package org.infinispan.util.concurrent.jdk8backported;

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

import java.util.concurrent.Future;

import java.io.Serializable;
import java.util.Collection;
import java.util.List;
import java.util.RandomAccess;
import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.ReentrantLock;
import java.lang.reflect.Constructor;

/**
 * Abstract base class for tasks that run within a {@link ForkJoinPool}.
 * A {@code ForkJoinTask} is a thread-like entity that is much
 * lighter weight than a normal thread.  Huge numbers of tasks and
 * subtasks may be hosted by a small number of actual threads in a
 * ForkJoinPool, at the price of some usage limitations.
 *
 * <p>A "main" {@code ForkJoinTask} begins execution when submitted
 * to a {@link ForkJoinPool}.  Once started, it will usually in turn
 * start other subtasks.  As indicated by the name of this class,
 * many programs using {@code ForkJoinTask} employ only methods
 * {@link #fork} and {@link #join}, or derivatives such as {@link
 * #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also
 * provides a number of other methods that can come into play in
 * advanced usages, as well as extension mechanics that allow
 * support of new forms of fork/join processing.
 *
 * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.
 * The efficiency of {@code ForkJoinTask}s stems from a set of
 * restrictions (that are only partially statically enforceable)
 * reflecting their main use as computational tasks calculating pure
 * functions or operating on purely isolated objects.  The primary
 * coordination mechanisms are {@link #fork}, that arranges
 * asynchronous execution, and {@link #join}, that doesn't proceed
 * until the task's result has been computed.  Computations should
 * ideally avoid {@code synchronized} methods or blocks, and should
 * minimize other blocking synchronization apart from joining other
 * tasks or using synchronizers such as Phasers that are advertised to
 * cooperate with fork/join scheduling. Subdividable tasks should also
 * not perform blocking IO, and should ideally access variables that
 * are completely independent of those accessed by other running
 * tasks. These guidelines are loosely enforced by not permitting
 * checked exceptions such as {@code IOExceptions} to be
 * thrown. However, computations may still encounter unchecked
 * exceptions, that are rethrown to callers attempting to join
 * them. These exceptions may additionally include {@link
 * RejectedExecutionException} stemming from internal resource
 * exhaustion, such as failure to allocate internal task
 * queues. Rethrown exceptions behave in the same way as regular
 * exceptions, but, when possible, contain stack traces (as displayed
 * for example using {@code ex.printStackTrace()}) of both the thread
 * that initiated the computation as well as the thread actually
 * encountering the exception; minimally only the latter.
 *
 * <p>It is possible to define and use ForkJoinTasks that may block,
 * but doing do requires three further considerations: (1) Completion
 * of few if any <em>other</em> tasks should be dependent on a task
 * that blocks on external synchronization or IO. Event-style async
 * tasks that are never joined (for example, those subclassing {@link
 * CountedCompleter}) often fall into this category.  (2) To minimize
 * resource impact, tasks should be small; ideally performing only the
 * (possibly) blocking action. (3) Unless the {@link
 * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly
 * blocked tasks is known to be less than the pool's {@link
 * ForkJoinPool#getParallelism} level, the pool cannot guarantee that
 * enough threads will be available to ensure progress or good
 * performance.
 *
 * <p>The primary method for awaiting completion and extracting
 * results of a task is {@link #join}, but there are several variants:
 * The {@link Future#get} methods support interruptible and/or timed
 * waits for completion and report results using {@code Future}
 * conventions. Method {@link #invoke} is semantically
 * equivalent to {@code fork(); join()} but always attempts to begin
 * execution in the current thread. The "<em>quiet</em>" forms of
 * these methods do not extract results or report exceptions. These
 * may be useful when a set of tasks are being executed, and you need
 * to delay processing of results or exceptions until all complete.
 * Method {@code invokeAll} (available in multiple versions)
 * performs the most common form of parallel invocation: forking a set
 * of tasks and joining them all.
 *
 * <p>In the most typical usages, a fork-join pair act like a call
 * (fork) and return (join) from a parallel recursive function. As is
 * the case with other forms of recursive calls, returns (joins)
 * should be performed innermost-first. For example, {@code a.fork();
 * b.fork(); b.join(); a.join();} is likely to be substantially more
 * efficient than joining {@code a} before {@code b}.
 *
 * <p>The execution status of tasks may be queried at several levels
 * of detail: {@link #isDone} is true if a task completed in any way
 * (including the case where a task was cancelled without executing);
 * {@link #isCompletedNormally} is true if a task completed without
 * cancellation or encountering an exception; {@link #isCancelled} is
 * true if the task was cancelled (in which case {@link #getException}
 * returns a {@link java.util.concurrent.CancellationException}); and
 * {@link #isCompletedAbnormally} is true if a task was either
 * cancelled or encountered an exception, in which case {@link
 * #getException} will return either the encountered exception or
 * {@link java.util.concurrent.CancellationException}.
 *
 * <p>The ForkJoinTask class is not usually directly subclassed.
 * Instead, you subclass one of the abstract classes that support a
 * particular style of fork/join processing, typically {@link
 * RecursiveAction} for most computations that do not return results,
 * {@link RecursiveTask} for those that do, and {@link
 * CountedCompleter} for those in which completed actions trigger
 * other actions.  Normally, a concrete ForkJoinTask subclass declares
 * fields comprising its parameters, established in a constructor, and
 * then defines a {@code compute} method that somehow uses the control
 * methods supplied by this base class. While these methods have
 * {@code public} access (to allow instances of different task
 * subclasses to call each other's methods), some of them may only be
 * called from within other ForkJoinTasks (as may be determined using
 * method {@link #inForkJoinPool}).  Attempts to invoke them in other
 * contexts result in exceptions or errors, possibly including {@code
 * ClassCastException}.
 *
 * <p>Method {@link #join} and its variants are appropriate for use
 * only when completion dependencies are acyclic; that is, the
 * parallel computation can be described as a directed acyclic graph
 * (DAG). Otherwise, executions may encounter a form of deadlock as
 * tasks cyclically wait for each other.  However, this framework
 * supports other methods and techniques (for example the use of
 * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
 * may be of use in constructing custom subclasses for problems that
 * are not statically structured as DAGs. To support such usages a
 * ForkJoinTask may be atomically <em>tagged</em> with a {@code short}
 * value using {@link #setForkJoinTaskTag} or {@link
 * #compareAndSetForkJoinTaskTag} and checked using {@link
 * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use
 * these {@code protected} methods or tags for any purpose, but they
 * may be of use in the construction of specialized subclasses.  For
 * example, parallel graph traversals can use the supplied methods to
 * avoid revisiting nodes/tasks that have already been processed.
 * (Method names for tagging are bulky in part to encourage definition
 * of methods that reflect their usage patterns.)
 *
 * <p>Most base support methods are {@code final}, to prevent
 * overriding of implementations that are intrinsically tied to the
 * underlying lightweight task scheduling framework.  Developers
 * creating new basic styles of fork/join processing should minimally
 * implement {@code protected} methods {@link #exec}, {@link
 * #setRawResult}, and {@link #getRawResult}, while also introducing
 * an abstract computational method that can be implemented in its
 * subclasses, possibly relying on other {@code protected} methods
 * provided by this class.
 *
 * <p>ForkJoinTasks should perform relatively small amounts of
 * computation. Large tasks should be split into smaller subtasks,
 * usually via recursive decomposition. As a very rough rule of thumb,
 * a task should perform more than 100 and less than 10000 basic
 * computational steps, and should avoid indefinite looping. If tasks
 * are too big, then parallelism cannot improve throughput. If too
 * small, then memory and internal task maintenance overhead may
 * overwhelm processing.
 *
 * <p>This class provides {@code adapt} methods for {@link Runnable}
 * and {@link Callable}, that may be of use when mixing execution of
 * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are
 * of this form, consider using a pool constructed in <em>asyncMode</em>.
 *
 * <p>ForkJoinTasks are {@code Serializable}, which enables them to be
 * used in extensions such as remote execution frameworks. It is
 * sensible to serialize tasks only before or after, but not during,
 * execution. Serialization is not relied on during execution itself.
 *
 * @since 1.7
 * @author Doug Lea
 */
public abstract class ForkJoinTask<V> implements Future<V>, Serializable {

   /*
   * See the internal documentation of class ForkJoinPool for a
   * general implementation overview.  ForkJoinTasks are mainly
   * responsible for maintaining their "status" field amidst relays
   * to methods in ForkJoinWorkerThread and ForkJoinPool.
   *
   * The methods of this class are more-or-less layered into
   * (1) basic status maintenance
   * (2) execution and awaiting completion
   * (3) user-level methods that additionally report results.
   * This is sometimes hard to see because this file orders exported
   * methods in a way that flows well in javadocs.
   */

   /*
   * The status field holds run control status bits packed into a
   * single int to minimize footprint and to ensure atomicity (via
   * CAS).  Status is initially zero, and takes on nonnegative
   * values until completed, upon which status (anded with
   * DONE_MASK) holds value NORMAL, CANCELLED, or EXCEPTIONAL. Tasks
   * undergoing blocking waits by other threads have the SIGNAL bit
   * set.  Completion of a stolen task with SIGNAL set awakens any
   * waiters via notifyAll. Even though suboptimal for some
   * purposes, we use basic builtin wait/notify to take advantage of
   * "monitor inflation" in JVMs that we would otherwise need to
   * emulate to avoid adding further per-task bookkeeping overhead.
   * We want these monitors to be "fat", i.e., not use biasing or
   * thin-lock techniques, so use some odd coding idioms that tend
   * to avoid them, mainly by arranging that every synchronized
   * block performs a wait, notifyAll or both.
   *
   * These control bits occupy only (some of) the upper half (16
   * bits) of status field. The lower bits are used for user-defined
   * tags.
   */

   /** The run status of this task */
   volatile int status; // accessed directly by pool and workers
   static final int DONE_MASK   = 0xf0000000;  // mask out non-completion bits
   static final int NORMAL      = 0xf0000000;  // must be negative
   static final int CANCELLED   = 0xc0000000;  // must be < NORMAL
   static final int EXCEPTIONAL = 0x80000000;  // must be < CANCELLED
   static final int SIGNAL      = 0x00010000;  // must be >= 1 << 16
   static final int SMASK       = 0x0000ffff;  // short bits for tags

   /**
    * Marks completion and wakes up threads waiting to join this
    * task.
    *
    * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
    * @return completion status on exit
    */
   private int setCompletion(int completion) {
      for (int s;;) {
         if ((s = status) < 0)
            return s;
         if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
            if ((s >>> 16) != 0)
               synchronized (this) { notifyAll(); }
            return completion;
         }
      }
   }

   /**
    * Primary execution method for stolen tasks. Unless done, calls
    * exec and records status if completed, but doesn't wait for
    * completion otherwise.
    *
    * @return status on exit from this method
    */
   final int doExec() {
      int s; boolean completed;
      if ((s = status) >= 0) {
         try {
            completed = exec();
         } catch (Throwable rex) {
            return setExceptionalCompletion(rex);
         }
         if (completed)
            s = setCompletion(NORMAL);
      }
      return s;
   }

   /**
    * Tries to set SIGNAL status unless already completed. Used by
    * ForkJoinPool. Other variants are directly incorporated into
    * externalAwaitDone etc.
    *
    * @return true if successful
    */
   final boolean trySetSignal() {
      int s = status;
      return s >= 0 && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL);
   }

   /**
    * Blocks a non-worker-thread until completion.
    * @return status upon completion
    */
   private int externalAwaitDone() {
      boolean interrupted = false;
      int s;
      while ((s = status) >= 0) {
         if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
            synchronized (this) {
               if (status >= 0) {
                  try {
                     wait();
                  } catch (InterruptedException ie) {
                     interrupted = true;
                  }
               }
               else
                  notifyAll();
            }
         }
      }
      if (interrupted)
         Thread.currentThread().interrupt();
      return s;
   }

   /**
    * Blocks a non-worker-thread until completion or interruption.
    */
   private int externalInterruptibleAwaitDone() throws InterruptedException {
      int s;
      if (Thread.interrupted())
         throw new InterruptedException();
      while ((s = status) >= 0) {
         if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
            synchronized (this) {
               if (status >= 0)
                  wait();
               else
                  notifyAll();
            }
         }
      }
      return s;
   }

   /**
    * Implementation for join, get, quietlyJoin. Directly handles
    * only cases of already-completed, external wait, and
    * unfork+exec.  Others are relayed to ForkJoinPool.awaitJoin.
    *
    * @return status upon completion
    */
   private int doJoin() {
      int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
      if ((s = status) >= 0) {
         if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
            if (!(w = (wt = (ForkJoinWorkerThread)t).workQueue).
                  tryUnpush(this) || (s = doExec()) >= 0)
               s = wt.pool.awaitJoin(w, this);
         }
         else
            s = externalAwaitDone();
      }
      return s;
   }

   /**
    * Implementation for invoke, quietlyInvoke.
    *
    * @return status upon completion
    */
   private int doInvoke() {
      int s; Thread t; ForkJoinWorkerThread wt;
      if ((s = doExec()) >= 0) {
         if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
            s = (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue,
                                                              this);
         else
            s = externalAwaitDone();
      }
      return s;
   }

   // Exception table support

   /**
    * Table of exceptions thrown by tasks, to enable reporting by
    * callers. Because exceptions are rare, we don't directly keep
    * them with task objects, but instead use a weak ref table.  Note
    * that cancellation exceptions don't appear in the table, but are
    * instead recorded as status values.
    *
    * Note: These statics are initialized below in static block.
    */
   private static final ExceptionNode[] exceptionTable;
   private static final ReentrantLock exceptionTableLock;
   private static final ReferenceQueue<Object> exceptionTableRefQueue;

   /**
    * Fixed capacity for exceptionTable.
    */
   private static final int EXCEPTION_MAP_CAPACITY = 32;

   /**
    * Key-value nodes for exception table.  The chained hash table
    * uses identity comparisons, full locking, and weak references
    * for keys. The table has a fixed capacity because it only
    * maintains task exceptions long enough for joiners to access
    * them, so should never become very large for sustained
    * periods. However, since we do not know when the last joiner
    * completes, we must use weak references and expunge them. We do
    * so on each operation (hence full locking). Also, some thread in
    * any ForkJoinPool will call helpExpungeStaleExceptions when its
    * pool becomes isQuiescent.
    */
   static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> {
      final Throwable ex;
      ExceptionNode next;
      final long thrower;  // use id not ref to avoid weak cycles
      ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {
         super(task, exceptionTableRefQueue);
         this.ex = ex;
         this.next = next;
         this.thrower = Thread.currentThread().getId();
      }
   }

   /**
    * Records exception and sets status.
    *
    * @return status on exit
    */
   final int recordExceptionalCompletion(Throwable ex) {
      int s;
      if ((s = status) >= 0) {
         int h = System.identityHashCode(this);
         final ReentrantLock lock = exceptionTableLock;
         lock.lock();
         try {
            expungeStaleExceptions();
            ExceptionNode[] t = exceptionTable;
            int i = h & (t.length - 1);
            for (ExceptionNode e = t[i]; ; e = e.next) {
               if (e == null) {
                  t[i] = new ExceptionNode(this, ex, t[i]);
                  break;
               }
               if (e.get() == this) // already present
                  break;
            }
         } finally {
            lock.unlock();
         }
         s = setCompletion(EXCEPTIONAL);
      }
      return s;
   }

   /**
    * Records exception and possibly propagates
    *
    * @return status on exit
    */
   private int setExceptionalCompletion(Throwable ex) {
      int s = recordExceptionalCompletion(ex);
      if ((s & DONE_MASK) == EXCEPTIONAL)
         internalPropagateException(ex);
      return s;
   }

   /**
    * Hook for exception propagation support for tasks with completers.
    */
   void internalPropagateException(Throwable ex) {
   }

   /**
    * Cancels, ignoring any exceptions thrown by cancel. Used during
    * worker and pool shutdown. Cancel is spec'ed not to throw any
    * exceptions, but if it does anyway, we have no recourse during
    * shutdown, so guard against this case.
    */
   static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
      if (t != null && t.status >= 0) {
         try {
            t.cancel(false);
         } catch (Throwable ignore) {
         }
      }
   }

   /**
    * Removes exception node and clears status
    */
   private void clearExceptionalCompletion() {
      int h = System.identityHashCode(this);
      final ReentrantLock lock = exceptionTableLock;
      lock.lock();
      try {
         ExceptionNode[] t = exceptionTable;
         int i = h & (t.length - 1);
         ExceptionNode e = t[i];
         ExceptionNode pred = null;
         while (e != null) {
            ExceptionNode next = e.next;
            if (e.get() == this) {
               if (pred == null)
                  t[i] = next;
               else
                  pred.next = next;
               break;
            }
            pred = e;
            e = next;
         }
         expungeStaleExceptions();
         status = 0;
      } finally {
         lock.unlock();
      }
   }

   /**
    * Returns a rethrowable exception for the given task, if
    * available. To provide accurate stack traces, if the exception
    * was not thrown by the current thread, we try to create a new
    * exception of the same type as the one thrown, but with the
    * recorded exception as its cause. If there is no such
    * constructor, we instead try to use a no-arg constructor,
    * followed by initCause, to the same effect. If none of these
    * apply, or any fail due to other exceptions, we return the
    * recorded exception, which is still correct, although it may
    * contain a misleading stack trace.
    *
    * @return the exception, or null if none
    */
   private Throwable getThrowableException() {
      if ((status & DONE_MASK) != EXCEPTIONAL)
         return null;
      int h = System.identityHashCode(this);
      ExceptionNode e;
      final ReentrantLock lock = exceptionTableLock;
      lock.lock();
      try {
         expungeStaleExceptions();
         ExceptionNode[] t = exceptionTable;
         e = t[h & (t.length - 1)];
         while (e != null && e.get() != this)
            e = e.next;
      } finally {
         lock.unlock();
      }
      Throwable ex;
      if (e == null || (ex = e.ex) == null)
         return null;
      if (false && e.thrower != Thread.currentThread().getId()) {
         Class<? extends Throwable> ec = ex.getClass();
         try {
            Constructor<?> noArgCtor = null;
            Constructor<?>[] cs = ec.getConstructors();// public ctors only
            for (int i = 0; i < cs.length; ++i) {
               Constructor<?> c = cs[i];
               Class<?>[] ps = c.getParameterTypes();
               if (ps.length == 0)
                  noArgCtor = c;
               else if (ps.length == 1 && ps[0] == Throwable.class)
                  return (Throwable)(c.newInstance(ex));
            }
            if (noArgCtor != null) {
               Throwable wx = (Throwable)(noArgCtor.newInstance());
               wx.initCause(ex);
               return wx;
            }
         } catch (Exception ignore) {
         }
      }
      return ex;
   }

   /**
    * Poll stale refs and remove them. Call only while holding lock.
    */
   private static void expungeStaleExceptions() {
      for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
         if (x instanceof ExceptionNode) {
            ForkJoinTask<?> key = ((ExceptionNode)x).get();
            ExceptionNode[] t = exceptionTable;
            int i = System.identityHashCode(key) & (t.length - 1);
            ExceptionNode e = t[i];
            ExceptionNode pred = null;
            while (e != null) {
               ExceptionNode next = e.next;
               if (e == x) {
                  if (pred == null)
                     t[i] = next;
                  else
                     pred.next = next;
                  break;
               }
               pred = e;
               e = next;
            }
         }
      }
   }

   /**
    * If lock is available, poll stale refs and remove them.
    * Called from ForkJoinPool when pools become quiescent.
    */
   static final void helpExpungeStaleExceptions() {
      final ReentrantLock lock = exceptionTableLock;
      if (lock.tryLock()) {
         try {
            expungeStaleExceptions();
         } finally {
            lock.unlock();
         }
      }
   }

   /**
    * Throws exception, if any, associated with the given status.
    */
   private void reportException(int s) {
      Throwable ex = ((s == CANCELLED) ?  new CancellationException() :
                            (s == EXCEPTIONAL) ? getThrowableException() :
                                  null);
      if (ex != null)
         U.throwException(ex);
   }

   // public methods

   /**
    * Arranges to asynchronously execute this task.  While it is not
    * necessarily enforced, it is a usage error to fork a task more
    * than once unless it has completed and been reinitialized.
    * Subsequent modifications to the state of this task or any data
    * it operates on are not necessarily consistently observable by
    * any thread other than the one executing it unless preceded by a
    * call to {@link #join} or related methods, or a call to {@link
    * #isDone} returning {@code true}.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @return {@code this}, to simplify usage
    */
   public final ForkJoinTask<V> fork() {
      ((ForkJoinWorkerThread)Thread.currentThread()).workQueue.push(this);
      return this;
   }

   /**
    * Returns the result of the computation when it {@link #isDone is
    * done}.  This method differs from {@link #get()} in that
    * abnormal completion results in {@code RuntimeException} or
    * {@code Error}, not {@code ExecutionException}, and that
    * interrupts of the calling thread do <em>not</em> cause the
    * method to abruptly return by throwing {@code
    * InterruptedException}.
    *
    * @return the computed result
    */
   public final V join() {
      int s;
      if ((s = doJoin() & DONE_MASK) != NORMAL)
         reportException(s);
      return getRawResult();
   }

   /**
    * Commences performing this task, awaits its completion if
    * necessary, and returns its result, or throws an (unchecked)
    * {@code RuntimeException} or {@code Error} if the underlying
    * computation did so.
    *
    * @return the computed result
    */
   public final V invoke() {
      int s;
      if ((s = doInvoke() & DONE_MASK) != NORMAL)
         reportException(s);
      return getRawResult();
   }

   /**
    * Forks the given tasks, returning when {@code isDone} holds for
    * each task or an (unchecked) exception is encountered, in which
    * case the exception is rethrown. If more than one task
    * encounters an exception, then this method throws any one of
    * these exceptions. If any task encounters an exception, the
    * other may be cancelled. However, the execution status of
    * individual tasks is not guaranteed upon exceptional return. The
    * status of each task may be obtained using {@link
    * #getException()} and related methods to check if they have been
    * cancelled, completed normally or exceptionally, or left
    * unprocessed.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @param t1 the first task
    * @param t2 the second task
    * @throws NullPointerException if any task is null
    */
   public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
      int s1, s2;
      t2.fork();
      if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL)
         t1.reportException(s1);
      if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL)
         t2.reportException(s2);
   }

   /**
    * Forks the given tasks, returning when {@code isDone} holds for
    * each task or an (unchecked) exception is encountered, in which
    * case the exception is rethrown. If more than one task
    * encounters an exception, then this method throws any one of
    * these exceptions. If any task encounters an exception, others
    * may be cancelled. However, the execution status of individual
    * tasks is not guaranteed upon exceptional return. The status of
    * each task may be obtained using {@link #getException()} and
    * related methods to check if they have been cancelled, completed
    * normally or exceptionally, or left unprocessed.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @param tasks the tasks
    * @throws NullPointerException if any task is null
    */
   public static void invokeAll(ForkJoinTask<?>... tasks) {
      Throwable ex = null;
      int last = tasks.length - 1;
      for (int i = last; i >= 0; --i) {
         ForkJoinTask<?> t = tasks[i];
         if (t == null) {
            if (ex == null)
               ex = new NullPointerException();
         }
         else if (i != 0)
            t.fork();
         else if (t.doInvoke() < NORMAL && ex == null)
            ex = t.getException();
      }
      for (int i = 1; i <= last; ++i) {
         ForkJoinTask<?> t = tasks[i];
         if (t != null) {
            if (ex != null)
               t.cancel(false);
            else if (t.doJoin() < NORMAL)
               ex = t.getException();
         }
      }
      if (ex != null)
         U.throwException(ex);
   }

   /**
    * Forks all tasks in the specified collection, returning when
    * {@code isDone} holds for each task or an (unchecked) exception
    * is encountered, in which case the exception is rethrown. If
    * more than one task encounters an exception, then this method
    * throws any one of these exceptions. If any task encounters an
    * exception, others may be cancelled. However, the execution
    * status of individual tasks is not guaranteed upon exceptional
    * return. The status of each task may be obtained using {@link
    * #getException()} and related methods to check if they have been
    * cancelled, completed normally or exceptionally, or left
    * unprocessed.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @param tasks the collection of tasks
    * @return the tasks argument, to simplify usage
    * @throws NullPointerException if tasks or any element are null
    */
   public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
      if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
         invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()]));
         return tasks;
      }
      @SuppressWarnings("unchecked")
      List<? extends ForkJoinTask<?>> ts =
            (List<? extends ForkJoinTask<?>>) tasks;
      Throwable ex = null;
      int last = ts.size() - 1;
      for (int i = last; i >= 0; --i) {
         ForkJoinTask<?> t = ts.get(i);
         if (t == null) {
            if (ex == null)
               ex = new NullPointerException();
         }
         else if (i != 0)
            t.fork();
         else if (t.doInvoke() < NORMAL && ex == null)
            ex = t.getException();
      }
      for (int i = 1; i <= last; ++i) {
         ForkJoinTask<?> t = ts.get(i);
         if (t != null) {
            if (ex != null)
               t.cancel(false);
            else if (t.doJoin() < NORMAL)
               ex = t.getException();
         }
      }
      if (ex != null)
         U.throwException(ex);
      return tasks;
   }

   /**
    * Attempts to cancel execution of this task. This attempt will
    * fail if the task has already completed or could not be
    * cancelled for some other reason. If successful, and this task
    * has not started when {@code cancel} is called, execution of
    * this task is suppressed. After this method returns
    * successfully, unless there is an intervening call to {@link
    * #reinitialize}, subsequent calls to {@link #isCancelled},
    * {@link #isDone}, and {@code cancel} will return {@code true}
    * and calls to {@link #join} and related methods will result in
    * {@code CancellationException}.
    *
    * <p>This method may be overridden in subclasses, but if so, must
    * still ensure that these properties hold. In particular, the
    * {@code cancel} method itself must not throw exceptions.
    *
    * <p>This method is designed to be invoked by <em>other</em>
    * tasks. To terminate the current task, you can just return or
    * throw an unchecked exception from its computation method, or
    * invoke {@link #completeExceptionally}.
    *
    * @param mayInterruptIfRunning this value has no effect in the
    * default implementation because interrupts are not used to
    * control cancellation.
    *
    * @return {@code true} if this task is now cancelled
    */
   public boolean cancel(boolean mayInterruptIfRunning) {
      return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
   }

   public final boolean isDone() {
      return status < 0;
   }

   public final boolean isCancelled() {
      return (status & DONE_MASK) == CANCELLED;
   }

   /**
    * Returns {@code true} if this task threw an exception or was cancelled.
    *
    * @return {@code true} if this task threw an exception or was cancelled
    */
   public final boolean isCompletedAbnormally() {
      return status < NORMAL;
   }

   /**
    * Returns {@code true} if this task completed without throwing an
    * exception and was not cancelled.
    *
    * @return {@code true} if this task completed without throwing an
    * exception and was not cancelled
    */
   public final boolean isCompletedNormally() {
      return (status & DONE_MASK) == NORMAL;
   }

   /**
    * Returns the exception thrown by the base computation, or a
    * {@code CancellationException} if cancelled, or {@code null} if
    * none or if the method has not yet completed.
    *
    * @return the exception, or {@code null} if none
    */
   public final Throwable getException() {
      int s = status & DONE_MASK;
      return ((s >= NORMAL)    ? null :
                    (s == CANCELLED) ? new CancellationException() :
                          getThrowableException());
   }

   /**
    * Completes this task abnormally, and if not already aborted or
    * cancelled, causes it to throw the given exception upon
    * {@code join} and related operations. This method may be used
    * to induce exceptions in asynchronous tasks, or to force
    * completion of tasks that would not otherwise complete.  Its use
    * in other situations is discouraged.  This method is
    * overridable, but overridden versions must invoke {@code super}
    * implementation to maintain guarantees.
    *
    * @param ex the exception to throw. If this exception is not a
    * {@code RuntimeException} or {@code Error}, the actual exception
    * thrown will be a {@code RuntimeException} with cause {@code ex}.
    */
   public void completeExceptionally(Throwable ex) {
      setExceptionalCompletion((ex instanceof RuntimeException) ||
                                     (ex instanceof Error) ? ex :
                                     new RuntimeException(ex));
   }

   /**
    * Completes this task, and if not already aborted or cancelled,
    * returning the given value as the result of subsequent
    * invocations of {@code join} and related operations. This method
    * may be used to provide results for asynchronous tasks, or to
    * provide alternative handling for tasks that would not otherwise
    * complete normally. Its use in other situations is
    * discouraged. This method is overridable, but overridden
    * versions must invoke {@code super} implementation to maintain
    * guarantees.
    *
    * @param value the result value for this task
    */
   public void complete(V value) {
      try {
         setRawResult(value);
      } catch (Throwable rex) {
         setExceptionalCompletion(rex);
         return;
      }
      setCompletion(NORMAL);
   }

   /**
    * Completes this task normally without setting a value. The most
    * recent value established by {@link #setRawResult} (or {@code
    * null} by default) will be returned as the result of subsequent
    * invocations of {@code join} and related operations.
    *
    * @since 1.8
    */
   public final void quietlyComplete() {
      setCompletion(NORMAL);
   }

   /**
    * Waits if necessary for the computation to complete, and then
    * retrieves its result.
    *
    * @return the computed result
    * @throws CancellationException if the computation was cancelled
    * @throws ExecutionException if the computation threw an
    * exception
    * @throws InterruptedException if the current thread is not a
    * member of a ForkJoinPool and was interrupted while waiting
    */
   public final V get() throws InterruptedException, ExecutionException {
      int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
            doJoin() : externalInterruptibleAwaitDone();
      Throwable ex;
      if ((s &= DONE_MASK) == CANCELLED)
         throw new CancellationException();
      if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
         throw new ExecutionException(ex);
      return getRawResult();
   }

   /**
    * Waits if necessary for at most the given time for the computation
    * to complete, and then retrieves its result, if available.
    *
    * @param timeout the maximum time to wait
    * @param unit the time unit of the timeout argument
    * @return the computed result
    * @throws CancellationException if the computation was cancelled
    * @throws ExecutionException if the computation threw an
    * exception
    * @throws InterruptedException if the current thread is not a
    * member of a ForkJoinPool and was interrupted while waiting
    * @throws TimeoutException if the wait timed out
    */
   public final V get(long timeout, TimeUnit unit)
         throws InterruptedException, ExecutionException, TimeoutException {
      if (Thread.interrupted())
         throw new InterruptedException();
      // Messy in part because we measure in nanosecs, but wait in millisecs
      int s; long ns, ms;
      if ((s = status) >= 0 && (ns = unit.toNanos(timeout)) > 0L) {
         long deadline = System.nanoTime() + ns;
         ForkJoinPool p = null;
         ForkJoinPool.WorkQueue w = null;
         Thread t = Thread.currentThread();
         if (t instanceof ForkJoinWorkerThread) {
            ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
            p = wt.pool;
            w = wt.workQueue;
            s = p.helpJoinOnce(w, this); // no retries on failure
         }
         boolean canBlock = false;
         boolean interrupted = false;
         try {
            while ((s = status) >= 0) {
               if (w != null && w.runState < 0)
                  cancelIgnoringExceptions(this);
               else if (!canBlock) {
                  if (p == null || p.tryCompensate(this, null))
                     canBlock = true;
               }
               else {
                  if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L &&
                        U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
                     synchronized (this) {
                        if (status >= 0) {
                           try {
                              wait(ms);
                           } catch (InterruptedException ie) {
                              if (p == null)
                                 interrupted = true;
                           }
                        }
                        else
                           notifyAll();
                     }
                  }
                  if ((s = status) < 0 || interrupted ||
                        (ns = deadline - System.nanoTime()) <= 0L)
                     break;
               }
            }
         } finally {
            if (p != null && canBlock)
               p.incrementActiveCount();
         }
         if (interrupted)
            throw new InterruptedException();
      }
      if ((s &= DONE_MASK) != NORMAL) {
         Throwable ex;
         if (s == CANCELLED)
            throw new CancellationException();
         if (s != EXCEPTIONAL)
            throw new TimeoutException();
         if ((ex = getThrowableException()) != null)
            throw new ExecutionException(ex);
      }
      return getRawResult();
   }

   /**
    * Joins this task, without returning its result or throwing its
    * exception. This method may be useful when processing
    * collections of tasks when some have been cancelled or otherwise
    * known to have aborted.
    */
   public final void quietlyJoin() {
      doJoin();
   }

   /**
    * Commences performing this task and awaits its completion if
    * necessary, without returning its result or throwing its
    * exception.
    */
   public final void quietlyInvoke() {
      doInvoke();
   }

   /**
    * Possibly executes tasks until the pool hosting the current task
    * {@link ForkJoinPool#isQuiescent is quiescent}. This method may
    * be of use in designs in which many tasks are forked, but none
    * are explicitly joined, instead executing them until all are
    * processed.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    */
   public static void helpQuiesce() {
      ForkJoinWorkerThread wt =
            (ForkJoinWorkerThread)Thread.currentThread();
      wt.pool.helpQuiescePool(wt.workQueue);
   }

   /**
    * Resets the internal bookkeeping state of this task, allowing a
    * subsequent {@code fork}. This method allows repeated reuse of
    * this task, but only if reuse occurs when this task has either
    * never been forked, or has been forked, then completed and all
    * outstanding joins of this task have also completed. Effects
    * under any other usage conditions are not guaranteed.
    * This method may be useful when executing
    * pre-constructed trees of subtasks in loops.
    *
    * <p>Upon completion of this method, {@code isDone()} reports
    * {@code false}, and {@code getException()} reports {@code
    * null}. However, the value returned by {@code getRawResult} is
    * unaffected. To clear this value, you can invoke {@code
    * setRawResult(null)}.
    */
   public void reinitialize() {
      if ((status & DONE_MASK) == EXCEPTIONAL)
         clearExceptionalCompletion();
      else
         status = 0;
   }

   /**
    * Returns the pool hosting the current task execution, or null
    * if this task is executing outside of any ForkJoinPool.
    *
    * @see #inForkJoinPool
    * @return the pool, or {@code null} if none
    */
   public static ForkJoinPool getPool() {
      Thread t = Thread.currentThread();
      return (t instanceof ForkJoinWorkerThread) ?
            ((ForkJoinWorkerThread) t).pool : null;
   }

   /**
    * Returns {@code true} if the current thread is a {@link
    * ForkJoinWorkerThread} executing as a ForkJoinPool computation.
    *
    * @return {@code true} if the current thread is a {@link
    * ForkJoinWorkerThread} executing as a ForkJoinPool computation,
    * or {@code false} otherwise
    */
   public static boolean inForkJoinPool() {
      return Thread.currentThread() instanceof ForkJoinWorkerThread;
   }

   /**
    * Tries to unschedule this task for execution. This method will
    * typically succeed if this task is the most recently forked task
    * by the current thread, and has not commenced executing in
    * another thread.  This method may be useful when arranging
    * alternative local processing of tasks that could have been, but
    * were not, stolen.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @return {@code true} if unforked
    */
   public boolean tryUnfork() {
      return ((ForkJoinWorkerThread)Thread.currentThread())
            .workQueue.tryUnpush(this);
   }

   /**
    * Returns an estimate of the number of tasks that have been
    * forked by the current worker thread but not yet executed. This
    * value may be useful for heuristic decisions about whether to
    * fork other tasks.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @return the number of tasks
    */
   public static int getQueuedTaskCount() {
      return ((ForkJoinWorkerThread) Thread.currentThread())
            .workQueue.queueSize();
   }

   /**
    * Returns an estimate of how many more locally queued tasks are
    * held by the current worker thread than there are other worker
    * threads that might steal them.  This value may be useful for
    * heuristic decisions about whether to fork other tasks. In many
    * usages of ForkJoinTasks, at steady state, each worker should
    * aim to maintain a small constant surplus (for example, 3) of
    * tasks, and to process computations locally if this threshold is
    * exceeded.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @return the surplus number of tasks, which may be negative
    */
   public static int getSurplusQueuedTaskCount() {
      /*
      * The aim of this method is to return 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.
      */
      ForkJoinWorkerThread wt =
            (ForkJoinWorkerThread)Thread.currentThread();
      return wt.workQueue.queueSize() - wt.pool.idlePerActive();
   }

   // Extension methods

   /**
    * Returns the result that would be returned by {@link #join}, even
    * if this task completed abnormally, or {@code null} if this task
    * is not known to have been completed.  This method is designed
    * to aid debugging, as well as to support extensions. Its use in
    * any other context is discouraged.
    *
    * @return the result, or {@code null} if not completed
    */
   public abstract V getRawResult();

   /**
    * Forces the given value to be returned as a result.  This method
    * is designed to support extensions, and should not in general be
    * called otherwise.
    *
    * @param value the value
    */
   protected abstract void setRawResult(V value);

   /**
    * Immediately performs the base action of this task and returns
    * true if, upon return from this method, this task is guaranteed
    * to have completed normally. This method may return false
    * otherwise, to indicate that this task is not necessarily
    * complete (or is not known to be complete), for example in
    * asynchronous actions that require explicit invocations of
    * completion methods. This method may also throw an (unchecked)
    * exception to indicate abnormal exit. This method is designed to
    * support extensions, and should not in general be called
    * otherwise.
    *
    * @return {@code true} if this task is known to have completed normally
    */
   protected abstract boolean exec();

   /**
    * Returns, but does not unschedule or execute, a task queued by
    * the current thread but not yet executed, if one is immediately
    * available. There is no guarantee that this task will actually
    * be polled or executed next. Conversely, this method may return
    * null even if a task exists but cannot be accessed without
    * contention with other threads.  This method is designed
    * primarily to support extensions, and is unlikely to be useful
    * otherwise.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @return the next task, or {@code null} if none are available
    */
   protected static ForkJoinTask<?> peekNextLocalTask() {
      return ((ForkJoinWorkerThread) Thread.currentThread()).workQueue.peek();
   }

   /**
    * Unschedules and returns, without executing, the next task
    * queued by the current thread but not yet executed.  This method
    * is designed primarily to support extensions, and is unlikely to
    * be useful otherwise.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @return the next task, or {@code null} if none are available
    */
   protected static ForkJoinTask<?> pollNextLocalTask() {
      return ((ForkJoinWorkerThread) Thread.currentThread())
            .workQueue.nextLocalTask();
   }

   /**
    * Unschedules and returns, without executing, the next task
    * queued by the current thread but not yet executed, if one is
    * available, or if not available, a task that was forked by some
    * other thread, if available. Availability may be transient, so a
    * {@code null} result does not necessarily imply quiescence
    * of the pool this task is operating in.  This method is designed
    * primarily to support extensions, and is unlikely to be useful
    * otherwise.
    *
    * <p>This method may be invoked only from within {@code
    * ForkJoinPool} computations (as may be determined using method
    * {@link #inForkJoinPool}).  Attempts to invoke in other contexts
    * result in exceptions or errors, possibly including {@code
    * ClassCastException}.
    *
    * @return a task, or {@code null} if none are available
    */
   protected static ForkJoinTask<?> pollTask() {
      ForkJoinWorkerThread wt =
            (ForkJoinWorkerThread)Thread.currentThread();
      return wt.pool.nextTaskFor(wt.workQueue);
   }

   // tag operations

   /**
    * Returns the tag for this task.
    *
    * @return the tag for this task
    * @since 1.8
    */
   public final short getForkJoinTaskTag() {
      return (short)status;
   }

   /**
    * Atomically sets the tag value for this task.
    *
    * @param tag the tag value
    * @return the previous value of the tag
    * @since 1.8
    */
   public final short setForkJoinTaskTag(short tag) {
      for (int s;;) {
         if (U.compareAndSwapInt(this, STATUS, s = status,
                                 (s & ~SMASK) | (tag & SMASK)))
            return (short)s;
      }
   }

   /**
    * Atomically conditionally sets the tag value for this task.
    * Among other applications, tags can be used as visit markers
    * in tasks operating on graphs, as in methods that check: {@code
    * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))}
    * before processing, otherwise exiting because the node has
    * already been visited.
    *
    * @param e the expected tag value
    * @param tag the new tag value
    * @return true if successful; i.e., the current value was
    * equal to e and is now tag.
    * @since 1.8
    */
   public final boolean compareAndSetForkJoinTaskTag(short e, short tag) {
      for (int s;;) {
         if ((short)(s = status) != e)
            return false;
         if (U.compareAndSwapInt(this, STATUS, s,
                                 (s & ~SMASK) | (tag & SMASK)))
            return true;
      }
   }

   /**
    * Adaptor for Runnables. This implements RunnableFuture
    * to be compliant with AbstractExecutorService constraints
    * when used in ForkJoinPool.
    */
   static final class AdaptedRunnable<T> extends ForkJoinTask<T>
         implements RunnableFuture<T> {
      final Runnable runnable;
      T result;
      AdaptedRunnable(Runnable runnable, T result) {
         if (runnable == null) throw new NullPointerException();
         this.runnable = runnable;
         this.result = result; // OK to set this even before completion
      }
      public final T getRawResult() { return result; }
      public final void setRawResult(T v) { result = v; }
      public final boolean exec() { runnable.run(); return true; }
      public final void run() { invoke(); }
      private static final long serialVersionUID = 5232453952276885070L;
   }

   /**
    * Adaptor for Runnables without results
    */
   static final class AdaptedRunnableAction extends ForkJoinTask<Void>
         implements RunnableFuture<Void> {
      final Runnable runnable;
      AdaptedRunnableAction(Runnable runnable) {
         if (runnable == null) throw new NullPointerException();
         this.runnable = runnable;
      }
      public final Void getRawResult() { return null; }
      public final void setRawResult(Void v) { }
      public final boolean exec() { runnable.run(); return true; }
      public final void run() { invoke(); }
      private static final long serialVersionUID = 5232453952276885070L;
   }

   /**
    * Adaptor for Callables
    */
   static final class AdaptedCallable<T> extends ForkJoinTask<T>
         implements RunnableFuture<T> {
      final Callable<? extends T> callable;
      T result;
      AdaptedCallable(Callable<? extends T> callable) {
         if (callable == null) throw new NullPointerException();
         this.callable = callable;
      }
      public final T getRawResult() { return result; }
      public final void setRawResult(T v) { result = v; }
      public final boolean exec() {
         try {
            result = callable.call();
            return true;
         } catch (Error err) {
            throw err;
         } catch (RuntimeException rex) {
            throw rex;
         } catch (Exception ex) {
            throw new RuntimeException(ex);
         }
      }
      public final void run() { invoke(); }
      private static final long serialVersionUID = 2838392045355241008L;
   }

   /**
    * Returns a new {@code ForkJoinTask} that performs the {@code run}
    * method of the given {@code Runnable} as its action, and returns
    * a null result upon {@link #join}.
    *
    * @param runnable the runnable action
    * @return the task
    */
   public static ForkJoinTask<?> adapt(Runnable runnable) {
      return new AdaptedRunnableAction(runnable);
   }

   /**
    * Returns a new {@code ForkJoinTask} that performs the {@code run}
    * method of the given {@code Runnable} as its action, and returns
    * the given result upon {@link #join}.
    *
    * @param runnable the runnable action
    * @param result the result upon completion
    * @return the task
    */
   public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
      return new AdaptedRunnable<T>(runnable, result);
   }

   /**
    * Returns a new {@code ForkJoinTask} that performs the {@code call}
    * method of the given {@code Callable} as its action, and returns
    * its result upon {@link #join}, translating any checked exceptions
    * encountered into {@code RuntimeException}.
    *
    * @param callable the callable action
    * @return the task
    */
   public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
      return new AdaptedCallable<T>(callable);
   }

   // Serialization support

   private static final long serialVersionUID = -7721805057305804111L;

   /**
    * Saves this task to a stream (that is, serializes it).
    *
    * @serialData the current run status and the exception thrown
    * during execution, or {@code null} if none
    */
   private void writeObject(java.io.ObjectOutputStream s)
         throws java.io.IOException {
      s.defaultWriteObject();
      s.writeObject(getException());
   }

   /**
    * Reconstitutes this task from a stream (that is, deserializes it).
    */
   private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOException, ClassNotFoundException {
      s.defaultReadObject();
      Object ex = s.readObject();
      if (ex != null)
         setExceptionalCompletion((Throwable)ex);
   }

   // Unsafe mechanics
   private static final sun.misc.Unsafe U;
   private static final long STATUS;
   static {
      exceptionTableLock = new ReentrantLock();
      exceptionTableRefQueue = new ReferenceQueue<Object>();
      exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
      try {
         U = getUnsafe();
         STATUS = U.objectFieldOffset
               (ForkJoinTask.class.getDeclaredField("status"));
      } catch (Exception e) {
         throw new Error(e);
      }
   }

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