTermination && runState < 0) {
              tryTerminate(false, false); // help terminate
              throw new RejectedExecutionException();
          }
      }
  
      // Creating, registering and deregistering workers
  
      /**
       * Tries to construct and start one worker. Assumes that total
       * count has already been incremented as a reservation.  Invokes
       * deregisterWorker on any failure.
       *
       * @param isSpare true if this is a spare thread
       * @return true if successful
       */
      private boolean createWorker(boolean isSpare) {
          ForkJoinWorkerThreadFactory fac = factory;
          Throwable ex = null;
          ForkJoinWorkerThread wt = null;
          WorkQueue q;
          try {
              if (fac != null && (wt = fac.newThread(this)) != null) {
                  if (isSpare && (q = wt.workQueue) != null)
                      q.config |= SPARE_WORKER;
                  wt.start();
                  return true;
              }
          } catch (Throwable rex) {
              ex = rex;
          }
          deregisterWorker(wt, ex);
          return false;
      }
  
      /**
       * Tries to add one worker, incrementing ctl counts before doing
       * so, relying on createWorker to back out on failure.
       *
       * @param c incoming ctl value, with total count negative and no
       * idle workers.  On CAS failure, c is refreshed and retried if
       * this holds (otherwise, a new worker is not needed).
       */
      private void tryAddWorker(long c) {
          do {
              long nc = ((AC_MASK & (c + AC_UNIT)) |
                         (TC_MASK & (c + TC_UNIT)));
              if (ctl == c && U.compareAndSwapLong(this, CTL, c, nc)) {
                  createWorker(false);
                  break;
              }
          } while (((c = ctl) & ADD_WORKER) != 0L && (int)c == 0);
      }
  
      /**
       * Callback from ForkJoinWorkerThread constructor to establish and
       * record its WorkQueue.
       *
       * @param wt the worker thread
       * @return the worker's queue
       */
      final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
          UncaughtExceptionHandler handler;
          AuxState aux;
          wt.setDaemon(true);                           // configure thread
          if ((handler = ueh) != null)
              wt.setUncaughtExceptionHandler(handler);
          WorkQueue w = new WorkQueue(this, wt);
          int i = 0;                                    // assign a pool index
          int mode = config & MODE_MASK;
          if ((aux = auxState) != null) {
              aux.lock();
              try {
                  int s = (int)(aux.indexSeed += SEED_INCREMENT), n, m;
                  WorkQueue[] ws = workQueues;
                  if (ws != null && (n = ws.length) > 0) {
                      i = (m = n - 1) & ((s << 1) | 1); // odd-numbered indices
                      if (ws[i] != null) {              // collision
                          int probes = 0;               // step by approx half n
                          int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
                          while (ws[i = (i + step) & m] != null) {
                              if (++probes >= n) {
                                  workQueues = ws = Arrays.copyOf(ws, n <<= 1);
                                  m = n - 1;
                                  probes = 0;
                              }
                          }
                      }
                      w.hint = s;                       // use as random seed
                      w.config = i | mode;
                      w.scanState = i | (s & 0x7fff0000); // random seq bits
                      ws[i] = w;
                  }
              } finally {
                  aux.unlock();
              }
          }
          wt.setName(workerNamePrefix.concat(Integer.toString(i >>> 1)));
          return w;
      }
  
      /**
       * Final callback from terminating worker, as well as upon failure
       * to construct or start a worker.  Removes record of worker from
       * array, and adjusts counts. If pool is shutting down, tries to
       * complete termination.
       *
       * @param wt the worker thread, or null if construction failed
       * @param ex the exception causing failure, or null if none
       */
      final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
          WorkQueue w = null;
          if (wt != null && (w = wt.workQueue) != null) {
              AuxState aux; WorkQueue[] ws;          // remove index from array
              int idx = w.config & SMASK;
              int ns = w.nsteals;
              if ((aux = auxState) != null) {
                  aux.lock();
                  try {
                      if ((ws = workQueues) != null && ws.length > idx &&
                          ws[idx] == w)
                          ws[idx] = null;
                      aux.stealCount += ns;
                  } finally {
                      aux.unlock();
                  }
              }
          }
          if (w == null || (w.config & UNREGISTERED) == 0) { // else pre-adjusted
              long c;                                   // decrement counts
              do {} while (!U.compareAndSwapLong
                           (this, CTL, c = ctl, ((AC_MASK & (c - AC_UNIT)) |
                                                 (TC_MASK & (c - TC_UNIT)) |
                                                 (SP_MASK & c))));
          }
          if (w != null) {
              w.currentSteal = null;
              w.qlock = -1;                             // ensure set
              w.cancelAll();                            // cancel remaining tasks
          }
          while (tryTerminate(false, false) >= 0) {     // possibly replace
              WorkQueue[] ws; int wl, sp; long c;
              if (w == null || w.array == null ||
                  (ws = workQueues) == null || (wl = ws.length) <= 0)
                  break;
              else if ((sp = (int)(c = ctl)) != 0) {    // wake up replacement
                  if (tryRelease(c, ws[(wl - 1) & sp], AC_UNIT))
                      break;
              }
              else if (ex != null && (c & ADD_WORKER) != 0L) {
                  tryAddWorker(c);                      // create replacement
                  break;
              }
              else                                      // don't need replacement
                  break;
          }
          if (ex == null)                               // help clean on way out
              ForkJoinTask.helpExpungeStaleExceptions();
          else                                          // rethrow
              ForkJoinTask.rethrow(ex);
      }
  
      // Signalling
  
      /**
       * Tries to create or activate a worker if too few are active.
       */
      final void signalWork() {
          for (;;) {
              long c; int sp, i; WorkQueue v; WorkQueue[] ws;
              if ((c = ctl) >= 0L)                      // enough workers
                  break;
              else if ((sp = (int)c) == 0) {            // no idle workers
                  if ((c & ADD_WORKER) != 0L)           // too few workers
                      tryAddWorker(c);
                  break;
              }
              else if ((ws = workQueues) == null)
                  break;                                // unstarted/terminated
              else if (ws.length <= (i = sp & SMASK))
                  break;                                // terminated
              else if ((v = ws[i]) == null)
                  break;                                // terminating
              else {
                  int ns = sp & ~UNSIGNALLED;
                  int vs = v.scanState;
                  long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + AC_UNIT));
                  if (sp == vs && U.compareAndSwapLong(this, CTL, c, nc)) {
                      v.scanState = ns;
                      LockSupport.unpark(v.parker);
                      break;
                  }
              }
          }
      }
  
      /**
       * Signals and releases worker v if it is top of idle worker
       * stack.  This performs a one-shot version of signalWork only if
       * there is (apparently) at least one idle worker.
       *
       * @param c incoming ctl value
       * @param v if non-null, a worker
       * @param inc the increment to active count (zero when compensating)
       * @return true if successful
       */
      private boolean tryRelease(long c, WorkQueue v, long inc) {
          int sp = (int)c, ns = sp & ~UNSIGNALLED;
          if (v != null) {
              int vs = v.scanState;
              long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + inc));
              if (sp == vs && U.compareAndSwapLong(this, CTL, c, nc)) {
                  v.scanState = ns;
                  LockSupport.unpark(v.parker);
                  return true;
              }
          }
          return false;
      }
  
      /**
       * With approx probability of a missed signal, tries (once) to
       * reactivate worker w (or some other worker), failing if stale or
       * known to be already active.
       *
       * @param w the worker
       * @param ws the workQueue array to use
       * @param r random seed
       */
      private void tryReactivate(WorkQueue w, WorkQueue[] ws, int r) {
          long c; int sp, wl; WorkQueue v;
          if ((sp = (int)(c = ctl)) != 0 && w != null &&
              ws != null && (wl = ws.length) > 0 &&
              ((sp ^ r) & SS_SEQ) == 0 &&
              (v = ws[(wl - 1) & sp]) != null) {
              long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + AC_UNIT));
              int ns = sp & ~UNSIGNALLED;
              if (w.scanState < 0 &&
                  v.scanState == sp &&
                  U.compareAndSwapLong(this, CTL, c, nc)) {
                  v.scanState = ns;
                  LockSupport.unpark(v.parker);
              }
          }
      }
  
      /**
       * If worker w exists and is active, enqueues and sets status to inactive.
       *
       * @param w the worker
       * @param ss current (non-negative) scanState
       */
      private void inactivate(WorkQueue w, int ss) {
          int ns = (ss + SS_SEQ) | UNSIGNALLED;
          long lc = ns & SP_MASK, nc, c;
          if (w != null) {
              w.scanState = ns;
              do {
                  nc = lc | (UC_MASK & ((c = ctl) - AC_UNIT));
                  w.stackPred = (int)c;
              } while (!U.compareAndSwapLong(this, CTL, c, nc));
          }
      }
  
      /**
       * Possibly blocks worker w waiting for signal, or returns
       * negative status if the worker should terminate. May return
       * without status change if multiple stale unparks and/or
       * interrupts occur.
       *
       * @param w the calling worker
       * @return negative if w should terminate
       */
      private int awaitWork(WorkQueue w) {
          int stat = 0;
          if (w != null && w.scanState < 0) {
              long c = ctl;
              if ((int)(c >> AC_SHIFT) + (config & SMASK) <= 0)
                  stat = timedAwaitWork(w, c);     // possibly quiescent
              else if ((runState & STOP) != 0)
                  stat = w.qlock = -1;             // pool terminating
              else if (w.scanState < 0) {
                  w.parker = Thread.currentThread();
                  if (w.scanState < 0)             // recheck after write
                      LockSupport.park(this);
                  w.parker = null;
                  if ((runState & STOP) != 0)
                      stat = w.qlock = -1;         // recheck
                  else if (w.scanState < 0)
                      Thread.interrupted();        // clear status
              }
          }
          return stat;
      }
  
      /**
       * Possibly triggers shutdown and tries (once) to block worker
       * when pool is (or may be) quiescent. Waits up to a duration
       * determined by number of workers.  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
       * @return negative if w should terminate
       */
      private int timedAwaitWork(WorkQueue w, long c) {
          int stat = 0;
          int scale = 1 - (short)(c >>> TC_SHIFT);
          long deadline = (((scale <= 0) ? 1 : scale) * IDLE_TIMEOUT_MS +
                           System.currentTimeMillis());
          if ((runState >= 0 || (stat = tryTerminate(false, false)) > 0) &&
              w != null && w.scanState < 0) {
              int ss; AuxState aux;
              w.parker = Thread.currentThread();
              if (w.scanState < 0)
                  LockSupport.parkUntil(this, deadline);
              w.parker = null;
              if ((runState & STOP) != 0)
                  stat = w.qlock = -1;         // pool terminating
              else if ((ss = w.scanState) < 0 && !Thread.interrupted() &&
                       (int)c == ss && (aux = auxState) != null && ctl == c &&
                       deadline - System.currentTimeMillis() <= TIMEOUT_SLOP_MS) {
                  aux.lock();
                  try {                        // pre-deregister
                      WorkQueue[] ws;
                      int cfg = w.config, idx = cfg & SMASK;
                      long nc = ((UC_MASK & (c - TC_UNIT)) |
                                 (SP_MASK & w.stackPred));
                      if ((runState & STOP) == 0 &&
                          (ws = workQueues) != null &&
                          idx < ws.length && idx >= 0 && ws[idx] == w &&
                          U.compareAndSwapLong(this, CTL, c, nc)) {
                          ws[idx] = null;
                          w.config = cfg | UNREGISTERED;
                          stat = w.qlock = -1;
                      }
                  } finally {
                      aux.unlock();
                  }
              }
          }
          return stat;
      }
  
      /**
       * If the given worker is a spare with no queued tasks, and there
       * are enough existing workers, drops it from ctl counts and sets
       * its state to terminated.
       *
       * @param w the calling worker -- must be a spare
       * @return true if dropped (in which case it must not process more tasks)
       */
      private boolean tryDropSpare(WorkQueue w) {
          if (w != null && w.isEmpty()) {           // no local tasks
              long c; int sp, wl; WorkQueue[] ws; WorkQueue v;
              while ((short)((c = ctl) >> TC_SHIFT) > 0 &&
                     ((sp = (int)c) != 0 || (int)(c >> AC_SHIFT) > 0) &&
                     (ws = workQueues) != null && (wl = ws.length) > 0) {
                  boolean dropped, canDrop;
                  if (sp == 0) {                    // no queued workers
                      long nc = ((AC_MASK & (c - AC_UNIT)) |
                                 (TC_MASK & (c - TC_UNIT)) | (SP_MASK & c));
                      dropped = U.compareAndSwapLong(this, CTL, c, nc);
                  }
                  else if (
                      (v = ws[(wl - 1) & sp]) == null || v.scanState != sp)
                      dropped = false;              // stale; retry
                  else {
                      long nc = v.stackPred & SP_MASK;
                      if (w == v || w.scanState >= 0) {
                          canDrop = true;           // w unqueued or topmost
                          nc |= ((AC_MASK & c) |    // ensure replacement
                                 (TC_MASK & (c - TC_UNIT)));
                      }
                      else {                        // w may be queued
                          canDrop = false;          // help uncover
                          nc |= ((AC_MASK & (c + AC_UNIT)) |
                                 (TC_MASK & c));
                      }
                      if (U.compareAndSwapLong(this, CTL, c, nc)) {
                          v.scanState = sp & ~UNSIGNALLED;
                          LockSupport.unpark(v.parker);
                          dropped = canDrop;
                      }
                      else
                          dropped = false;
                  }
                  if (dropped) {                    // pre-deregister
                      int cfg = w.config, idx = cfg & SMASK;
                      if (idx >= 0 && idx < ws.length && ws[idx] == w)
                          ws[idx] = null;
                      w.config = cfg | UNREGISTERED;
                      w.qlock = -1;
                      return true;
                  }
              }
          }
          return false;
      }
  
      /**
       * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
       */
      final void runWorker(WorkQueue w) {
          w.growArray();                                  // allocate queue
          int bound = (w.config & SPARE_WORKER) != 0 ? 0 : POLL_LIMIT;
          long seed = w.hint * 0xdaba0b6eb09322e3L;       // initial random seed
          if ((runState & STOP) == 0) {
              for (long r = (seed == 0L) ? 1L : seed;;) { // ensure nonzero
                  if (bound == 0 && tryDropSpare(w))
                      break;
                  // high bits of prev seed for step; current low bits for idx
                  int step = (int)(r >>> 48) | 1;
                  r ^= r >>> 12; r ^= r << 25; r ^= r >>> 27; // xorshift
                  if (scan(w, bound, step, (int)r) < 0 && awaitWork(w) < 0)
                      break;
              }
          }
      }
  
      // Scanning for tasks
  
      /**
       * Repeatedly scans for and tries to steal and execute (via
       * workQueue.runTask) a queued task. Each scan traverses queues in
       * pseudorandom permutation. Upon finding a non-empty queue, makes
       * at most the given bound attempts to re-poll (fewer if
       * contended) on the same queue before returning (impossible
       * scanState value) 0 to restart scan. Else returns after at least
       * 1 and at most 32 full scans.
       *
       * @param w the worker (via its WorkQueue)
       * @param bound repoll bound as bitmask (0 if spare)
       * @param step (circular) index increment per iteration (must be odd)
       * @param r a random seed for origin index
       * @return negative if should await signal
       */
      private int scan(WorkQueue w, int bound, int step, int r) {
          int stat = 0, wl; WorkQueue[] ws;
          if ((ws = workQueues) != null && w != null && (wl = ws.length) > 0) {
              for (int m = wl - 1,
                       origin = m & r, idx = origin,
                       npolls = 0,
                       ss = w.scanState;;) {         // negative if inactive
                  WorkQueue q; ForkJoinTask<?>[] a; int b, al;
                  if ((q = ws[idx]) != null && (b = q.base) - q.top < 0 &&
                      (a = q.array) != null && (al = a.length) > 0) {
                      int index = (al - 1) & b;
                      long offset = ((long)index << ASHIFT) + ABASE;
                      ForkJoinTask<?> t = (ForkJoinTask<?>)
                          U.getObjectVolatile(a, offset);
                      if (t == null)
                          break;                     // empty or busy
                      else if (b++ != q.base)
                          break;                     // busy
                      else if (ss < 0) {
                          tryReactivate(w, ws, r);
                          break;                     // retry upon rescan
                      }
                      else if (!U.compareAndSwapObject(a, offset, t, null))
                          break;                     // contended
                      else {
                          q.base = b;
                          w.currentSteal = t;
                          if (b != q.top)            // propagate signal
                              signalWork();
                          w.runTask(t);
                          if (++npolls > bound)
                              break;
                      }
                  }
                  else if (npolls != 0)              // rescan
                      break;
                  else if ((idx = (idx + step) & m) == origin) {
                      if (ss < 0) {                  // await signal
                          stat = ss;
                          break;
                      }
                      else if (r >= 0) {
                          inactivate(w, ss);
                          break;
                      }
                      else
                          r <<= 1;                   // at most 31 rescans
                  }
              }
          }
          return stat;
      }
  
      // Joining tasks
  
      /**
       * Tries to steal and run tasks within the target's computation.
       * Uses a variant of the top-level algorithm, restricted to tasks
       * with the given task as ancestor: It prefers taking and running
       * eligible tasks popped from the worker's own queue (via
       * popCC). Otherwise it scans others, randomly moving on
       * contention or execution, deciding to give up based on a
       * checksum (via return codes from pollAndExecCC). The maxTasks
       * argument supports external usages; internal calls use zero,
       * allowing unbounded steps (external calls trap non-positive
       * values).
       *
       * @param w caller
       * @param maxTasks if non-zero, the maximum number of other tasks to run
       * @return task status on exit
       */
      final int helpComplete(WorkQueue w, CountedCompleter<?> task,
                             int maxTasks) {
          WorkQueue[] ws; int s = 0, wl;
          if ((ws = workQueues) != null && (wl = ws.length) > 1 &&
              task != null && w != null) {
              for (int m = wl - 1,
                       mode = w.config,
                       r = ~mode,                  // scanning seed
                       origin = r & m, k = origin, // first queue to scan
                       step = 3,                   // first scan step
                       h = 1,                      // 1:ran, >1:contended, <0:hash
                       oldSum = 0, checkSum = 0;;) {
                  CountedCompleter<?> p; WorkQueue q; int i;
                  if ((s = task.status) < 0)
                      break;
                  if (h == 1 && (p = w.popCC(task, mode)) != null) {
                      p.doExec();                  // run local task
                      if (maxTasks != 0 && --maxTasks == 0)
                          break;
                      origin = k;                  // reset
                      oldSum = checkSum = 0;
                  }
                  else {                           // poll other worker queues
                      if ((i = k | 1) < 0 || i > m || (q = ws[i]) == null)
                          h = 0;
                      else if ((h = q.pollAndExecCC(task)) < 0)
                          checkSum += h;
                      if (h > 0) {
                          if (h == 1 && maxTasks != 0 && --maxTasks == 0)
                              break;
                          step = (r >>> 16) | 3;
                          r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
                          k = origin = r & m;      // move and restart
                          oldSum = checkSum = 0;
                      }
                      else if ((k = (k + step) & m) == origin) {
                          if (oldSum == (oldSum = checkSum))
                              break;
                          checkSum = 0;
                      }
                  }
              }
          }
          return s;
      }
  
      /**
       * 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.
       *
       * @param w caller
       * @param task the task to join
       */
      private void helpStealer(WorkQueue w, ForkJoinTask<?> task) {
          if (task != null && w != null) {
              ForkJoinTask<?> ps = w.currentSteal;
              WorkQueue[] ws; int wl, oldSum = 0;
              outer: while (w.tryRemoveAndExec(task) && task.status >= 0 &&
                            (ws = workQueues) != null && (wl = ws.length) > 0) {
                  ForkJoinTask<?> subtask;
                  int m = wl - 1, checkSum = 0;          // for stability check
                  WorkQueue j = w, v;                    // v is subtask stealer
                  descent: for (subtask = task; subtask.status >= 0; ) {
                      for (int h = j.hint | 1, k = 0, i;;) {
                          if ((v = ws[i = (h + (k << 1)) & m]) != null) {
                              if (v.currentSteal == subtask) {
                                  j.hint = i;
                                  break;
                              }
                              checkSum += v.base;
                          }
                          if (++k > m)                   // can't find stealer
                              break outer;
                      }
  
                      for (;;) {                         // help v or descend
                          ForkJoinTask<?>[] a; int b, al;
                          if (subtask.status < 0)        // too late to help
                              break descent;
                          checkSum += (b = v.base);
                          ForkJoinTask<?> next = v.currentJoin;
                          ForkJoinTask<?> t = null;
                          if ((a = v.array) != null && (al = a.length) > 0) {
                              int index = (al - 1) & b;
                              long offset = ((long)index << ASHIFT) + ABASE;
                              t = (ForkJoinTask<?>)
                                  U.getObjectVolatile(a, offset);
                              if (t != null && b++ == v.base) {
                                  if (j.currentJoin != subtask ||
                                      v.currentSteal != subtask ||
                                      subtask.status < 0)
                                      break descent;     // stale
                                  if (U.compareAndSwapObject(a, offset, t, null)) {
                                      v.base = b;
                                      w.currentSteal = t;
                                      for (int top = w.top;;) {
                                          t.doExec();    // help
                                          w.currentSteal = ps;
                                          if (task.status < 0)
                                              break outer;
                                          if (w.top == top)
                                              break;     // run local tasks
                                          if ((t = w.pop()) == null)
                                              break descent;
                                          w.currentSteal = t;
                                      }
                                  }
                              }
                          }
                          if (t == null && b == v.base && b - v.top >= 0) {
                              if ((subtask = next) == null) {  // try to descend
                                  if (next == v.currentJoin &&
                                      oldSum == (oldSum = checkSum))
                                      break outer;
                                  break descent;
                              }
                              j = v;
                              break;
                          }
                      }
                  }
              }
          }
      }
  
      /**
       * Tries to decrement active count (sometimes implicitly) and
       * possibly release or create a compensating worker in preparation
       * for blocking. Returns false (retryable by caller), on
       * contention, detected staleness, instability, or termination.
       *
       * @param w caller
       */
      private boolean tryCompensate(WorkQueue w) {
          boolean canBlock; int wl;
          long c = ctl;
          WorkQueue[] ws = workQueues;
          int pc = config & SMASK;
          int ac = pc + (int)(c >> AC_SHIFT);
          int tc = pc + (short)(c >> TC_SHIFT);
          if (w == null || w.qlock < 0 || pc == 0 ||  // terminating or disabled
              ws == null || (wl = ws.length) <= 0)
              canBlock = false;
          else {
              int m = wl - 1, sp;
              boolean busy = true;                    // validate ac
              for (int i = 0; i <= m; ++i) {
                  int k; WorkQueue v;
                  if ((k = (i << 1) | 1) <= m && k >= 0 && (v = ws[k]) != null &&
                      v.scanState >= 0 && v.currentSteal == null) {
                      busy = false;
                      break;
                  }
              }
              if (!busy || ctl != c)
                  canBlock = false;                   // unstable or stale
              else if ((sp = (int)c) != 0)            // release idle worker
                  canBlock = tryRelease(c, ws[m & sp], 0L);
              else if (tc >= pc && ac > 1 && w.isEmpty()) {
                  long nc = ((AC_MASK & (c - AC_UNIT)) |
                             (~AC_MASK & c));         // uncompensated
                  canBlock = U.compareAndSwapLong(this, CTL, c, nc);
              }
              else if (tc >= MAX_CAP ||
                       (this == common && tc >= pc + COMMON_MAX_SPARES))
                  throw new RejectedExecutionException(
                      "Thread limit exceeded replacing blocked worker");
              else {                                  // similar to tryAddWorker
                  boolean isSpare = (tc >= pc);
                  long nc = (AC_MASK & c) | (TC_MASK & (c + TC_UNIT));
                  canBlock = (U.compareAndSwapLong(this, CTL, c, nc) &&
                              createWorker(isSpare)); // throws on exception
              }
          }
          return canBlock;
      }
  
      /**
       * Helps and/or blocks until the given task is done or timeout.
       *
       * @param w caller
       * @param task the task
       * @param deadline for timed waits, if nonzero
       * @return task status on exit
       */
      final int awaitJoin(WorkQueue w, ForkJoinTask<?> task, long deadline) {
          int s = 0;
          if (w != null) {
              ForkJoinTask<?> prevJoin = w.currentJoin;
              if (task != null && (s = task.status) >= 0) {
                  w.currentJoin = task;
                  CountedCompleter<?> cc = (task instanceof CountedCompleter) ?
                      (CountedCompleter<?>)task : null;
                  for (;;) {
                      if (cc != null)
                          helpComplete(w, cc, 0);
                      else
                          helpStealer(w, task);
                      if ((s = task.status) < 0)
                          break;
                      long ms, ns;
                      if (deadline == 0L)
                          ms = 0L;
                      else if ((ns = deadline - System.nanoTime()) <= 0L)
                          break;
                      else if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) <= 0L)
                          ms = 1L;
                      if (tryCompensate(w)) {
                          task.internalWait(ms);
                          U.getAndAddLong(this, CTL, AC_UNIT);
                      }
                      if ((s = task.status) < 0)
                          break;
                  }
                  w.currentJoin = prevJoin;
              }
          }
          return s;
      }
  
      // Specialized scanning
  
      /**
       * Returns a (probably) non-empty steal queue, if one is found
       * during a 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[] ws; int wl;  // one-shot version of scan loop
          int r = ThreadLocalRandom.nextSecondarySeed();
          if ((ws = workQueues) != null && (wl = ws.length) > 0) {
              int m = wl - 1, origin = r & m;
              for (int k = origin, oldSum = 0, checkSum = 0;;) {
                  WorkQueue q; int b;
                  if ((q = ws[k]) != null) {
                      if ((b = q.base) - q.top < 0)
                          return q;
                      checkSum += b;
                  }
                  if ((k = (k + 1) & m) == origin) {
                      if (oldSum == (oldSum = checkSum))
                          break;
                      checkSum = 0;
                  }
              }
          }
          return null;
      }
  
      /**
       * 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) {
          ForkJoinTask<?> ps = w.currentSteal; // save context
          int wc = w.config;
          for (boolean active = true;;) {
              long c; WorkQueue q; ForkJoinTask<?> t;
              if (wc >= 0 && (t = w.pop()) != null) { // run locals if LIFO
                  (w.currentSteal = t).doExec();
                  w.currentSteal = ps;
              }
              else if ((q = findNonEmptyStealQueue()) != null) {
                  if (!active) {      // re-establish active count
                      active = true;
                      U.getAndAddLong(this, CTL, AC_UNIT);
                  }
                  if ((t = q.pollAt(q.base)) != null) {
                      (w.currentSteal = t).doExec();
                      w.currentSteal = ps;
                      if (++w.nsteals < 0)
                          w.transferStealCount(this);
                  }
              }
              else if (active) {      // decrement active count without queuing
                  long nc = (AC_MASK & ((c = ctl) - AC_UNIT)) | (~AC_MASK & c);
                  if (U.compareAndSwapLong(this, CTL, c, nc))
                      active = false;
              }
              else if ((int)((c = ctl) >> AC_SHIFT) + (config & SMASK) <= 0 &&
                       U.compareAndSwapLong(this, CTL, c, 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;
              if ((t = w.nextLocalTask()) != null)
                  return t;
              if ((q = findNonEmptyStealQueue()) == null)
                  return null;
              if ((t = q.pollAt(q.base)) != null)
                  return t;
          }
      }
  
      /**
       * Returns a cheap heuristic guide for task partitioning when
       * programmers, frameworks, tools, or languages have little or no
       * idea about task granularity.  In essence, by offering this
       * method, we ask users only about tradeoffs in overhead vs
       * expected throughput and its variance, rather than how finely to
       * partition tasks.
       *
       * In a steady state strict (tree-structured) computation, each
       * thread makes available for stealing enough tasks for other
       * threads to remain active. Inductively, if all threads play by
       * the same rules, each thread should make available only a
       * constant number of tasks.
       *
       * The minimum useful constant is just 1. But using a value of 1
       * would require immediate replenishment upon each steal to
       * maintain enough tasks, which is infeasible.  Further,
       * partitionings/granularities of offered tasks should minimize
       * steal rates, which in general means that threads nearer the top
       * of computation tree should generate more than those nearer the
       * bottom. In perfect steady state, each thread is at
       * approximately the same level of computation tree. However,
       * producing extra tasks amortizes the uncertainty of progress and
       * diffusion assumptions.
       *
       * So, users will want to use values larger (but not much larger)
       * than 1 to both smooth over transient shortages and hedge
       * against uneven progress; as traded off against the cost of
       * extra task overhead. We leave the user to pick a threshold
       * value to compare with the results of this call to guide
       * decisions, but recommend values such as 3.
       *
       * When all threads are active, it is on average OK to estimate
       * surplus strictly locally. In steady-state, if one thread is
       * maintaining say 2 surplus tasks, then so are others. So we can
       * just use estimated queue length.  However, this strategy alone
       * leads to serious mis-estimates in some non-steady-state
       * conditions (ramp-up, ramp-down, other stalls). We can detect
       * many of these by further considering the number of "idle"
       * threads, that are known to have zero queued tasks, so
       * compensate by a factor of (#idle/#active) threads.
       */
      static int getSurplusQueuedTaskCount() {
          Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
          if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
              int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).config & SMASK;
              int n = (q = wt.workQueue).top - q.base;
              int a = (int)(pool.ctl >> AC_SHIFT) + p;
              return n - (a > (p >>>= 1) ? 0 :
                          a > (p >>>= 1) ? 1 :
                          a > (p >>>= 1) ? 2 :
                          a > (p >>>= 1) ? 4 :
                          8);
          }
          return 0;
      }
  
      //  Termination
  
      /**
       * Possibly initiates and/or completes termination.
       *
       * @param now if true, unconditionally terminate, else only
       * if no work and no active workers
       * @param enable if true, terminate when next possible
       * @return -1: terminating/terminated, 0: retry if internal caller, else 1
       */
      private int tryTerminate(boolean now, boolean enable) {
          int rs; // 3 phases: try to set SHUTDOWN, then STOP, then TERMINATED
  
          while ((rs = runState) >= 0) {
              if (!enable || this == common)        // cannot shutdown
                  return 1;
              else if (rs == 0)
                  tryInitialize(false);             // ensure initialized
              else
                  U.compareAndSwapInt(this, RUNSTATE, rs, rs | SHUTDOWN);
          }
  
          if ((rs & STOP) == 0) {                   // try to initiate termination
              if (!now) {                           // check quiescence
                  for (long oldSum = 0L;;) {        // repeat until stable
                      WorkQueue[] ws; WorkQueue w; int b;
                      long checkSum = ctl;
                      if ((int)(checkSum >> AC_SHIFT) + (config & SMASK) > 0)
                          return 0;                 // still active workers
                      if ((ws = workQueues) != null) {
                          for (int i = 0; i < ws.length; ++i) {
                              if ((w = ws[i]) != null) {
                                  checkSum += (b = w.base);
                                  if (w.currentSteal != null || b != w.top)
                                      return 0;     // retry if internal caller
                              }
                          }
                      }
                      if (oldSum == (oldSum = checkSum))
                          break;
                  }
              }
              do {} while (!U.compareAndSwapInt(this, RUNSTATE,
                                                rs = runState, rs | STOP));
          }
  
          for (long oldSum = 0L;;) {                // repeat until stable
              WorkQueue[] ws; WorkQueue w; ForkJoinWorkerThread wt;
              long checkSum = ctl;
              if ((ws = workQueues) != null) {      // help terminate others
                  for (int i = 0; i < ws.length; ++i) {
                      if ((w = ws[i]) != null) {
                          w.cancelAll();            // clear queues
                          checkSum += w.base;
                          if (w.qlock >= 0) {
                              w.qlock = -1;         // racy set OK
                              if ((wt = w.owner) != null) {
                                  try {             // unblock join or park
                                      wt.interrupt();
                                  } catch (Throwable ignore) {
                                  }
                              }
                          }
                      }
                  }
              }
              if (oldSum == (oldSum = checkSum))
                  break;
          }
  
          if ((short)(ctl >>> TC_SHIFT) + (config & SMASK) <= 0) {
              runState = (STARTED | SHUTDOWN | STOP | TERMINATED); // final write
              synchronized (this) {
                  notifyAll();                      // for awaitTermination
              }
          }
  
          return -1;
      }
  
      // External operations
  
      /**
       * Constructs and tries to install a new external queue,
       * failing if the workQueues array already has a queue at
       * the given index.
       *
       * @param index the index of the new queue
       */
      private void tryCreateExternalQueue(int index) {
          AuxState aux;
          if ((aux = auxState) != null && index >= 0) {
              WorkQueue q = new WorkQueue(this, null);
              q.config = index;
              q.scanState = ~UNSIGNALLED;
              q.qlock = 1;                   // lock queue
              boolean installed = false;
              aux.lock();
              try {                          // lock pool to install
                  WorkQueue[] ws;
                  if ((ws = workQueues) != null && index < ws.length &&
                      ws[index] == null) {
                      ws[index] = q;         // else throw away
                      installed = true;
                  }
              } finally {
                  aux.unlock();
              }
              if (installed) {
                  try {
                      q.growArray();
                  } finally {
                      q.qlock = 0;
                  }
              }
          }
      }
  
      /**
       * Adds the given task to a submission queue at submitter's
       * current queue. Also performs secondary initialization upon the
       * first submission of the first task to the pool, and detects
       * first submission by an external thread and creates a new shared
       * queue if the one at index if empty or contended.
       *
       * @param task the task. Caller must ensure non-null.
       */
      final void externalPush(ForkJoinTask<?> task) {
          int r;                            // initialize caller's probe
          if ((r = ThreadLocalRandom.getProbe()) == 0) {
              ThreadLocalRandom.localInit();
              r = ThreadLocalRandom.getProbe();
          }
          for (;;) {
              WorkQueue q; int wl, k, stat;
              int rs = runState;
              WorkQueue[] ws = workQueues;
              if (rs <= 0 || ws == null || (wl = ws.length) <= 0)
                  tryInitialize(true);
              else if ((q = ws[k = (wl - 1) & r & SQMASK]) == null)
                  tryCreateExternalQueue(k);
              else if ((stat = q.sharedPush(task)) < 0)
                  break;
              else if (stat == 0) {
                  signalWork();
                  break;
              }
              else                          // move if busy
                  r = ThreadLocalRandom.advanceProbe(r);
          }
      }
  
      /**
       * Pushes a possibly-external submission.
       */
      private <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task) {
          Thread t; ForkJoinWorkerThread w; WorkQueue q;
          if (task == null)
              throw new NullPointerException();
          if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
              (w = (ForkJoinWorkerThread)t).pool == this &&
              (q = w.workQueue) != null)
              q.push(task);
          else
              externalPush(task);
          return task;
      }
  
      /**
       * Returns common pool queue for an external thread.
       */
      static WorkQueue commonSubmitterQueue() {
          ForkJoinPool p = common;
          int r = ThreadLocalRandom.getProbe();
          WorkQueue[] ws; int wl;
          return (p != null && (ws = p.workQueues) != null &&
                  (wl = ws.length) > 0) ?
              ws[(wl - 1) & r & SQMASK] : null;
      }
  
      /**
       * Performs tryUnpush for an external submitter.
       */
      final boolean tryExternalUnpush(ForkJoinTask<?> task) {
          int r = ThreadLocalRandom.getProbe();
          WorkQueue[] ws; WorkQueue w; int wl;
          return ((ws = workQueues) != null &&
                  (wl = ws.length) > 0 &&
                  (w = ws[(wl - 1) & r & SQMASK]) != null &&
                  w.trySharedUnpush(task));
      }
  
      /**
       * Performs helpComplete for an external submitter.
       */
      final int externalHelpComplete(CountedCompleter<?> task, int maxTasks) {
          WorkQueue[] ws; int wl;
          int r = ThreadLocalRandom.getProbe();
          return ((ws = workQueues) != null && (wl = ws.length) > 0) ?
              helpComplete(ws[(wl - 1) & r & SQMASK], task, maxTasks) : 0;
      }
  
      // Exported methods
  
      // Constructors
  
      /**
       * Creates a {@code ForkJoinPool} with parallelism equal to {@link
       * 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
       *         RuntimePermission}{@code ("modifyThread")}
       */
      public ForkJoinPool() {
          this(Math.min(MAX_CAP, 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
       *         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 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
       *         RuntimePermission}{@code ("modifyThread")}
       */
      public ForkJoinPool(int parallelism,
                          ForkJoinWorkerThreadFactory factory,
                          UncaughtExceptionHandler handler,
                          boolean asyncMode) {
          this(checkParallelism(parallelism),
               checkFactory(factory),
               handler,
               asyncMode ? FIFO_QUEUE : LIFO_QUEUE,
               "ForkJoinPool-" + nextPoolId() + "-worker-");
          checkPermission();
      }
  
      private static int checkParallelism(int parallelism) {
          if (parallelism <= 0 || parallelism > MAX_CAP)
              throw new IllegalArgumentException();
          return parallelism;
      }
  
      private static ForkJoinWorkerThreadFactory checkFactory
          (ForkJoinWorkerThreadFactory factory) {
          if (factory == null)
              throw new NullPointerException();
          return factory;
      }
  
      /**
       * Creates a {@code ForkJoinPool} with the given parameters, without
       * any security checks or parameter validation.  Invoked directly by
       * makeCommonPool.
       */
      private ForkJoinPool(int parallelism,
                           ForkJoinWorkerThreadFactory factory,
                           UncaughtExceptionHandler handler,
                           int mode,
                           String workerNamePrefix) {
          this.workerNamePrefix = workerNamePrefix;
          this.factory = factory;
          this.ueh = handler;
          this.config = (parallelism & SMASK) | mode;
          long np = (long)(-parallelism); // offset ctl counts
          this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
      }
  
      /**
       * Returns the common pool instance. This pool is statically
       * constructed; its run state is unaffected by attempts to {@link
       * #shutdown} or {@link #shutdownNow}. However this pool and any
       * ongoing processing are automatically terminated upon program
       * {@link System#exit}.  Any program that relies on asynchronous
       * task processing to complete before program termination should
       * invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence},
       * before exit.
       *
       * @return the common pool instance
       * @since 1.8
       */
      public static ForkJoinPool commonPool() {
          // assert common != null : "static init error";
          return common;
      }
  
      // 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
       * @param <T> the type of the task's result
       * @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();
          externalSubmit(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) {
          externalSubmit(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.RunnableExecuteAction(task);
          externalSubmit(job);
      }
  
      /**
       * Submits a ForkJoinTask for execution.
       *
       * @param task the task to submit
       * @param <T> the type of the task's result
       * @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) {
          return externalSubmit(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) {
          return externalSubmit(new ForkJoinTask.AdaptedCallable<T>(task));
      }
  
      /**
       * @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) {
          return externalSubmit(new ForkJoinTask.AdaptedRunnable<T>(task, result));
      }
  
      /**
       * @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);
          return externalSubmit(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.
          ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());
  
          try {
              for (Callable<T> t : tasks) {
                  ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
                  futures.add(f);
                  externalSubmit(f);
              }
              for (int i = 0, size = futures.size(); i < size; i++)
                  ((ForkJoinTask<?>)futures.get(i)).quietlyJoin();
              return futures;
          } catch (Throwable t) {
              for (int i = 0, size = futures.size(); i < size; i++)
                  futures.get(i).cancel(false);
              throw t;
          }
      }
  
      /**
       * 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 UncaughtExceptionHandler getUncaughtExceptionHandler() {
          return ueh;
      }
  
      /**
       * Returns the targeted parallelism level of this pool.
       *
       * @return the targeted parallelism level of this pool
       */
      public int getParallelism() {
          int par;
          return ((par = config & SMASK) > 0) ? par : 1;
      }
  
      /**
       * Returns the targeted parallelism level of the common pool.
       *
       * @return the targeted parallelism level of the common pool
       * @since 1.8
       */
      public static int getCommonPoolParallelism() {
          return COMMON_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 (config & SMASK) + (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 (config & FIFO_QUEUE) != 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 = (config & SMASK) + (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 (config & SMASK) + (int)(ctl >> AC_SHIFT) <= 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() {
          AuxState sc = auxState;
          long count = (sc == null) ? 0L : sc.stealCount;
          WorkQueue[] ws; WorkQueue w;
          if ((ws = workQueues) != null) {
              for (int i = 1; i < ws.length; i += 2) {
                  if ((w = ws[i]) != null)
                      count += w.nsteals;
              }
          }
          return count;
      }
  
      /**
       * 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; int wl; WorkQueue w; ForkJoinTask<?> t;
          int r = ThreadLocalRandom.nextSecondarySeed();
          if ((ws = workQueues) != null && (wl = ws.length) > 0) {
              for (int m = wl - 1, i = 0; i < wl; ++i) {
                  if ((w = ws[(i << 1) & m]) != 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;
          AuxState sc = auxState;
          long st = (sc == null) ? 0L : sc.stealCount;
          long c = ctl;
          WorkQueue[] ws; WorkQueue w;
          if ((ws = workQueues) != null) {
              for (int i = 0; i < ws.length; ++i) {
                  if ((w = ws[i]) != null) {
                      int size = w.queueSize();
                      if ((i & 1) == 0)
                          qs += size;
                      else {
                          qt += size;
                          st += w.nsteals;
                          if (w.isApparentlyUnblocked())
                              ++rc;
                      }
                  }
              }
          }
          int pc = (config & SMASK);
          int tc = pc + (short)(c >>> TC_SHIFT);
          int ac = pc + (int)(c >> AC_SHIFT);
          if (ac < 0) // ignore transient negative
              ac = 0;
          int rs = runState;
          String level = ((rs & TERMINATED) != 0 ? "Terminated" :
                          (rs & STOP)       != 0 ? "Terminating" :
                          (rs & SHUTDOWN)   != 0 ? "Shutting down" :
                          "Running");
          return super.toString() +
              "[" + level +
              ", parallelism = " + pc +
              ", size = " + tc +
              ", active = " + ac +
              ", running = " + rc +
              ", steals = " + st +
              ", tasks = " + qt +
              ", submissions = " + qs +
              "]";
      }
  
      /**
       * Possibly initiates an orderly shutdown in which previously
       * submitted tasks are executed, but no new tasks will be
       * accepted. Invocation has no effect on execution state if this
       * is the {@link #commonPool()}, and no additional effect if
       * already shut down.  Tasks that are in the process of being
       * submitted concurrently during the course of this method may or
       * may not be rejected.
       *
       * @throws SecurityException if a security manager exists and
       *         the caller is not permitted to modify threads
       *         because it does not hold {@link
       *         RuntimePermission}{@code ("modifyThread")}
       */
      public void shutdown() {
          checkPermission();
          tryTerminate(false, true);
      }
  
      /**
       * Possibly attempts to cancel and/or stop all tasks, and reject
       * all subsequently submitted tasks.  Invocation has no effect on
       * execution state if this is the {@link #commonPool()}, and no
       * additional effect if already shut down. Otherwise, tasks that
       * are in the process of being submitted or executed concurrently
       * during the course of this method may or may not be
       * rejected. This method cancels both existing and unexecuted
       * tasks, in order to permit termination in the presence of task
       * dependencies. So the method always returns an empty list
       * (unlike the case for some other Executors).
       *
       * @return an empty list
       * @throws SecurityException if a security manager exists and
       *         the caller is not permitted to modify threads
       *         because it does not hold {@link
       *         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() {
          return (runState & TERMINATED) != 0;
      }
  
      /**
       * 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 I/O,
       * causing this executor not to properly terminate. (See the
       * advisory notes for class {@link ForkJoinTask} stating that
       * tasks should not normally entail blocking operations.  But if
       * they do, they must abort them on interrupt.)
       *
       * @return {@code true} if terminating but not yet terminated
       */
      public boolean isTerminating() {
          int rs = runState;
          return (rs & STOP) != 0 && (rs & TERMINATED) == 0;
      }
  
      /**
       * 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 & SHUTDOWN) != 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. Because the {@link
       * #commonPool()} never terminates until program shutdown, when
       * applied to the common pool, this method is equivalent to {@link
       * #awaitQuiescence(long, TimeUnit)} but always returns {@code false}.
       *
       * @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 {
          if (Thread.interrupted())
              throw new InterruptedException();
          if (this == common) {
              awaitQuiescence(timeout, unit);
              return false;
          }
          long nanos = unit.toNanos(timeout);
          if (isTerminated())
              return true;
          if (nanos <= 0L)
              return false;
          long deadline = System.nanoTime() + nanos;
          synchronized (this) {
              for (;;) {
                  if (isTerminated())
                      return true;
                  if (nanos <= 0L)
                      return false;
                  long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
                  wait(millis > 0L ? millis : 1L);
                  nanos = deadline - System.nanoTime();
              }
          }
      }
  
      /**
       * If called by a ForkJoinTask operating in this pool, equivalent
       * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise,
       * waits and/or attempts to assist performing tasks until this
       * pool {@link #isQuiescent} or the indicated timeout elapses.
       *
       * @param timeout the maximum time to wait
       * @param unit the time unit of the timeout argument
       * @return {@code true} if quiescent; {@code false} if the
       * timeout elapsed.
       */
      public boolean awaitQuiescence(long timeout, TimeUnit unit) {
          long nanos = unit.toNanos(timeout);
          ForkJoinWorkerThread wt;
          Thread thread = Thread.currentThread();
          if ((thread instanceof ForkJoinWorkerThread) &&
              (wt = (ForkJoinWorkerThread)thread).pool == this) {
              helpQuiescePool(wt.workQueue);
              return true;
          }
          long startTime = System.nanoTime();
          WorkQueue[] ws;
          int r = 0, wl;
          boolean found = true;
          while (!isQuiescent() && (ws = workQueues) != null &&
                 (wl = ws.length) > 0) {
              if (!found) {
                  if ((System.nanoTime() - startTime) > nanos)
                      return false;
                  Thread.yield(); // cannot block
              }
              found = false;
              for (int m = wl - 1, j = (m + 1) << 2; j >= 0; --j) {
                  ForkJoinTask<?> t; WorkQueue q; int b, k;
                  if ((k = r++ & m) <= m && k >= 0 && (q = ws[k]) != null &&
                      (b = q.base) - q.top < 0) {
                      found = true;
                      if ((t = q.pollAt(b)) != null)
                          t.doExec();
                      break;
                  }
              }
          }
          return true;
      }
  
      /**
       * Waits and/or attempts to assist performing tasks indefinitely
       * until the {@link #commonPool()} {@link #isQuiescent}.
       */
      static void quiesceCommonPool() {
          common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
      }
  
      /**
       * Interface for extending managed parallelism for tasks running
       * in {@link ForkJoinPool}s.
       *
       * <p>A {@code ManagedBlocker} provides two methods.  Method
       * {@link #isReleasable} must return {@code true} if blocking is
       * not necessary. Method {@link #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(ManagedBlocker)}.
       * 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.
           * @return {@code true} if blocking is unnecessary
           */
          boolean isReleasable();
      }
  
      /**
       * Runs the given possibly blocking task.  When {@linkplain
       * ForkJoinTask#inForkJoinPool() running in a ForkJoinPool}, this
       * method possibly arranges for a spare thread to be activated if
       * necessary to ensure sufficient parallelism while the current
       * thread is blocked in {@link ManagedBlocker#block blocker.block()}.
       *
       * <p>This method repeatedly calls {@code blocker.isReleasable()} and
       * {@code blocker.block()} until either method returns {@code true}.
       * Every call to {@code blocker.block()} is preceded by a call to
       * {@code blocker.isReleasable()} that returned {@code false}.
       *
       * <p>If not running in a ForkJoinPool, this method is
       * behaviorally equivalent to
       * <pre> {@code
       * while (!blocker.isReleasable())
       *   if (blocker.block())
       *     break;}</pre>
       *
       * If running in a ForkJoinPool, the pool may first be expanded to
       * ensure sufficient parallelism available during the call to
       * {@code blocker.block()}.
       *
       * @param blocker the blocker task
       * @throws InterruptedException if {@code blocker.block()} did so
       */
      public static void managedBlock(ManagedBlocker blocker)
          throws InterruptedException {
          ForkJoinPool p;
          ForkJoinWorkerThread wt;
          Thread t = Thread.currentThread();
          if ((t instanceof ForkJoinWorkerThread) &&
              (p = (wt = (ForkJoinWorkerThread)t).pool) != null) {
              WorkQueue w = wt.workQueue;
              while (!blocker.isReleasable()) {
                  if (p.tryCompensate(w)) {
                      try {
                          do {} while (!blocker.isReleasable() &&
                                       !blocker.block());
                      } finally {
                          U.getAndAddLong(p, CTL, AC_UNIT);
                      }
                      break;
                  }
              }
          }
          else {
              do {} while (!blocker.isReleasable() &&
                           !blocker.block());
          }
      }
  
      // 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 = sun.misc.Unsafe.getUnsafe();
      private static final long CTL;
      private static final long RUNSTATE;
      private static final int ABASE;
      private static final int ASHIFT;
  
      static {
          try {
              CTL = U.objectFieldOffset
                  (ForkJoinPool.class.getDeclaredField("ctl"));
              RUNSTATE = U.objectFieldOffset
                  (ForkJoinPool.class.getDeclaredField("runState"));
              ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
              int scale = U.arrayIndexScale(ForkJoinTask[].class);
              if ((scale & (scale - 1)) != 0)
                  throw new Error("array index scale not a power of two");
              ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
          } catch (ReflectiveOperationException e) {
              throw new Error(e);
          }
  
          // Reduce the risk of rare disastrous classloading in first call to
          // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
          Class<?> ensureLoaded = LockSupport.class;
  
          int commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
          try {
              String p = System.getProperty
                  ("java.util.concurrent.ForkJoinPool.common.maximumSpares");
              if (p != null)
                  commonMaxSpares = Integer.parseInt(p);
          } catch (Exception ignore) {}
          COMMON_MAX_SPARES = commonMaxSpares;
  
          defaultForkJoinWorkerThreadFactory =
              new DefaultForkJoinWorkerThreadFactory();
          modifyThreadPermission = new RuntimePermission("modifyThread");
  
          common = java.security.AccessController.doPrivileged
              (new java.security.PrivilegedAction<ForkJoinPool>() {
                  public ForkJoinPool run() { return makeCommonPool(); }});
  
          // report 1 even if threads disabled
          COMMON_PARALLELISM = Math.max(common.config & SMASK, 1);
      }
  
      /**
       * Creates and returns the common pool, respecting user settings
       * specified via system properties.
       */
      static ForkJoinPool makeCommonPool() {
          int parallelism = -1;
          ForkJoinWorkerThreadFactory factory = null;
          UncaughtExceptionHandler handler = null;
          try {  // ignore exceptions in accessing/parsing properties
              String pp = System.getProperty
                  ("java.util.concurrent.ForkJoinPool.common.parallelism");
              String fp = System.getProperty
                  ("java.util.concurrent.ForkJoinPool.common.threadFactory");
              String hp = System.getProperty
                  ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
              if (pp != null)
                  parallelism = Integer.parseInt(pp);
              if (fp != null)
                  factory = ((ForkJoinWorkerThreadFactory)ClassLoader.
                             getSystemClassLoader().loadClass(fp).newInstance());
              if (hp != null)
                  handler = ((UncaughtExceptionHandler)ClassLoader.
                             getSystemClassLoader().loadClass(hp).newInstance());
          } catch (Exception ignore) {
          }
          if (factory == null) {
              if (System.getSecurityManager() == null)
                  factory = defaultForkJoinWorkerThreadFactory;
              else // use security-managed default
                  factory = new InnocuousForkJoinWorkerThreadFactory();
          }
          if (parallelism < 0 && // default 1 less than #cores
              (parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0)
              parallelism = 1;
          if (parallelism > MAX_CAP)
              parallelism = MAX_CAP;
          return new ForkJoinPool(parallelism, factory, handler, LIFO_QUEUE,
                                  "ForkJoinPool.commonPool-worker-");
      }
  
      /**
       * Factory for innocuous worker threads.
       */
      private static final class InnocuousForkJoinWorkerThreadFactory
          implements ForkJoinWorkerThreadFactory {
  
          /**
           * An ACC to restrict permissions for the factory itself.
           * The constructed workers have no permissions set.
           */
          private static final AccessControlContext innocuousAcc;
          static {
              Permissions innocuousPerms = new Permissions();
              innocuousPerms.add(modifyThreadPermission);
              innocuousPerms.add(new RuntimePermission(
                                     "enableContextClassLoaderOverride"));
              innocuousPerms.add(new RuntimePermission(
                                     "modifyThreadGroup"));
              innocuousAcc = new AccessControlContext(new ProtectionDomain[] {
                      new ProtectionDomain(null, innocuousPerms)
                  });
          }
  
          public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
              return java.security.AccessController.doPrivileged(
                  new java.security.PrivilegedAction<ForkJoinWorkerThread>() {
                      public ForkJoinWorkerThread run() {
                          return new ForkJoinWorkerThread.
                              InnocuousForkJoinWorkerThread(pool);
                      }}, innocuousAcc);
          }
      }
  
  }
  