Imported Upstream version 1.0.3upstream/1.0.3

12 files changed, 10376 insertions, 0 deletions

diff --git a/src/main/java/jsr166y/ConcurrentLinkedDeque.java b/src/main/java/jsr166y/ConcurrentLinkedDeque.java
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+/*
+ * Written by Doug Lea and Martin Buchholz with assistance from members of
+ * JCP JSR-166 Expert Group and released to the public domain, as explained
+ * at http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+import java.util.AbstractCollection;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Deque;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+import java.util.Queue;
+
+/**
+ * An unbounded concurrent {@linkplain Deque deque} based on linked nodes.
+ * Concurrent insertion, removal, and access operations execute safely
+ * across multiple threads.
+ * A {@code ConcurrentLinkedDeque} is an appropriate choice when
+ * many threads will share access to a common collection.
+ * Like most other concurrent collection implementations, this class
+ * does not permit the use of {@code null} elements.
+ *
+ * <p>Iterators are <i>weakly consistent</i>, returning elements
+ * reflecting the state of the deque at some point at or since the
+ * creation of the iterator.  They do <em>not</em> throw {@link
+ * java.util.ConcurrentModificationException
+ * ConcurrentModificationException}, and may proceed concurrently with
+ * other operations.
+ *
+ * <p>Beware that, unlike in most collections, the {@code size} method
+ * is <em>NOT</em> a constant-time operation. Because of the
+ * asynchronous nature of these deques, determining the current number
+ * of elements requires a traversal of the elements, and so may report
+ * inaccurate results if this collection is modified during traversal.
+ * Additionally, the bulk operations {@code addAll},
+ * {@code removeAll}, {@code retainAll}, {@code containsAll},
+ * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
+ * to be performed atomically. For example, an iterator operating
+ * concurrently with an {@code addAll} operation might view only some
+ * of the added elements.
+ *
+ * <p>This class and its iterator implement all of the <em>optional</em>
+ * methods of the {@link Deque} and {@link Iterator} interfaces.
+ *
+ * <p>Memory consistency effects: As with other concurrent collections,
+ * actions in a thread prior to placing an object into a
+ * {@code ConcurrentLinkedDeque}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions subsequent to the access or removal of that element from
+ * the {@code ConcurrentLinkedDeque} in another thread.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.7
+ * @author Doug Lea
+ * @author Martin Buchholz
+ * @param <E> the type of elements held in this collection
+ */
+public class ConcurrentLinkedDeque<E>
+    extends AbstractCollection<E>
+    implements Deque<E>, java.io.Serializable {
+
+    /*
+     * This is an implementation of a concurrent lock-free deque
+     * supporting interior removes but not interior insertions, as
+     * required to support the entire Deque interface.
+     *
+     * We extend the techniques developed for ConcurrentLinkedQueue and
+     * LinkedTransferQueue (see the internal docs for those classes).
+     * Understanding the ConcurrentLinkedQueue implementation is a
+     * prerequisite for understanding the implementation of this class.
+     *
+     * The data structure is a symmetrical doubly-linked "GC-robust"
+     * linked list of nodes.  We minimize the number of volatile writes
+     * using two techniques: advancing multiple hops with a single CAS
+     * and mixing volatile and non-volatile writes of the same memory
+     * locations.
+     *
+     * A node contains the expected E ("item") and links to predecessor
+     * ("prev") and successor ("next") nodes:
+     *
+     * class Node<E> { volatile Node<E> prev, next; volatile E item; }
+     *
+     * A node p is considered "live" if it contains a non-null item
+     * (p.item != null).  When an item is CASed to null, the item is
+     * atomically logically deleted from the collection.
+     *
+     * At any time, there is precisely one "first" node with a null
+     * prev reference that terminates any chain of prev references
+     * starting at a live node.  Similarly there is precisely one
+     * "last" node terminating any chain of next references starting at
+     * a live node.  The "first" and "last" nodes may or may not be live.
+     * The "first" and "last" nodes are always mutually reachable.
+     *
+     * A new element is added atomically by CASing the null prev or
+     * next reference in the first or last node to a fresh node
+     * containing the element.  The element's node atomically becomes
+     * "live" at that point.
+     *
+     * A node is considered "active" if it is a live node, or the
+     * first or last node.  Active nodes cannot be unlinked.
+     *
+     * A "self-link" is a next or prev reference that is the same node:
+     *   p.prev == p  or  p.next == p
+     * Self-links are used in the node unlinking process.  Active nodes
+     * never have self-links.
+     *
+     * A node p is active if and only if:
+     *
+     * p.item != null ||
+     * (p.prev == null && p.next != p) ||
+     * (p.next == null && p.prev != p)
+     *
+     * The deque object has two node references, "head" and "tail".
+     * The head and tail are only approximations to the first and last
+     * nodes of the deque.  The first node can always be found by
+     * following prev pointers from head; likewise for tail.  However,
+     * it is permissible for head and tail to be referring to deleted
+     * nodes that have been unlinked and so may not be reachable from
+     * any live node.
+     *
+     * There are 3 stages of node deletion;
+     * "logical deletion", "unlinking", and "gc-unlinking".
+     *
+     * 1. "logical deletion" by CASing item to null atomically removes
+     * the element from the collection, and makes the containing node
+     * eligible for unlinking.
+     *
+     * 2. "unlinking" makes a deleted node unreachable from active
+     * nodes, and thus eventually reclaimable by GC.  Unlinked nodes
+     * may remain reachable indefinitely from an iterator.
+     *
+     * Physical node unlinking is merely an optimization (albeit a
+     * critical one), and so can be performed at our convenience.  At
+     * any time, the set of live nodes maintained by prev and next
+     * links are identical, that is, the live nodes found via next
+     * links from the first node is equal to the elements found via
+     * prev links from the last node.  However, this is not true for
+     * nodes that have already been logically deleted - such nodes may
+     * be reachable in one direction only.
+     *
+     * 3. "gc-unlinking" takes unlinking further by making active
+     * nodes unreachable from deleted nodes, making it easier for the
+     * GC to reclaim future deleted nodes.  This step makes the data
+     * structure "gc-robust", as first described in detail by Boehm
+     * (http://portal.acm.org/citation.cfm?doid=503272.503282).
+     *
+     * GC-unlinked nodes may remain reachable indefinitely from an
+     * iterator, but unlike unlinked nodes, are never reachable from
+     * head or tail.
+     *
+     * Making the data structure GC-robust will eliminate the risk of
+     * unbounded memory retention with conservative GCs and is likely
+     * to improve performance with generational GCs.
+     *
+     * When a node is dequeued at either end, e.g. via poll(), we would
+     * like to break any references from the node to active nodes.  We
+     * develop further the use of self-links that was very effective in
+     * other concurrent collection classes.  The idea is to replace
+     * prev and next pointers with special values that are interpreted
+     * to mean off-the-list-at-one-end.  These are approximations, but
+     * good enough to preserve the properties we want in our
+     * traversals, e.g. we guarantee that a traversal will never visit
+     * the same element twice, but we don't guarantee whether a
+     * traversal that runs out of elements will be able to see more
+     * elements later after enqueues at that end.  Doing gc-unlinking
+     * safely is particularly tricky, since any node can be in use
+     * indefinitely (for example by an iterator).  We must ensure that
+     * the nodes pointed at by head/tail never get gc-unlinked, since
+     * head/tail are needed to get "back on track" by other nodes that
+     * are gc-unlinked.  gc-unlinking accounts for much of the
+     * implementation complexity.
+     *
+     * Since neither unlinking nor gc-unlinking are necessary for
+     * correctness, there are many implementation choices regarding
+     * frequency (eagerness) of these operations.  Since volatile
+     * reads are likely to be much cheaper than CASes, saving CASes by
+     * unlinking multiple adjacent nodes at a time may be a win.
+     * gc-unlinking can be performed rarely and still be effective,
+     * since it is most important that long chains of deleted nodes
+     * are occasionally broken.
+     *
+     * The actual representation we use is that p.next == p means to
+     * goto the first node (which in turn is reached by following prev
+     * pointers from head), and p.next == null && p.prev == p means
+     * that the iteration is at an end and that p is a (static final)
+     * dummy node, NEXT_TERMINATOR, and not the last active node.
+     * Finishing the iteration when encountering such a TERMINATOR is
+     * good enough for read-only traversals, so such traversals can use
+     * p.next == null as the termination condition.  When we need to
+     * find the last (active) node, for enqueueing a new node, we need
+     * to check whether we have reached a TERMINATOR node; if so,
+     * restart traversal from tail.
+     *
+     * The implementation is completely directionally symmetrical,
+     * except that most public methods that iterate through the list
+     * follow next pointers ("forward" direction).
+     *
+     * We believe (without full proof) that all single-element deque
+     * operations (e.g., addFirst, peekLast, pollLast) are linearizable
+     * (see Herlihy and Shavit's book).  However, some combinations of
+     * operations are known not to be linearizable.  In particular,
+     * when an addFirst(A) is racing with pollFirst() removing B, it is
+     * possible for an observer iterating over the elements to observe
+     * A B C and subsequently observe A C, even though no interior
+     * removes are ever performed.  Nevertheless, iterators behave
+     * reasonably, providing the "weakly consistent" guarantees.
+     *
+     * Empirically, microbenchmarks suggest that this class adds about
+     * 40% overhead relative to ConcurrentLinkedQueue, which feels as
+     * good as we can hope for.
+     */
+
+    private static final long serialVersionUID = 876323262645176354L;
+
+    /**
+     * A node from which the first node on list (that is, the unique node p
+     * with p.prev == null && p.next != p) can be reached in O(1) time.
+     * Invariants:
+     * - the first node is always O(1) reachable from head via prev links
+     * - all live nodes are reachable from the first node via succ()
+     * - head != null
+     * - (tmp = head).next != tmp || tmp != head
+     * - head is never gc-unlinked (but may be unlinked)
+     * Non-invariants:
+     * - head.item may or may not be null
+     * - head may not be reachable from the first or last node, or from tail
+     */
+    private transient volatile Node<E> head;
+
+    /**
+     * A node from which the last node on list (that is, the unique node p
+     * with p.next == null && p.prev != p) can be reached in O(1) time.
+     * Invariants:
+     * - the last node is always O(1) reachable from tail via next links
+     * - all live nodes are reachable from the last node via pred()
+     * - tail != null
+     * - tail is never gc-unlinked (but may be unlinked)
+     * Non-invariants:
+     * - tail.item may or may not be null
+     * - tail may not be reachable from the first or last node, or from head
+     */
+    private transient volatile Node<E> tail;
+
+    private static final Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR;
+
+    @SuppressWarnings("unchecked")
+    Node<E> prevTerminator() {
+        return (Node<E>) PREV_TERMINATOR;
+    }
+
+    @SuppressWarnings("unchecked")
+    Node<E> nextTerminator() {
+        return (Node<E>) NEXT_TERMINATOR;
+    }
+
+    static final class Node<E> {
+        volatile Node<E> prev;
+        volatile E item;
+        volatile Node<E> next;
+
+        Node() {  // default constructor for NEXT_TERMINATOR, PREV_TERMINATOR
+        }
+
+        /**
+         * Constructs a new node.  Uses relaxed write because item can
+         * only be seen after publication via casNext or casPrev.
+         */
+        Node(E item) {
+            UNSAFE.putObject(this, itemOffset, item);
+        }
+
+        boolean casItem(E cmp, E val) {
+            return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
+        }
+
+        void lazySetNext(Node<E> val) {
+            UNSAFE.putOrderedObject(this, nextOffset, val);
+        }
+
+        boolean casNext(Node<E> cmp, Node<E> val) {
+            return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
+        }
+
+        void lazySetPrev(Node<E> val) {
+            UNSAFE.putOrderedObject(this, prevOffset, val);
+        }
+
+        boolean casPrev(Node<E> cmp, Node<E> val) {
+            return UNSAFE.compareAndSwapObject(this, prevOffset, cmp, val);
+        }
+
+        // Unsafe mechanics
+
+        private static final sun.misc.Unsafe UNSAFE;
+        private static final long prevOffset;
+        private static final long itemOffset;
+        private static final long nextOffset;
+
+        static {
+            try {
+                UNSAFE = getUnsafe();
+                Class<?> k = Node.class;
+                prevOffset = UNSAFE.objectFieldOffset
+                    (k.getDeclaredField("prev"));
+                itemOffset = UNSAFE.objectFieldOffset
+                    (k.getDeclaredField("item"));
+                nextOffset = UNSAFE.objectFieldOffset
+                    (k.getDeclaredField("next"));
+            } catch (Exception e) {
+                throw new Error(e);
+            }
+        }
+    }
+
+    /**
+     * Links e as first element.
+     */
+    private void linkFirst(E e) {
+        checkNotNull(e);
+        final Node<E> newNode = new Node<E>(e);
+
+        restartFromHead:
+        for (;;)
+            for (Node<E> h = head, p = h, q;;) {
+                if ((q = p.prev) != null &&
+                    (q = (p = q).prev) != null)
+                    // Check for head updates every other hop.
+                    // If p == q, we are sure to follow head instead.
+                    p = (h != (h = head)) ? h : q;
+                else if (p.next == p) // PREV_TERMINATOR
+                    continue restartFromHead;
+                else {
+                    // p is first node
+                    newNode.lazySetNext(p); // CAS piggyback
+                    if (p.casPrev(null, newNode)) {
+                        // Successful CAS is the linearization point
+                        // for e to become an element of this deque,
+                        // and for newNode to become "live".
+                        if (p != h) // hop two nodes at a time
+                            casHead(h, newNode);  // Failure is OK.
+                        return;
+                    }
+                    // Lost CAS race to another thread; re-read prev
+                }
+            }
+    }
+
+    /**
+     * Links e as last element.
+     */
+    private void linkLast(E e) {
+        checkNotNull(e);
+        final Node<E> newNode = new Node<E>(e);
+
+        restartFromTail:
+        for (;;)
+            for (Node<E> t = tail, p = t, q;;) {
+                if ((q = p.next) != null &&
+                    (q = (p = q).next) != null)
+                    // Check for tail updates every other hop.
+                    // If p == q, we are sure to follow tail instead.
+                    p = (t != (t = tail)) ? t : q;
+                else if (p.prev == p) // NEXT_TERMINATOR
+                    continue restartFromTail;
+                else {
+                    // p is last node
+                    newNode.lazySetPrev(p); // CAS piggyback
+                    if (p.casNext(null, newNode)) {
+                        // Successful CAS is the linearization point
+                        // for e to become an element of this deque,
+                        // and for newNode to become "live".
+                        if (p != t) // hop two nodes at a time
+                            casTail(t, newNode);  // Failure is OK.
+                        return;
+                    }
+                    // Lost CAS race to another thread; re-read next
+                }
+            }
+    }
+
+    private static final int HOPS = 2;
+
+    /**
+     * Unlinks non-null node x.
+     */
+    void unlink(Node<E> x) {
+        // assert x != null;
+        // assert x.item == null;
+        // assert x != PREV_TERMINATOR;
+        // assert x != NEXT_TERMINATOR;
+
+        final Node<E> prev = x.prev;
+        final Node<E> next = x.next;
+        if (prev == null) {
+            unlinkFirst(x, next);
+        } else if (next == null) {
+            unlinkLast(x, prev);
+        } else {
+            // Unlink interior node.
+            //
+            // This is the common case, since a series of polls at the
+            // same end will be "interior" removes, except perhaps for
+            // the first one, since end nodes cannot be unlinked.
+            //
+            // At any time, all active nodes are mutually reachable by
+            // following a sequence of either next or prev pointers.
+            //
+            // Our strategy is to find the unique active predecessor
+            // and successor of x.  Try to fix up their links so that
+            // they point to each other, leaving x unreachable from
+            // active nodes.  If successful, and if x has no live
+            // predecessor/successor, we additionally try to gc-unlink,
+            // leaving active nodes unreachable from x, by rechecking
+            // that the status of predecessor and successor are
+            // unchanged and ensuring that x is not reachable from
+            // tail/head, before setting x's prev/next links to their
+            // logical approximate replacements, self/TERMINATOR.
+            Node<E> activePred, activeSucc;
+            boolean isFirst, isLast;
+            int hops = 1;
+
+            // Find active predecessor
+            for (Node<E> p = prev; ; ++hops) {
+                if (p.item != null) {
+                    activePred = p;
+                    isFirst = false;
+                    break;
+                }
+                Node<E> q = p.prev;
+                if (q == null) {
+                    if (p.next == p)
+                        return;
+                    activePred = p;
+                    isFirst = true;
+                    break;
+                }
+                else if (p == q)
+                    return;
+                else
+                    p = q;
+            }
+
+            // Find active successor
+            for (Node<E> p = next; ; ++hops) {
+                if (p.item != null) {
+                    activeSucc = p;
+                    isLast = false;
+                    break;
+                }
+                Node<E> q = p.next;
+                if (q == null) {
+                    if (p.prev == p)
+                        return;
+                    activeSucc = p;
+                    isLast = true;
+                    break;
+                }
+                else if (p == q)
+                    return;
+                else
+                    p = q;
+            }
+
+            // TODO: better HOP heuristics
+            if (hops < HOPS
+                // always squeeze out interior deleted nodes
+                && (isFirst | isLast))
+                return;
+
+            // Squeeze out deleted nodes between activePred and
+            // activeSucc, including x.
+            skipDeletedSuccessors(activePred);
+            skipDeletedPredecessors(activeSucc);
+
+            // Try to gc-unlink, if possible
+            if ((isFirst | isLast) &&
+
+                // Recheck expected state of predecessor and successor
+                (activePred.next == activeSucc) &&
+                (activeSucc.prev == activePred) &&
+                (isFirst ? activePred.prev == null : activePred.item != null) &&
+                (isLast  ? activeSucc.next == null : activeSucc.item != null)) {
+
+                updateHead(); // Ensure x is not reachable from head
+                updateTail(); // Ensure x is not reachable from tail
+
+                // Finally, actually gc-unlink
+                x.lazySetPrev(isFirst ? prevTerminator() : x);
+                x.lazySetNext(isLast  ? nextTerminator() : x);
+            }
+        }
+    }
+
+    /**
+     * Unlinks non-null first node.
+     */
+    private void unlinkFirst(Node<E> first, Node<E> next) {
+        // assert first != null;
+        // assert next != null;
+        // assert first.item == null;
+        for (Node<E> o = null, p = next, q;;) {
+            if (p.item != null || (q = p.next) == null) {
+                if (o != null && p.prev != p && first.casNext(next, p)) {
+                    skipDeletedPredecessors(p);
+                    if (first.prev == null &&
+                        (p.next == null || p.item != null) &&
+                        p.prev == first) {
+
+                        updateHead(); // Ensure o is not reachable from head
+                        updateTail(); // Ensure o is not reachable from tail
+
+                        // Finally, actually gc-unlink
+                        o.lazySetNext(o);
+                        o.lazySetPrev(prevTerminator());
+                    }
+                }
+                return;
+            }
+            else if (p == q)
+                return;
+            else {
+                o = p;
+                p = q;
+            }
+        }
+    }
+
+    /**
+     * Unlinks non-null last node.
+     */
+    private void unlinkLast(Node<E> last, Node<E> prev) {
+        // assert last != null;
+        // assert prev != null;
+        // assert last.item == null;
+        for (Node<E> o = null, p = prev, q;;) {
+            if (p.item != null || (q = p.prev) == null) {
+                if (o != null && p.next != p && last.casPrev(prev, p)) {
+                    skipDeletedSuccessors(p);
+                    if (last.next == null &&
+                        (p.prev == null || p.item != null) &&
+                        p.next == last) {
+
+                        updateHead(); // Ensure o is not reachable from head
+                        updateTail(); // Ensure o is not reachable from tail
+
+                        // Finally, actually gc-unlink
+                        o.lazySetPrev(o);
+                        o.lazySetNext(nextTerminator());
+                    }
+                }
+                return;
+            }
+            else if (p == q)
+                return;
+            else {
+                o = p;
+                p = q;
+            }
+        }
+    }
+
+    /**
+     * Guarantees that any node which was unlinked before a call to
+     * this method will be unreachable from head after it returns.
+     * Does not guarantee to eliminate slack, only that head will
+     * point to a node that was active while this method was running.
+     */
+    private final void updateHead() {
+        // Either head already points to an active node, or we keep
+        // trying to cas it to the first node until it does.
+        Node<E> h, p, q;
+        restartFromHead:
+        while ((h = head).item == null && (p = h.prev) != null) {
+            for (;;) {
+                if ((q = p.prev) == null ||
+                    (q = (p = q).prev) == null) {
+                    // It is possible that p is PREV_TERMINATOR,
+                    // but if so, the CAS is guaranteed to fail.
+                    if (casHead(h, p))
+                        return;
+                    else
+                        continue restartFromHead;
+                }
+                else if (h != head)
+                    continue restartFromHead;
+                else
+                    p = q;
+            }
+        }
+    }
+
+    /**
+     * Guarantees that any node which was unlinked before a call to
+     * this method will be unreachable from tail after it returns.
+     * Does not guarantee to eliminate slack, only that tail will
+     * point to a node that was active while this method was running.
+     */
+    private final void updateTail() {
+        // Either tail already points to an active node, or we keep
+        // trying to cas it to the last node until it does.
+        Node<E> t, p, q;
+        restartFromTail:
+        while ((t = tail).item == null && (p = t.next) != null) {
+            for (;;) {
+                if ((q = p.next) == null ||
+                    (q = (p = q).next) == null) {
+                    // It is possible that p is NEXT_TERMINATOR,
+                    // but if so, the CAS is guaranteed to fail.
+                    if (casTail(t, p))
+                        return;
+                    else
+                        continue restartFromTail;
+                }
+                else if (t != tail)
+                    continue restartFromTail;
+                else
+                    p = q;
+            }
+        }
+    }
+
+    private void skipDeletedPredecessors(Node<E> x) {
+        whileActive:
+        do {
+            Node<E> prev = x.prev;
+            // assert prev != null;
+            // assert x != NEXT_TERMINATOR;
+            // assert x != PREV_TERMINATOR;
+            Node<E> p = prev;
+            findActive:
+            for (;;) {
+                if (p.item != null)
+                    break findActive;
+                Node<E> q = p.prev;
+                if (q == null) {
+                    if (p.next == p)
+                        continue whileActive;
+                    break findActive;
+                }
+                else if (p == q)
+                    continue whileActive;
+                else
+                    p = q;
+            }
+
+            // found active CAS target
+            if (prev == p || x.casPrev(prev, p))
+                return;
+
+        } while (x.item != null || x.next == null);
+    }
+
+    private void skipDeletedSuccessors(Node<E> x) {
+        whileActive:
+        do {
+            Node<E> next = x.next;
+            // assert next != null;
+            // assert x != NEXT_TERMINATOR;
+            // assert x != PREV_TERMINATOR;
+            Node<E> p = next;
+            findActive:
+            for (;;) {
+                if (p.item != null)
+                    break findActive;
+                Node<E> q = p.next;
+                if (q == null) {
+                    if (p.prev == p)
+                        continue whileActive;
+                    break findActive;
+                }
+                else if (p == q)
+                    continue whileActive;
+                else
+                    p = q;
+            }
+
+            // found active CAS target
+            if (next == p || x.casNext(next, p))
+                return;
+
+        } while (x.item != null || x.prev == null);
+    }
+
+    /**
+     * Returns the successor of p, or the first node if p.next has been
+     * linked to self, which will only be true if traversing with a
+     * stale pointer that is now off the list.
+     */
+    final Node<E> succ(Node<E> p) {
+        // TODO: should we skip deleted nodes here?
+        Node<E> q = p.next;
+        return (p == q) ? first() : q;
+    }
+
+    /**
+     * Returns the predecessor of p, or the last node if p.prev has been
+     * linked to self, which will only be true if traversing with a
+     * stale pointer that is now off the list.
+     */
+    final Node<E> pred(Node<E> p) {
+        Node<E> q = p.prev;
+        return (p == q) ? last() : q;
+    }
+
+    /**
+     * Returns the first node, the unique node p for which:
+     *     p.prev == null && p.next != p
+     * The returned node may or may not be logically deleted.
+     * Guarantees that head is set to the returned node.
+     */
+    Node<E> first() {
+        restartFromHead:
+        for (;;)
+            for (Node<E> h = head, p = h, q;;) {
+                if ((q = p.prev) != null &&
+                    (q = (p = q).prev) != null)
+                    // Check for head updates every other hop.
+                    // If p == q, we are sure to follow head instead.
+                    p = (h != (h = head)) ? h : q;
+                else if (p == h
+                         // It is possible that p is PREV_TERMINATOR,
+                         // but if so, the CAS is guaranteed to fail.
+                         || casHead(h, p))
+                    return p;
+                else
+                    continue restartFromHead;
+            }
+    }
+
+    /**
+     * Returns the last node, the unique node p for which:
+     *     p.next == null && p.prev != p
+     * The returned node may or may not be logically deleted.
+     * Guarantees that tail is set to the returned node.
+     */
+    Node<E> last() {
+        restartFromTail:
+        for (;;)
+            for (Node<E> t = tail, p = t, q;;) {
+                if ((q = p.next) != null &&
+                    (q = (p = q).next) != null)
+                    // Check for tail updates every other hop.
+                    // If p == q, we are sure to follow tail instead.
+                    p = (t != (t = tail)) ? t : q;
+                else if (p == t
+                         // It is possible that p is NEXT_TERMINATOR,
+                         // but if so, the CAS is guaranteed to fail.
+                         || casTail(t, p))
+                    return p;
+                else
+                    continue restartFromTail;
+            }
+    }
+
+    // Minor convenience utilities
+
+    /**
+     * Throws NullPointerException if argument is null.
+     *
+     * @param v the element
+     */
+    private static void checkNotNull(Object v) {
+        if (v == null)
+            throw new NullPointerException();
+    }
+
+    /**
+     * Returns element unless it is null, in which case throws
+     * NoSuchElementException.
+     *
+     * @param v the element
+     * @return the element
+     */
+    private E screenNullResult(E v) {
+        if (v == null)
+            throw new NoSuchElementException();
+        return v;
+    }
+
+    /**
+     * Creates an array list and fills it with elements of this list.
+     * Used by toArray.
+     *
+     * @return the array list
+     */
+    private ArrayList<E> toArrayList() {
+        ArrayList<E> list = new ArrayList<E>();
+        for (Node<E> p = first(); p != null; p = succ(p)) {
+            E item = p.item;
+            if (item != null)
+                list.add(item);
+        }
+        return list;
+    }
+
+    /**
+     * Constructs an empty deque.
+     */
+    public ConcurrentLinkedDeque() {
+        head = tail = new Node<E>(null);
+    }
+
+    /**
+     * Constructs a deque initially containing the elements of
+     * the given collection, added in traversal order of the
+     * collection's iterator.
+     *
+     * @param c the collection of elements to initially contain
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public ConcurrentLinkedDeque(Collection<? extends E> c) {
+        // Copy c into a private chain of Nodes
+        Node<E> h = null, t = null;
+        for (E e : c) {
+            checkNotNull(e);
+            Node<E> newNode = new Node<E>(e);
+            if (h == null)
+                h = t = newNode;
+            else {
+                t.lazySetNext(newNode);
+                newNode.lazySetPrev(t);
+                t = newNode;
+            }
+        }
+        initHeadTail(h, t);
+    }
+
+    /**
+     * Initializes head and tail, ensuring invariants hold.
+     */
+    private void initHeadTail(Node<E> h, Node<E> t) {
+        if (h == t) {
+            if (h == null)
+                h = t = new Node<E>(null);
+            else {
+                // Avoid edge case of a single Node with non-null item.
+                Node<E> newNode = new Node<E>(null);
+                t.lazySetNext(newNode);
+                newNode.lazySetPrev(t);
+                t = newNode;
+            }
+        }
+        head = h;
+        tail = t;
+    }
+
+    /**
+     * Inserts the specified element at the front of this deque.
+     * As the deque is unbounded, this method will never throw
+     * {@link IllegalStateException}.
+     *
+     * @throws NullPointerException if the specified element is null
+     */
+    public void addFirst(E e) {
+        linkFirst(e);
+    }
+
+    /**
+     * Inserts the specified element at the end of this deque.
+     * As the deque is unbounded, this method will never throw
+     * {@link IllegalStateException}.
+     *
+     * <p>This method is equivalent to {@link #add}.
+     *
+     * @throws NullPointerException if the specified element is null
+     */
+    public void addLast(E e) {
+        linkLast(e);
+    }
+
+    /**
+     * Inserts the specified element at the front of this deque.
+     * As the deque is unbounded, this method will never return {@code false}.
+     *
+     * @return {@code true} (as specified by {@link Deque#offerFirst})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offerFirst(E e) {
+        linkFirst(e);
+        return true;
+    }
+
+    /**
+     * Inserts the specified element at the end of this deque.
+     * As the deque is unbounded, this method will never return {@code false}.
+     *
+     * <p>This method is equivalent to {@link #add}.
+     *
+     * @return {@code true} (as specified by {@link Deque#offerLast})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offerLast(E e) {
+        linkLast(e);
+        return true;
+    }
+
+    public E peekFirst() {
+        for (Node<E> p = first(); p != null; p = succ(p)) {
+            E item = p.item;
+            if (item != null)
+                return item;
+        }
+        return null;
+    }
+
+    public E peekLast() {
+        for (Node<E> p = last(); p != null; p = pred(p)) {
+            E item = p.item;
+            if (item != null)
+                return item;
+        }
+        return null;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E getFirst() {
+        return screenNullResult(peekFirst());
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E getLast() {
+        return screenNullResult(peekLast());
+    }
+
+    public E pollFirst() {
+        for (Node<E> p = first(); p != null; p = succ(p)) {
+            E item = p.item;
+            if (item != null && p.casItem(item, null)) {
+                unlink(p);
+                return item;
+            }
+        }
+        return null;
+    }
+
+    public E pollLast() {
+        for (Node<E> p = last(); p != null; p = pred(p)) {
+            E item = p.item;
+            if (item != null && p.casItem(item, null)) {
+                unlink(p);
+                return item;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E removeFirst() {
+        return screenNullResult(pollFirst());
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E removeLast() {
+        return screenNullResult(pollLast());
+    }
+
+    // *** Queue and stack methods ***
+
+    /**
+     * Inserts the specified element at the tail of this deque.
+     * As the deque is unbounded, this method will never return {@code false}.
+     *
+     * @return {@code true} (as specified by {@link Queue#offer})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        return offerLast(e);
+    }
+
+    /**
+     * Inserts the specified element at the tail of this deque.
+     * As the deque is unbounded, this method will never throw
+     * {@link IllegalStateException} or return {@code false}.
+     *
+     * @return {@code true} (as specified by {@link Collection#add})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean add(E e) {
+        return offerLast(e);
+    }
+
+    public E poll()           { return pollFirst(); }
+    public E remove()         { return removeFirst(); }
+    public E peek()           { return peekFirst(); }
+    public E element()        { return getFirst(); }
+    public void push(E e)     { addFirst(e); }
+    public E pop()            { return removeFirst(); }
+
+    /**
+     * Removes the first element {@code e} such that
+     * {@code o.equals(e)}, if such an element exists in this deque.
+     * If the deque does not contain the element, it is unchanged.
+     *
+     * @param o element to be removed from this deque, if present
+     * @return {@code true} if the deque contained the specified element
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean removeFirstOccurrence(Object o) {
+        checkNotNull(o);
+        for (Node<E> p = first(); p != null; p = succ(p)) {
+            E item = p.item;
+            if (item != null && o.equals(item) && p.casItem(item, null)) {
+                unlink(p);
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Removes the last element {@code e} such that
+     * {@code o.equals(e)}, if such an element exists in this deque.
+     * If the deque does not contain the element, it is unchanged.
+     *
+     * @param o element to be removed from this deque, if present
+     * @return {@code true} if the deque contained the specified element
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean removeLastOccurrence(Object o) {
+        checkNotNull(o);
+        for (Node<E> p = last(); p != null; p = pred(p)) {
+            E item = p.item;
+            if (item != null && o.equals(item) && p.casItem(item, null)) {
+                unlink(p);
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Returns {@code true} if this deque contains at least one
+     * element {@code e} such that {@code o.equals(e)}.
+     *
+     * @param o element whose presence in this deque is to be tested
+     * @return {@code true} if this deque contains the specified element
+     */
+    public boolean contains(Object o) {
+        if (o == null) return false;
+        for (Node<E> p = first(); p != null; p = succ(p)) {
+            E item = p.item;
+            if (item != null && o.equals(item))
+                return true;
+        }
+        return false;
+    }
+
+    /**
+     * Returns {@code true} if this collection contains no elements.
+     *
+     * @return {@code true} if this collection contains no elements
+     */
+    public boolean isEmpty() {
+        return peekFirst() == null;
+    }
+
+    /**
+     * Returns the number of elements in this deque.  If this deque
+     * contains more than {@code Integer.MAX_VALUE} elements, it
+     * returns {@code Integer.MAX_VALUE}.
+     *
+     * <p>Beware that, unlike in most collections, this method is
+     * <em>NOT</em> a constant-time operation. Because of the
+     * asynchronous nature of these deques, determining the current
+     * number of elements requires traversing them all to count them.
+     * Additionally, it is possible for the size to change during
+     * execution of this method, in which case the returned result
+     * will be inaccurate. Thus, this method is typically not very
+     * useful in concurrent applications.
+     *
+     * @return the number of elements in this deque
+     */
+    public int size() {
+        int count = 0;
+        for (Node<E> p = first(); p != null; p = succ(p))
+            if (p.item != null)
+                // Collection.size() spec says to max out
+                if (++count == Integer.MAX_VALUE)
+                    break;
+        return count;
+    }
+
+    /**
+     * Removes the first element {@code e} such that
+     * {@code o.equals(e)}, if such an element exists in this deque.
+     * If the deque does not contain the element, it is unchanged.
+     *
+     * @param o element to be removed from this deque, if present
+     * @return {@code true} if the deque contained the specified element
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean remove(Object o) {
+        return removeFirstOccurrence(o);
+    }
+
+    /**
+     * Appends all of the elements in the specified collection to the end of
+     * this deque, in the order that they are returned by the specified
+     * collection's iterator.  Attempts to {@code addAll} of a deque to
+     * itself result in {@code IllegalArgumentException}.
+     *
+     * @param c the elements to be inserted into this deque
+     * @return {@code true} if this deque changed as a result of the call
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     * @throws IllegalArgumentException if the collection is this deque
+     */
+    public boolean addAll(Collection<? extends E> c) {
+        if (c == this)
+            // As historically specified in AbstractQueue#addAll
+            throw new IllegalArgumentException();
+
+        // Copy c into a private chain of Nodes
+        Node<E> beginningOfTheEnd = null, last = null;
+        for (E e : c) {
+            checkNotNull(e);
+            Node<E> newNode = new Node<E>(e);
+            if (beginningOfTheEnd == null)
+                beginningOfTheEnd = last = newNode;
+            else {
+                last.lazySetNext(newNode);
+                newNode.lazySetPrev(last);
+                last = newNode;
+            }
+        }
+        if (beginningOfTheEnd == null)
+            return false;
+
+        // Atomically append the chain at the tail of this collection
+        restartFromTail:
+        for (;;)
+            for (Node<E> t = tail, p = t, q;;) {
+                if ((q = p.next) != null &&
+                    (q = (p = q).next) != null)
+                    // Check for tail updates every other hop.
+                    // If p == q, we are sure to follow tail instead.
+                    p = (t != (t = tail)) ? t : q;
+                else if (p.prev == p) // NEXT_TERMINATOR
+                    continue restartFromTail;
+                else {
+                    // p is last node
+                    beginningOfTheEnd.lazySetPrev(p); // CAS piggyback
+                    if (p.casNext(null, beginningOfTheEnd)) {
+                        // Successful CAS is the linearization point
+                        // for all elements to be added to this deque.
+                        if (!casTail(t, last)) {
+                            // Try a little harder to update tail,
+                            // since we may be adding many elements.
+                            t = tail;
+                            if (last.next == null)
+                                casTail(t, last);
+                        }
+                        return true;
+                    }
+                    // Lost CAS race to another thread; re-read next
+                }
+            }
+    }
+
+    /**
+     * Removes all of the elements from this deque.
+     */
+    public void clear() {
+        while (pollFirst() != null)
+            ;
+    }
+
+    /**
+     * Returns an array containing all of the elements in this deque, in
+     * proper sequence (from first to last element).
+     *
+     * <p>The returned array will be "safe" in that no references to it are
+     * maintained by this deque.  (In other words, this method must allocate
+     * a new array).  The caller is thus free to modify the returned array.
+     *
+     * <p>This method acts as bridge between array-based and collection-based
+     * APIs.
+     *
+     * @return an array containing all of the elements in this deque
+     */
+    public Object[] toArray() {
+        return toArrayList().toArray();
+    }
+
+    /**
+     * Returns an array containing all of the elements in this deque,
+     * in proper sequence (from first to last element); the runtime
+     * type of the returned array is that of the specified array.  If
+     * the deque fits in the specified array, it is returned therein.
+     * Otherwise, a new array is allocated with the runtime type of
+     * the specified array and the size of this deque.
+     *
+     * <p>If this deque fits in the specified array with room to spare
+     * (i.e., the array has more elements than this deque), the element in
+     * the array immediately following the end of the deque is set to
+     * {@code null}.
+     *
+     * <p>Like the {@link #toArray()} method, this method acts as
+     * bridge between array-based and collection-based APIs.  Further,
+     * this method allows precise control over the runtime type of the
+     * output array, and may, under certain circumstances, be used to
+     * save allocation costs.
+     *
+     * <p>Suppose {@code x} is a deque known to contain only strings.
+     * The following code can be used to dump the deque into a newly
+     * allocated array of {@code String}:
+     *
+     *  <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
+     *
+     * Note that {@code toArray(new Object[0])} is identical in function to
+     * {@code toArray()}.
+     *
+     * @param a the array into which the elements of the deque are to
+     *          be stored, if it is big enough; otherwise, a new array of the
+     *          same runtime type is allocated for this purpose
+     * @return an array containing all of the elements in this deque
+     * @throws ArrayStoreException if the runtime type of the specified array
+     *         is not a supertype of the runtime type of every element in
+     *         this deque
+     * @throws NullPointerException if the specified array is null
+     */
+    public <T> T[] toArray(T[] a) {
+        return toArrayList().toArray(a);
+    }
+
+    /**
+     * Returns an iterator over the elements in this deque in proper sequence.
+     * The elements will be returned in order from first (head) to last (tail).
+     *
+     * <p>The returned iterator is a "weakly consistent" iterator that
+     * will never throw {@link java.util.ConcurrentModificationException
+     * ConcurrentModificationException}, and guarantees to traverse
+     * elements as they existed upon construction of the iterator, and
+     * may (but is not guaranteed to) reflect any modifications
+     * subsequent to construction.
+     *
+     * @return an iterator over the elements in this deque in proper sequence
+     */
+    public Iterator<E> iterator() {
+        return new Itr();
+    }
+
+    /**
+     * Returns an iterator over the elements in this deque in reverse
+     * sequential order.  The elements will be returned in order from
+     * last (tail) to first (head).
+     *
+     * <p>The returned iterator is a "weakly consistent" iterator that
+     * will never throw {@link java.util.ConcurrentModificationException
+     * ConcurrentModificationException}, and guarantees to traverse
+     * elements as they existed upon construction of the iterator, and
+     * may (but is not guaranteed to) reflect any modifications
+     * subsequent to construction.
+     *
+     * @return an iterator over the elements in this deque in reverse order
+     */
+    public Iterator<E> descendingIterator() {
+        return new DescendingItr();
+    }
+
+    private abstract class AbstractItr implements Iterator<E> {
+        /**
+         * Next node to return item for.
+         */
+        private Node<E> nextNode;
+
+        /**
+         * nextItem holds on to item fields because once we claim
+         * that an element exists in hasNext(), we must return it in
+         * the following next() call even if it was in the process of
+         * being removed when hasNext() was called.
+         */
+        private E nextItem;
+
+        /**
+         * Node returned by most recent call to next. Needed by remove.
+         * Reset to null if this element is deleted by a call to remove.
+         */
+        private Node<E> lastRet;
+
+        abstract Node<E> startNode();
+        abstract Node<E> nextNode(Node<E> p);
+
+        AbstractItr() {
+            advance();
+        }
+
+        /**
+         * Sets nextNode and nextItem to next valid node, or to null
+         * if no such.
+         */
+        private void advance() {
+            lastRet = nextNode;
+
+            Node<E> p = (nextNode == null) ? startNode() : nextNode(nextNode);
+            for (;; p = nextNode(p)) {
+                if (p == null) {
+                    // p might be active end or TERMINATOR node; both are OK
+                    nextNode = null;
+                    nextItem = null;
+                    break;
+                }
+                E item = p.item;
+                if (item != null) {
+                    nextNode = p;
+                    nextItem = item;
+                    break;
+                }
+            }
+        }
+
+        public boolean hasNext() {
+            return nextItem != null;
+        }
+
+        public E next() {
+            E item = nextItem;
+            if (item == null) throw new NoSuchElementException();
+            advance();
+            return item;
+        }
+
+        public void remove() {
+            Node<E> l = lastRet;
+            if (l == null) throw new IllegalStateException();
+            l.item = null;
+            unlink(l);
+            lastRet = null;
+        }
+    }
+
+    /** Forward iterator */
+    private class Itr extends AbstractItr {
+        Node<E> startNode() { return first(); }
+        Node<E> nextNode(Node<E> p) { return succ(p); }
+    }
+
+    /** Descending iterator */
+    private class DescendingItr extends AbstractItr {
+        Node<E> startNode() { return last(); }
+        Node<E> nextNode(Node<E> p) { return pred(p); }
+    }
+
+    /**
+     * Saves the state to a stream (that is, serializes it).
+     *
+     * @serialData All of the elements (each an {@code E}) in
+     * the proper order, followed by a null
+     * @param s the stream
+     */
+    private void writeObject(java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+
+        // Write out any hidden stuff
+        s.defaultWriteObject();
+
+        // Write out all elements in the proper order.
+        for (Node<E> p = first(); p != null; p = succ(p)) {
+            E item = p.item;
+            if (item != null)
+                s.writeObject(item);
+        }
+
+        // Use trailing null as sentinel
+        s.writeObject(null);
+    }
+
+    /**
+     * Reconstitutes the instance from a stream (that is, deserializes it).
+     * @param s the stream
+     */
+    private void readObject(java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        s.defaultReadObject();
+
+        // Read in elements until trailing null sentinel found
+        Node<E> h = null, t = null;
+        Object item;
+        while ((item = s.readObject()) != null) {
+            @SuppressWarnings("unchecked")
+            Node<E> newNode = new Node<E>((E) item);
+            if (h == null)
+                h = t = newNode;
+            else {
+                t.lazySetNext(newNode);
+                newNode.lazySetPrev(t);
+                t = newNode;
+            }
+        }
+        initHeadTail(h, t);
+    }
+
+
+    private boolean casHead(Node<E> cmp, Node<E> val) {
+        return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
+    }
+
+    private boolean casTail(Node<E> cmp, Node<E> val) {
+        return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
+    }
+
+    // Unsafe mechanics
+
+    private static final sun.misc.Unsafe UNSAFE;
+    private static final long headOffset;
+    private static final long tailOffset;
+    static {
+        PREV_TERMINATOR = new Node<Object>();
+        PREV_TERMINATOR.next = PREV_TERMINATOR;
+        NEXT_TERMINATOR = new Node<Object>();
+        NEXT_TERMINATOR.prev = NEXT_TERMINATOR;
+        try {
+            UNSAFE = getUnsafe();
+            Class<?> k = ConcurrentLinkedDeque.class;
+            headOffset = UNSAFE.objectFieldOffset
+                (k.getDeclaredField("head"));
+            tailOffset = UNSAFE.objectFieldOffset
+                (k.getDeclaredField("tail"));
+        } 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
+     */
+    static sun.misc.Unsafe getUnsafe() {
+        try {
+            return sun.misc.Unsafe.getUnsafe();
+        } catch (SecurityException tryReflectionInstead) {}
+        try {
+            return java.security.AccessController.doPrivileged
+            (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
+                public sun.misc.Unsafe run() throws Exception {
+                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
+                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
+                        f.setAccessible(true);
+                        Object x = f.get(null);
+                        if (k.isInstance(x))
+                            return k.cast(x);
+                    }
+                    throw new NoSuchFieldError("the Unsafe");
+                }});
+        } catch (java.security.PrivilegedActionException e) {
+            throw new RuntimeException("Could not initialize intrinsics",
+                                       e.getCause());
+        }
+    }
+}
diff --git a/src/main/java/jsr166y/CountedCompleter.java b/src/main/java/jsr166y/CountedCompleter.java
new file mode 100644
index 0000000..7e74240
--- /dev/null
+++ b/src/main/java/jsr166y/CountedCompleter.java
@@ -0,0 +1,744 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+/**
+ * A {@link ForkJoinTask} with a completion action performed when
+ * triggered and there are no remaining pending
+ * actions. CountedCompleters are in general more robust in the
+ * presence of subtask stalls and blockage than are other forms of
+ * ForkJoinTasks, but are less intuitive to program.  Uses of
+ * CountedCompleter are similar to those of other completion based
+ * components (such as {@link java.nio.channels.CompletionHandler})
+ * except that multiple <em>pending</em> completions may be necessary
+ * to trigger the completion action {@link #onCompletion}, not just one.
+ * Unless initialized otherwise, the {@linkplain #getPendingCount pending
+ * count} starts at zero, but may be (atomically) changed using
+ * methods {@link #setPendingCount}, {@link #addToPendingCount}, and
+ * {@link #compareAndSetPendingCount}. Upon invocation of {@link
+ * #tryComplete}, if the pending action count is nonzero, it is
+ * decremented; otherwise, the completion action is performed, and if
+ * this completer itself has a completer, the process is continued
+ * with its completer.  As is the case with related synchronization
+ * components such as {@link java.util.concurrent.Phaser Phaser} and
+ * {@link java.util.concurrent.Semaphore Semaphore}, these methods
+ * affect only internal counts; they do not establish any further
+ * internal bookkeeping. In particular, the identities of pending
+ * tasks are not maintained. As illustrated below, you can create
+ * subclasses that do record some or all pending tasks or their
+ * results when needed.  As illustrated below, utility methods
+ * supporting customization of completion traversals are also
+ * provided. However, because CountedCompleters provide only basic
+ * synchronization mechanisms, it may be useful to create further
+ * abstract subclasses that maintain linkages, fields, and additional
+ * support methods appropriate for a set of related usages.
+ *
+ * <p>A concrete CountedCompleter class must define method {@link
+ * #compute}, that should in most cases (as illustrated below), invoke
+ * {@code tryComplete()} once before returning. The class may also
+ * optionally override method {@link #onCompletion} to perform an
+ * action upon normal completion, and method {@link
+ * #onExceptionalCompletion} to perform an action upon any exception.
+ *
+ * <p>CountedCompleters most often do not bear results, in which case
+ * they are normally declared as {@code CountedCompleter<Void>}, and
+ * will always return {@code null} as a result value.  In other cases,
+ * you should override method {@link #getRawResult} to provide a
+ * result from {@code join(), invoke()}, and related methods.  In
+ * general, this method should return the value of a field (or a
+ * function of one or more fields) of the CountedCompleter object that
+ * holds the result upon completion. Method {@link #setRawResult} by
+ * default plays no role in CountedCompleters.  It is possible, but
+ * rarely applicable, to override this method to maintain other
+ * objects or fields holding result data.
+ *
+ * <p>A CountedCompleter that does not itself have a completer (i.e.,
+ * one for which {@link #getCompleter} returns {@code null}) can be
+ * used as a regular ForkJoinTask with this added functionality.
+ * However, any completer that in turn has another completer serves
+ * only as an internal helper for other computations, so its own task
+ * status (as reported in methods such as {@link ForkJoinTask#isDone})
+ * is arbitrary; this status changes only upon explicit invocations of
+ * {@link #complete}, {@link ForkJoinTask#cancel}, {@link
+ * ForkJoinTask#completeExceptionally} or upon exceptional completion
+ * of method {@code compute}. Upon any exceptional completion, the
+ * exception may be relayed to a task's completer (and its completer,
+ * and so on), if one exists and it has not otherwise already
+ * completed. Similarly, cancelling an internal CountedCompleter has
+ * only a local effect on that completer, so is not often useful.
+ *
+ * <p><b>Sample Usages.</b>
+ *
+ * <p><b>Parallel recursive decomposition.</b> CountedCompleters may
+ * be arranged in trees similar to those often used with {@link
+ * RecursiveAction}s, although the constructions involved in setting
+ * them up typically vary. Here, the completer of each task is its
+ * parent in the computation tree. Even though they entail a bit more
+ * bookkeeping, CountedCompleters may be better choices when applying
+ * a possibly time-consuming operation (that cannot be further
+ * subdivided) to each element of an array or collection; especially
+ * when the operation takes a significantly different amount of time
+ * to complete for some elements than others, either because of
+ * intrinsic variation (for example I/O) or auxiliary effects such as
+ * garbage collection.  Because CountedCompleters provide their own
+ * continuations, other threads need not block waiting to perform
+ * them.
+ *
+ * <p>For example, here is an initial version of a class that uses
+ * divide-by-two recursive decomposition to divide work into single
+ * pieces (leaf tasks). Even when work is split into individual calls,
+ * tree-based techniques are usually preferable to directly forking
+ * leaf tasks, because they reduce inter-thread communication and
+ * improve load balancing. In the recursive case, the second of each
+ * pair of subtasks to finish triggers completion of its parent
+ * (because no result combination is performed, the default no-op
+ * implementation of method {@code onCompletion} is not overridden). A
+ * static utility method sets up the base task and invokes it
+ * (here, implicitly using the {@link ForkJoinPool#commonPool()}).
+ *
+ * <pre> {@code
+ * class MyOperation<E> { void apply(E e) { ... }  }
+ *
+ * class ForEach<E> extends CountedCompleter<Void> {
+ *
+ *   public static <E> void forEach(E[] array, MyOperation<E> op) {
+ *     new ForEach<E>(null, array, op, 0, array.length).invoke();
+ *   }
+ *
+ *   final E[] array; final MyOperation<E> op; final int lo, hi;
+ *   ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
+ *     super(p);
+ *     this.array = array; this.op = op; this.lo = lo; this.hi = hi;
+ *   }
+ *
+ *   public void compute() { // version 1
+ *     if (hi - lo >= 2) {
+ *       int mid = (lo + hi) >>> 1;
+ *       setPendingCount(2); // must set pending count before fork
+ *       new ForEach(this, array, op, mid, hi).fork(); // right child
+ *       new ForEach(this, array, op, lo, mid).fork(); // left child
+ *     }
+ *     else if (hi > lo)
+ *       op.apply(array[lo]);
+ *     tryComplete();
+ *   }
+ * }}</pre>
+ *
+ * This design can be improved by noticing that in the recursive case,
+ * the task has nothing to do after forking its right task, so can
+ * directly invoke its left task before returning. (This is an analog
+ * of tail recursion removal.)  Also, because the task returns upon
+ * executing its left task (rather than falling through to invoke
+ * {@code tryComplete}) the pending count is set to one:
+ *
+ * <pre> {@code
+ * class ForEach<E> ...
+ *   public void compute() { // version 2
+ *     if (hi - lo >= 2) {
+ *       int mid = (lo + hi) >>> 1;
+ *       setPendingCount(1); // only one pending
+ *       new ForEach(this, array, op, mid, hi).fork(); // right child
+ *       new ForEach(this, array, op, lo, mid).compute(); // direct invoke
+ *     }
+ *     else {
+ *       if (hi > lo)
+ *         op.apply(array[lo]);
+ *       tryComplete();
+ *     }
+ *   }
+ * }</pre>
+ *
+ * As a further improvement, notice that the left task need not even
+ * exist.  Instead of creating a new one, we can iterate using the
+ * original task, and add a pending count for each fork. Additionally,
+ * because no task in this tree implements an {@link #onCompletion}
+ * method, {@code tryComplete()} can be replaced with {@link
+ * #propagateCompletion}.
+ *
+ * <pre> {@code
+ * class ForEach<E> ...
+ *   public void compute() { // version 3
+ *     int l = lo,  h = hi;
+ *     while (h - l >= 2) {
+ *       int mid = (l + h) >>> 1;
+ *       addToPendingCount(1);
+ *       new ForEach(this, array, op, mid, h).fork(); // right child
+ *       h = mid;
+ *     }
+ *     if (h > l)
+ *       op.apply(array[l]);
+ *     propagateCompletion();
+ *   }
+ * }</pre>
+ *
+ * Additional improvements of such classes might entail precomputing
+ * pending counts so that they can be established in constructors,
+ * specializing classes for leaf steps, subdividing by say, four,
+ * instead of two per iteration, and using an adaptive threshold
+ * instead of always subdividing down to single elements.
+ *
+ * <p><b>Searching.</b> A tree of CountedCompleters can search for a
+ * value or property in different parts of a data structure, and
+ * report a result in an {@link
+ * java.util.concurrent.atomic.AtomicReference AtomicReference} as
+ * soon as one is found. The others can poll the result to avoid
+ * unnecessary work. (You could additionally {@linkplain #cancel
+ * cancel} other tasks, but it is usually simpler and more efficient
+ * to just let them notice that the result is set and if so skip
+ * further processing.)  Illustrating again with an array using full
+ * partitioning (again, in practice, leaf tasks will almost always
+ * process more than one element):
+ *
+ * <pre> {@code
+ * class Searcher<E> extends CountedCompleter<E> {
+ *   final E[] array; final AtomicReference<E> result; final int lo, hi;
+ *   Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) {
+ *     super(p);
+ *     this.array = array; this.result = result; this.lo = lo; this.hi = hi;
+ *   }
+ *   public E getRawResult() { return result.get(); }
+ *   public void compute() { // similar to ForEach version 3
+ *     int l = lo,  h = hi;
+ *     while (result.get() == null && h >= l) {
+ *       if (h - l >= 2) {
+ *         int mid = (l + h) >>> 1;
+ *         addToPendingCount(1);
+ *         new Searcher(this, array, result, mid, h).fork();
+ *         h = mid;
+ *       }
+ *       else {
+ *         E x = array[l];
+ *         if (matches(x) && result.compareAndSet(null, x))
+ *           quietlyCompleteRoot(); // root task is now joinable
+ *         break;
+ *       }
+ *     }
+ *     tryComplete(); // normally complete whether or not found
+ *   }
+ *   boolean matches(E e) { ... } // return true if found
+ *
+ *   public static <E> E search(E[] array) {
+ *       return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke();
+ *   }
+ * }}</pre>
+ *
+ * In this example, as well as others in which tasks have no other
+ * effects except to compareAndSet a common result, the trailing
+ * unconditional invocation of {@code tryComplete} could be made
+ * conditional ({@code if (result.get() == null) tryComplete();})
+ * because no further bookkeeping is required to manage completions
+ * once the root task completes.
+ *
+ * <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
+ * results of multiple subtasks usually need to access these results
+ * in method {@link #onCompletion}. As illustrated in the following
+ * class (that performs a simplified form of map-reduce where mappings
+ * and reductions are all of type {@code E}), one way to do this in
+ * divide and conquer designs is to have each subtask record its
+ * sibling, so that it can be accessed in method {@code onCompletion}.
+ * This technique applies to reductions in which the order of
+ * combining left and right results does not matter; ordered
+ * reductions require explicit left/right designations.  Variants of
+ * other streamlinings seen in the above examples may also apply.
+ *
+ * <pre> {@code
+ * class MyMapper<E> { E apply(E v) {  ...  } }
+ * class MyReducer<E> { E apply(E x, E y) {  ...  } }
+ * class MapReducer<E> extends CountedCompleter<E> {
+ *   final E[] array; final MyMapper<E> mapper;
+ *   final MyReducer<E> reducer; final int lo, hi;
+ *   MapReducer<E> sibling;
+ *   E result;
+ *   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
+ *              MyReducer<E> reducer, int lo, int hi) {
+ *     super(p);
+ *     this.array = array; this.mapper = mapper;
+ *     this.reducer = reducer; this.lo = lo; this.hi = hi;
+ *   }
+ *   public void compute() {
+ *     if (hi - lo >= 2) {
+ *       int mid = (lo + hi) >>> 1;
+ *       MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
+ *       MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
+ *       left.sibling = right;
+ *       right.sibling = left;
+ *       setPendingCount(1); // only right is pending
+ *       right.fork();
+ *       left.compute();     // directly execute left
+ *     }
+ *     else {
+ *       if (hi > lo)
+ *           result = mapper.apply(array[lo]);
+ *       tryComplete();
+ *     }
+ *   }
+ *   public void onCompletion(CountedCompleter<?> caller) {
+ *     if (caller != this) {
+ *       MapReducer<E> child = (MapReducer<E>)caller;
+ *       MapReducer<E> sib = child.sibling;
+ *       if (sib == null || sib.result == null)
+ *         result = child.result;
+ *       else
+ *         result = reducer.apply(child.result, sib.result);
+ *     }
+ *   }
+ *   public E getRawResult() { return result; }
+ *
+ *   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
+ *     return new MapReducer<E>(null, array, mapper, reducer,
+ *                              0, array.length).invoke();
+ *   }
+ * }}</pre>
+ *
+ * Here, method {@code onCompletion} takes a form common to many
+ * completion designs that combine results. This callback-style method
+ * is triggered once per task, in either of the two different contexts
+ * in which the pending count is, or becomes, zero: (1) by a task
+ * itself, if its pending count is zero upon invocation of {@code
+ * tryComplete}, or (2) by any of its subtasks when they complete and
+ * decrement the pending count to zero. The {@code caller} argument
+ * distinguishes cases.  Most often, when the caller is {@code this},
+ * no action is necessary. Otherwise the caller argument can be used
+ * (usually via a cast) to supply a value (and/or links to other
+ * values) to be combined.  Assuming proper use of pending counts, the
+ * actions inside {@code onCompletion} occur (once) upon completion of
+ * a task and its subtasks. No additional synchronization is required
+ * within this method to ensure thread safety of accesses to fields of
+ * this task or other completed tasks.
+ *
+ * <p><b>Completion Traversals</b>. If using {@code onCompletion} to
+ * process completions is inapplicable or inconvenient, you can use
+ * methods {@link #firstComplete} and {@link #nextComplete} to create
+ * custom traversals.  For example, to define a MapReducer that only
+ * splits out right-hand tasks in the form of the third ForEach
+ * example, the completions must cooperatively reduce along
+ * unexhausted subtask links, which can be done as follows:
+ *
+ * <pre> {@code
+ * class MapReducer<E> extends CountedCompleter<E> { // version 2
+ *   final E[] array; final MyMapper<E> mapper;
+ *   final MyReducer<E> reducer; final int lo, hi;
+ *   MapReducer<E> forks, next; // record subtask forks in list
+ *   E result;
+ *   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
+ *              MyReducer<E> reducer, int lo, int hi, MapReducer<E> next) {
+ *     super(p);
+ *     this.array = array; this.mapper = mapper;
+ *     this.reducer = reducer; this.lo = lo; this.hi = hi;
+ *     this.next = next;
+ *   }
+ *   public void compute() {
+ *     int l = lo,  h = hi;
+ *     while (h - l >= 2) {
+ *       int mid = (l + h) >>> 1;
+ *       addToPendingCount(1);
+ *       (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork;
+ *       h = mid;
+ *     }
+ *     if (h > l)
+ *       result = mapper.apply(array[l]);
+ *     // process completions by reducing along and advancing subtask links
+ *     for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
+ *       for (MapReducer t = (MapReducer)c, s = t.forks;  s != null; s = t.forks = s.next)
+ *         t.result = reducer.apply(t.result, s.result);
+ *     }
+ *   }
+ *   public E getRawResult() { return result; }
+ *
+ *   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
+ *     return new MapReducer<E>(null, array, mapper, reducer,
+ *                              0, array.length, null).invoke();
+ *   }
+ * }}</pre>
+ *
+ * <p><b>Triggers.</b> Some CountedCompleters are themselves never
+ * forked, but instead serve as bits of plumbing in other designs;
+ * including those in which the completion of one of more async tasks
+ * triggers another async task. For example:
+ *
+ * <pre> {@code
+ * class HeaderBuilder extends CountedCompleter<...> { ... }
+ * class BodyBuilder extends CountedCompleter<...> { ... }
+ * class PacketSender extends CountedCompleter<...> {
+ *   PacketSender(...) { super(null, 1); ... } // trigger on second completion
+ *   public void compute() { } // never called
+ *   public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
+ * }
+ * // sample use:
+ * PacketSender p = new PacketSender();
+ * new HeaderBuilder(p, ...).fork();
+ * new BodyBuilder(p, ...).fork();
+ * }</pre>
+ *
+ * @since 1.8
+ * @author Doug Lea
+ */
+public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
+    private static final long serialVersionUID = 5232453752276485070L;
+
+    /** This task's completer, or null if none */
+    final CountedCompleter<?> completer;
+    /** The number of pending tasks until completion */
+    volatile int pending;
+
+    /**
+     * Creates a new CountedCompleter with the given completer
+     * and initial pending count.
+     *
+     * @param completer this task's completer, or {@code null} if none
+     * @param initialPendingCount the initial pending count
+     */
+    protected CountedCompleter(CountedCompleter<?> completer,
+                               int initialPendingCount) {
+        this.completer = completer;
+        this.pending = initialPendingCount;
+    }
+
+    /**
+     * Creates a new CountedCompleter with the given completer
+     * and an initial pending count of zero.
+     *
+     * @param completer this task's completer, or {@code null} if none
+     */
+    protected CountedCompleter(CountedCompleter<?> completer) {
+        this.completer = completer;
+    }
+
+    /**
+     * Creates a new CountedCompleter with no completer
+     * and an initial pending count of zero.
+     */
+    protected CountedCompleter() {
+        this.completer = null;
+    }
+
+    /**
+     * The main computation performed by this task.
+     */
+    public abstract void compute();
+
+    /**
+     * Performs an action when method {@link #tryComplete} is invoked
+     * and the pending count is zero, or when the unconditional
+     * method {@link #complete} is invoked.  By default, this method
+     * does nothing. You can distinguish cases by checking the
+     * identity of the given caller argument. If not equal to {@code
+     * this}, then it is typically a subtask that may contain results
+     * (and/or links to other results) to combine.
+     *
+     * @param caller the task invoking this method (which may
+     * be this task itself)
+     */
+    public void onCompletion(CountedCompleter<?> caller) {
+    }
+
+    /**
+     * Performs an action when method {@link #completeExceptionally}
+     * is invoked or method {@link #compute} throws an exception, and
+     * this task has not otherwise already completed normally. On
+     * entry to this method, this task {@link
+     * ForkJoinTask#isCompletedAbnormally}.  The return value of this
+     * method controls further propagation: If {@code true} and this
+     * task has a completer, then this completer is also completed
+     * exceptionally.  The default implementation of this method does
+     * nothing except return {@code true}.
+     *
+     * @param ex the exception
+     * @param caller the task invoking this method (which may
+     * be this task itself)
+     * @return true if this exception should be propagated to this
+     * task's completer, if one exists
+     */
+    public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
+        return true;
+    }
+
+    /**
+     * Returns the completer established in this task's constructor,
+     * or {@code null} if none.
+     *
+     * @return the completer
+     */
+    public final CountedCompleter<?> getCompleter() {
+        return completer;
+    }
+
+    /**
+     * Returns the current pending count.
+     *
+     * @return the current pending count
+     */
+    public final int getPendingCount() {
+        return pending;
+    }
+
+    /**
+     * Sets the pending count to the given value.
+     *
+     * @param count the count
+     */
+    public final void setPendingCount(int count) {
+        pending = count;
+    }
+
+    /**
+     * Adds (atomically) the given value to the pending count.
+     *
+     * @param delta the value to add
+     */
+    public final void addToPendingCount(int delta) {
+        int c; // note: can replace with intrinsic in jdk8
+        do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
+    }
+
+    /**
+     * Sets (atomically) the pending count to the given count only if
+     * it currently holds the given expected value.
+     *
+     * @param expected the expected value
+     * @param count the new value
+     * @return true if successful
+     */
+    public final boolean compareAndSetPendingCount(int expected, int count) {
+        return U.compareAndSwapInt(this, PENDING, expected, count);
+    }
+
+    /**
+     * If the pending count is nonzero, (atomically) decrements it.
+     *
+     * @return the initial (undecremented) pending count holding on entry
+     * to this method
+     */
+    public final int decrementPendingCountUnlessZero() {
+        int c;
+        do {} while ((c = pending) != 0 &&
+                     !U.compareAndSwapInt(this, PENDING, c, c - 1));
+        return c;
+    }
+
+    /**
+     * Returns the root of the current computation; i.e., this
+     * task if it has no completer, else its completer's root.
+     *
+     * @return the root of the current computation
+     */
+    public final CountedCompleter<?> getRoot() {
+        CountedCompleter<?> a = this, p;
+        while ((p = a.completer) != null)
+            a = p;
+        return a;
+    }
+
+    /**
+     * If the pending count is nonzero, decrements the count;
+     * otherwise invokes {@link #onCompletion} and then similarly
+     * tries to complete this task's completer, if one exists,
+     * else marks this task as complete.
+     */
+    public final void tryComplete() {
+        CountedCompleter<?> a = this, s = a;
+        for (int c;;) {
+            if ((c = a.pending) == 0) {
+                a.onCompletion(s);
+                if ((a = (s = a).completer) == null) {
+                    s.quietlyComplete();
+                    return;
+                }
+            }
+            else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
+                return;
+        }
+    }
+
+    /**
+     * Equivalent to {@link #tryComplete} but does not invoke {@link
+     * #onCompletion} along the completion path: If the pending count
+     * is nonzero, decrements the count; otherwise, similarly tries to
+     * complete this task's completer, if one exists, else marks this
+     * task as complete. This method may be useful in cases where
+     * {@code onCompletion} should not, or need not, be invoked for
+     * each completer in a computation.
+     */
+    public final void propagateCompletion() {
+        CountedCompleter<?> a = this, s = a;
+        for (int c;;) {
+            if ((c = a.pending) == 0) {
+                if ((a = (s = a).completer) == null) {
+                    s.quietlyComplete();
+                    return;
+                }
+            }
+            else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
+                return;
+        }
+    }
+
+    /**
+     * Regardless of pending count, invokes {@link #onCompletion},
+     * marks this task as complete and further triggers {@link
+     * #tryComplete} on this task's completer, if one exists.  The
+     * given rawResult is used as an argument to {@link #setRawResult}
+     * before invoking {@link #onCompletion} or marking this task as
+     * complete; its value is meaningful only for classes overriding
+     * {@code setRawResult}.
+     *
+     * <p>This method may be useful when forcing completion as soon as
+     * any one (versus all) of several subtask results are obtained.
+     * However, in the common (and recommended) case in which {@code
+     * setRawResult} is not overridden, this effect can be obtained
+     * more simply using {@code quietlyCompleteRoot();}.
+     *
+     * @param rawResult the raw result
+     */
+    public void complete(T rawResult) {
+        CountedCompleter<?> p;
+        setRawResult(rawResult);
+        onCompletion(this);
+        quietlyComplete();
+        if ((p = completer) != null)
+            p.tryComplete();
+    }
+
+
+    /**
+     * If this task's pending count is zero, returns this task;
+     * otherwise decrements its pending count and returns {@code
+     * null}. This method is designed to be used with {@link
+     * #nextComplete} in completion traversal loops.
+     *
+     * @return this task, if pending count was zero, else {@code null}
+     */
+    public final CountedCompleter<?> firstComplete() {
+        for (int c;;) {
+            if ((c = pending) == 0)
+                return this;
+            else if (U.compareAndSwapInt(this, PENDING, c, c - 1))
+                return null;
+        }
+    }
+
+    /**
+     * If this task does not have a completer, invokes {@link
+     * ForkJoinTask#quietlyComplete} and returns {@code null}.  Or, if
+     * this task's pending count is non-zero, decrements its pending
+     * count and returns {@code null}.  Otherwise, returns the
+     * completer.  This method can be used as part of a completion
+     * traversal loop for homogeneous task hierarchies:
+     *
+     * <pre> {@code
+     * for (CountedCompleter<?> c = firstComplete();
+     *      c != null;
+     *      c = c.nextComplete()) {
+     *   // ... process c ...
+     * }}</pre>
+     *
+     * @return the completer, or {@code null} if none
+     */
+    public final CountedCompleter<?> nextComplete() {
+        CountedCompleter<?> p;
+        if ((p = completer) != null)
+            return p.firstComplete();
+        else {
+            quietlyComplete();
+            return null;
+        }
+    }
+
+    /**
+     * Equivalent to {@code getRoot().quietlyComplete()}.
+     */
+    public final void quietlyCompleteRoot() {
+        for (CountedCompleter<?> a = this, p;;) {
+            if ((p = a.completer) == null) {
+                a.quietlyComplete();
+                return;
+            }
+            a = p;
+        }
+    }
+
+    /**
+     * Supports ForkJoinTask exception propagation.
+     */
+    void internalPropagateException(Throwable ex) {
+        CountedCompleter<?> a = this, s = a;
+        while (a.onExceptionalCompletion(ex, s) &&
+               (a = (s = a).completer) != null && a.status >= 0)
+            a.recordExceptionalCompletion(ex);
+    }
+
+    /**
+     * Implements execution conventions for CountedCompleters.
+     */
+    protected final boolean exec() {
+        compute();
+        return false;
+    }
+
+    /**
+     * Returns the result of the computation.  By default,
+     * returns {@code null}, which is appropriate for {@code Void}
+     * actions, but in other cases should be overridden, almost
+     * always to return a field or function of a field that
+     * holds the result upon completion.
+     *
+     * @return the result of the computation
+     */
+    public T getRawResult() { return null; }
+
+    /**
+     * A method that result-bearing CountedCompleters may optionally
+     * use to help maintain result data.  By default, does nothing.
+     * Overrides are not recommended. However, if this method is
+     * overridden to update existing objects or fields, then it must
+     * in general be defined to be thread-safe.
+     */
+    protected void setRawResult(T t) { }
+
+    // Unsafe mechanics
+    private static final sun.misc.Unsafe U;
+    private static final long PENDING;
+    static {
+        try {
+            U = getUnsafe();
+            PENDING = U.objectFieldOffset
+                (CountedCompleter.class.getDeclaredField("pending"));
+        } catch (Exception e) {
+            throw new Error(e);
+        }
+    }
+
+    /**
+     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
+     * Replace with a simple call to Unsafe.getUnsafe when integrating
+     * into a jdk.
+     *
+     * @return a sun.misc.Unsafe
+     */
+    private static sun.misc.Unsafe getUnsafe() {
+        try {
+            return sun.misc.Unsafe.getUnsafe();
+        } catch (SecurityException tryReflectionInstead) {}
+        try {
+            return java.security.AccessController.doPrivileged
+            (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
+                public sun.misc.Unsafe run() throws Exception {
+                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
+                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
+                        f.setAccessible(true);
+                        Object x = f.get(null);
+                        if (k.isInstance(x))
+                            return k.cast(x);
+                    }
+                    throw new NoSuchFieldError("the Unsafe");
+                }});
+        } catch (java.security.PrivilegedActionException e) {
+            throw new RuntimeException("Could not initialize intrinsics",
+                                       e.getCause());
+        }
+    }
+}
diff --git a/src/main/java/jsr166y/ForkJoinPool.java b/src/main/java/jsr166y/ForkJoinPool.java
new file mode 100644
index 0000000..211722d
--- /dev/null
+++ b/src/main/java/jsr166y/ForkJoinPool.java
@@ -0,0 +1,3427 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.concurrent.AbstractExecutorService;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Future;
+import java.util.concurrent.RejectedExecutionException;
+import java.util.concurrent.RunnableFuture;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * 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>
+ *  <tr>
+ *    <td></td>
+ *    <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td>
+ *    <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td> <b>Arrange async execution</td>
+ *    <td> {@link #execute(ForkJoinTask)}</td>
+ *    <td> {@link ForkJoinTask#fork}</td>
+ *  </tr>
+ *  <tr>
+ *    <td> <b>Await and obtain result</td>
+ *    <td> {@link #invoke(ForkJoinTask)}</td>
+ *    <td> {@link ForkJoinTask#invoke}</td>
+ *  </tr>
+ *  <tr>
+ *    <td> <b>Arrange exec and obtain Future</td>
+ *    <td> {@link #submit(ForkJoinTask)}</td>
+ *    <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
+ *  </tr>
+ * </table>
+ *
+ * <p>The common pool is by default constructed with default
+ * parameters, but these may be controlled by setting three {@link
+ * System#getProperty system properties} with prefix {@code
+ * java.util.concurrent.ForkJoinPool.common}: {@code parallelism} --
+ * an integer greater than zero, {@code threadFactory} -- the class
+ * name of a {@link ForkJoinWorkerThreadFactory}, and {@code
+ * exceptionHandler} -- the class name of a {@link
+ * java.lang.Thread.UncaughtExceptionHandler
+ * Thread.UncaughtExceptionHandler}. Upon any error in establishing
+ * these settings, default parameters are used.
+ *
+ * <p><b>Implementation notes</b>: This implementation restricts the
+ * maximum number of running threads to 32767. Attempts to create
+ * pools with greater than the maximum number result in
+ * {@code IllegalArgumentException}.
+ *
+ * <p>This implementation rejects submitted tasks (that is, by throwing
+ * {@link RejectedExecutionException}) only when the pool is shut down
+ * or internal resources have been exhausted.
+ *
+ * @since 1.7
+ * @author Doug Lea
+ */
+public class ForkJoinPool extends AbstractExecutorService {
+
+    /*
+     * Implementation Overview
+     *
+     * This class and its nested classes provide the main
+     * functionality and control for a set of worker threads:
+     * Submissions from non-FJ threads enter into submission queues.
+     * Workers take these tasks and typically split them into subtasks
+     * that may be stolen by other workers.  Preference rules give
+     * first priority to processing tasks from their own queues (LIFO
+     * or FIFO, depending on mode), then to randomized FIFO steals of
+     * tasks in other queues.
+     *
+     * WorkQueues
+     * ==========
+     *
+     * Most operations occur within work-stealing queues (in nested
+     * class WorkQueue).  These are special forms of Deques that
+     * support only three of the four possible end-operations -- push,
+     * pop, and poll (aka steal), under the further constraints that
+     * push and pop are called only from the owning thread (or, as
+     * extended here, under a lock), while poll may be called from
+     * other threads.  (If you are unfamiliar with them, you probably
+     * want to read Herlihy and Shavit's book "The Art of
+     * Multiprocessor programming", chapter 16 describing these in
+     * more detail before proceeding.)  The main work-stealing queue
+     * design is roughly similar to those in the papers "Dynamic
+     * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
+     * (http://research.sun.com/scalable/pubs/index.html) and
+     * "Idempotent work stealing" by Michael, Saraswat, and Vechev,
+     * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
+     * The main differences ultimately stem from GC requirements that
+     * we null out taken slots as soon as we can, to maintain as small
+     * a footprint as possible even in programs generating huge
+     * numbers of tasks. To accomplish this, we shift the CAS
+     * arbitrating pop vs poll (steal) from being on the indices
+     * ("base" and "top") to the slots themselves.  So, both a
+     * successful pop and poll mainly entail a CAS of a slot from
+     * non-null to null.  Because we rely on CASes of references, we
+     * do not need tag bits on base or top.  They are simple ints as
+     * used in any circular array-based queue (see for example
+     * ArrayDeque).  Updates to the indices must still be ordered in a
+     * way that guarantees that top == base means the queue is empty,
+     * but otherwise may err on the side of possibly making the queue
+     * appear nonempty when a push, pop, or poll have not fully
+     * committed. Note that this means that the poll operation,
+     * considered individually, is not wait-free. One thief cannot
+     * successfully continue until another in-progress one (or, if
+     * previously empty, a push) completes.  However, in the
+     * aggregate, we ensure at least probabilistic non-blockingness.
+     * If an attempted steal fails, a thief always chooses a different
+     * random victim target to try next. So, in order for one thief to
+     * progress, it suffices for any in-progress poll or new push on
+     * any empty queue to complete. (This is why we normally use
+     * method pollAt and its variants that try once at the apparent
+     * base index, else consider alternative actions, rather than
+     * method poll.)
+     *
+     * This approach also enables support of a user mode in which local
+     * task processing is in FIFO, not LIFO order, simply by using
+     * poll rather than pop.  This can be useful in message-passing
+     * frameworks in which tasks are never joined.  However neither
+     * mode considers affinities, loads, cache localities, etc, so
+     * rarely provide the best possible performance on a given
+     * machine, but portably provide good throughput by averaging over
+     * these factors.  (Further, even if we did try to use such
+     * information, we do not usually have a basis for exploiting it.
+     * For example, some sets of tasks profit from cache affinities,
+     * but others are harmed by cache pollution effects.)
+     *
+     * WorkQueues are also used in a similar way for tasks submitted
+     * to the pool. We cannot mix these tasks in the same queues used
+     * for work-stealing (this would contaminate lifo/fifo
+     * processing). Instead, we randomly associate submission queues
+     * with submitting threads, using a form of hashing.  The
+     * ThreadLocal Submitter class contains a value initially used as
+     * a hash code for choosing existing queues, but may be randomly
+     * repositioned upon contention with other submitters.  In
+     * essence, submitters act like workers except that they are
+     * restricted to executing local tasks that they submitted (or in
+     * the case of CountedCompleters, others with the same root task).
+     * However, because most shared/external queue operations are more
+     * expensive than internal, and because, at steady state, external
+     * submitters will compete for CPU with workers, ForkJoinTask.join
+     * and related methods disable them from repeatedly helping to
+     * process tasks if all workers are active.  Insertion of tasks in
+     * shared mode requires a lock (mainly to protect in the case of
+     * resizing) but we use only a simple spinlock (using bits in
+     * field qlock), because submitters encountering a busy queue move
+     * on to try or create other queues -- they block only when
+     * creating and registering new queues.
+     *
+     * Management
+     * ==========
+     *
+     * The main throughput advantages of work-stealing stem from
+     * decentralized control -- workers mostly take tasks from
+     * themselves or each other. We cannot negate this in the
+     * implementation of other management responsibilities. The main
+     * tactic for avoiding bottlenecks is packing nearly all
+     * essentially atomic control state into two volatile variables
+     * that are by far most often read (not written) as status and
+     * consistency checks.
+     *
+     * Field "ctl" contains 64 bits holding all the information needed
+     * to atomically decide to add, inactivate, enqueue (on an event
+     * queue), dequeue, and/or re-activate workers.  To enable this
+     * packing, we restrict maximum parallelism to (1<<15)-1 (which is
+     * far in excess of normal operating range) to allow ids, counts,
+     * and their negations (used for thresholding) to fit into 16bit
+     * fields.
+     *
+     * Field "plock" is a form of sequence lock with a saturating
+     * shutdown bit (similarly for per-queue "qlocks"), mainly
+     * protecting updates to the workQueues array, as well as to
+     * enable shutdown.  When used as a lock, it is normally only very
+     * briefly held, so is nearly always available after at most a
+     * brief spin, but we use a monitor-based backup strategy to
+     * block when needed.
+     *
+     * Recording WorkQueues.  WorkQueues are recorded in the
+     * "workQueues" array that is created upon first use and expanded
+     * if necessary.  Updates to the array while recording new workers
+     * and unrecording terminated ones are protected from each other
+     * by a lock but the array is otherwise concurrently readable, and
+     * accessed directly.  To simplify index-based operations, the
+     * array size is always a power of two, and all readers must
+     * tolerate null slots. Worker queues are at odd indices. Shared
+     * (submission) queues are at even indices, up to a maximum of 64
+     * slots, to limit growth even if array needs to expand to add
+     * more workers. Grouping them together in this way simplifies and
+     * speeds up task scanning.
+     *
+     * All worker thread creation is on-demand, triggered by task
+     * submissions, replacement of terminated workers, and/or
+     * compensation for blocked workers. However, all other support
+     * code is set up to work with other policies.  To ensure that we
+     * do not hold on to worker references that would prevent GC, ALL
+     * accesses to workQueues are via indices into the workQueues
+     * array (which is one source of some of the messy code
+     * constructions here). In essence, the workQueues array serves as
+     * a weak reference mechanism. Thus for example the wait queue
+     * field of ctl stores indices, not references.  Access to the
+     * workQueues in associated methods (for example signalWork) must
+     * both index-check and null-check the IDs. All such accesses
+     * ignore bad IDs by returning out early from what they are doing,
+     * since this can only be associated with termination, in which
+     * case it is OK to give up.  All uses of the workQueues array
+     * also check that it is non-null (even if previously
+     * non-null). This allows nulling during termination, which is
+     * currently not necessary, but remains an option for
+     * resource-revocation-based shutdown schemes. It also helps
+     * reduce JIT issuance of uncommon-trap code, which tends to
+     * unnecessarily complicate control flow in some methods.
+     *
+     * Event Queuing. Unlike HPC work-stealing frameworks, we cannot
+     * let workers spin indefinitely scanning for tasks when none can
+     * be found immediately, and we cannot start/resume workers unless
+     * there appear to be tasks available.  On the other hand, we must
+     * quickly prod them into action when new tasks are submitted or
+     * generated. In many usages, ramp-up time to activate workers is
+     * the main limiting factor in overall performance (this is
+     * compounded at program start-up by JIT compilation and
+     * allocation). So we try to streamline this as much as possible.
+     * We park/unpark workers after placing in an event wait queue
+     * when they cannot find work. This "queue" is actually a simple
+     * Treiber stack, headed by the "id" field of ctl, plus a 15bit
+     * counter value (that reflects the number of times a worker has
+     * been inactivated) to avoid ABA effects (we need only as many
+     * version numbers as worker threads). Successors are held in
+     * field WorkQueue.nextWait.  Queuing deals with several intrinsic
+     * races, mainly that a task-producing thread can miss seeing (and
+     * signalling) another thread that gave up looking for work but
+     * has not yet entered the wait queue. We solve this by requiring
+     * a full sweep of all workers (via repeated calls to method
+     * scan()) both before and after a newly waiting worker is added
+     * to the wait queue. During a rescan, the worker might release
+     * some other queued worker rather than itself, which has the same
+     * net effect. Because enqueued workers may actually be rescanning
+     * rather than waiting, we set and clear the "parker" field of
+     * WorkQueues to reduce unnecessary calls to unpark.  (This
+     * requires a secondary recheck to avoid missed signals.)  Note
+     * the unusual conventions about Thread.interrupts surrounding
+     * parking and other blocking: Because interrupts are used solely
+     * to alert threads to check termination, which is checked anyway
+     * upon blocking, we clear status (using Thread.interrupted)
+     * before any call to park, so that park does not immediately
+     * return due to status being set via some other unrelated call to
+     * interrupt in user code.
+     *
+     * Signalling.  We create or wake up workers only when there
+     * appears to be at least one task they might be able to find and
+     * execute. However, many other threads may notice the same task
+     * and each signal to wake up a thread that might take it. So in
+     * general, pools will be over-signalled.  When a submission is
+     * added or another worker adds a task to a queue that has fewer
+     * than two tasks, they signal waiting workers (or trigger
+     * creation of new ones if fewer than the given parallelism level
+     * -- signalWork), and may leave a hint to the unparked worker to
+     * help signal others upon wakeup).  These primary signals are
+     * buttressed by others (see method helpSignal) whenever other
+     * threads scan for work or do not have a task to process.  On
+     * most platforms, signalling (unpark) overhead time is noticeably
+     * long, and the time between signalling a thread and it actually
+     * making progress can be very noticeably long, so it is worth
+     * offloading these delays from critical paths as much as
+     * possible.
+     *
+     * Trimming workers. To release resources after periods of lack of
+     * use, a worker starting to wait when the pool is quiescent will
+     * time out and terminate if the pool has remained quiescent for a
+     * given period -- a short period if there are more threads than
+     * parallelism, longer as the number of threads decreases. This
+     * will slowly propagate, eventually terminating all workers after
+     * periods of non-use.
+     *
+     * Shutdown and Termination. A call to shutdownNow atomically sets
+     * a plock bit and then (non-atomically) sets each worker's
+     * qlock status, cancels all unprocessed tasks, and wakes up
+     * all waiting workers.  Detecting whether termination should
+     * commence after a non-abrupt shutdown() call requires more work
+     * and bookkeeping. We need consensus about quiescence (i.e., that
+     * there is no more work). The active count provides a primary
+     * indication but non-abrupt shutdown still requires a rechecking
+     * scan for any workers that are inactive but not queued.
+     *
+     * Joining Tasks
+     * =============
+     *
+     * Any of several actions may be taken when one worker is waiting
+     * to join a task stolen (or always held) by another.  Because we
+     * are multiplexing many tasks on to a pool of workers, we can't
+     * just let them block (as in Thread.join).  We also cannot just
+     * reassign the joiner's run-time stack with another and replace
+     * it later, which would be a form of "continuation", that even if
+     * possible is not necessarily a good idea since we sometimes need
+     * both an unblocked task and its continuation to progress.
+     * Instead we combine two tactics:
+     *
+     *   Helping: Arranging for the joiner to execute some task that it
+     *      would be running if the steal had not occurred.
+     *
+     *   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 tryHelpStealer entails a form of "linear"
+     * helping: Each worker records (in field currentSteal) the most
+     * recent task it stole from some other worker. Plus, it records
+     * (in field currentJoin) the task it is currently actively
+     * joining. Method tryHelpStealer uses these markers to try to
+     * find a worker to help (i.e., steal back a task from and execute
+     * it) that could hasten completion of the actively joined task.
+     * In essence, the joiner executes a task that would be on its own
+     * local deque had the to-be-joined task not been stolen. This may
+     * be seen as a conservative variant of the approach in Wagner &
+     * Calder "Leapfrogging: a portable technique for implementing
+     * efficient futures" SIGPLAN Notices, 1993
+     * (http://portal.acm.org/citation.cfm?id=155354). It differs in
+     * that: (1) We only maintain dependency links across workers upon
+     * steals, rather than use per-task bookkeeping.  This sometimes
+     * requires a linear scan of workQueues array to locate stealers,
+     * but often doesn't because stealers leave hints (that may become
+     * stale/wrong) of where to locate them.  It is only a hint
+     * because a worker might have had multiple steals and the hint
+     * records only one of them (usually the most current).  Hinting
+     * isolates cost to when it is needed, rather than adding to
+     * per-task overhead.  (2) It is "shallow", ignoring nesting and
+     * potentially cyclic mutual steals.  (3) It is intentionally
+     * racy: field currentJoin is updated only while actively joining,
+     * which means that we miss links in the chain during long-lived
+     * tasks, GC stalls etc (which is OK since blocking in such cases
+     * is usually a good idea).  (4) We bound the number of attempts
+     * to find work (see MAX_HELP) and fall back to suspending the
+     * worker and if necessary replacing it with another.
+     *
+     * Helping actions for CountedCompleters are much simpler: Method
+     * helpComplete can take and execute any task with the same root
+     * as the task being waited on. However, this still entails some
+     * traversal of completer chains, so is less efficient than using
+     * CountedCompleters without explicit joins.
+     *
+     * It is impossible to keep exactly the target parallelism number
+     * of threads running at any given time.  Determining the
+     * existence of conservatively safe helping targets, the
+     * availability of already-created spares, and the apparent need
+     * to create new spares are all racy, so we rely on multiple
+     * retries of each.  Compensation in the apparent absence of
+     * helping opportunities is challenging to control on JVMs, where
+     * GC and other activities can stall progress of tasks that in
+     * turn stall out many other dependent tasks, without us being
+     * able to determine whether they will ever require compensation.
+     * Even though work-stealing otherwise encounters little
+     * degradation in the presence of more threads than cores,
+     * aggressively adding new threads in such cases entails risk of
+     * unwanted positive feedback control loops in which more threads
+     * cause more dependent stalls (as well as delayed progress of
+     * unblocked threads to the point that we know they are available)
+     * leading to more situations requiring more threads, and so
+     * on. This aspect of control can be seen as an (analytically
+     * intractable) game with an opponent that may choose the worst
+     * (for us) active thread to stall at any time.  We take several
+     * precautions to bound losses (and thus bound gains), mainly in
+     * methods tryCompensate and awaitJoin.
+     *
+     * Common Pool
+     * ===========
+     *
+     * The static 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
+     * fullExternalPush during the first submission to the pool.
+     *
+     * When external threads submit to the common pool, they can
+     * perform some subtask processing (see externalHelpJoin and
+     * related methods).  We do not need to record whether these
+     * submissions are to the common pool -- if not, externalHelpJoin
+     * returns quickly (at the most helping to signal some common pool
+     * workers). These submitters would otherwise be blocked waiting
+     * for completion, so the extra effort (with liberally sprinkled
+     * task status checks) in inapplicable cases amounts to an odd
+     * form of limited spin-wait before blocking in ForkJoinTask.join.
+     *
+     * Style notes
+     * ===========
+     *
+     * There is a lot of representation-level coupling among classes
+     * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask.  The
+     * fields of WorkQueue maintain data structures managed by
+     * ForkJoinPool, so are directly accessed.  There is little point
+     * trying to reduce this, since any associated future changes in
+     * representations will need to be accompanied by algorithmic
+     * changes anyway. Several methods intrinsically sprawl because
+     * they must accumulate sets of consistent reads of volatiles held
+     * in local variables.  Methods signalWork() and scan() are the
+     * main bottlenecks, so are especially heavily
+     * micro-optimized/mangled.  There are lots of inline assignments
+     * (of form "while ((local = field) != 0)") which are usually the
+     * simplest way to ensure the required read orderings (which are
+     * sometimes critical). This leads to a "C"-like style of listing
+     * declarations of these locals at the heads of methods or blocks.
+     * There are several occurrences of the unusual "do {} while
+     * (!cas...)"  which is the simplest way to force an update of a
+     * CAS'ed variable. There are also other coding oddities (including
+     * several unnecessary-looking hoisted null checks) that help
+     * some methods perform reasonably even when interpreted (not
+     * compiled).
+     *
+     * The order of declarations in this file is:
+     * (1) Static utility functions
+     * (2) Nested (static) classes
+     * (3) Static fields
+     * (4) Fields, along with constants used when unpacking some of them
+     * (5) Internal control methods
+     * (6) Callbacks and other support for ForkJoinTask methods
+     * (7) Exported methods
+     * (8) Static block initializing statics in minimally dependent order
+     */
+
+    // Static utilities
+
+    /**
+     * If there is a security manager, makes sure caller has
+     * permission to modify threads.
+     */
+    private static void checkPermission() {
+        SecurityManager security = System.getSecurityManager();
+        if (security != null)
+            security.checkPermission(modifyThreadPermission);
+    }
+
+    // Nested classes
+
+    /**
+     * Factory for creating new {@link ForkJoinWorkerThread}s.
+     * A {@code ForkJoinWorkerThreadFactory} must be defined and used
+     * for {@code ForkJoinWorkerThread} subclasses that extend base
+     * functionality or initialize threads with different contexts.
+     */
+    public static interface ForkJoinWorkerThreadFactory {
+        /**
+         * Returns a new worker thread operating in the given pool.
+         *
+         * @param pool the pool this thread works in
+         * @throws NullPointerException if the pool is null
+         */
+        public ForkJoinWorkerThread newThread(ForkJoinPool pool);
+    }
+
+    /**
+     * Default ForkJoinWorkerThreadFactory implementation; creates a
+     * new ForkJoinWorkerThread.
+     */
+    static final class DefaultForkJoinWorkerThreadFactory
+        implements ForkJoinWorkerThreadFactory {
+        public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
+            return new ForkJoinWorkerThread(pool);
+        }
+    }
+
+    /**
+     * Per-thread records for threads that submit to pools. Currently
+     * holds only pseudo-random seed / index that is used to choose
+     * submission queues in method externalPush. In the future, this may
+     * also incorporate a means to implement different task rejection
+     * and resubmission policies.
+     *
+     * Seeds for submitters and workers/workQueues work in basically
+     * the same way but are initialized and updated using slightly
+     * different mechanics. Both are initialized using the same
+     * approach as in class ThreadLocal, where successive values are
+     * unlikely to collide with previous values. Seeds are then
+     * randomly modified upon collisions using xorshifts, which
+     * requires a non-zero seed.
+     */
+    static final class Submitter {
+        int seed;
+        Submitter(int s) { seed = s; }
+    }
+
+    /**
+     * Class for artificial tasks that are used to replace the target
+     * of local joins if they are removed from an interior queue slot
+     * in WorkQueue.tryRemoveAndExec. We don't need the proxy to
+     * actually do anything beyond having a unique identity.
+     */
+    static final class EmptyTask extends ForkJoinTask<Void> {
+        private static final long serialVersionUID = -7721805057305804111L;
+        EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
+        public final Void getRawResult() { return null; }
+        public final void setRawResult(Void x) {}
+        public final boolean exec() { return true; }
+    }
+
+    /**
+     * Queues supporting work-stealing as well as external task
+     * submission. See above for main rationale and algorithms.
+     * Implementation relies heavily on "Unsafe" intrinsics
+     * and selective use of "volatile":
+     *
+     * Field "base" is the index (mod array.length) of the least valid
+     * queue slot, which is always the next position to steal (poll)
+     * from if nonempty. Reads and writes require volatile orderings
+     * but not CAS, because updates are only performed after slot
+     * CASes.
+     *
+     * Field "top" is the index (mod array.length) of the next queue
+     * slot to push to or pop from. It is written only by owner thread
+     * for push, or under lock for external/shared push, and accessed
+     * by other threads only after reading (volatile) base.  Both top
+     * and base are allowed to wrap around on overflow, but (top -
+     * base) (or more commonly -(base - top) to force volatile read of
+     * base before top) still estimates size. The lock ("qlock") is
+     * forced to -1 on termination, causing all further lock attempts
+     * to fail. (Note: we don't need CAS for termination state because
+     * upon pool shutdown, all shared-queues will stop being used
+     * anyway.)  Nearly all lock bodies are set up so that exceptions
+     * within lock bodies are "impossible" (modulo JVM errors that
+     * would cause failure anyway.)
+     *
+     * The array slots are read and written using the emulation of
+     * volatiles/atomics provided by Unsafe. Insertions must in
+     * general use putOrderedObject as a form of releasing store to
+     * ensure that all writes to the task object are ordered before
+     * its publication in the queue.  All removals entail a CAS to
+     * null.  The array is always a power of two. To ensure safety of
+     * Unsafe array operations, all accesses perform explicit null
+     * checks and implicit bounds checks via power-of-two masking.
+     *
+     * In addition to basic queuing support, this class contains
+     * fields described elsewhere to control execution. It turns out
+     * to work better memory-layout-wise to include them in this class
+     * rather than a separate class.
+     *
+     * Performance on most platforms is very sensitive to placement of
+     * instances of both WorkQueues and their arrays -- we absolutely
+     * do not want multiple WorkQueue instances or multiple queue
+     * arrays sharing cache lines. (It would be best for queue objects
+     * and their arrays to share, but there is nothing available to
+     * help arrange that).  Unfortunately, because they are recorded
+     * in a common array, WorkQueue instances are often moved to be
+     * adjacent by garbage collectors. To reduce impact, we use field
+     * padding that works OK on common platforms; this effectively
+     * trades off slightly slower average field access for the sake of
+     * avoiding really bad worst-case access. (Until better JVM
+     * support is in place, this padding is dependent on transient
+     * properties of JVM field layout rules.) We also take care in
+     * allocating, sizing and resizing the array. Non-shared queue
+     * arrays are initialized by workers before use. Others are
+     * allocated on first use.
+     */
+    static final class WorkQueue {
+        /**
+         * Capacity of work-stealing queue array upon initialization.
+         * Must be a power of two; at least 4, but should be larger to
+         * reduce or eliminate cacheline sharing among queues.
+         * Currently, it is much larger, as a partial workaround for
+         * the fact that JVMs often place arrays in locations that
+         * share GC bookkeeping (especially cardmarks) such that
+         * per-write accesses encounter serious memory contention.
+         */
+        static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
+
+        /**
+         * Maximum size for queue arrays. Must be a power of two less
+         * than or equal to 1 << (31 - width of array entry) to ensure
+         * lack of wraparound of index calculations, but defined to a
+         * value a bit less than this to help users trap runaway
+         * programs before saturating systems.
+         */
+        static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M
+
+        // Heuristic padding to ameliorate unfortunate memory placements
+        volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06;
+
+        int seed;                  // for random scanning; initialize nonzero
+        volatile int eventCount;   // encoded inactivation count; < 0 if inactive
+        int nextWait;              // encoded record of next event waiter
+        int hint;                  // steal or signal hint (index)
+        int poolIndex;             // index of this queue in pool (or 0)
+        final int mode;            // 0: lifo, > 0: fifo, < 0: shared
+        int nsteals;               // number of steals
+        volatile int qlock;        // 1: locked, -1: terminate; else 0
+        volatile int base;         // index of next slot for poll
+        int top;                   // index of next slot for push
+        ForkJoinTask<?>[] array;   // the elements (initially unallocated)
+        final ForkJoinPool pool;   // the containing pool (may be null)
+        final ForkJoinWorkerThread owner; // owning thread or null if shared
+        volatile Thread parker;    // == owner during call to park; else null
+        volatile ForkJoinTask<?> currentJoin;  // task being joined in awaitJoin
+        ForkJoinTask<?> currentSteal; // current non-local task being executed
+
+        volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
+        volatile Object pad18, pad19, pad1a, pad1b, pad1c, pad1d;
+
+        WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner, int mode,
+                  int seed) {
+            this.pool = pool;
+            this.owner = owner;
+            this.mode = mode;
+            this.seed = seed;
+            // Place indices in the center of array (that is not yet allocated)
+            base = top = INITIAL_QUEUE_CAPACITY >>> 1;
+        }
+
+        /**
+         * Returns the approximate number of tasks in the queue.
+         */
+        final int queueSize() {
+            int n = base - top;       // non-owner callers must read base first
+            return (n >= 0) ? 0 : -n; // ignore transient negative
+        }
+
+       /**
+         * Provides a more accurate estimate of whether this queue has
+         * any tasks than does queueSize, by checking whether a
+         * near-empty queue has at least one unclaimed task.
+         */
+        final boolean isEmpty() {
+            ForkJoinTask<?>[] a; int m, s;
+            int n = base - (s = top);
+            return (n >= 0 ||
+                    (n == -1 &&
+                     ((a = array) == null ||
+                      (m = a.length - 1) < 0 ||
+                      U.getObject
+                      (a, (long)((m & (s - 1)) << ASHIFT) + ABASE) == null)));
+        }
+
+        /**
+         * Pushes a task. Call only by owner in unshared queues.  (The
+         * shared-queue version is embedded in method externalPush.)
+         *
+         * @param task the task. Caller must ensure non-null.
+         * @throws RejectedExecutionException if array cannot be resized
+         */
+        final void push(ForkJoinTask<?> task) {
+            ForkJoinTask<?>[] a; ForkJoinPool p;
+            int s = top, m, n;
+            if ((a = array) != null) {    // ignore if queue removed
+                int j = (((m = a.length - 1) & s) << ASHIFT) + ABASE;
+                U.putOrderedObject(a, j, task);
+                if ((n = (top = s + 1) - base) <= 2) {
+                    if ((p = pool) != null)
+                        p.signalWork(this);
+                }
+                else if (n >= m)
+                    growArray();
+            }
+        }
+
+       /**
+         * Initializes or doubles the capacity of array. Call either
+         * by owner or with lock held -- it is OK for base, but not
+         * top, to move while resizings are in progress.
+         */
+        final ForkJoinTask<?>[] growArray() {
+            ForkJoinTask<?>[] oldA = array;
+            int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
+            if (size > MAXIMUM_QUEUE_CAPACITY)
+                throw new RejectedExecutionException("Queue capacity exceeded");
+            int oldMask, t, b;
+            ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
+            if (oldA != null && (oldMask = oldA.length - 1) >= 0 &&
+                (t = top) - (b = base) > 0) {
+                int mask = size - 1;
+                do {
+                    ForkJoinTask<?> x;
+                    int oldj = ((b & oldMask) << ASHIFT) + ABASE;
+                    int j    = ((b &    mask) << ASHIFT) + ABASE;
+                    x = (ForkJoinTask<?>)U.getObjectVolatile(oldA, oldj);
+                    if (x != null &&
+                        U.compareAndSwapObject(oldA, oldj, x, null))
+                        U.putObjectVolatile(a, j, x);
+                } while (++b != t);
+            }
+            return a;
+        }
+
+        /**
+         * Takes next task, if one exists, in LIFO order.  Call only
+         * by owner in unshared queues.
+         */
+        final ForkJoinTask<?> pop() {
+            ForkJoinTask<?>[] a; ForkJoinTask<?> t; int m;
+            if ((a = array) != null && (m = a.length - 1) >= 0) {
+                for (int s; (s = top - 1) - base >= 0;) {
+                    long j = ((m & s) << ASHIFT) + ABASE;
+                    if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null)
+                        break;
+                    if (U.compareAndSwapObject(a, j, t, null)) {
+                        top = s;
+                        return t;
+                    }
+                }
+            }
+            return null;
+        }
+
+        /**
+         * Takes a task in FIFO order if b is base of queue and a task
+         * can be claimed without contention. Specialized versions
+         * appear in ForkJoinPool methods scan and tryHelpStealer.
+         */
+        final ForkJoinTask<?> pollAt(int b) {
+            ForkJoinTask<?> t; ForkJoinTask<?>[] a;
+            if ((a = array) != null) {
+                int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                if ((t = (ForkJoinTask<?>)U.getObjectVolatile(a, j)) != null &&
+                    base == b &&
+                    U.compareAndSwapObject(a, j, t, null)) {
+                    base = b + 1;
+                    return t;
+                }
+            }
+            return null;
+        }
+
+        /**
+         * Takes next task, if one exists, in FIFO order.
+         */
+        final ForkJoinTask<?> poll() {
+            ForkJoinTask<?>[] a; int b; ForkJoinTask<?> t;
+            while ((b = base) - top < 0 && (a = array) != null) {
+                int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
+                if (t != null) {
+                    if (base == b &&
+                        U.compareAndSwapObject(a, j, t, null)) {
+                        base = b + 1;
+                        return t;
+                    }
+                }
+                else if (base == b) {
+                    if (b + 1 == top)
+                        break;
+                    Thread.yield(); // wait for lagging update (very rare)
+                }
+            }
+            return null;
+        }
+
+        /**
+         * Takes next task, if one exists, in order specified by mode.
+         */
+        final ForkJoinTask<?> nextLocalTask() {
+            return mode == 0 ? pop() : poll();
+        }
+
+        /**
+         * Returns next task, if one exists, in order specified by mode.
+         */
+        final ForkJoinTask<?> peek() {
+            ForkJoinTask<?>[] a = array; int m;
+            if (a == null || (m = a.length - 1) < 0)
+                return null;
+            int i = mode == 0 ? top - 1 : base;
+            int j = ((i & m) << ASHIFT) + ABASE;
+            return (ForkJoinTask<?>)U.getObjectVolatile(a, j);
+        }
+
+        /**
+         * Pops the given task only if it is at the current top.
+         * (A shared version is available only via FJP.tryExternalUnpush)
+         */
+        final boolean tryUnpush(ForkJoinTask<?> t) {
+            ForkJoinTask<?>[] a; int s;
+            if ((a = array) != null && (s = top) != base &&
+                U.compareAndSwapObject
+                (a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
+                top = s;
+                return true;
+            }
+            return false;
+        }
+
+        /**
+         * Removes and cancels all known tasks, ignoring any exceptions.
+         */
+        final void cancelAll() {
+            ForkJoinTask.cancelIgnoringExceptions(currentJoin);
+            ForkJoinTask.cancelIgnoringExceptions(currentSteal);
+            for (ForkJoinTask<?> t; (t = poll()) != null; )
+                ForkJoinTask.cancelIgnoringExceptions(t);
+        }
+
+        /**
+         * Computes next value for random probes.  Scans don't require
+         * a very high quality generator, but also not a crummy one.
+         * Marsaglia xor-shift is cheap and works well enough.  Note:
+         * This is manually inlined in its usages in ForkJoinPool to
+         * avoid writes inside busy scan loops.
+         */
+        final int nextSeed() {
+            int r = seed;
+            r ^= r << 13;
+            r ^= r >>> 17;
+            return seed = r ^= r << 5;
+        }
+
+        // Specialized execution methods
+
+        /**
+         * Pops and runs tasks until empty.
+         */
+        private void popAndExecAll() {
+            // A bit faster than repeated pop calls
+            ForkJoinTask<?>[] a; int m, s; long j; ForkJoinTask<?> t;
+            while ((a = array) != null && (m = a.length - 1) >= 0 &&
+                   (s = top - 1) - base >= 0 &&
+                   (t = ((ForkJoinTask<?>)
+                         U.getObject(a, j = ((m & s) << ASHIFT) + ABASE)))
+                   != null) {
+                if (U.compareAndSwapObject(a, j, t, null)) {
+                    top = s;
+                    t.doExec();
+                }
+            }
+        }
+
+        /**
+         * Polls and runs tasks until empty.
+         */
+        private void pollAndExecAll() {
+            for (ForkJoinTask<?> t; (t = poll()) != null;)
+                t.doExec();
+        }
+
+        /**
+         * If present, removes from queue and executes the given task,
+         * or any other cancelled task. Returns (true) on any CAS
+         * or consistency check failure so caller can retry.
+         *
+         * @return false if no progress can be made, else true
+         */
+        final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
+            boolean stat = true, removed = false, empty = true;
+            ForkJoinTask<?>[] a; int m, s, b, n;
+            if ((a = array) != null && (m = a.length - 1) >= 0 &&
+                (n = (s = top) - (b = base)) > 0) {
+                for (ForkJoinTask<?> t;;) {           // traverse from s to b
+                    int j = ((--s & m) << ASHIFT) + ABASE;
+                    t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
+                    if (t == null)                    // inconsistent length
+                        break;
+                    else if (t == task) {
+                        if (s + 1 == top) {           // pop
+                            if (!U.compareAndSwapObject(a, j, task, null))
+                                break;
+                            top = s;
+                            removed = true;
+                        }
+                        else if (base == b)           // replace with proxy
+                            removed = U.compareAndSwapObject(a, j, task,
+                                                             new EmptyTask());
+                        break;
+                    }
+                    else if (t.status >= 0)
+                        empty = false;
+                    else if (s + 1 == top) {          // pop and throw away
+                        if (U.compareAndSwapObject(a, j, t, null))
+                            top = s;
+                        break;
+                    }
+                    if (--n == 0) {
+                        if (!empty && base == b)
+                            stat = false;
+                        break;
+                    }
+                }
+            }
+            if (removed)
+                task.doExec();
+            return stat;
+        }
+
+        /**
+         * Polls for and executes the given task or any other task in
+         * its CountedCompleter computation.
+         */
+        final boolean pollAndExecCC(ForkJoinTask<?> root) {
+            ForkJoinTask<?>[] a; int b; Object o;
+            outer: while ((b = base) - top < 0 && (a = array) != null) {
+                long j = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                if ((o = U.getObject(a, j)) == null ||
+                    !(o instanceof CountedCompleter))
+                    break;
+                for (CountedCompleter<?> t = (CountedCompleter<?>)o, r = t;;) {
+                    if (r == root) {
+                        if (base == b &&
+                            U.compareAndSwapObject(a, j, t, null)) {
+                            base = b + 1;
+                            t.doExec();
+                            return true;
+                        }
+                        else
+                            break; // restart
+                    }
+                    if ((r = r.completer) == null)
+                        break outer; // not part of root computation
+                }
+            }
+            return false;
+        }
+
+        /**
+         * Executes a top-level task and any local tasks remaining
+         * after execution.
+         */
+        final void runTask(ForkJoinTask<?> t) {
+            if (t != null) {
+                (currentSteal = t).doExec();
+                currentSteal = null;
+                ++nsteals;
+                if (base - top < 0) {       // process remaining local tasks
+                    if (mode == 0)
+                        popAndExecAll();
+                    else
+                        pollAndExecAll();
+                }
+            }
+        }
+
+        /**
+         * Executes a non-top-level (stolen) task.
+         */
+        final void runSubtask(ForkJoinTask<?> t) {
+            if (t != null) {
+                ForkJoinTask<?> ps = currentSteal;
+                (currentSteal = t).doExec();
+                currentSteal = ps;
+            }
+        }
+
+        /**
+         * Returns true if owned and not known to be blocked.
+         */
+        final boolean isApparentlyUnblocked() {
+            Thread wt; Thread.State s;
+            return (eventCount >= 0 &&
+                    (wt = owner) != null &&
+                    (s = wt.getState()) != Thread.State.BLOCKED &&
+                    s != Thread.State.WAITING &&
+                    s != Thread.State.TIMED_WAITING);
+        }
+
+        // Unsafe mechanics
+        private static final sun.misc.Unsafe U;
+        private static final long QLOCK;
+        private static final int ABASE;
+        private static final int ASHIFT;
+        static {
+            try {
+                U = getUnsafe();
+                Class<?> k = WorkQueue.class;
+                Class<?> ak = ForkJoinTask[].class;
+                QLOCK = U.objectFieldOffset
+                    (k.getDeclaredField("qlock"));
+                ABASE = U.arrayBaseOffset(ak);
+                int scale = U.arrayIndexScale(ak);
+                if ((scale & (scale - 1)) != 0)
+                    throw new Error("data type scale not a power of two");
+                ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
+            } catch (Exception 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;
+
+    /**
+     * Per-thread submission bookkeeping. Shared across all pools
+     * to reduce ThreadLocal pollution and because random motion
+     * to avoid contention in one pool is likely to hold for others.
+     * Lazily initialized on first submission (but null-checked
+     * in other contexts to avoid unnecessary initialization).
+     */
+    static final ThreadLocal<Submitter> submitters;
+
+    /**
+     * Permission required for callers of methods that may start or
+     * kill threads.
+     */
+    private 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. Must equal common.parallelism.
+     */
+    static final int commonParallelism;
+
+    /**
+     * 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 constants
+
+    /**
+     * Initial timeout value (in nanoseconds) for the thread
+     * triggering quiescence to park waiting for new work. On timeout,
+     * the thread will instead try to shrink the number of
+     * workers. The value should be large enough to avoid overly
+     * aggressive shrinkage during most transient stalls (long GCs
+     * etc).
+     */
+    private static final long IDLE_TIMEOUT      = 2000L * 1000L * 1000L; // 2sec
+
+    /**
+     * Timeout value when there are more threads than parallelism level
+     */
+    private static final long FAST_IDLE_TIMEOUT =  200L * 1000L * 1000L;
+
+    /**
+     * Tolerance for idle timeouts, to cope with timer undershoots
+     */
+    private static final long TIMEOUT_SLOP = 2000000L;
+
+    /**
+     * The maximum stolen->joining link depth allowed in method
+     * tryHelpStealer.  Must be a power of two.  Depths for legitimate
+     * chains are unbounded, but we use a fixed constant to avoid
+     * (otherwise unchecked) cycles and to bound staleness of
+     * traversal parameters at the expense of sometimes blocking when
+     * we could be helping.
+     */
+    private static final int MAX_HELP = 64;
+
+    /**
+     * Increment for seed generators. See class ThreadLocal for
+     * explanation.
+     */
+    private static final int SEED_INCREMENT = 0x61c88647;
+
+    /*
+     * Bits and masks for control variables
+     *
+     * Field ctl is a long packed with:
+     * AC: Number of active running workers minus target parallelism (16 bits)
+     * TC: Number of total workers minus target parallelism (16 bits)
+     * ST: true if pool is terminating (1 bit)
+     * EC: the wait count of top waiting thread (15 bits)
+     * ID: poolIndex of top of Treiber stack of waiters (16 bits)
+     *
+     * When convenient, we can extract the upper 32 bits of counts and
+     * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e =
+     * (int)ctl.  The ec field is never accessed alone, but always
+     * together with id and st. The offsets of counts by the target
+     * parallelism and the positionings of fields makes it possible to
+     * perform the most common checks via sign tests of fields: When
+     * ac is negative, there are not enough active workers, when tc is
+     * negative, there are not enough total workers, and when e is
+     * negative, the pool is terminating.  To deal with these possibly
+     * negative fields, we use casts in and out of "short" and/or
+     * signed shifts to maintain signedness.
+     *
+     * When a thread is queued (inactivated), its eventCount field is
+     * set negative, which is the only way to tell if a worker is
+     * prevented from executing tasks, even though it must continue to
+     * scan for them to avoid queuing races. Note however that
+     * eventCount updates lag releases so usage requires care.
+     *
+     * Field plock is an int packed with:
+     * SHUTDOWN: true if shutdown is enabled (1 bit)
+     * SEQ:  a sequence lock, with PL_LOCK bit set if locked (30 bits)
+     * SIGNAL: set when threads may be waiting on the lock (1 bit)
+     *
+     * The sequence number enables simple consistency checks:
+     * Staleness of read-only operations on the workQueues array can
+     * be checked by comparing plock before vs after the reads.
+     */
+
+    // bit positions/shifts for fields
+    private static final int  AC_SHIFT   = 48;
+    private static final int  TC_SHIFT   = 32;
+    private static final int  ST_SHIFT   = 31;
+    private static final int  EC_SHIFT   = 16;
+
+    // bounds
+    private static final int  SMASK      = 0xffff;  // short bits
+    private static final int  MAX_CAP    = 0x7fff;  // max #workers - 1
+    private static final int  EVENMASK   = 0xfffe;  // even short bits
+    private static final int  SQMASK     = 0x007e;  // max 64 (even) slots
+    private static final int  SHORT_SIGN = 1 << 15;
+    private static final int  INT_SIGN   = 1 << 31;
+
+    // masks
+    private static final long STOP_BIT   = 0x0001L << ST_SHIFT;
+    private static final long AC_MASK    = ((long)SMASK) << AC_SHIFT;
+    private static final long TC_MASK    = ((long)SMASK) << TC_SHIFT;
+
+    // units for incrementing and decrementing
+    private static final long TC_UNIT    = 1L << TC_SHIFT;
+    private static final long AC_UNIT    = 1L << AC_SHIFT;
+
+    // masks and units for dealing with u = (int)(ctl >>> 32)
+    private static final int  UAC_SHIFT  = AC_SHIFT - 32;
+    private static final int  UTC_SHIFT  = TC_SHIFT - 32;
+    private static final int  UAC_MASK   = SMASK << UAC_SHIFT;
+    private static final int  UTC_MASK   = SMASK << UTC_SHIFT;
+    private static final int  UAC_UNIT   = 1 << UAC_SHIFT;
+    private static final int  UTC_UNIT   = 1 << UTC_SHIFT;
+
+    // masks and units for dealing with e = (int)ctl
+    private static final int E_MASK      = 0x7fffffff; // no STOP_BIT
+    private static final int E_SEQ       = 1 << EC_SHIFT;
+
+    // plock bits
+    private static final int SHUTDOWN    = 1 << 31;
+    private static final int PL_LOCK     = 2;
+    private static final int PL_SIGNAL   = 1;
+    private static final int PL_SPINS    = 1 << 8;
+
+    // access mode for WorkQueue
+    static final int LIFO_QUEUE          =  0;
+    static final int FIFO_QUEUE          =  1;
+    static final int SHARED_QUEUE        = -1;
+
+    // bounds for #steps in scan loop -- must be power 2 minus 1
+    private static final int MIN_SCAN    = 0x1ff;   // cover estimation slop
+    private static final int MAX_SCAN    = 0x1ffff; // 4 * max workers
+
+    // Instance fields
+
+    /*
+     * Field layout of this class tends to matter more than one would
+     * like. Runtime layout order is only loosely related to
+     * declaration order and may differ across JVMs, but the following
+     * empirically works OK on current JVMs.
+     */
+
+    // Heuristic padding to ameliorate unfortunate memory placements
+    volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06;
+
+    volatile long stealCount;                  // collects worker counts
+    volatile long ctl;                         // main pool control
+    volatile int plock;                        // shutdown status and seqLock
+    volatile int indexSeed;                    // worker/submitter index seed
+    final int config;                          // mode and parallelism level
+    WorkQueue[] workQueues;                    // main registry
+    final ForkJoinWorkerThreadFactory factory;
+    final Thread.UncaughtExceptionHandler ueh; // per-worker UEH
+    final String workerNamePrefix;             // to create worker name string
+
+    volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
+    volatile Object pad18, pad19, pad1a, pad1b;
+
+    /**
+     * Acquires the plock lock to protect worker array and related
+     * updates. This method is called only if an initial CAS on plock
+     * fails. This acts as a spinlock for normal cases, but falls back
+     * to builtin monitor to block when (rarely) needed. This would be
+     * a terrible idea for a highly contended lock, but works fine as
+     * a more conservative alternative to a pure spinlock.
+     */
+    private int acquirePlock() {
+        int spins = PL_SPINS, r = 0, ps, nps;
+        for (;;) {
+            if (((ps = plock) & PL_LOCK) == 0 &&
+                U.compareAndSwapInt(this, PLOCK, ps, nps = ps + PL_LOCK))
+                return nps;
+            else if (r == 0) { // randomize spins if possible
+                Thread t = Thread.currentThread(); WorkQueue w; Submitter z;
+                if ((t instanceof ForkJoinWorkerThread) &&
+                    (w = ((ForkJoinWorkerThread)t).workQueue) != null)
+                    r = w.seed;
+                else if ((z = submitters.get()) != null)
+                    r = z.seed;
+                else
+                    r = 1;
+            }
+            else if (spins >= 0) {
+                r ^= r << 1; r ^= r >>> 3; r ^= r << 10; // xorshift
+                if (r >= 0)
+                    --spins;
+            }
+            else if (U.compareAndSwapInt(this, PLOCK, ps, ps | PL_SIGNAL)) {
+                synchronized (this) {
+                    if ((plock & PL_SIGNAL) != 0) {
+                        try {
+                            wait();
+                        } catch (InterruptedException ie) {
+                            try {
+                                Thread.currentThread().interrupt();
+                            } catch (SecurityException ignore) {
+                            }
+                        }
+                    }
+                    else
+                        notifyAll();
+                }
+            }
+        }
+    }
+
+    /**
+     * Unlocks and signals any thread waiting for plock. Called only
+     * when CAS of seq value for unlock fails.
+     */
+    private void releasePlock(int ps) {
+        plock = ps;
+        synchronized (this) { notifyAll(); }
+    }
+
+    /**
+     * Tries to create and start one worker if fewer than target
+     * parallelism level exist. Adjusts counts etc on failure.
+     */
+    private void tryAddWorker() {
+        long c; int u;
+        while ((u = (int)((c = ctl) >>> 32)) < 0 &&
+               (u & SHORT_SIGN) != 0 && (int)c == 0) {
+            long nc = (long)(((u + UTC_UNIT) & UTC_MASK) |
+                             ((u + UAC_UNIT) & UAC_MASK)) << 32;
+            if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                ForkJoinWorkerThreadFactory fac;
+                Throwable ex = null;
+                ForkJoinWorkerThread wt = null;
+                try {
+                    if ((fac = factory) != null &&
+                        (wt = fac.newThread(this)) != null) {
+                        wt.start();
+                        break;
+                    }
+                } catch (Throwable e) {
+                    ex = e;
+                }
+                deregisterWorker(wt, ex);
+                break;
+            }
+        }
+    }
+
+    //  Registering and deregistering workers
+
+    /**
+     * Callback from ForkJoinWorkerThread to establish and record its
+     * WorkQueue. To avoid scanning bias due to packing entries in
+     * front of the workQueues array, we treat the array as a simple
+     * power-of-two hash table using per-thread seed as hash,
+     * expanding as needed.
+     *
+     * @param wt the worker thread
+     * @return the worker's queue
+     */
+    final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
+        Thread.UncaughtExceptionHandler handler; WorkQueue[] ws; int s, ps;
+        wt.setDaemon(true);
+        if ((handler = ueh) != null)
+            wt.setUncaughtExceptionHandler(handler);
+        do {} while (!U.compareAndSwapInt(this, INDEXSEED, s = indexSeed,
+                                          s += SEED_INCREMENT) ||
+                     s == 0); // skip 0
+        WorkQueue w = new WorkQueue(this, wt, config >>> 16, s);
+        if (((ps = plock) & PL_LOCK) != 0 ||
+            !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+            ps = acquirePlock();
+        int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+        try {
+            if ((ws = workQueues) != null) {    // skip if shutting down
+                int n = ws.length, m = n - 1;
+                int r = (s << 1) | 1;           // use odd-numbered indices
+                if (ws[r &= m] != null) {       // collision
+                    int probes = 0;             // step by approx half size
+                    int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
+                    while (ws[r = (r + step) & m] != null) {
+                        if (++probes >= n) {
+                            workQueues = ws = Arrays.copyOf(ws, n <<= 1);
+                            m = n - 1;
+                            probes = 0;
+                        }
+                    }
+                }
+                w.eventCount = w.poolIndex = r; // volatile write orders
+                ws[r] = w;
+            }
+        } finally {
+            if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+                releasePlock(nps);
+        }
+        wt.setName(workerNamePrefix.concat(Integer.toString(w.poolIndex)));
+        return w;
+    }
+
+    /**
+     * Final callback from terminating worker, as well as upon failure
+     * to construct or start a worker.  Removes record of worker from
+     * array, and adjusts counts. If pool is shutting down, tries to
+     * complete termination.
+     *
+     * @param wt the worker thread or null if construction failed
+     * @param ex the exception causing failure, or null if none
+     */
+    final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
+        WorkQueue w = null;
+        if (wt != null && (w = wt.workQueue) != null) {
+            int ps;
+            w.qlock = -1;                // ensure set
+            long ns = w.nsteals, sc;     // collect steal count
+            do {} while (!U.compareAndSwapLong(this, STEALCOUNT,
+                                               sc = stealCount, sc + ns));
+            if (((ps = plock) & PL_LOCK) != 0 ||
+                !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+                ps = acquirePlock();
+            int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+            try {
+                int idx = w.poolIndex;
+                WorkQueue[] ws = workQueues;
+                if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w)
+                    ws[idx] = null;
+            } finally {
+                if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+                    releasePlock(nps);
+            }
+        }
+
+        long c;                          // adjust ctl counts
+        do {} while (!U.compareAndSwapLong
+                     (this, CTL, c = ctl, (((c - AC_UNIT) & AC_MASK) |
+                                           ((c - TC_UNIT) & TC_MASK) |
+                                           (c & ~(AC_MASK|TC_MASK)))));
+
+        if (!tryTerminate(false, false) && w != null && w.array != null) {
+            w.cancelAll();               // cancel remaining tasks
+            WorkQueue[] ws; WorkQueue v; Thread p; int u, i, e;
+            while ((u = (int)((c = ctl) >>> 32)) < 0 && (e = (int)c) >= 0) {
+                if (e > 0) {             // activate or create replacement
+                    if ((ws = workQueues) == null ||
+                        (i = e & SMASK) >= ws.length ||
+                        (v = ws[i]) == null)
+                        break;
+                    long nc = (((long)(v.nextWait & E_MASK)) |
+                               ((long)(u + UAC_UNIT) << 32));
+                    if (v.eventCount != (e | INT_SIGN))
+                        break;
+                    if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                        v.eventCount = (e + E_SEQ) & E_MASK;
+                        if ((p = v.parker) != null)
+                            U.unpark(p);
+                        break;
+                    }
+                }
+                else {
+                    if ((short)u < 0)
+                        tryAddWorker();
+                    break;
+                }
+            }
+        }
+        if (ex == null)                     // help clean refs on way out
+            ForkJoinTask.helpExpungeStaleExceptions();
+        else                                // rethrow
+            ForkJoinTask.rethrow(ex);
+    }
+
+    // Submissions
+
+    /**
+     * Unless shutting down, adds the given task to a submission queue
+     * at submitter's current queue index (modulo submission
+     * range). Only the most common path is directly handled in this
+     * method. All others are relayed to fullExternalPush.
+     *
+     * @param task the task. Caller must ensure non-null.
+     */
+    final void externalPush(ForkJoinTask<?> task) {
+        WorkQueue[] ws; WorkQueue q; Submitter z; int m; ForkJoinTask<?>[] a;
+        if ((z = submitters.get()) != null && plock > 0 &&
+            (ws = workQueues) != null && (m = (ws.length - 1)) >= 0 &&
+            (q = ws[m & z.seed & SQMASK]) != null &&
+            U.compareAndSwapInt(q, QLOCK, 0, 1)) { // lock
+            int b = q.base, s = q.top, n, an;
+            if ((a = q.array) != null && (an = a.length) > (n = s + 1 - b)) {
+                int j = (((an - 1) & s) << ASHIFT) + ABASE;
+                U.putOrderedObject(a, j, task);
+                q.top = s + 1;                     // push on to deque
+                q.qlock = 0;
+                if (n <= 2)
+                    signalWork(q);
+                return;
+            }
+            q.qlock = 0;
+        }
+        fullExternalPush(task);
+    }
+
+    /**
+     * Full version of externalPush. This method is called, among
+     * other times, upon the first submission of the first task to the
+     * pool, so must perform secondary initialization.  It also
+     * detects first submission by an external thread by looking up
+     * its ThreadLocal, and creates a new shared queue if the one at
+     * index if empty or contended. The plock lock body must be
+     * exception-free (so no try/finally) so we optimistically
+     * allocate new queues outside the lock and throw them away if
+     * (very rarely) not needed.
+     *
+     * Secondary initialization occurs when plock is zero, to create
+     * workQueue array and set plock to a valid value.  This lock body
+     * must also be exception-free. Because the plock seq value can
+     * eventually wrap around zero, this method harmlessly fails to
+     * reinitialize if workQueues exists, while still advancing plock.
+     */
+    private void fullExternalPush(ForkJoinTask<?> task) {
+        int r = 0; // random index seed
+        for (Submitter z = submitters.get();;) {
+            WorkQueue[] ws; WorkQueue q; int ps, m, k;
+            if (z == null) {
+                if (U.compareAndSwapInt(this, INDEXSEED, r = indexSeed,
+                                        r += SEED_INCREMENT) && r != 0)
+                    submitters.set(z = new Submitter(r));
+            }
+            else if (r == 0) {                  // move to a different index
+                r = z.seed;
+                r ^= r << 13;                   // same xorshift as WorkQueues
+                r ^= r >>> 17;
+                z.seed = r ^ (r << 5);
+            }
+            else if ((ps = plock) < 0)
+                throw new RejectedExecutionException();
+            else if (ps == 0 || (ws = workQueues) == null ||
+                     (m = ws.length - 1) < 0) { // initialize workQueues
+                int p = config & SMASK;         // find power of two table size
+                int n = (p > 1) ? p - 1 : 1;    // ensure at least 2 slots
+                n |= n >>> 1; n |= n >>> 2;  n |= n >>> 4;
+                n |= n >>> 8; n |= n >>> 16; n = (n + 1) << 1;
+                WorkQueue[] nws = ((ws = workQueues) == null || ws.length == 0 ?
+                                   new WorkQueue[n] : null);
+                if (((ps = plock) & PL_LOCK) != 0 ||
+                    !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+                    ps = acquirePlock();
+                if (((ws = workQueues) == null || ws.length == 0) && nws != null)
+                    workQueues = nws;
+                int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+                if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+                    releasePlock(nps);
+            }
+            else if ((q = ws[k = r & m & SQMASK]) != null) {
+                if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+                    ForkJoinTask<?>[] a = q.array;
+                    int s = q.top;
+                    boolean submitted = false;
+                    try {                      // locked version of push
+                        if ((a != null && a.length > s + 1 - q.base) ||
+                            (a = q.growArray()) != null) {   // must presize
+                            int j = (((a.length - 1) & s) << ASHIFT) + ABASE;
+                            U.putOrderedObject(a, j, task);
+                            q.top = s + 1;
+                            submitted = true;
+                        }
+                    } finally {
+                        q.qlock = 0;  // unlock
+                    }
+                    if (submitted) {
+                        signalWork(q);
+                        return;
+                    }
+                }
+                r = 0; // move on failure
+            }
+            else if (((ps = plock) & PL_LOCK) == 0) { // create new queue
+                q = new WorkQueue(this, null, SHARED_QUEUE, r);
+                if (((ps = plock) & PL_LOCK) != 0 ||
+                    !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+                    ps = acquirePlock();
+                if ((ws = workQueues) != null && k < ws.length && ws[k] == null)
+                    ws[k] = q;
+                int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+                if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+                    releasePlock(nps);
+            }
+            else
+                r = 0; // try elsewhere while lock held
+        }
+    }
+
+    // Maintaining ctl counts
+
+    /**
+     * Increments active count; mainly called upon return from blocking.
+     */
+    final void incrementActiveCount() {
+        long c;
+        do {} while (!U.compareAndSwapLong(this, CTL, c = ctl, c + AC_UNIT));
+    }
+
+    /**
+     * Tries to create or activate a worker if too few are active.
+     *
+     * @param q the (non-null) queue holding tasks to be signalled
+     */
+    final void signalWork(WorkQueue q) {
+        int hint = q.poolIndex;
+        long c; int e, u, i, n; WorkQueue[] ws; WorkQueue w; Thread p;
+        while ((u = (int)((c = ctl) >>> 32)) < 0) {
+            if ((e = (int)c) > 0) {
+                if ((ws = workQueues) != null && ws.length > (i = e & SMASK) &&
+                    (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) {
+                    long nc = (((long)(w.nextWait & E_MASK)) |
+                               ((long)(u + UAC_UNIT) << 32));
+                    if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                        w.hint = hint;
+                        w.eventCount = (e + E_SEQ) & E_MASK;
+                        if ((p = w.parker) != null)
+                            U.unpark(p);
+                        break;
+                    }
+                    if (q.top - q.base <= 0)
+                        break;
+                }
+                else
+                    break;
+            }
+            else {
+                if ((short)u < 0)
+                    tryAddWorker();
+                break;
+            }
+        }
+    }
+
+    // Scanning for tasks
+
+    /**
+     * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
+     */
+    final void runWorker(WorkQueue w) {
+        w.growArray(); // allocate queue
+        do { w.runTask(scan(w)); } while (w.qlock >= 0);
+    }
+
+    /**
+     * Scans for and, if found, returns one task, else possibly
+     * inactivates the worker. This method operates on single reads of
+     * volatile state and is designed to be re-invoked continuously,
+     * in part because it returns upon detecting inconsistencies,
+     * contention, or state changes that indicate possible success on
+     * re-invocation.
+     *
+     * The scan searches for tasks across queues (starting at a random
+     * index, and relying on registerWorker to irregularly scatter
+     * them within array to avoid bias), checking each at least twice.
+     * The scan terminates upon either finding a non-empty queue, or
+     * completing the sweep. If the worker is not inactivated, it
+     * takes and returns a task from this queue. Otherwise, if not
+     * activated, it signals workers (that may include itself) and
+     * returns so caller can retry. Also returns for true if the
+     * worker array may have changed during an empty scan.  On failure
+     * to find a task, we take one of the following actions, after
+     * which the caller will retry calling this method unless
+     * terminated.
+     *
+     * * If pool is terminating, terminate the worker.
+     *
+     * * If not already enqueued, try to inactivate and enqueue the
+     * worker on wait queue. Or, if inactivating has caused the pool
+     * to be quiescent, relay to idleAwaitWork to possibly shrink
+     * pool.
+     *
+     * * If already enqueued and none of the above apply, possibly
+     * park awaiting signal, else lingering to help scan and signal.
+     *
+     * * If a non-empty queue discovered or left as a hint,
+     * help wake up other workers before return.
+     *
+     * @param w the worker (via its WorkQueue)
+     * @return a task or null if none found
+     */
+    private final ForkJoinTask<?> scan(WorkQueue w) {
+        WorkQueue[] ws; int m;
+        int ps = plock;                          // read plock before ws
+        if (w != null && (ws = workQueues) != null && (m = ws.length - 1) >= 0) {
+            int ec = w.eventCount;               // ec is negative if inactive
+            int r = w.seed; r ^= r << 13; r ^= r >>> 17; w.seed = r ^= r << 5;
+            w.hint = -1;                         // update seed and clear hint
+            int j = ((m + m + 1) | MIN_SCAN) & MAX_SCAN;
+            do {
+                WorkQueue q; ForkJoinTask<?>[] a; int b;
+                if ((q = ws[(r + j) & m]) != null && (b = q.base) - q.top < 0 &&
+                    (a = q.array) != null) {     // probably nonempty
+                    int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                    ForkJoinTask<?> t = (ForkJoinTask<?>)
+                        U.getObjectVolatile(a, i);
+                    if (q.base == b && ec >= 0 && t != null &&
+                        U.compareAndSwapObject(a, i, t, null)) {
+                        if ((q.base = b + 1) - q.top < 0)
+                            signalWork(q);
+                        return t;                // taken
+                    }
+                    else if ((ec < 0 || j < m) && (int)(ctl >> AC_SHIFT) <= 0) {
+                        w.hint = (r + j) & m;    // help signal below
+                        break;                   // cannot take
+                    }
+                }
+            } while (--j >= 0);
+
+            int h, e, ns; long c, sc; WorkQueue q;
+            if ((ns = w.nsteals) != 0) {
+                if (U.compareAndSwapLong(this, STEALCOUNT,
+                                         sc = stealCount, sc + ns))
+                    w.nsteals = 0;               // collect steals and rescan
+            }
+            else if (plock != ps)                // consistency check
+                ;                                // skip
+            else if ((e = (int)(c = ctl)) < 0)
+                w.qlock = -1;                    // pool is terminating
+            else {
+                if ((h = w.hint) < 0) {
+                    if (ec >= 0) {               // try to enqueue/inactivate
+                        long nc = (((long)ec |
+                                    ((c - AC_UNIT) & (AC_MASK|TC_MASK))));
+                        w.nextWait = e;          // link and mark inactive
+                        w.eventCount = ec | INT_SIGN;
+                        if (ctl != c || !U.compareAndSwapLong(this, CTL, c, nc))
+                            w.eventCount = ec;   // unmark on CAS failure
+                        else if ((int)(c >> AC_SHIFT) == 1 - (config & SMASK))
+                            idleAwaitWork(w, nc, c);
+                    }
+                    else if (w.eventCount < 0 && ctl == c) {
+                        Thread wt = Thread.currentThread();
+                        Thread.interrupted();    // clear status
+                        U.putObject(wt, PARKBLOCKER, this);
+                        w.parker = wt;           // emulate LockSupport.park
+                        if (w.eventCount < 0)    // recheck
+                            U.park(false, 0L);   // block
+                        w.parker = null;
+                        U.putObject(wt, PARKBLOCKER, null);
+                    }
+                }
+                if ((h >= 0 || (h = w.hint) >= 0) &&
+                    (ws = workQueues) != null && h < ws.length &&
+                    (q = ws[h]) != null) {      // signal others before retry
+                    WorkQueue v; Thread p; int u, i, s;
+                    for (int n = (config & SMASK) - 1;;) {
+                        int idleCount = (w.eventCount < 0) ? 0 : -1;
+                        if (((s = idleCount - q.base + q.top) <= n &&
+                             (n = s) <= 0) ||
+                            (u = (int)((c = ctl) >>> 32)) >= 0 ||
+                            (e = (int)c) <= 0 || m < (i = e & SMASK) ||
+                            (v = ws[i]) == null)
+                            break;
+                        long nc = (((long)(v.nextWait & E_MASK)) |
+                                   ((long)(u + UAC_UNIT) << 32));
+                        if (v.eventCount != (e | INT_SIGN) ||
+                            !U.compareAndSwapLong(this, CTL, c, nc))
+                            break;
+                        v.hint = h;
+                        v.eventCount = (e + E_SEQ) & E_MASK;
+                        if ((p = v.parker) != null)
+                            U.unpark(p);
+                        if (--n <= 0)
+                            break;
+                    }
+                }
+            }
+        }
+        return null;
+    }
+
+    /**
+     * If inactivating worker w has caused the pool to become
+     * quiescent, checks for pool termination, and, so long as this is
+     * not the only worker, waits for event for up to a given
+     * duration.  On timeout, if ctl has not changed, terminates the
+     * worker, which will in turn wake up another worker to possibly
+     * repeat this process.
+     *
+     * @param w the calling worker
+     * @param currentCtl the ctl value triggering possible quiescence
+     * @param prevCtl the ctl value to restore if thread is terminated
+     */
+    private void idleAwaitWork(WorkQueue w, long currentCtl, long prevCtl) {
+        if (w != null && w.eventCount < 0 &&
+            !tryTerminate(false, false) && (int)prevCtl != 0 &&
+            ctl == currentCtl) {
+            int dc = -(short)(currentCtl >>> TC_SHIFT);
+            long parkTime = dc < 0 ? FAST_IDLE_TIMEOUT: (dc + 1) * IDLE_TIMEOUT;
+            long deadline = System.nanoTime() + parkTime - TIMEOUT_SLOP;
+            Thread wt = Thread.currentThread();
+            while (ctl == currentCtl) {
+                Thread.interrupted();  // timed variant of version in scan()
+                U.putObject(wt, PARKBLOCKER, this);
+                w.parker = wt;
+                if (ctl == currentCtl)
+                    U.park(false, parkTime);
+                w.parker = null;
+                U.putObject(wt, PARKBLOCKER, null);
+                if (ctl != currentCtl)
+                    break;
+                if (deadline - System.nanoTime() <= 0L &&
+                    U.compareAndSwapLong(this, CTL, currentCtl, prevCtl)) {
+                    w.eventCount = (w.eventCount + E_SEQ) | E_MASK;
+                    w.hint = -1;
+                    w.qlock = -1;   // shrink
+                    break;
+                }
+            }
+        }
+    }
+
+    /**
+     * Scans through queues looking for work while joining a task; if
+     * any present, signals. May return early if more signalling is
+     * detectably unneeded.
+     *
+     * @param task return early if done
+     * @param origin an index to start scan
+     */
+    private void helpSignal(ForkJoinTask<?> task, int origin) {
+        WorkQueue[] ws; WorkQueue w; Thread p; long c; int m, u, e, i, s;
+        if (task != null && task.status >= 0 &&
+            (u = (int)(ctl >>> 32)) < 0 && (u >> UAC_SHIFT) < 0 &&
+            (ws = workQueues) != null && (m = ws.length - 1) >= 0) {
+            outer: for (int k = origin, j = m; j >= 0; --j) {
+                WorkQueue q = ws[k++ & m];
+                for (int n = m;;) { // limit to at most m signals
+                    if (task.status < 0)
+                        break outer;
+                    if (q == null ||
+                        ((s = -q.base + q.top) <= n && (n = s) <= 0))
+                        break;
+                    if ((u = (int)((c = ctl) >>> 32)) >= 0 ||
+                        (e = (int)c) <= 0 || m < (i = e & SMASK) ||
+                        (w = ws[i]) == null)
+                        break outer;
+                    long nc = (((long)(w.nextWait & E_MASK)) |
+                               ((long)(u + UAC_UNIT) << 32));
+                    if (w.eventCount != (e | INT_SIGN))
+                        break outer;
+                    if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                        w.eventCount = (e + E_SEQ) & E_MASK;
+                        if ((p = w.parker) != null)
+                            U.unpark(p);
+                        if (--n <= 0)
+                            break;
+                    }
+                }
+            }
+        }
+    }
+
+    /**
+     * Tries to locate and execute tasks for a stealer of the given
+     * task, or in turn one of its stealers, Traces currentSteal ->
+     * currentJoin links looking for a thread working on a descendant
+     * of the given task and with a non-empty queue to steal back and
+     * execute tasks from. The first call to this method upon a
+     * waiting join will often entail scanning/search, (which is OK
+     * because the joiner has nothing better to do), but this method
+     * leaves hints in workers to speed up subsequent calls. The
+     * implementation is very branchy to cope with potential
+     * inconsistencies or loops encountering chains that are stale,
+     * unknown, or so long that they are likely cyclic.
+     *
+     * @param joiner the joining worker
+     * @param task the task to join
+     * @return 0 if no progress can be made, negative if task
+     * known complete, else positive
+     */
+    private int tryHelpStealer(WorkQueue joiner, ForkJoinTask<?> task) {
+        int stat = 0, steps = 0;                    // bound to avoid cycles
+        if (joiner != null && task != null) {       // hoist null checks
+            restart: for (;;) {
+                ForkJoinTask<?> subtask = task;     // current target
+                for (WorkQueue j = joiner, v;;) {   // v is stealer of subtask
+                    WorkQueue[] ws; int m, s, h;
+                    if ((s = task.status) < 0) {
+                        stat = s;
+                        break restart;
+                    }
+                    if ((ws = workQueues) == null || (m = ws.length - 1) <= 0)
+                        break restart;              // shutting down
+                    if ((v = ws[h = (j.hint | 1) & m]) == null ||
+                        v.currentSteal != subtask) {
+                        for (int origin = h;;) {    // find stealer
+                            if (((h = (h + 2) & m) & 15) == 1 &&
+                                (subtask.status < 0 || j.currentJoin != subtask))
+                                continue restart;   // occasional staleness check
+                            if ((v = ws[h]) != null &&
+                                v.currentSteal == subtask) {
+                                j.hint = h;        // save hint
+                                break;
+                            }
+                            if (h == origin)
+                                break restart;      // cannot find stealer
+                        }
+                    }
+                    for (;;) { // help stealer or descend to its stealer
+                        ForkJoinTask[] a;  int b;
+                        if (subtask.status < 0)     // surround probes with
+                            continue restart;       //   consistency checks
+                        if ((b = v.base) - v.top < 0 && (a = v.array) != null) {
+                            int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
+                            ForkJoinTask<?> t =
+                                (ForkJoinTask<?>)U.getObjectVolatile(a, i);
+                            if (subtask.status < 0 || j.currentJoin != subtask ||
+                                v.currentSteal != subtask)
+                                continue restart;   // stale
+                            stat = 1;               // apparent progress
+                            if (t != null && v.base == b &&
+                                U.compareAndSwapObject(a, i, t, null)) {
+                                v.base = b + 1;     // help stealer
+                                joiner.runSubtask(t);
+                            }
+                            else if (v.base == b && ++steps == MAX_HELP)
+                                break restart;      // v apparently stalled
+                        }
+                        else {                      // empty -- try to descend
+                            ForkJoinTask<?> next = v.currentJoin;
+                            if (subtask.status < 0 || j.currentJoin != subtask ||
+                                v.currentSteal != subtask)
+                                continue restart;   // stale
+                            else if (next == null || ++steps == MAX_HELP)
+                                break restart;      // dead-end or maybe cyclic
+                            else {
+                                subtask = next;
+                                j = v;
+                                break;
+                            }
+                        }
+                    }
+                }
+            }
+        }
+        return stat;
+    }
+
+    /**
+     * Analog of tryHelpStealer for CountedCompleters. Tries to steal
+     * and run tasks within the target's computation.
+     *
+     * @param task the task to join
+     * @param mode if shared, exit upon completing any task
+     * if all workers are active
+     */
+    private int helpComplete(ForkJoinTask<?> task, int mode) {
+        WorkQueue[] ws; WorkQueue q; int m, n, s, u;
+        if (task != null && (ws = workQueues) != null &&
+            (m = ws.length - 1) >= 0) {
+            for (int j = 1, origin = j;;) {
+                if ((s = task.status) < 0)
+                    return s;
+                if ((q = ws[j & m]) != null && q.pollAndExecCC(task)) {
+                    origin = j;
+                    if (mode == SHARED_QUEUE &&
+                        ((u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0))
+                        break;
+                }
+                else if ((j = (j + 2) & m) == origin)
+                    break;
+            }
+        }
+        return 0;
+    }
+
+    /**
+     * Tries to decrement active count (sometimes implicitly) and
+     * possibly release or create a compensating worker in preparation
+     * for blocking. Fails on contention or termination. Otherwise,
+     * adds a new thread if no idle workers are available and pool
+     * may become starved.
+     */
+    final boolean tryCompensate() {
+        int pc = config & SMASK, e, i, tc; long c;
+        WorkQueue[] ws; WorkQueue w; Thread p;
+        if ((ws = workQueues) != null && (e = (int)(c = ctl)) >= 0) {
+            if (e != 0 && (i = e & SMASK) < ws.length &&
+                (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) {
+                long nc = ((long)(w.nextWait & E_MASK) |
+                           (c & (AC_MASK|TC_MASK)));
+                if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                    w.eventCount = (e + E_SEQ) & E_MASK;
+                    if ((p = w.parker) != null)
+                        U.unpark(p);
+                    return true;   // replace with idle worker
+                }
+            }
+            else if ((tc = (short)(c >>> TC_SHIFT)) >= 0 &&
+                     (int)(c >> AC_SHIFT) + pc > 1) {
+                long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
+                if (U.compareAndSwapLong(this, CTL, c, nc))
+                    return true;   // no compensation
+            }
+            else if (tc + pc < MAX_CAP) {
+                long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
+                if (U.compareAndSwapLong(this, CTL, c, nc)) {
+                    ForkJoinWorkerThreadFactory fac;
+                    Throwable ex = null;
+                    ForkJoinWorkerThread wt = null;
+                    try {
+                        if ((fac = factory) != null &&
+                            (wt = fac.newThread(this)) != null) {
+                            wt.start();
+                            return true;
+                        }
+                    } catch (Throwable rex) {
+                        ex = rex;
+                    }
+                    deregisterWorker(wt, ex); // clean up and return false
+                }
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Helps and/or blocks until the given task is done.
+     *
+     * @param joiner the joining worker
+     * @param task the task
+     * @return task status on exit
+     */
+    final int awaitJoin(WorkQueue joiner, ForkJoinTask<?> task) {
+        int s = 0;
+        if (joiner != null && task != null && (s = task.status) >= 0) {
+            ForkJoinTask<?> prevJoin = joiner.currentJoin;
+            joiner.currentJoin = task;
+            do {} while ((s = task.status) >= 0 && !joiner.isEmpty() &&
+                         joiner.tryRemoveAndExec(task)); // process local tasks
+            if (s >= 0 && (s = task.status) >= 0) {
+                helpSignal(task, joiner.poolIndex);
+                if ((s = task.status) >= 0 &&
+                    (task instanceof CountedCompleter))
+                    s = helpComplete(task, LIFO_QUEUE);
+            }
+            while (s >= 0 && (s = task.status) >= 0) {
+                if ((!joiner.isEmpty() ||           // try helping
+                     (s = tryHelpStealer(joiner, task)) == 0) &&
+                    (s = task.status) >= 0) {
+                    helpSignal(task, joiner.poolIndex);
+                    if ((s = task.status) >= 0 && tryCompensate()) {
+                        if (task.trySetSignal() && (s = task.status) >= 0) {
+                            synchronized (task) {
+                                if (task.status >= 0) {
+                                    try {                // see ForkJoinTask
+                                        task.wait();     //  for explanation
+                                    } catch (InterruptedException ie) {
+                                    }
+                                }
+                                else
+                                    task.notifyAll();
+                            }
+                        }
+                        long c;                          // re-activate
+                        do {} while (!U.compareAndSwapLong
+                                     (this, CTL, c = ctl, c + AC_UNIT));
+                    }
+                }
+            }
+            joiner.currentJoin = prevJoin;
+        }
+        return s;
+    }
+
+    /**
+     * Stripped-down variant of awaitJoin used by timed joins. Tries
+     * to help join only while there is continuous progress. (Caller
+     * will then enter a timed wait.)
+     *
+     * @param joiner the joining worker
+     * @param task the task
+     */
+    final void helpJoinOnce(WorkQueue joiner, ForkJoinTask<?> task) {
+        int s;
+        if (joiner != null && task != null && (s = task.status) >= 0) {
+            ForkJoinTask<?> prevJoin = joiner.currentJoin;
+            joiner.currentJoin = task;
+            do {} while ((s = task.status) >= 0 && !joiner.isEmpty() &&
+                         joiner.tryRemoveAndExec(task));
+            if (s >= 0 && (s = task.status) >= 0) {
+                helpSignal(task, joiner.poolIndex);
+                if ((s = task.status) >= 0 &&
+                    (task instanceof CountedCompleter))
+                    s = helpComplete(task, LIFO_QUEUE);
+            }
+            if (s >= 0 && joiner.isEmpty()) {
+                do {} while (task.status >= 0 &&
+                             tryHelpStealer(joiner, task) > 0);
+            }
+            joiner.currentJoin = prevJoin;
+        }
+    }
+
+    /**
+     * 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.
+     * @param r a (random) seed for scanning
+     */
+    private WorkQueue findNonEmptyStealQueue(int r) {
+        for (;;) {
+            int ps = plock, m; WorkQueue[] ws; WorkQueue q;
+            if ((ws = workQueues) != null && (m = ws.length - 1) >= 0) {
+                for (int j = (m + 1) << 2; j >= 0; --j) {
+                    if ((q = ws[(((r + j) << 1) | 1) & m]) != null &&
+                        q.base - q.top < 0)
+                        return q;
+                }
+            }
+            if (plock == ps)
+                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) {
+        for (boolean active = true;;) {
+            long c; WorkQueue q; ForkJoinTask<?> t; int b;
+            while ((t = w.nextLocalTask()) != null) {
+                if (w.base - w.top < 0)
+                    signalWork(w);
+                t.doExec();
+            }
+            if ((q = findNonEmptyStealQueue(w.nextSeed())) != null) {
+                if (!active) {      // re-establish active count
+                    active = true;
+                    do {} while (!U.compareAndSwapLong
+                                 (this, CTL, c = ctl, c + AC_UNIT));
+                }
+                if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) {
+                    if (q.base - q.top < 0)
+                        signalWork(q);
+                    w.runSubtask(t);
+                }
+            }
+            else if (active) {      // decrement active count without queuing
+                long nc = (c = ctl) - AC_UNIT;
+                if ((int)(nc >> AC_SHIFT) + (config & SMASK) == 0)
+                    return;         // bypass decrement-then-increment
+                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))
+                return;
+        }
+    }
+
+    /**
+     * Gets and removes a local or stolen task for the given worker.
+     *
+     * @return a task, if available
+     */
+    final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
+        for (ForkJoinTask<?> t;;) {
+            WorkQueue q; int b;
+            if ((t = w.nextLocalTask()) != null)
+                return t;
+            if ((q = findNonEmptyStealQueue(w.nextSeed())) == null)
+                return null;
+            if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) {
+                if (q.base - q.top < 0)
+                    signalWork(q);
+                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.
+     *
+     * Note: The approximation of #busy workers as #active workers is
+     * not very good under current signalling scheme, and should be
+     * improved.
+     */
+    static int getSurplusQueuedTaskCount() {
+        Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
+        if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
+            int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).config & SMASK;
+            int n = (q = wt.workQueue).top - q.base;
+            int a = (int)(pool.ctl >> AC_SHIFT) + p;
+            return n - (a > (p >>>= 1) ? 0 :
+                        a > (p >>>= 1) ? 1 :
+                        a > (p >>>= 1) ? 2 :
+                        a > (p >>>= 1) ? 4 :
+                        8);
+        }
+        return 0;
+    }
+
+    //  Termination
+
+    /**
+     * Possibly initiates and/or completes termination.  The caller
+     * triggering termination runs three passes through workQueues:
+     * (0) Setting termination status, followed by wakeups of queued
+     * workers; (1) cancelling all tasks; (2) interrupting lagging
+     * threads (likely in external tasks, but possibly also blocked in
+     * joins).  Each pass repeats previous steps because of potential
+     * lagging thread creation.
+     *
+     * @param now if true, unconditionally terminate, else only
+     * if no work and no active workers
+     * @param enable if true, enable shutdown when next possible
+     * @return true if now terminating or terminated
+     */
+    private boolean tryTerminate(boolean now, boolean enable) {
+        int ps;
+        if (this == common)                    // cannot shut down
+            return false;
+        if ((ps = plock) >= 0) {                   // enable by setting plock
+            if (!enable)
+                return false;
+            if ((ps & PL_LOCK) != 0 ||
+                !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+                ps = acquirePlock();
+            int nps = ((ps + PL_LOCK) & ~SHUTDOWN) | SHUTDOWN;
+            if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+                releasePlock(nps);
+        }
+        for (long c;;) {
+            if (((c = ctl) & STOP_BIT) != 0) {     // already terminating
+                if ((short)(c >>> TC_SHIFT) == -(config & SMASK)) {
+                    synchronized (this) {
+                        notifyAll();               // signal when 0 workers
+                    }
+                }
+                return true;
+            }
+            if (!now) {                            // check if idle & no tasks
+                WorkQueue[] ws; WorkQueue w;
+                if ((int)(c >> AC_SHIFT) != -(config & SMASK))
+                    return false;
+                if ((ws = workQueues) != null) {
+                    for (int i = 0; i < ws.length; ++i) {
+                        if ((w = ws[i]) != null) {
+                            if (!w.isEmpty()) {    // signal unprocessed tasks
+                                signalWork(w);
+                                return false;
+                            }
+                            if ((i & 1) != 0 && w.eventCount >= 0)
+                                return false;      // unqueued inactive worker
+                        }
+                    }
+                }
+            }
+            if (U.compareAndSwapLong(this, CTL, c, c | STOP_BIT)) {
+                for (int pass = 0; pass < 3; ++pass) {
+                    WorkQueue[] ws; WorkQueue w; Thread wt;
+                    if ((ws = workQueues) != null) {
+                        int n = ws.length;
+                        for (int i = 0; i < n; ++i) {
+                            if ((w = ws[i]) != null) {
+                                w.qlock = -1;
+                                if (pass > 0) {
+                                    w.cancelAll();
+                                    if (pass > 1 && (wt = w.owner) != null) {
+                                        if (!wt.isInterrupted()) {
+                                            try {
+                                                wt.interrupt();
+                                            } catch (Throwable ignore) {
+                                            }
+                                        }
+                                        U.unpark(wt);
+                                    }
+                                }
+                            }
+                        }
+                        // Wake up workers parked on event queue
+                        int i, e; long cc; Thread p;
+                        while ((e = (int)(cc = ctl) & E_MASK) != 0 &&
+                               (i = e & SMASK) < n && i >= 0 &&
+                               (w = ws[i]) != null) {
+                            long nc = ((long)(w.nextWait & E_MASK) |
+                                       ((cc + AC_UNIT) & AC_MASK) |
+                                       (cc & (TC_MASK|STOP_BIT)));
+                            if (w.eventCount == (e | INT_SIGN) &&
+                                U.compareAndSwapLong(this, CTL, cc, nc)) {
+                                w.eventCount = (e + E_SEQ) & E_MASK;
+                                w.qlock = -1;
+                                if ((p = w.parker) != null)
+                                    U.unpark(p);
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    // external operations on common pool
+
+    /**
+     * Returns common pool queue for a thread that has submitted at
+     * least one task.
+     */
+    static WorkQueue commonSubmitterQueue() {
+        ForkJoinPool p; WorkQueue[] ws; int m; Submitter z;
+        return ((z = submitters.get()) != null &&
+                (p = common) != null &&
+                (ws = p.workQueues) != null &&
+                (m = ws.length - 1) >= 0) ?
+            ws[m & z.seed & SQMASK] : null;
+    }
+
+    /**
+     * Tries to pop the given task from submitter's queue in common pool.
+     */
+    static boolean tryExternalUnpush(ForkJoinTask<?> t) {
+        ForkJoinPool p; WorkQueue[] ws; WorkQueue q; Submitter z;
+        ForkJoinTask<?>[] a;  int m, s;
+        if (t != null &&
+            (z = submitters.get()) != null &&
+            (p = common) != null &&
+            (ws = p.workQueues) != null &&
+            (m = ws.length - 1) >= 0 &&
+            (q = ws[m & z.seed & SQMASK]) != null &&
+            (s = q.top) != q.base &&
+            (a = q.array) != null) {
+            long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
+            if (U.getObject(a, j) == t &&
+                U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+                if (q.array == a && q.top == s && // recheck
+                    U.compareAndSwapObject(a, j, t, null)) {
+                    q.top = s - 1;
+                    q.qlock = 0;
+                    return true;
+                }
+                q.qlock = 0;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Tries to pop and run local tasks within the same computation
+     * as the given root. On failure, tries to help complete from
+     * other queues via helpComplete.
+     */
+    private void externalHelpComplete(WorkQueue q, ForkJoinTask<?> root) {
+        ForkJoinTask<?>[] a; int m;
+        if (q != null && (a = q.array) != null && (m = (a.length - 1)) >= 0 &&
+            root != null && root.status >= 0) {
+            for (;;) {
+                int s, u; Object o; CountedCompleter<?> task = null;
+                if ((s = q.top) - q.base > 0) {
+                    long j = ((m & (s - 1)) << ASHIFT) + ABASE;
+                    if ((o = U.getObject(a, j)) != null &&
+                        (o instanceof CountedCompleter)) {
+                        CountedCompleter<?> t = (CountedCompleter<?>)o, r = t;
+                        do {
+                            if (r == root) {
+                                if (U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+                                    if (q.array == a && q.top == s &&
+                                        U.compareAndSwapObject(a, j, t, null)) {
+                                        q.top = s - 1;
+                                        task = t;
+                                    }
+                                    q.qlock = 0;
+                                }
+                                break;
+                            }
+                        } while ((r = r.completer) != null);
+                    }
+                }
+                if (task != null)
+                    task.doExec();
+                if (root.status < 0 ||
+                    (u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0)
+                    break;
+                if (task == null) {
+                    helpSignal(root, q.poolIndex);
+                    if (root.status >= 0)
+                        helpComplete(root, SHARED_QUEUE);
+                    break;
+                }
+            }
+        }
+    }
+
+    /**
+     * Tries to help execute or signal availability of the given task
+     * from submitter's queue in common pool.
+     */
+    static void externalHelpJoin(ForkJoinTask<?> t) {
+        // Some hard-to-avoid overlap with tryExternalUnpush
+        ForkJoinPool p; WorkQueue[] ws; WorkQueue q, w; Submitter z;
+        ForkJoinTask<?>[] a;  int m, s, n;
+        if (t != null &&
+            (z = submitters.get()) != null &&
+            (p = common) != null &&
+            (ws = p.workQueues) != null &&
+            (m = ws.length - 1) >= 0 &&
+            (q = ws[m & z.seed & SQMASK]) != null &&
+            (a = q.array) != null) {
+            int am = a.length - 1;
+            if ((s = q.top) != q.base) {
+                long j = ((am & (s - 1)) << ASHIFT) + ABASE;
+                if (U.getObject(a, j) == t &&
+                    U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+                    if (q.array == a && q.top == s &&
+                        U.compareAndSwapObject(a, j, t, null)) {
+                        q.top = s - 1;
+                        q.qlock = 0;
+                        t.doExec();
+                    }
+                    else
+                        q.qlock = 0;
+                }
+            }
+            if (t.status >= 0) {
+                if (t instanceof CountedCompleter)
+                    p.externalHelpComplete(q, t);
+                else
+                    p.helpSignal(t, q.poolIndex);
+            }
+        }
+    }
+
+    // Exported methods
+
+    // Constructors
+
+    /**
+     * Creates a {@code ForkJoinPool} with parallelism equal to {@link
+     * java.lang.Runtime#availableProcessors}, using the {@linkplain
+     * #defaultForkJoinWorkerThreadFactory default thread factory},
+     * no UncaughtExceptionHandler, and non-async LIFO processing mode.
+     *
+     * @throws SecurityException if a security manager exists and
+     *         the caller is not permitted to modify threads
+     *         because it does not hold {@link
+     *         java.lang.RuntimePermission}{@code ("modifyThread")}
+     */
+    public ForkJoinPool() {
+        this(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
+     *         java.lang.RuntimePermission}{@code ("modifyThread")}
+     */
+    public ForkJoinPool(int parallelism) {
+        this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
+    }
+
+    /**
+     * Creates a {@code ForkJoinPool} with the given parameters.
+     *
+     * @param parallelism the parallelism level. For default value,
+     * use {@link java.lang.Runtime#availableProcessors}.
+     * @param factory the factory for creating new threads. For default value,
+     * use {@link #defaultForkJoinWorkerThreadFactory}.
+     * @param handler the handler for internal worker threads that
+     * terminate due to unrecoverable errors encountered while executing
+     * tasks. For default value, use {@code null}.
+     * @param asyncMode if true,
+     * establishes local first-in-first-out scheduling mode for forked
+     * tasks that are never joined. This mode may be more appropriate
+     * than default locally stack-based mode in applications in which
+     * worker threads only process event-style asynchronous tasks.
+     * For default value, use {@code false}.
+     * @throws IllegalArgumentException if parallelism less than or
+     *         equal to zero, or greater than implementation limit
+     * @throws NullPointerException if the factory is null
+     * @throws SecurityException if a security manager exists and
+     *         the caller is not permitted to modify threads
+     *         because it does not hold {@link
+     *         java.lang.RuntimePermission}{@code ("modifyThread")}
+     */
+    public ForkJoinPool(int parallelism,
+                        ForkJoinWorkerThreadFactory factory,
+                        Thread.UncaughtExceptionHandler handler,
+                        boolean asyncMode) {
+        checkPermission();
+        if (factory == null)
+            throw new NullPointerException();
+        if (parallelism <= 0 || parallelism > MAX_CAP)
+            throw new IllegalArgumentException();
+        this.factory = factory;
+        this.ueh = handler;
+        this.config = parallelism | (asyncMode ? (FIFO_QUEUE << 16) : 0);
+        long np = (long)(-parallelism); // offset ctl counts
+        this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
+        int pn = nextPoolId();
+        StringBuilder sb = new StringBuilder("ForkJoinPool-");
+        sb.append(Integer.toString(pn));
+        sb.append("-worker-");
+        this.workerNamePrefix = sb.toString();
+    }
+
+    /**
+     * Constructor for common pool, suitable only for static initialization.
+     * Basically the same as above, but uses smallest possible initial footprint.
+     */
+    ForkJoinPool(int parallelism, long ctl,
+                 ForkJoinWorkerThreadFactory factory,
+                 Thread.UncaughtExceptionHandler handler) {
+        this.config = parallelism;
+        this.ctl = ctl;
+        this.factory = factory;
+        this.ueh = handler;
+        this.workerNamePrefix = "ForkJoinPool.commonPool-worker-";
+    }
+
+    /**
+     * Returns the common pool instance. This pool is statically
+     * constructed; its run state is unaffected by attempts to {@link
+     * #shutdown} or {@link #shutdownNow}. 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}, 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
+     * @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();
+        externalPush(task);
+        return task.join();
+    }
+
+    /**
+     * Arranges for (asynchronous) execution of the given task.
+     *
+     * @param task the task
+     * @throws NullPointerException if the task is null
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     */
+    public void execute(ForkJoinTask<?> task) {
+        if (task == null)
+            throw new NullPointerException();
+        externalPush(task);
+    }
+
+    // AbstractExecutorService methods
+
+    /**
+     * @throws NullPointerException if the task is null
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     */
+    public void execute(Runnable task) {
+        if (task == null)
+            throw new NullPointerException();
+        ForkJoinTask<?> job;
+        if (task instanceof ForkJoinTask<?>) // avoid re-wrap
+            job = (ForkJoinTask<?>) task;
+        else
+            job = new ForkJoinTask.AdaptedRunnableAction(task);
+        externalPush(job);
+    }
+
+    /**
+     * Submits a ForkJoinTask for execution.
+     *
+     * @param task the task to submit
+     * @return the task
+     * @throws NullPointerException if the task is null
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     */
+    public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
+        if (task == null)
+            throw new NullPointerException();
+        externalPush(task);
+        return task;
+    }
+
+    /**
+     * @throws NullPointerException if the task is null
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     */
+    public <T> ForkJoinTask<T> submit(Callable<T> task) {
+        ForkJoinTask<T> job = new ForkJoinTask.AdaptedCallable<T>(task);
+        externalPush(job);
+        return job;
+    }
+
+    /**
+     * @throws NullPointerException if the task is null
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     */
+    public <T> ForkJoinTask<T> submit(Runnable task, T result) {
+        ForkJoinTask<T> job = new ForkJoinTask.AdaptedRunnable<T>(task, result);
+        externalPush(job);
+        return job;
+    }
+
+    /**
+     * @throws NullPointerException if the task is null
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     */
+    public ForkJoinTask<?> submit(Runnable task) {
+        if (task == null)
+            throw new NullPointerException();
+        ForkJoinTask<?> job;
+        if (task instanceof ForkJoinTask<?>) // avoid re-wrap
+            job = (ForkJoinTask<?>) task;
+        else
+            job = new ForkJoinTask.AdaptedRunnableAction(task);
+        externalPush(job);
+        return job;
+    }
+
+    /**
+     * @throws NullPointerException       {@inheritDoc}
+     * @throws RejectedExecutionException {@inheritDoc}
+     */
+    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
+        // In previous versions of this class, this method constructed
+        // a task to run ForkJoinTask.invokeAll, but now external
+        // invocation of multiple tasks is at least as efficient.
+        ArrayList<Future<T>> futures = new ArrayList<Future<T>>(tasks.size());
+
+        boolean done = false;
+        try {
+            for (Callable<T> t : tasks) {
+                ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
+                futures.add(f);
+                externalPush(f);
+            }
+            for (int i = 0, size = futures.size(); i < size; i++)
+                ((ForkJoinTask<?>)futures.get(i)).quietlyJoin();
+            done = true;
+            return futures;
+        } finally {
+            if (!done)
+                for (int i = 0, size = futures.size(); i < size; i++)
+                    futures.get(i).cancel(false);
+        }
+    }
+
+    /**
+     * Returns the factory used for constructing new workers.
+     *
+     * @return the factory used for constructing new workers
+     */
+    public ForkJoinWorkerThreadFactory getFactory() {
+        return factory;
+    }
+
+    /**
+     * Returns the handler for internal worker threads that terminate
+     * due to unrecoverable errors encountered while executing tasks.
+     *
+     * @return the handler, or {@code null} if none
+     */
+    public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() {
+        return ueh;
+    }
+
+    /**
+     * Returns the targeted parallelism level of this pool.
+     *
+     * @return the targeted parallelism level of this pool
+     */
+    public int getParallelism() {
+        return config & SMASK;
+    }
+
+    /**
+     * 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 commonParallelism;
+    }
+
+    /**
+     * 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 >>> 16) == FIFO_QUEUE;
+    }
+
+    /**
+     * 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 (int)(ctl >> AC_SHIFT) + (config & SMASK) == 0;
+    }
+
+    /**
+     * Returns an estimate of the total number of tasks stolen from
+     * one thread's work queue by another. The reported value
+     * underestimates the actual total number of steals when the pool
+     * is not quiescent. This value may be useful for monitoring and
+     * tuning fork/join programs: in general, steal counts should be
+     * high enough to keep threads busy, but low enough to avoid
+     * overhead and contention across threads.
+     *
+     * @return the number of steals
+     */
+    public long getStealCount() {
+        long count = stealCount;
+        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; WorkQueue w; ForkJoinTask<?> t;
+        if ((ws = workQueues) != null) {
+            for (int i = 0; i < ws.length; i += 2) {
+                if ((w = ws[i]) != null && (t = w.poll()) != null)
+                    return t;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Removes all available unexecuted submitted and forked tasks
+     * from scheduling queues and adds them to the given collection,
+     * without altering their execution status. These may include
+     * artificially generated or wrapped tasks. This method is
+     * designed to be invoked only when the pool is known to be
+     * quiescent. Invocations at other times may not remove all
+     * tasks. A failure encountered while attempting to add elements
+     * to collection {@code c} may result in elements being in
+     * neither, either or both collections when the associated
+     * exception is thrown.  The behavior of this operation is
+     * undefined if the specified collection is modified while the
+     * operation is in progress.
+     *
+     * @param c the collection to transfer elements into
+     * @return the number of elements transferred
+     */
+    protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
+        int count = 0;
+        WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
+        if ((ws = workQueues) != null) {
+            for (int i = 0; i < ws.length; ++i) {
+                if ((w = ws[i]) != null) {
+                    while ((t = w.poll()) != null) {
+                        c.add(t);
+                        ++count;
+                    }
+                }
+            }
+        }
+        return count;
+    }
+
+    /**
+     * Returns a string identifying this pool, as well as its state,
+     * including indications of run state, parallelism level, and
+     * worker and task counts.
+     *
+     * @return a string identifying this pool, as well as its state
+     */
+    public String toString() {
+        // Use a single pass through workQueues to collect counts
+        long qt = 0L, qs = 0L; int rc = 0;
+        long st = stealCount;
+        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;
+        String level;
+        if ((c & STOP_BIT) != 0)
+            level = (tc == 0) ? "Terminated" : "Terminating";
+        else
+            level = plock < 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
+     *         java.lang.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
+     *         java.lang.RuntimePermission}{@code ("modifyThread")}
+     */
+    public List<Runnable> shutdownNow() {
+        checkPermission();
+        tryTerminate(true, true);
+        return Collections.emptyList();
+    }
+
+    /**
+     * Returns {@code true} if all tasks have completed following shut down.
+     *
+     * @return {@code true} if all tasks have completed following shut down
+     */
+    public boolean isTerminated() {
+        long c = ctl;
+        return ((c & STOP_BIT) != 0L &&
+                (short)(c >>> TC_SHIFT) == -(config & SMASK));
+    }
+
+    /**
+     * 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() {
+        long c = ctl;
+        return ((c & STOP_BIT) != 0L &&
+                (short)(c >>> TC_SHIFT) != -(config & SMASK));
+    }
+
+    /**
+     * Returns {@code true} if this pool has been shut down.
+     *
+     * @return {@code true} if this pool has been shut down
+     */
+    public boolean isShutdown() {
+        return plock < 0;
+    }
+
+    /**
+     * Blocks until all tasks have completed execution after a
+     * shutdown request, or the timeout occurs, or the current thread
+     * is interrupted, whichever happens first. Because the {@link
+     * #commonPool()} never terminates until program shutdown, when
+     * applied to the common pool, this method is equivalent to {@link
+     * #awaitQuiescence} 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;
+        long startTime = System.nanoTime();
+        boolean terminated = false;
+        synchronized (this) {
+            for (long waitTime = nanos, millis = 0L;;) {
+                if (terminated = isTerminated() ||
+                    waitTime <= 0L ||
+                    (millis = unit.toMillis(waitTime)) <= 0L)
+                    break;
+                wait(millis);
+                waitTime = nanos - (System.nanoTime() - startTime);
+            }
+        }
+        return terminated;
+    }
+
+    /**
+     * 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, m;
+        boolean found = true;
+        while (!isQuiescent() && (ws = workQueues) != null &&
+               (m = ws.length - 1) >= 0) {
+            if (!found) {
+                if ((System.nanoTime() - startTime) > nanos)
+                    return false;
+                Thread.yield(); // cannot block
+            }
+            found = false;
+            for (int j = (m + 1) << 2; j >= 0; --j) {
+                ForkJoinTask<?> t; WorkQueue q; int b;
+                if ((q = ws[r++ & m]) != null && (b = q.base) - q.top < 0) {
+                    found = true;
+                    if ((t = q.pollAt(b)) != null) {
+                        if (q.base - q.top < 0)
+                            signalWork(q);
+                        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
+     * {@code isReleasable} must return {@code true} if blocking is
+     * not necessary. Method {@code block} blocks the current thread
+     * if necessary (perhaps internally invoking {@code isReleasable}
+     * before actually blocking). These actions are performed by any
+     * thread invoking {@link ForkJoinPool#managedBlock}.  The
+     * unusual methods in this API accommodate synchronizers that may,
+     * but don't usually, block for long periods. Similarly, they
+     * allow more efficient internal handling of cases in which
+     * additional workers may be, but usually are not, needed to
+     * ensure sufficient parallelism.  Toward this end,
+     * implementations of method {@code isReleasable} must be amenable
+     * to repeated invocation.
+     *
+     * <p>For example, here is a ManagedBlocker based on a
+     * ReentrantLock:
+     *  <pre> {@code
+     * class ManagedLocker implements ManagedBlocker {
+     *   final ReentrantLock lock;
+     *   boolean hasLock = false;
+     *   ManagedLocker(ReentrantLock lock) { this.lock = lock; }
+     *   public boolean block() {
+     *     if (!hasLock)
+     *       lock.lock();
+     *     return true;
+     *   }
+     *   public boolean isReleasable() {
+     *     return hasLock || (hasLock = lock.tryLock());
+     *   }
+     * }}</pre>
+     *
+     * <p>Here is a class that possibly blocks waiting for an
+     * item on a given queue:
+     *  <pre> {@code
+     * class QueueTaker<E> implements ManagedBlocker {
+     *   final BlockingQueue<E> queue;
+     *   volatile E item = null;
+     *   QueueTaker(BlockingQueue<E> q) { this.queue = q; }
+     *   public boolean block() throws InterruptedException {
+     *     if (item == null)
+     *       item = queue.take();
+     *     return true;
+     *   }
+     *   public boolean isReleasable() {
+     *     return item != null || (item = queue.poll()) != null;
+     *   }
+     *   public E getItem() { // call after pool.managedBlock completes
+     *     return item;
+     *   }
+     * }}</pre>
+     */
+    public static interface ManagedBlocker {
+        /**
+         * Possibly blocks the current thread, for example waiting for
+         * a lock or condition.
+         *
+         * @return {@code true} if no additional blocking is necessary
+         * (i.e., if isReleasable would return true)
+         * @throws InterruptedException if interrupted while waiting
+         * (the method is not required to do so, but is allowed to)
+         */
+        boolean block() throws InterruptedException;
+
+        /**
+         * Returns {@code true} if blocking is unnecessary.
+         */
+        boolean isReleasable();
+    }
+
+    /**
+     * Blocks in accord with the given blocker.  If the current thread
+     * is a {@link ForkJoinWorkerThread}, this method possibly
+     * arranges for a spare thread to be activated if necessary to
+     * ensure sufficient parallelism while the current thread is blocked.
+     *
+     * <p>If the caller is not a {@link ForkJoinTask}, this method is
+     * behaviorally equivalent to
+     *  <pre> {@code
+     * while (!blocker.isReleasable())
+     *   if (blocker.block())
+     *     return;
+     * }</pre>
+     *
+     * If the caller is a {@code ForkJoinTask}, then the pool may
+     * first be expanded to ensure parallelism, and later adjusted.
+     *
+     * @param blocker the blocker
+     * @throws InterruptedException if blocker.block did so
+     */
+    public static void managedBlock(ManagedBlocker blocker)
+        throws InterruptedException {
+        Thread t = Thread.currentThread();
+        if (t instanceof ForkJoinWorkerThread) {
+            ForkJoinPool p = ((ForkJoinWorkerThread)t).pool;
+            while (!blocker.isReleasable()) { // variant of helpSignal
+                WorkQueue[] ws; WorkQueue q; int m, u;
+                if ((ws = p.workQueues) != null && (m = ws.length - 1) >= 0) {
+                    for (int i = 0; i <= m; ++i) {
+                        if (blocker.isReleasable())
+                            return;
+                        if ((q = ws[i]) != null && q.base - q.top < 0) {
+                            p.signalWork(q);
+                            if ((u = (int)(p.ctl >>> 32)) >= 0 ||
+                                (u >> UAC_SHIFT) >= 0)
+                                break;
+                        }
+                    }
+                }
+                if (p.tryCompensate()) {
+                    try {
+                        do {} while (!blocker.isReleasable() &&
+                                     !blocker.block());
+                    } finally {
+                        p.incrementActiveCount();
+                    }
+                    break;
+                }
+            }
+        }
+        else {
+            do {} while (!blocker.isReleasable() &&
+                         !blocker.block());
+        }
+    }
+
+    // AbstractExecutorService overrides.  These rely on undocumented
+    // fact that ForkJoinTask.adapt returns ForkJoinTasks that also
+    // implement RunnableFuture.
+
+    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
+        return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
+    }
+
+    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
+        return new ForkJoinTask.AdaptedCallable<T>(callable);
+    }
+
+    // Unsafe mechanics
+    private static final sun.misc.Unsafe U;
+    private static final long CTL;
+    private static final long PARKBLOCKER;
+    private static final int ABASE;
+    private static final int ASHIFT;
+    private static final long STEALCOUNT;
+    private static final long PLOCK;
+    private static final long INDEXSEED;
+    private static final long QLOCK;
+
+    static {
+        // initialize field offsets for CAS etc
+        try {
+            U = getUnsafe();
+            Class<?> k = ForkJoinPool.class;
+            CTL = U.objectFieldOffset
+                (k.getDeclaredField("ctl"));
+            STEALCOUNT = U.objectFieldOffset
+                (k.getDeclaredField("stealCount"));
+            PLOCK = U.objectFieldOffset
+                (k.getDeclaredField("plock"));
+            INDEXSEED = U.objectFieldOffset
+                (k.getDeclaredField("indexSeed"));
+            Class<?> tk = Thread.class;
+            PARKBLOCKER = U.objectFieldOffset
+                (tk.getDeclaredField("parkBlocker"));
+            Class<?> wk = WorkQueue.class;
+            QLOCK = U.objectFieldOffset
+                (wk.getDeclaredField("qlock"));
+            Class<?> ak = ForkJoinTask[].class;
+            ABASE = U.arrayBaseOffset(ak);
+            int scale = U.arrayIndexScale(ak);
+            if ((scale & (scale - 1)) != 0)
+                throw new Error("data type scale not a power of two");
+            ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
+        } catch (Exception e) {
+            throw new Error(e);
+        }
+
+        submitters = new ThreadLocal<Submitter>();
+        ForkJoinWorkerThreadFactory fac = defaultForkJoinWorkerThreadFactory =
+            new DefaultForkJoinWorkerThreadFactory();
+        modifyThreadPermission = new RuntimePermission("modifyThread");
+
+        /*
+         * Establish common pool parameters.  For extra caution,
+         * computations to set up common pool state are here; the
+         * constructor just assigns these values to fields.
+         */
+
+        int par = 0;
+        Thread.UncaughtExceptionHandler handler = null;
+        try {  // TBD: limit or report ignored exceptions?
+            String pp = System.getProperty
+                ("java.util.concurrent.ForkJoinPool.common.parallelism");
+            String hp = System.getProperty
+                ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
+            String fp = System.getProperty
+                ("java.util.concurrent.ForkJoinPool.common.threadFactory");
+            if (fp != null)
+                fac = ((ForkJoinWorkerThreadFactory)ClassLoader.
+                       getSystemClassLoader().loadClass(fp).newInstance());
+            if (hp != null)
+                handler = ((Thread.UncaughtExceptionHandler)ClassLoader.
+                           getSystemClassLoader().loadClass(hp).newInstance());
+            if (pp != null)
+                par = Integer.parseInt(pp);
+        } catch (Exception ignore) {
+        }
+
+        if (par <= 0)
+            par = Runtime.getRuntime().availableProcessors();
+        if (par > MAX_CAP)
+            par = MAX_CAP;
+        commonParallelism = par;
+        long np = (long)(-par); // precompute initial ctl value
+        long ct = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
+
+        common = new ForkJoinPool(par, ct, fac, handler);
+    }
+
+    /**
+     * 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 tryReflectionInstead) {}
+        try {
+            return java.security.AccessController.doPrivileged
+            (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
+                public sun.misc.Unsafe run() throws Exception {
+                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
+                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
+                        f.setAccessible(true);
+                        Object x = f.get(null);
+                        if (k.isInstance(x))
+                            return k.cast(x);
+                    }
+                    throw new NoSuchFieldError("the Unsafe");
+                }});
+        } catch (java.security.PrivilegedActionException e) {
+            throw new RuntimeException("Could not initialize intrinsics",
+                                       e.getCause());
+        }
+    }
+}
diff --git a/src/main/java/jsr166y/ForkJoinTask.java b/src/main/java/jsr166y/ForkJoinTask.java
new file mode 100644
index 0000000..ab56eca
--- /dev/null
+++ b/src/main/java/jsr166y/ForkJoinTask.java
@@ -0,0 +1,1509 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+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 it is
+ * explicitly submitted to a {@link ForkJoinPool}, or, if not already
+ * engaged in a ForkJoin computation, commenced in the {@link
+ * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or
+ * related methods.  Once started, it will usually in turn start other
+ * subtasks.  As indicated by the name of this class, many programs
+ * using {@code ForkJoinTask} employ only methods {@link #fork} and
+ * {@link #join}, or derivatives such as {@link
+ * #invokeAll(ForkJoinTask...) invokeAll}.  However, this class also
+ * provides a number of other methods that can come into play in
+ * advanced usages, as well as extension mechanics that allow support
+ * of new forms of fork/join processing.
+ *
+ * <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 I/O, and should ideally access variables that
+ * are completely independent of those accessed by other running
+ * tasks. These guidelines are loosely enforced by not permitting
+ * checked exceptions such as {@code IOExceptions} to be
+ * thrown. However, computations may still encounter unchecked
+ * exceptions, that are rethrown to callers attempting to join
+ * them. These exceptions may additionally include {@link
+ * RejectedExecutionException} stemming from internal resource
+ * exhaustion, such as failure to allocate internal task
+ * queues. Rethrown exceptions behave in the same way as regular
+ * exceptions, but, when possible, contain stack traces (as displayed
+ * for example using {@code ex.printStackTrace()}) of both the thread
+ * that initiated the computation as well as the thread actually
+ * encountering the exception; minimally only the latter.
+ *
+ * <p>It is possible to define and use ForkJoinTasks that may block,
+ * but doing do requires three further considerations: (1) Completion
+ * of few if any <em>other</em> tasks should be dependent on a task
+ * that blocks on external synchronization or I/O. Event-style async
+ * tasks that are never joined (for example, those subclassing {@link
+ * CountedCompleter}) often fall into this category.  (2) To minimize
+ * resource impact, tasks should be small; ideally performing only the
+ * (possibly) blocking action. (3) Unless the {@link
+ * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly
+ * blocked tasks is known to be less than the pool's {@link
+ * ForkJoinPool#getParallelism} level, the pool cannot guarantee that
+ * enough threads will be available to ensure progress or good
+ * performance.
+ *
+ * <p>The primary method for awaiting completion and extracting
+ * results of a task is {@link #join}, but there are several variants:
+ * 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.
+ *
+ * <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() {
+        int s;
+        ForkJoinPool.externalHelpJoin(this);
+        boolean interrupted = false;
+        while ((s = status) >= 0) {
+            if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
+                synchronized (this) {
+                    if (status >= 0) {
+                        try {
+                            wait();
+                        } catch (InterruptedException ie) {
+                            interrupted = true;
+                        }
+                    }
+                    else
+                        notifyAll();
+                }
+            }
+        }
+        if (interrupted)
+            Thread.currentThread().interrupt();
+        return s;
+    }
+
+    /**
+     * Blocks a non-worker-thread until completion or interruption.
+     */
+    private int externalInterruptibleAwaitDone() throws InterruptedException {
+        int s;
+        if (Thread.interrupted())
+            throw new InterruptedException();
+        ForkJoinPool.externalHelpJoin(this);
+        while ((s = status) >= 0) {
+            if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
+                synchronized (this) {
+                    if (status >= 0)
+                        wait();
+                    else
+                        notifyAll();
+                }
+            }
+        }
+        return s;
+    }
+
+
+    /**
+     * 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;
+        return (s = status) < 0 ? s :
+            ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+            (w = (wt = (ForkJoinWorkerThread)t).workQueue).
+            tryUnpush(this) && (s = doExec()) < 0 ? s :
+            wt.pool.awaitJoin(w, this) :
+            externalAwaitDone();
+    }
+
+    /**
+     * Implementation for invoke, quietlyInvoke.
+     *
+     * @return status upon completion
+     */
+    private int doInvoke() {
+        int s; Thread t; ForkJoinWorkerThread wt;
+        return (s = doExec()) < 0 ? s :
+            ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+            (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this) :
+            externalAwaitDone();
+    }
+
+    // Exception table support
+
+    /**
+     * 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
+        final int hashCode;  // store task hashCode before weak ref disappears
+        ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) {
+            super(task, exceptionTableRefQueue);
+            this.ex = ex;
+            this.next = next;
+            this.thrower = Thread.currentThread().getId();
+            this.hashCode = System.identityHashCode(task);
+        }
+    }
+
+    /**
+     * 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) {
+                int hashCode = ((ExceptionNode)x).hashCode;
+                ExceptionNode[] t = exceptionTable;
+                int i = hashCode & (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();
+            }
+        }
+    }
+
+    /**
+     * A version of "sneaky throw" to relay exceptions
+     */
+    static void rethrow(final Throwable ex) {
+        if (ex != null) {
+            if (ex instanceof Error)
+                throw (Error)ex;
+            if (ex instanceof RuntimeException)
+                throw (RuntimeException)ex;
+            ForkJoinTask.<RuntimeException>uncheckedThrow(ex);
+        }
+    }
+
+    /**
+     * The sneaky part of sneaky throw, relying on generics
+     * limitations to evade compiler complaints about rethrowing
+     * unchecked exceptions
+     */
+    @SuppressWarnings("unchecked") static <T extends Throwable>
+        void uncheckedThrow(Throwable t) throws T {
+        if (t != null)
+            throw (T)t; // rely on vacuous cast
+    }
+
+    /**
+     * Throws exception, if any, associated with the given status.
+     */
+    private void reportException(int s) {
+        if (s == CANCELLED)
+            throw new CancellationException();
+        if (s == EXCEPTIONAL)
+            rethrow(getThrowableException());
+    }
+
+    // public methods
+
+    /**
+     * Arranges to asynchronously execute this task in the pool the
+     * current task is running in, if applicable, or using the {@link
+     * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.  While
+     * it is not necessarily enforced, it is a usage error to fork a
+     * task more than once unless it has completed and been
+     * reinitialized.  Subsequent modifications to the state of this
+     * task or any data it operates on are not necessarily
+     * consistently observable by any thread other than the one
+     * executing it unless preceded by a call to {@link #join} or
+     * related methods, or a call to {@link #isDone} returning {@code
+     * true}.
+     *
+     * @return {@code this}, to simplify usage
+     */
+    public final ForkJoinTask<V> fork() {
+        Thread t;
+        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
+            ((ForkJoinWorkerThread)t).workQueue.push(this);
+        else
+            ForkJoinPool.common.externalPush(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.
+     *
+     * @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.
+     *
+     * @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)
+            rethrow(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.
+     *
+     * @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)
+            rethrow(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 ms;
+        long ns = unit.toNanos(timeout);
+        if ((s = status) >= 0 && ns > 0L) {
+            long deadline = System.nanoTime() + ns;
+            ForkJoinPool p = null;
+            ForkJoinPool.WorkQueue w = null;
+            Thread t = Thread.currentThread();
+            if (t instanceof ForkJoinWorkerThread) {
+                ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
+                p = wt.pool;
+                w = wt.workQueue;
+                p.helpJoinOnce(w, this); // no retries on failure
+            }
+            else
+                ForkJoinPool.externalHelpJoin(this);
+            boolean canBlock = false;
+            boolean interrupted = false;
+            try {
+                while ((s = status) >= 0) {
+                    if (w != null && w.qlock < 0)
+                        cancelIgnoringExceptions(this);
+                    else if (!canBlock) {
+                        if (p == null || p.tryCompensate())
+                            canBlock = true;
+                    }
+                    else {
+                        if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L &&
+                            U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
+                            synchronized (this) {
+                                if (status >= 0) {
+                                    try {
+                                        wait(ms);
+                                    } catch (InterruptedException ie) {
+                                        if (p == null)
+                                            interrupted = true;
+                                    }
+                                }
+                                else
+                                    notifyAll();
+                            }
+                        }
+                        if ((s = status) < 0 || interrupted ||
+                            (ns = deadline - System.nanoTime()) <= 0L)
+                            break;
+                    }
+                }
+            } 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.
+     */
+    public static void helpQuiesce() {
+        Thread t;
+        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
+            ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
+            wt.pool.helpQuiescePool(wt.workQueue);
+        }
+        else
+            ForkJoinPool.quiesceCommonPool();
+    }
+
+    /**
+     * 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 (but is not guaranteed to) succeed if this task is
+     * the most recently forked task by the current thread, and has
+     * not commenced executing in another thread.  This method may be
+     * useful when arranging alternative local processing of tasks
+     * that could have been, but were not, stolen.
+     *
+     * @return {@code true} if unforked
+     */
+    public boolean tryUnfork() {
+        Thread t;
+        return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+                ((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) :
+                ForkJoinPool.tryExternalUnpush(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.
+     *
+     * @return the number of tasks
+     */
+    public static int getQueuedTaskCount() {
+        Thread t; ForkJoinPool.WorkQueue q;
+        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
+            q = ((ForkJoinWorkerThread)t).workQueue;
+        else
+            q = ForkJoinPool.commonSubmitterQueue();
+        return (q == null) ? 0 : q.queueSize();
+    }
+
+    /**
+     * Returns an estimate of how many more locally queued tasks are
+     * held by the current worker thread than there are other worker
+     * threads that might steal them, or zero if this thread is not
+     * operating in a ForkJoinPool. This value may be useful for
+     * heuristic decisions about whether to fork other tasks. In many
+     * usages of ForkJoinTasks, at steady state, each worker should
+     * aim to maintain a small constant surplus (for example, 3) of
+     * tasks, and to process computations locally if this threshold is
+     * exceeded.
+     *
+     * @return the surplus number of tasks, which may be negative
+     */
+    public static int getSurplusQueuedTaskCount() {
+        return ForkJoinPool.getSurplusQueuedTaskCount();
+    }
+
+    // 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.
+     *
+     * @return the next task, or {@code null} if none are available
+     */
+    protected static ForkJoinTask<?> peekNextLocalTask() {
+        Thread t; ForkJoinPool.WorkQueue q;
+        if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
+            q = ((ForkJoinWorkerThread)t).workQueue;
+        else
+            q = ForkJoinPool.commonSubmitterQueue();
+        return (q == null) ? null : q.peek();
+    }
+
+    /**
+     * Unschedules and returns, without executing, the next task
+     * queued by the current thread but not yet executed, if the
+     * current thread is operating in a ForkJoinPool.  This method is
+     * designed primarily to support extensions, and is unlikely to be
+     * useful otherwise.
+     *
+     * @return the next task, or {@code null} if none are available
+     */
+    protected static ForkJoinTask<?> pollNextLocalTask() {
+        Thread t;
+        return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+            ((ForkJoinWorkerThread)t).workQueue.nextLocalTask() :
+            null;
+    }
+
+    /**
+     * If the current thread is operating in a ForkJoinPool,
+     * unschedules and returns, without executing, the next task
+     * queued by the current thread but not yet executed, if one is
+     * available, or if not available, a task that was forked by some
+     * other thread, if available. Availability may be transient, so a
+     * {@code null} result does not necessarily imply quiescence of
+     * the pool this task is operating in.  This method is designed
+     * primarily to support extensions, and is unlikely to be useful
+     * otherwise.
+     *
+     * @return a task, or {@code null} if none are available
+     */
+    protected static ForkJoinTask<?> pollTask() {
+        Thread t; ForkJoinWorkerThread wt;
+        return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+            (wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) :
+            null;
+    }
+
+    // tag operations
+
+    /**
+     * Returns the tag for this task.
+     *
+     * @return the tag for this task
+     * @since 1.8
+     */
+    public final short getForkJoinTaskTag() {
+        return (short)status;
+    }
+
+    /**
+     * Atomically sets the tag value for this task.
+     *
+     * @param tag the tag value
+     * @return the previous value of the tag
+     * @since 1.8
+     */
+    public final short setForkJoinTaskTag(short tag) {
+        for (int s;;) {
+            if (U.compareAndSwapInt(this, STATUS, s = status,
+                                    (s & ~SMASK) | (tag & SMASK)))
+                return (short)s;
+        }
+    }
+
+    /**
+     * Atomically conditionally sets the tag value for this task.
+     * Among other applications, tags can be used as visit markers
+     * in tasks operating on graphs, as in methods that check: {@code
+     * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))}
+     * before processing, otherwise exiting because the node has
+     * already been visited.
+     *
+     * @param e the expected tag value
+     * @param tag the new tag value
+     * @return true if successful; i.e., the current value was
+     * equal to e and is now tag.
+     * @since 1.8
+     */
+    public final boolean compareAndSetForkJoinTaskTag(short e, short tag) {
+        for (int s;;) {
+            if ((short)(s = status) != e)
+                return false;
+            if (U.compareAndSwapInt(this, STATUS, s,
+                                    (s & ~SMASK) | (tag & SMASK)))
+                return true;
+        }
+    }
+
+    /**
+     * 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();
+            Class<?> k = ForkJoinTask.class;
+            STATUS = U.objectFieldOffset
+                (k.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 tryReflectionInstead) {}
+        try {
+            return java.security.AccessController.doPrivileged
+            (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
+                public sun.misc.Unsafe run() throws Exception {
+                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
+                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
+                        f.setAccessible(true);
+                        Object x = f.get(null);
+                        if (k.isInstance(x))
+                            return k.cast(x);
+                    }
+                    throw new NoSuchFieldError("the Unsafe");
+                }});
+        } catch (java.security.PrivilegedActionException e) {
+            throw new RuntimeException("Could not initialize intrinsics",
+                                       e.getCause());
+        }
+    }
+}
diff --git a/src/main/java/jsr166y/ForkJoinWorkerThread.java b/src/main/java/jsr166y/ForkJoinWorkerThread.java
new file mode 100644
index 0000000..647d0cf
--- /dev/null
+++ b/src/main/java/jsr166y/ForkJoinWorkerThread.java
@@ -0,0 +1,121 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+/**
+ * A thread managed by a {@link ForkJoinPool}, which executes
+ * {@link ForkJoinTask}s.
+ * This class is subclassable solely for the sake of adding
+ * functionality -- there are no overridable methods dealing with
+ * scheduling or execution.  However, you can override initialization
+ * and termination methods surrounding the main task processing loop.
+ * If you do create such a subclass, you will also need to supply a
+ * custom {@link ForkJoinPool.ForkJoinWorkerThreadFactory} to use it
+ * in a {@code ForkJoinPool}.
+ *
+ * @since 1.7
+ * @author Doug Lea
+ */
+public class ForkJoinWorkerThread extends Thread {
+    /*
+     * ForkJoinWorkerThreads are managed by ForkJoinPools and perform
+     * ForkJoinTasks. For explanation, see the internal documentation
+     * of class ForkJoinPool.
+     *
+     * This class just maintains links to its pool and WorkQueue.  The
+     * pool field is set immediately upon construction, but the
+     * workQueue field is not set until a call to registerWorker
+     * completes. This leads to a visibility race, that is tolerated
+     * by requiring that the workQueue field is only accessed by the
+     * owning thread.
+     */
+
+    final ForkJoinPool pool;                // the pool this thread works in
+    final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics
+
+    /**
+     * Creates a ForkJoinWorkerThread operating in the given pool.
+     *
+     * @param pool the pool this thread works in
+     * @throws NullPointerException if pool is null
+     */
+    protected ForkJoinWorkerThread(ForkJoinPool pool) {
+        // Use a placeholder until a useful name can be set in registerWorker
+        super("aForkJoinWorkerThread");
+        this.pool = pool;
+        this.workQueue = pool.registerWorker(this);
+    }
+
+    /**
+     * Returns the pool hosting this thread.
+     *
+     * @return the pool
+     */
+    public ForkJoinPool getPool() {
+        return pool;
+    }
+
+    /**
+     * Returns the index number of this thread in its pool.  The
+     * returned value ranges from zero to the maximum number of
+     * threads (minus one) that have ever been created in the pool.
+     * This method may be useful for applications that track status or
+     * collect results per-worker rather than per-task.
+     *
+     * @return the index number
+     */
+    public int getPoolIndex() {
+        return workQueue.poolIndex;
+    }
+
+    /**
+     * Initializes internal state after construction but before
+     * processing any tasks. If you override this method, you must
+     * invoke {@code super.onStart()} at the beginning of the method.
+     * Initialization requires care: Most fields must have legal
+     * default values, to ensure that attempted accesses from other
+     * threads work correctly even before this thread starts
+     * processing tasks.
+     */
+    protected void onStart() {
+    }
+
+    /**
+     * Performs cleanup associated with termination of this worker
+     * thread.  If you override this method, you must invoke
+     * {@code super.onTermination} at the end of the overridden method.
+     *
+     * @param exception the exception causing this thread to abort due
+     * to an unrecoverable error, or {@code null} if completed normally
+     */
+    protected void onTermination(Throwable exception) {
+    }
+
+    /**
+     * This method is required to be public, but should never be
+     * called explicitly. It performs the main run loop to execute
+     * {@link ForkJoinTask}s.
+     */
+    public void run() {
+        Throwable exception = null;
+        try {
+            onStart();
+            pool.runWorker(workQueue);
+        } catch (Throwable ex) {
+            exception = ex;
+        } finally {
+            try {
+                onTermination(exception);
+            } catch (Throwable ex) {
+                if (exception == null)
+                    exception = ex;
+            } finally {
+                pool.deregisterWorker(this, exception);
+            }
+        }
+    }
+}
diff --git a/src/main/java/jsr166y/LinkedTransferQueue.java b/src/main/java/jsr166y/LinkedTransferQueue.java
new file mode 100644
index 0000000..b29a86c
--- /dev/null
+++ b/src/main/java/jsr166y/LinkedTransferQueue.java
@@ -0,0 +1,1353 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+import java.util.AbstractQueue;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+import java.util.Queue;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.LockSupport;
+
+/**
+ * An unbounded {@link TransferQueue} based on linked nodes.
+ * This queue orders elements FIFO (first-in-first-out) with respect
+ * to any given producer.  The <em>head</em> of the queue is that
+ * element that has been on the queue the longest time for some
+ * producer.  The <em>tail</em> of the queue is that element that has
+ * been on the queue the shortest time for some producer.
+ *
+ * <p>Beware that, unlike in most collections, the {@code size} method
+ * is <em>NOT</em> a constant-time operation. Because of the
+ * asynchronous nature of these queues, determining the current number
+ * of elements requires a traversal of the elements, and so may report
+ * inaccurate results if this collection is modified during traversal.
+ * Additionally, the bulk operations {@code addAll},
+ * {@code removeAll}, {@code retainAll}, {@code containsAll},
+ * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
+ * to be performed atomically. For example, an iterator operating
+ * concurrently with an {@code addAll} operation might view only some
+ * of the added elements.
+ *
+ * <p>This class and its iterator implement all of the
+ * <em>optional</em> methods of the {@link Collection} and {@link
+ * Iterator} interfaces.
+ *
+ * <p>Memory consistency effects: As with other concurrent
+ * collections, actions in a thread prior to placing an object into a
+ * {@code LinkedTransferQueue}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions subsequent to the access or removal of that element from
+ * the {@code LinkedTransferQueue} in another thread.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.7
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public class LinkedTransferQueue<E> extends AbstractQueue<E>
+    implements TransferQueue<E>, java.io.Serializable {
+    private static final long serialVersionUID = -3223113410248163686L;
+
+    /*
+     * *** Overview of Dual Queues with Slack ***
+     *
+     * Dual Queues, introduced by Scherer and Scott
+     * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are
+     * (linked) queues in which nodes may represent either data or
+     * requests.  When a thread tries to enqueue a data node, but
+     * encounters a request node, it instead "matches" and removes it;
+     * and vice versa for enqueuing requests. Blocking Dual Queues
+     * arrange that threads enqueuing unmatched requests block until
+     * other threads provide the match. Dual Synchronous Queues (see
+     * Scherer, Lea, & Scott
+     * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
+     * additionally arrange that threads enqueuing unmatched data also
+     * block.  Dual Transfer Queues support all of these modes, as
+     * dictated by callers.
+     *
+     * A FIFO dual queue may be implemented using a variation of the
+     * Michael & Scott (M&S) lock-free queue algorithm
+     * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).
+     * It maintains two pointer fields, "head", pointing to a
+     * (matched) node that in turn points to the first actual
+     * (unmatched) queue node (or null if empty); and "tail" that
+     * points to the last node on the queue (or again null if
+     * empty). For example, here is a possible queue with four data
+     * elements:
+     *
+     *  head                tail
+     *    |                   |
+     *    v                   v
+     *    M -> U -> U -> U -> U
+     *
+     * The M&S queue algorithm is known to be prone to scalability and
+     * overhead limitations when maintaining (via CAS) these head and
+     * tail pointers. This has led to the development of
+     * contention-reducing variants such as elimination arrays (see
+     * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
+     * optimistic back pointers (see Ladan-Mozes & Shavit
+     * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
+     * However, the nature of dual queues enables a simpler tactic for
+     * improving M&S-style implementations when dual-ness is needed.
+     *
+     * In a dual queue, each node must atomically maintain its match
+     * status. While there are other possible variants, we implement
+     * this here as: for a data-mode node, matching entails CASing an
+     * "item" field from a non-null data value to null upon match, and
+     * vice-versa for request nodes, CASing from null to a data
+     * value. (Note that the linearization properties of this style of
+     * queue are easy to verify -- elements are made available by
+     * linking, and unavailable by matching.) Compared to plain M&S
+     * queues, this property of dual queues requires one additional
+     * successful atomic operation per enq/deq pair. But it also
+     * enables lower cost variants of queue maintenance mechanics. (A
+     * variation of this idea applies even for non-dual queues that
+     * support deletion of interior elements, such as
+     * j.u.c.ConcurrentLinkedQueue.)
+     *
+     * Once a node is matched, its match status can never again
+     * change.  We may thus arrange that the linked list of them
+     * contain a prefix of zero or more matched nodes, followed by a
+     * suffix of zero or more unmatched nodes. (Note that we allow
+     * both the prefix and suffix to be zero length, which in turn
+     * means that we do not use a dummy header.)  If we were not
+     * concerned with either time or space efficiency, we could
+     * correctly perform enqueue and dequeue operations by traversing
+     * from a pointer to the initial node; CASing the item of the
+     * first unmatched node on match and CASing the next field of the
+     * trailing node on appends. (Plus some special-casing when
+     * initially empty).  While this would be a terrible idea in
+     * itself, it does have the benefit of not requiring ANY atomic
+     * updates on head/tail fields.
+     *
+     * We introduce here an approach that lies between the extremes of
+     * never versus always updating queue (head and tail) pointers.
+     * This offers a tradeoff between sometimes requiring extra
+     * traversal steps to locate the first and/or last unmatched
+     * nodes, versus the reduced overhead and contention of fewer
+     * updates to queue pointers. For example, a possible snapshot of
+     * a queue is:
+     *
+     *  head           tail
+     *    |              |
+     *    v              v
+     *    M -> M -> U -> U -> U -> U
+     *
+     * The best value for this "slack" (the targeted maximum distance
+     * between the value of "head" and the first unmatched node, and
+     * similarly for "tail") is an empirical matter. We have found
+     * that using very small constants in the range of 1-3 work best
+     * over a range of platforms. Larger values introduce increasing
+     * costs of cache misses and risks of long traversal chains, while
+     * smaller values increase CAS contention and overhead.
+     *
+     * Dual queues with slack differ from plain M&S dual queues by
+     * virtue of only sometimes updating head or tail pointers when
+     * matching, appending, or even traversing nodes; in order to
+     * maintain a targeted slack.  The idea of "sometimes" may be
+     * operationalized in several ways. The simplest is to use a
+     * per-operation counter incremented on each traversal step, and
+     * to try (via CAS) to update the associated queue pointer
+     * whenever the count exceeds a threshold. Another, that requires
+     * more overhead, is to use random number generators to update
+     * with a given probability per traversal step.
+     *
+     * In any strategy along these lines, because CASes updating
+     * fields may fail, the actual slack may exceed targeted
+     * slack. However, they may be retried at any time to maintain
+     * targets.  Even when using very small slack values, this
+     * approach works well for dual queues because it allows all
+     * operations up to the point of matching or appending an item
+     * (hence potentially allowing progress by another thread) to be
+     * read-only, thus not introducing any further contention. As
+     * described below, we implement this by performing slack
+     * maintenance retries only after these points.
+     *
+     * As an accompaniment to such techniques, traversal overhead can
+     * be further reduced without increasing contention of head
+     * pointer updates: Threads may sometimes shortcut the "next" link
+     * path from the current "head" node to be closer to the currently
+     * known first unmatched node, and similarly for tail. Again, this
+     * may be triggered with using thresholds or randomization.
+     *
+     * These ideas must be further extended to avoid unbounded amounts
+     * of costly-to-reclaim garbage caused by the sequential "next"
+     * links of nodes starting at old forgotten head nodes: As first
+     * described in detail by Boehm
+     * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC
+     * delays noticing that any arbitrarily old node has become
+     * garbage, all newer dead nodes will also be unreclaimed.
+     * (Similar issues arise in non-GC environments.)  To cope with
+     * this in our implementation, upon CASing to advance the head
+     * pointer, we set the "next" link of the previous head to point
+     * only to itself; thus limiting the length of connected dead lists.
+     * (We also take similar care to wipe out possibly garbage
+     * retaining values held in other Node fields.)  However, doing so
+     * adds some further complexity to traversal: If any "next"
+     * pointer links to itself, it indicates that the current thread
+     * has lagged behind a head-update, and so the traversal must
+     * continue from the "head".  Traversals trying to find the
+     * current tail starting from "tail" may also encounter
+     * self-links, in which case they also continue at "head".
+     *
+     * It is tempting in slack-based scheme to not even use CAS for
+     * updates (similarly to Ladan-Mozes & Shavit). However, this
+     * cannot be done for head updates under the above link-forgetting
+     * mechanics because an update may leave head at a detached node.
+     * And while direct writes are possible for tail updates, they
+     * increase the risk of long retraversals, and hence long garbage
+     * chains, which can be much more costly than is worthwhile
+     * considering that the cost difference of performing a CAS vs
+     * write is smaller when they are not triggered on each operation
+     * (especially considering that writes and CASes equally require
+     * additional GC bookkeeping ("write barriers") that are sometimes
+     * more costly than the writes themselves because of contention).
+     *
+     * *** Overview of implementation ***
+     *
+     * We use a threshold-based approach to updates, with a slack
+     * threshold of two -- that is, we update head/tail when the
+     * current pointer appears to be two or more steps away from the
+     * first/last node. The slack value is hard-wired: a path greater
+     * than one is naturally implemented by checking equality of
+     * traversal pointers except when the list has only one element,
+     * in which case we keep slack threshold at one. Avoiding tracking
+     * explicit counts across method calls slightly simplifies an
+     * already-messy implementation. Using randomization would
+     * probably work better if there were a low-quality dirt-cheap
+     * per-thread one available, but even ThreadLocalRandom is too
+     * heavy for these purposes.
+     *
+     * With such a small slack threshold value, it is not worthwhile
+     * to augment this with path short-circuiting (i.e., unsplicing
+     * interior nodes) except in the case of cancellation/removal (see
+     * below).
+     *
+     * We allow both the head and tail fields to be null before any
+     * nodes are enqueued; initializing upon first append.  This
+     * simplifies some other logic, as well as providing more
+     * efficient explicit control paths instead of letting JVMs insert
+     * implicit NullPointerExceptions when they are null.  While not
+     * currently fully implemented, we also leave open the possibility
+     * of re-nulling these fields when empty (which is complicated to
+     * arrange, for little benefit.)
+     *
+     * All enqueue/dequeue operations are handled by the single method
+     * "xfer" with parameters indicating whether to act as some form
+     * of offer, put, poll, take, or transfer (each possibly with
+     * timeout). The relative complexity of using one monolithic
+     * method outweighs the code bulk and maintenance problems of
+     * using separate methods for each case.
+     *
+     * Operation consists of up to three phases. The first is
+     * implemented within method xfer, the second in tryAppend, and
+     * the third in method awaitMatch.
+     *
+     * 1. Try to match an existing node
+     *
+     *    Starting at head, skip already-matched nodes until finding
+     *    an unmatched node of opposite mode, if one exists, in which
+     *    case matching it and returning, also if necessary updating
+     *    head to one past the matched node (or the node itself if the
+     *    list has no other unmatched nodes). If the CAS misses, then
+     *    a loop retries advancing head by two steps until either
+     *    success or the slack is at most two. By requiring that each
+     *    attempt advances head by two (if applicable), we ensure that
+     *    the slack does not grow without bound. Traversals also check
+     *    if the initial head is now off-list, in which case they
+     *    start at the new head.
+     *
+     *    If no candidates are found and the call was untimed
+     *    poll/offer, (argument "how" is NOW) return.
+     *
+     * 2. Try to append a new node (method tryAppend)
+     *
+     *    Starting at current tail pointer, find the actual last node
+     *    and try to append a new node (or if head was null, establish
+     *    the first node). Nodes can be appended only if their
+     *    predecessors are either already matched or are of the same
+     *    mode. If we detect otherwise, then a new node with opposite
+     *    mode must have been appended during traversal, so we must
+     *    restart at phase 1. The traversal and update steps are
+     *    otherwise similar to phase 1: Retrying upon CAS misses and
+     *    checking for staleness.  In particular, if a self-link is
+     *    encountered, then we can safely jump to a node on the list
+     *    by continuing the traversal at current head.
+     *
+     *    On successful append, if the call was ASYNC, return.
+     *
+     * 3. Await match or cancellation (method awaitMatch)
+     *
+     *    Wait for another thread to match node; instead cancelling if
+     *    the current thread was interrupted or the wait timed out. On
+     *    multiprocessors, we use front-of-queue spinning: If a node
+     *    appears to be the first unmatched node in the queue, it
+     *    spins a bit before blocking. In either case, before blocking
+     *    it tries to unsplice any nodes between the current "head"
+     *    and the first unmatched node.
+     *
+     *    Front-of-queue spinning vastly improves performance of
+     *    heavily contended queues. And so long as it is relatively
+     *    brief and "quiet", spinning does not much impact performance
+     *    of less-contended queues.  During spins threads check their
+     *    interrupt status and generate a thread-local random number
+     *    to decide to occasionally perform a Thread.yield. While
+     *    yield has underdefined specs, we assume that it might help,
+     *    and will not hurt, in limiting impact of spinning on busy
+     *    systems.  We also use smaller (1/2) spins for nodes that are
+     *    not known to be front but whose predecessors have not
+     *    blocked -- these "chained" spins avoid artifacts of
+     *    front-of-queue rules which otherwise lead to alternating
+     *    nodes spinning vs blocking. Further, front threads that
+     *    represent phase changes (from data to request node or vice
+     *    versa) compared to their predecessors receive additional
+     *    chained spins, reflecting longer paths typically required to
+     *    unblock threads during phase changes.
+     *
+     *
+     * ** Unlinking removed interior nodes **
+     *
+     * In addition to minimizing garbage retention via self-linking
+     * described above, we also unlink removed interior nodes. These
+     * may arise due to timed out or interrupted waits, or calls to
+     * remove(x) or Iterator.remove.  Normally, given a node that was
+     * at one time known to be the predecessor of some node s that is
+     * to be removed, we can unsplice s by CASing the next field of
+     * its predecessor if it still points to s (otherwise s must
+     * already have been removed or is now offlist). But there are two
+     * situations in which we cannot guarantee to make node s
+     * unreachable in this way: (1) If s is the trailing node of list
+     * (i.e., with null next), then it is pinned as the target node
+     * for appends, so can only be removed later after other nodes are
+     * appended. (2) We cannot necessarily unlink s given a
+     * predecessor node that is matched (including the case of being
+     * cancelled): the predecessor may already be unspliced, in which
+     * case some previous reachable node may still point to s.
+     * (For further explanation see Herlihy & Shavit "The Art of
+     * Multiprocessor Programming" chapter 9).  Although, in both
+     * cases, we can rule out the need for further action if either s
+     * or its predecessor are (or can be made to be) at, or fall off
+     * from, the head of list.
+     *
+     * Without taking these into account, it would be possible for an
+     * unbounded number of supposedly removed nodes to remain
+     * reachable.  Situations leading to such buildup are uncommon but
+     * can occur in practice; for example when a series of short timed
+     * calls to poll repeatedly time out but never otherwise fall off
+     * the list because of an untimed call to take at the front of the
+     * queue.
+     *
+     * When these cases arise, rather than always retraversing the
+     * entire list to find an actual predecessor to unlink (which
+     * won't help for case (1) anyway), we record a conservative
+     * estimate of possible unsplice failures (in "sweepVotes").
+     * We trigger a full sweep when the estimate exceeds a threshold
+     * ("SWEEP_THRESHOLD") indicating the maximum number of estimated
+     * removal failures to tolerate before sweeping through, unlinking
+     * cancelled nodes that were not unlinked upon initial removal.
+     * We perform sweeps by the thread hitting threshold (rather than
+     * background threads or by spreading work to other threads)
+     * because in the main contexts in which removal occurs, the
+     * caller is already timed-out, cancelled, or performing a
+     * potentially O(n) operation (e.g. remove(x)), none of which are
+     * time-critical enough to warrant the overhead that alternatives
+     * would impose on other threads.
+     *
+     * Because the sweepVotes estimate is conservative, and because
+     * nodes become unlinked "naturally" as they fall off the head of
+     * the queue, and because we allow votes to accumulate even while
+     * sweeps are in progress, there are typically significantly fewer
+     * such nodes than estimated.  Choice of a threshold value
+     * balances the likelihood of wasted effort and contention, versus
+     * providing a worst-case bound on retention of interior nodes in
+     * quiescent queues. The value defined below was chosen
+     * empirically to balance these under various timeout scenarios.
+     *
+     * Note that we cannot self-link unlinked interior nodes during
+     * sweeps. However, the associated garbage chains terminate when
+     * some successor ultimately falls off the head of the list and is
+     * self-linked.
+     */
+
+    /** True if on multiprocessor */
+    private static final boolean MP =
+        Runtime.getRuntime().availableProcessors() > 1;
+
+    /**
+     * The number of times to spin (with randomly interspersed calls
+     * to Thread.yield) on multiprocessor before blocking when a node
+     * is apparently the first waiter in the queue.  See above for
+     * explanation. Must be a power of two. The value is empirically
+     * derived -- it works pretty well across a variety of processors,
+     * numbers of CPUs, and OSes.
+     */
+    private static final int FRONT_SPINS   = 1 << 7;
+
+    /**
+     * The number of times to spin before blocking when a node is
+     * preceded by another node that is apparently spinning.  Also
+     * serves as an increment to FRONT_SPINS on phase changes, and as
+     * base average frequency for yielding during spins. Must be a
+     * power of two.
+     */
+    private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
+
+    /**
+     * The maximum number of estimated removal failures (sweepVotes)
+     * to tolerate before sweeping through the queue unlinking
+     * cancelled nodes that were not unlinked upon initial
+     * removal. See above for explanation. The value must be at least
+     * two to avoid useless sweeps when removing trailing nodes.
+     */
+    static final int SWEEP_THRESHOLD = 32;
+
+    /**
+     * Queue nodes. Uses Object, not E, for items to allow forgetting
+     * them after use.  Relies heavily on Unsafe mechanics to minimize
+     * unnecessary ordering constraints: Writes that are intrinsically
+     * ordered wrt other accesses or CASes use simple relaxed forms.
+     */
+    static final class Node {
+        final boolean isData;   // false if this is a request node
+        volatile Object item;   // initially non-null if isData; CASed to match
+        volatile Node next;
+        volatile Thread waiter; // null until waiting
+
+        // CAS methods for fields
+        final boolean casNext(Node cmp, Node val) {
+            return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
+        }
+
+        final boolean casItem(Object cmp, Object val) {
+            // assert cmp == null || cmp.getClass() != Node.class;
+            return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
+        }
+
+        /**
+         * Constructs a new node.  Uses relaxed write because item can
+         * only be seen after publication via casNext.
+         */
+        Node(Object item, boolean isData) {
+            UNSAFE.putObject(this, itemOffset, item); // relaxed write
+            this.isData = isData;
+        }
+
+        /**
+         * Links node to itself to avoid garbage retention.  Called
+         * only after CASing head field, so uses relaxed write.
+         */
+        final void forgetNext() {
+            UNSAFE.putObject(this, nextOffset, this);
+        }
+
+        /**
+         * Sets item to self and waiter to null, to avoid garbage
+         * retention after matching or cancelling. Uses relaxed writes
+         * because order is already constrained in the only calling
+         * contexts: item is forgotten only after volatile/atomic
+         * mechanics that extract items.  Similarly, clearing waiter
+         * follows either CAS or return from park (if ever parked;
+         * else we don't care).
+         */
+        final void forgetContents() {
+            UNSAFE.putObject(this, itemOffset, this);
+            UNSAFE.putObject(this, waiterOffset, null);
+        }
+
+        /**
+         * Returns true if this node has been matched, including the
+         * case of artificial matches due to cancellation.
+         */
+        final boolean isMatched() {
+            Object x = item;
+            return (x == this) || ((x == null) == isData);
+        }
+
+        /**
+         * Returns true if this is an unmatched request node.
+         */
+        final boolean isUnmatchedRequest() {
+            return !isData && item == null;
+        }
+
+        /**
+         * Returns true if a node with the given mode cannot be
+         * appended to this node because this node is unmatched and
+         * has opposite data mode.
+         */
+        final boolean cannotPrecede(boolean haveData) {
+            boolean d = isData;
+            Object x;
+            return d != haveData && (x = item) != this && (x != null) == d;
+        }
+
+        /**
+         * Tries to artificially match a data node -- used by remove.
+         */
+        final boolean tryMatchData() {
+            // assert isData;
+            Object x = item;
+            if (x != null && x != this && casItem(x, null)) {
+                LockSupport.unpark(waiter);
+                return true;
+            }
+            return false;
+        }
+
+        private static final long serialVersionUID = -3375979862319811754L;
+
+        // Unsafe mechanics
+        private static final sun.misc.Unsafe UNSAFE;
+        private static final long itemOffset;
+        private static final long nextOffset;
+        private static final long waiterOffset;
+        static {
+            try {
+                UNSAFE = getUnsafe();
+                Class<?> k = Node.class;
+                itemOffset = UNSAFE.objectFieldOffset
+                    (k.getDeclaredField("item"));
+                nextOffset = UNSAFE.objectFieldOffset
+                    (k.getDeclaredField("next"));
+                waiterOffset = UNSAFE.objectFieldOffset
+                    (k.getDeclaredField("waiter"));
+            } catch (Exception e) {
+                throw new Error(e);
+            }
+        }
+    }
+
+    /** head of the queue; null until first enqueue */
+    transient volatile Node head;
+
+    /** tail of the queue; null until first append */
+    private transient volatile Node tail;
+
+    /** The number of apparent failures to unsplice removed nodes */
+    private transient volatile int sweepVotes;
+
+    // CAS methods for fields
+    private boolean casTail(Node cmp, Node val) {
+        return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
+    }
+
+    private boolean casHead(Node cmp, Node val) {
+        return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
+    }
+
+    private boolean casSweepVotes(int cmp, int val) {
+        return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val);
+    }
+
+    /*
+     * Possible values for "how" argument in xfer method.
+     */
+    private static final int NOW   = 0; // for untimed poll, tryTransfer
+    private static final int ASYNC = 1; // for offer, put, add
+    private static final int SYNC  = 2; // for transfer, take
+    private static final int TIMED = 3; // for timed poll, tryTransfer
+
+    @SuppressWarnings("unchecked")
+    static <E> E cast(Object item) {
+        // assert item == null || item.getClass() != Node.class;
+        return (E) item;
+    }
+
+    /**
+     * Implements all queuing methods. See above for explanation.
+     *
+     * @param e the item or null for take
+     * @param haveData true if this is a put, else a take
+     * @param how NOW, ASYNC, SYNC, or TIMED
+     * @param nanos timeout in nanosecs, used only if mode is TIMED
+     * @return an item if matched, else e
+     * @throws NullPointerException if haveData mode but e is null
+     */
+    private E xfer(E e, boolean haveData, int how, long nanos) {
+        if (haveData && (e == null))
+            throw new NullPointerException();
+        Node s = null;                        // the node to append, if needed
+
+        retry:
+        for (;;) {                            // restart on append race
+
+            for (Node h = head, p = h; p != null;) { // find & match first node
+                boolean isData = p.isData;
+                Object item = p.item;
+                if (item != p && (item != null) == isData) { // unmatched
+                    if (isData == haveData)   // can't match
+                        break;
+                    if (p.casItem(item, e)) { // match
+                        for (Node q = p; q != h;) {
+                            Node n = q.next;  // update by 2 unless singleton
+                            if (head == h && casHead(h, n == null ? q : n)) {
+                                h.forgetNext();
+                                break;
+                            }                 // advance and retry
+                            if ((h = head)   == null ||
+                                (q = h.next) == null || !q.isMatched())
+                                break;        // unless slack < 2
+                        }
+                        LockSupport.unpark(p.waiter);
+                        return LinkedTransferQueue.<E>cast(item);
+                    }
+                }
+                Node n = p.next;
+                p = (p != n) ? n : (h = head); // Use head if p offlist
+            }
+
+            if (how != NOW) {                 // No matches available
+                if (s == null)
+                    s = new Node(e, haveData);
+                Node pred = tryAppend(s, haveData);
+                if (pred == null)
+                    continue retry;           // lost race vs opposite mode
+                if (how != ASYNC)
+                    return awaitMatch(s, pred, e, (how == TIMED), nanos);
+            }
+            return e; // not waiting
+        }
+    }
+
+    /**
+     * Tries to append node s as tail.
+     *
+     * @param s the node to append
+     * @param haveData true if appending in data mode
+     * @return null on failure due to losing race with append in
+     * different mode, else s's predecessor, or s itself if no
+     * predecessor
+     */
+    private Node tryAppend(Node s, boolean haveData) {
+        for (Node t = tail, p = t;;) {        // move p to last node and append
+            Node n, u;                        // temps for reads of next & tail
+            if (p == null && (p = head) == null) {
+                if (casHead(null, s))
+                    return s;                 // initialize
+            }
+            else if (p.cannotPrecede(haveData))
+                return null;                  // lost race vs opposite mode
+            else if ((n = p.next) != null)    // not last; keep traversing
+                p = p != t && t != (u = tail) ? (t = u) : // stale tail
+                    (p != n) ? n : null;      // restart if off list
+            else if (!p.casNext(null, s))
+                p = p.next;                   // re-read on CAS failure
+            else {
+                if (p != t) {                 // update if slack now >= 2
+                    while ((tail != t || !casTail(t, s)) &&
+                           (t = tail)   != null &&
+                           (s = t.next) != null && // advance and retry
+                           (s = s.next) != null && s != t);
+                }
+                return p;
+            }
+        }
+    }
+
+    /**
+     * Spins/yields/blocks until node s is matched or caller gives up.
+     *
+     * @param s the waiting node
+     * @param pred the predecessor of s, or s itself if it has no
+     * predecessor, or null if unknown (the null case does not occur
+     * in any current calls but may in possible future extensions)
+     * @param e the comparison value for checking match
+     * @param timed if true, wait only until timeout elapses
+     * @param nanos timeout in nanosecs, used only if timed is true
+     * @return matched item, or e if unmatched on interrupt or timeout
+     */
+    private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
+        long lastTime = timed ? System.nanoTime() : 0L;
+        Thread w = Thread.currentThread();
+        int spins = -1; // initialized after first item and cancel checks
+        ThreadLocalRandom randomYields = null; // bound if needed
+
+        for (;;) {
+            Object item = s.item;
+            if (item != e) {                  // matched
+                // assert item != s;
+                s.forgetContents();           // avoid garbage
+                return LinkedTransferQueue.<E>cast(item);
+            }
+            if ((w.isInterrupted() || (timed && nanos <= 0)) &&
+                    s.casItem(e, s)) {        // cancel
+                unsplice(pred, s);
+                return e;
+            }
+
+            if (spins < 0) {                  // establish spins at/near front
+                if ((spins = spinsFor(pred, s.isData)) > 0)
+                    randomYields = ThreadLocalRandom.current();
+            }
+            else if (spins > 0) {             // spin
+                --spins;
+                if (randomYields.nextInt(CHAINED_SPINS) == 0)
+                    Thread.yield();           // occasionally yield
+            }
+            else if (s.waiter == null) {
+                s.waiter = w;                 // request unpark then recheck
+            }
+            else if (timed) {
+                long now = System.nanoTime();
+                if ((nanos -= now - lastTime) > 0)
+                    LockSupport.parkNanos(this, nanos);
+                lastTime = now;
+            }
+            else {
+                LockSupport.park(this);
+            }
+        }
+    }
+
+    /**
+     * Returns spin/yield value for a node with given predecessor and
+     * data mode. See above for explanation.
+     */
+    private static int spinsFor(Node pred, boolean haveData) {
+        if (MP && pred != null) {
+            if (pred.isData != haveData)      // phase change
+                return FRONT_SPINS + CHAINED_SPINS;
+            if (pred.isMatched())             // probably at front
+                return FRONT_SPINS;
+            if (pred.waiter == null)          // pred apparently spinning
+                return CHAINED_SPINS;
+        }
+        return 0;
+    }
+
+    /* -------------- Traversal methods -------------- */
+
+    /**
+     * Returns the successor of p, or the head node if p.next has been
+     * linked to self, which will only be true if traversing with a
+     * stale pointer that is now off the list.
+     */
+    final Node succ(Node p) {
+        Node next = p.next;
+        return (p == next) ? head : next;
+    }
+
+    /**
+     * Returns the first unmatched node of the given mode, or null if
+     * none.  Used by methods isEmpty, hasWaitingConsumer.
+     */
+    private Node firstOfMode(boolean isData) {
+        for (Node p = head; p != null; p = succ(p)) {
+            if (!p.isMatched())
+                return (p.isData == isData) ? p : null;
+        }
+        return null;
+    }
+
+    /**
+     * Returns the item in the first unmatched node with isData; or
+     * null if none.  Used by peek.
+     */
+    private E firstDataItem() {
+        for (Node p = head; p != null; p = succ(p)) {
+            Object item = p.item;
+            if (p.isData) {
+                if (item != null && item != p)
+                    return LinkedTransferQueue.<E>cast(item);
+            }
+            else if (item == null)
+                return null;
+        }
+        return null;
+    }
+
+    /**
+     * Traverses and counts unmatched nodes of the given mode.
+     * Used by methods size and getWaitingConsumerCount.
+     */
+    private int countOfMode(boolean data) {
+        int count = 0;
+        for (Node p = head; p != null; ) {
+            if (!p.isMatched()) {
+                if (p.isData != data)
+                    return 0;
+                if (++count == Integer.MAX_VALUE) // saturated
+                    break;
+            }
+            Node n = p.next;
+            if (n != p)
+                p = n;
+            else {
+                count = 0;
+                p = head;
+            }
+        }
+        return count;
+    }
+
+    final class Itr implements Iterator<E> {
+        private Node nextNode;   // next node to return item for
+        private E nextItem;      // the corresponding item
+        private Node lastRet;    // last returned node, to support remove
+        private Node lastPred;   // predecessor to unlink lastRet
+
+        /**
+         * Moves to next node after prev, or first node if prev null.
+         */
+        private void advance(Node prev) {
+            /*
+             * To track and avoid buildup of deleted nodes in the face
+             * of calls to both Queue.remove and Itr.remove, we must
+             * include variants of unsplice and sweep upon each
+             * advance: Upon Itr.remove, we may need to catch up links
+             * from lastPred, and upon other removes, we might need to
+             * skip ahead from stale nodes and unsplice deleted ones
+             * found while advancing.
+             */
+
+            Node r, b; // reset lastPred upon possible deletion of lastRet
+            if ((r = lastRet) != null && !r.isMatched())
+                lastPred = r;    // next lastPred is old lastRet
+            else if ((b = lastPred) == null || b.isMatched())
+                lastPred = null; // at start of list
+            else {
+                Node s, n;       // help with removal of lastPred.next
+                while ((s = b.next) != null &&
+                       s != b && s.isMatched() &&
+                       (n = s.next) != null && n != s)
+                    b.casNext(s, n);
+            }
+
+            this.lastRet = prev;
+
+            for (Node p = prev, s, n;;) {
+                s = (p == null) ? head : p.next;
+                if (s == null)
+                    break;
+                else if (s == p) {
+                    p = null;
+                    continue;
+                }
+                Object item = s.item;
+                if (s.isData) {
+                    if (item != null && item != s) {
+                        nextItem = LinkedTransferQueue.<E>cast(item);
+                        nextNode = s;
+                        return;
+                    }
+                }
+                else if (item == null)
+                    break;
+                // assert s.isMatched();
+                if (p == null)
+                    p = s;
+                else if ((n = s.next) == null)
+                    break;
+                else if (s == n)
+                    p = null;
+                else
+                    p.casNext(s, n);
+            }
+            nextNode = null;
+            nextItem = null;
+        }
+
+        Itr() {
+            advance(null);
+        }
+
+        public final boolean hasNext() {
+            return nextNode != null;
+        }
+
+        public final E next() {
+            Node p = nextNode;
+            if (p == null) throw new NoSuchElementException();
+            E e = nextItem;
+            advance(p);
+            return e;
+        }
+
+        public final void remove() {
+            final Node lastRet = this.lastRet;
+            if (lastRet == null)
+                throw new IllegalStateException();
+            this.lastRet = null;
+            if (lastRet.tryMatchData())
+                unsplice(lastPred, lastRet);
+        }
+    }
+
+    /* -------------- Removal methods -------------- */
+
+    /**
+     * Unsplices (now or later) the given deleted/cancelled node with
+     * the given predecessor.
+     *
+     * @param pred a node that was at one time known to be the
+     * predecessor of s, or null or s itself if s is/was at head
+     * @param s the node to be unspliced
+     */
+    final void unsplice(Node pred, Node s) {
+        s.forgetContents(); // forget unneeded fields
+        /*
+         * See above for rationale. Briefly: if pred still points to
+         * s, try to unlink s.  If s cannot be unlinked, because it is
+         * trailing node or pred might be unlinked, and neither pred
+         * nor s are head or offlist, add to sweepVotes, and if enough
+         * votes have accumulated, sweep.
+         */
+        if (pred != null && pred != s && pred.next == s) {
+            Node n = s.next;
+            if (n == null ||
+                (n != s && pred.casNext(s, n) && pred.isMatched())) {
+                for (;;) {               // check if at, or could be, head
+                    Node h = head;
+                    if (h == pred || h == s || h == null)
+                        return;          // at head or list empty
+                    if (!h.isMatched())
+                        break;
+                    Node hn = h.next;
+                    if (hn == null)
+                        return;          // now empty
+                    if (hn != h && casHead(h, hn))
+                        h.forgetNext();  // advance head
+                }
+                if (pred.next != pred && s.next != s) { // recheck if offlist
+                    for (;;) {           // sweep now if enough votes
+                        int v = sweepVotes;
+                        if (v < SWEEP_THRESHOLD) {
+                            if (casSweepVotes(v, v + 1))
+                                break;
+                        }
+                        else if (casSweepVotes(v, 0)) {
+                            sweep();
+                            break;
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    /**
+     * Unlinks matched (typically cancelled) nodes encountered in a
+     * traversal from head.
+     */
+    private void sweep() {
+        for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
+            if (!s.isMatched())
+                // Unmatched nodes are never self-linked
+                p = s;
+            else if ((n = s.next) == null) // trailing node is pinned
+                break;
+            else if (s == n)    // stale
+                // No need to also check for p == s, since that implies s == n
+                p = head;
+            else
+                p.casNext(s, n);
+        }
+    }
+
+    /**
+     * Main implementation of remove(Object)
+     */
+    private boolean findAndRemove(Object e) {
+        if (e != null) {
+            for (Node pred = null, p = head; p != null; ) {
+                Object item = p.item;
+                if (p.isData) {
+                    if (item != null && item != p && e.equals(item) &&
+                        p.tryMatchData()) {
+                        unsplice(pred, p);
+                        return true;
+                    }
+                }
+                else if (item == null)
+                    break;
+                pred = p;
+                if ((p = p.next) == pred) { // stale
+                    pred = null;
+                    p = head;
+                }
+            }
+        }
+        return false;
+    }
+
+
+    /**
+     * Creates an initially empty {@code LinkedTransferQueue}.
+     */
+    public LinkedTransferQueue() {
+    }
+
+    /**
+     * Creates a {@code LinkedTransferQueue}
+     * initially containing the elements of the given collection,
+     * added in traversal order of the collection's iterator.
+     *
+     * @param c the collection of elements to initially contain
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public LinkedTransferQueue(Collection<? extends E> c) {
+        this();
+        addAll(c);
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue.
+     * As the queue is unbounded, this method will never block.
+     *
+     * @throws NullPointerException if the specified element is null
+     */
+    public void put(E e) {
+        xfer(e, true, ASYNC, 0);
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue.
+     * As the queue is unbounded, this method will never block or
+     * return {@code false}.
+     *
+     * @return {@code true} (as specified by
+     *  {@link java.util.concurrent.BlockingQueue#offer(Object,long,TimeUnit)
+     *  BlockingQueue.offer})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e, long timeout, TimeUnit unit) {
+        xfer(e, true, ASYNC, 0);
+        return true;
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue.
+     * As the queue is unbounded, this method will never return {@code false}.
+     *
+     * @return {@code true} (as specified by {@link Queue#offer})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        xfer(e, true, ASYNC, 0);
+        return true;
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue.
+     * As the queue is unbounded, this method will never throw
+     * {@link IllegalStateException} or return {@code false}.
+     *
+     * @return {@code true} (as specified by {@link Collection#add})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean add(E e) {
+        xfer(e, true, ASYNC, 0);
+        return true;
+    }
+
+    /**
+     * Transfers the element to a waiting consumer immediately, if possible.
+     *
+     * <p>More precisely, transfers the specified element immediately
+     * if there exists a consumer already waiting to receive it (in
+     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
+     * otherwise returning {@code false} without enqueuing the element.
+     *
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean tryTransfer(E e) {
+        return xfer(e, true, NOW, 0) == null;
+    }
+
+    /**
+     * Transfers the element to a consumer, waiting if necessary to do so.
+     *
+     * <p>More precisely, transfers the specified element immediately
+     * if there exists a consumer already waiting to receive it (in
+     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
+     * else inserts the specified element at the tail of this queue
+     * and waits until the element is received by a consumer.
+     *
+     * @throws NullPointerException if the specified element is null
+     */
+    public void transfer(E e) throws InterruptedException {
+        if (xfer(e, true, SYNC, 0) != null) {
+            Thread.interrupted(); // failure possible only due to interrupt
+            throw new InterruptedException();
+        }
+    }
+
+    /**
+     * Transfers the element to a consumer if it is possible to do so
+     * before the timeout elapses.
+     *
+     * <p>More precisely, transfers the specified element immediately
+     * if there exists a consumer already waiting to receive it (in
+     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
+     * else inserts the specified element at the tail of this queue
+     * and waits until the element is received by a consumer,
+     * returning {@code false} if the specified wait time elapses
+     * before the element can be transferred.
+     *
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean tryTransfer(E e, long timeout, TimeUnit unit)
+        throws InterruptedException {
+        if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
+            return true;
+        if (!Thread.interrupted())
+            return false;
+        throw new InterruptedException();
+    }
+
+    public E take() throws InterruptedException {
+        E e = xfer(null, false, SYNC, 0);
+        if (e != null)
+            return e;
+        Thread.interrupted();
+        throw new InterruptedException();
+    }
+
+    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
+        E e = xfer(null, false, TIMED, unit.toNanos(timeout));
+        if (e != null || !Thread.interrupted())
+            return e;
+        throw new InterruptedException();
+    }
+
+    public E poll() {
+        return xfer(null, false, NOW, 0);
+    }
+
+    /**
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        int n = 0;
+        for (E e; (e = poll()) != null;) {
+            c.add(e);
+            ++n;
+        }
+        return n;
+    }
+
+    /**
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c, int maxElements) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        int n = 0;
+        for (E e; n < maxElements && (e = poll()) != null;) {
+            c.add(e);
+            ++n;
+        }
+        return n;
+    }
+
+    /**
+     * Returns an iterator over the elements in this queue in proper sequence.
+     * The elements will be returned in order from first (head) to last (tail).
+     *
+     * <p>The returned iterator is a "weakly consistent" iterator that
+     * will never throw {@link java.util.ConcurrentModificationException
+     * ConcurrentModificationException}, and guarantees to traverse
+     * elements as they existed upon construction of the iterator, and
+     * may (but is not guaranteed to) reflect any modifications
+     * subsequent to construction.
+     *
+     * @return an iterator over the elements in this queue in proper sequence
+     */
+    public Iterator<E> iterator() {
+        return new Itr();
+    }
+
+    public E peek() {
+        return firstDataItem();
+    }
+
+    /**
+     * Returns {@code true} if this queue contains no elements.
+     *
+     * @return {@code true} if this queue contains no elements
+     */
+    public boolean isEmpty() {
+        for (Node p = head; p != null; p = succ(p)) {
+            if (!p.isMatched())
+                return !p.isData;
+        }
+        return true;
+    }
+
+    public boolean hasWaitingConsumer() {
+        return firstOfMode(false) != null;
+    }
+
+    /**
+     * Returns the number of elements in this queue.  If this queue
+     * contains more than {@code Integer.MAX_VALUE} elements, returns
+     * {@code Integer.MAX_VALUE}.
+     *
+     * <p>Beware that, unlike in most collections, this method is
+     * <em>NOT</em> a constant-time operation. Because of the
+     * asynchronous nature of these queues, determining the current
+     * number of elements requires an O(n) traversal.
+     *
+     * @return the number of elements in this queue
+     */
+    public int size() {
+        return countOfMode(true);
+    }
+
+    public int getWaitingConsumerCount() {
+        return countOfMode(false);
+    }
+
+    /**
+     * Removes a single instance of the specified element from this queue,
+     * if it is present.  More formally, removes an element {@code e} such
+     * that {@code o.equals(e)}, if this queue contains one or more such
+     * elements.
+     * Returns {@code true} if this queue contained the specified element
+     * (or equivalently, if this queue changed as a result of the call).
+     *
+     * @param o element to be removed from this queue, if present
+     * @return {@code true} if this queue changed as a result of the call
+     */
+    public boolean remove(Object o) {
+        return findAndRemove(o);
+    }
+
+    /**
+     * Returns {@code true} if this queue contains the specified element.
+     * More formally, returns {@code true} if and only if this queue contains
+     * at least one element {@code e} such that {@code o.equals(e)}.
+     *
+     * @param o object to be checked for containment in this queue
+     * @return {@code true} if this queue contains the specified element
+     */
+    public boolean contains(Object o) {
+        if (o == null) return false;
+        for (Node p = head; p != null; p = succ(p)) {
+            Object item = p.item;
+            if (p.isData) {
+                if (item != null && item != p && o.equals(item))
+                    return true;
+            }
+            else if (item == null)
+                break;
+        }
+        return false;
+    }
+
+    /**
+     * Always returns {@code Integer.MAX_VALUE} because a
+     * {@code LinkedTransferQueue} is not capacity constrained.
+     *
+     * @return {@code Integer.MAX_VALUE} (as specified by
+     *         {@link java.util.concurrent.BlockingQueue#remainingCapacity()
+     *         BlockingQueue.remainingCapacity})
+     */
+    public int remainingCapacity() {
+        return Integer.MAX_VALUE;
+    }
+
+    /**
+     * Saves the state to a stream (that is, serializes it).
+     *
+     * @serialData All of the elements (each an {@code E}) in
+     * the proper order, followed by a null
+     * @param s the stream
+     */
+    private void writeObject(java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+        s.defaultWriteObject();
+        for (E e : this)
+            s.writeObject(e);
+        // Use trailing null as sentinel
+        s.writeObject(null);
+    }
+
+    /**
+     * Reconstitutes the Queue instance from a stream (that is,
+     * deserializes it).
+     *
+     * @param s the stream
+     */
+    private void readObject(java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        s.defaultReadObject();
+        for (;;) {
+            @SuppressWarnings("unchecked")
+            E item = (E) s.readObject();
+            if (item == null)
+                break;
+            else
+                offer(item);
+        }
+    }
+
+    // Unsafe mechanics
+
+    private static final sun.misc.Unsafe UNSAFE;
+    private static final long headOffset;
+    private static final long tailOffset;
+    private static final long sweepVotesOffset;
+    static {
+        try {
+            UNSAFE = getUnsafe();
+            Class<?> k = LinkedTransferQueue.class;
+            headOffset = UNSAFE.objectFieldOffset
+                (k.getDeclaredField("head"));
+            tailOffset = UNSAFE.objectFieldOffset
+                (k.getDeclaredField("tail"));
+            sweepVotesOffset = UNSAFE.objectFieldOffset
+                (k.getDeclaredField("sweepVotes"));
+        } 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
+     */
+    static sun.misc.Unsafe getUnsafe() {
+        try {
+            return sun.misc.Unsafe.getUnsafe();
+        } catch (SecurityException tryReflectionInstead) {}
+        try {
+            return java.security.AccessController.doPrivileged
+            (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
+                public sun.misc.Unsafe run() throws Exception {
+                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
+                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
+                        f.setAccessible(true);
+                        Object x = f.get(null);
+                        if (k.isInstance(x))
+                            return k.cast(x);
+                    }
+                    throw new NoSuchFieldError("the Unsafe");
+                }});
+        } catch (java.security.PrivilegedActionException e) {
+            throw new RuntimeException("Could not initialize intrinsics",
+                                       e.getCause());
+        }
+    }
+}
diff --git a/src/main/java/jsr166y/Phaser.java b/src/main/java/jsr166y/Phaser.java
new file mode 100644
index 0000000..45dd0ba
--- /dev/null
+++ b/src/main/java/jsr166y/Phaser.java
@@ -0,0 +1,1164 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.concurrent.locks.LockSupport;
+
+/**
+ * A reusable synchronization barrier, similar in functionality to
+ * {@link java.util.concurrent.CyclicBarrier CyclicBarrier} and
+ * {@link java.util.concurrent.CountDownLatch CountDownLatch}
+ * but supporting more flexible usage.
+ *
+ * <p><b>Registration.</b> Unlike the case for other barriers, the
+ * number of parties <em>registered</em> to synchronize on a phaser
+ * may vary over time.  Tasks may be registered at any time (using
+ * methods {@link #register}, {@link #bulkRegister}, or forms of
+ * constructors establishing initial numbers of parties), and
+ * optionally deregistered upon any arrival (using {@link
+ * #arriveAndDeregister}).  As is the case with most basic
+ * synchronization constructs, registration and deregistration affect
+ * only internal counts; they do not establish any further internal
+ * bookkeeping, so tasks cannot query whether they are registered.
+ * (However, you can introduce such bookkeeping by subclassing this
+ * class.)
+ *
+ * <p><b>Synchronization.</b> Like a {@code CyclicBarrier}, a {@code
+ * Phaser} may be repeatedly awaited.  Method {@link
+ * #arriveAndAwaitAdvance} has effect analogous to {@link
+ * java.util.concurrent.CyclicBarrier#await CyclicBarrier.await}. Each
+ * generation of a phaser has an associated phase number. The phase
+ * number starts at zero, and advances when all parties arrive at the
+ * phaser, wrapping around to zero after reaching {@code
+ * Integer.MAX_VALUE}. The use of phase numbers enables independent
+ * control of actions upon arrival at a phaser and upon awaiting
+ * others, via two kinds of methods that may be invoked by any
+ * registered party:
+ *
+ * <ul>
+ *
+ *   <li> <b>Arrival.</b> Methods {@link #arrive} and
+ *       {@link #arriveAndDeregister} record arrival.  These methods
+ *       do not block, but return an associated <em>arrival phase
+ *       number</em>; that is, the phase number of the phaser to which
+ *       the arrival applied. When the final party for a given phase
+ *       arrives, an optional action is performed and the phase
+ *       advances.  These actions are performed by the party
+ *       triggering a phase advance, and are arranged by overriding
+ *       method {@link #onAdvance(int, int)}, which also controls
+ *       termination. Overriding this method is similar to, but more
+ *       flexible than, providing a barrier action to a {@code
+ *       CyclicBarrier}.
+ *
+ *   <li> <b>Waiting.</b> Method {@link #awaitAdvance} requires an
+ *       argument indicating an arrival phase number, and returns when
+ *       the phaser advances to (or is already at) a different phase.
+ *       Unlike similar constructions using {@code CyclicBarrier},
+ *       method {@code awaitAdvance} continues to wait even if the
+ *       waiting thread is interrupted. Interruptible and timeout
+ *       versions are also available, but exceptions encountered while
+ *       tasks wait interruptibly or with timeout do not change the
+ *       state of the phaser. If necessary, you can perform any
+ *       associated recovery within handlers of those exceptions,
+ *       often after invoking {@code forceTermination}.  Phasers may
+ *       also be used by tasks executing in a {@link ForkJoinPool},
+ *       which will ensure sufficient parallelism to execute tasks
+ *       when others are blocked waiting for a phase to advance.
+ *
+ * </ul>
+ *
+ * <p><b>Termination.</b> A phaser may enter a <em>termination</em>
+ * state, that may be checked using method {@link #isTerminated}. Upon
+ * termination, all synchronization methods immediately return without
+ * waiting for advance, as indicated by a negative return value.
+ * Similarly, attempts to register upon termination have no effect.
+ * Termination is triggered when an invocation of {@code onAdvance}
+ * returns {@code true}. The default implementation returns {@code
+ * true} if a deregistration has caused the number of registered
+ * parties to become zero.  As illustrated below, when phasers control
+ * actions with a fixed number of iterations, it is often convenient
+ * to override this method to cause termination when the current phase
+ * number reaches a threshold. Method {@link #forceTermination} is
+ * also available to abruptly release waiting threads and allow them
+ * to terminate.
+ *
+ * <p><b>Tiering.</b> Phasers may be <em>tiered</em> (i.e.,
+ * constructed in tree structures) to reduce contention. Phasers with
+ * large numbers of parties that would otherwise experience heavy
+ * synchronization contention costs may instead be set up so that
+ * groups of sub-phasers share a common parent.  This may greatly
+ * increase throughput even though it incurs greater per-operation
+ * overhead.
+ *
+ * <p>In a tree of tiered phasers, registration and deregistration of
+ * child phasers with their parent are managed automatically.
+ * Whenever the number of registered parties of a child phaser becomes
+ * non-zero (as established in the {@link #Phaser(Phaser,int)}
+ * constructor, {@link #register}, or {@link #bulkRegister}), the
+ * child phaser is registered with its parent.  Whenever the number of
+ * registered parties becomes zero as the result of an invocation of
+ * {@link #arriveAndDeregister}, the child phaser is deregistered
+ * from its parent.
+ *
+ * <p><b>Monitoring.</b> While synchronization methods may be invoked
+ * only by registered parties, the current state of a phaser may be
+ * monitored by any caller.  At any given moment there are {@link
+ * #getRegisteredParties} parties in total, of which {@link
+ * #getArrivedParties} have arrived at the current phase ({@link
+ * #getPhase}).  When the remaining ({@link #getUnarrivedParties})
+ * parties arrive, the phase advances.  The values returned by these
+ * methods may reflect transient states and so are not in general
+ * useful for synchronization control.  Method {@link #toString}
+ * returns snapshots of these state queries in a form convenient for
+ * informal monitoring.
+ *
+ * <p><b>Sample usages:</b>
+ *
+ * <p>A {@code Phaser} may be used instead of a {@code CountDownLatch}
+ * to control a one-shot action serving a variable number of parties.
+ * The typical idiom is for the method setting this up to first
+ * register, then start the actions, then deregister, as in:
+ *
+ *  <pre> {@code
+ * void runTasks(List<Runnable> tasks) {
+ *   final Phaser phaser = new Phaser(1); // "1" to register self
+ *   // create and start threads
+ *   for (final Runnable task : tasks) {
+ *     phaser.register();
+ *     new Thread() {
+ *       public void run() {
+ *         phaser.arriveAndAwaitAdvance(); // await all creation
+ *         task.run();
+ *       }
+ *     }.start();
+ *   }
+ *
+ *   // allow threads to start and deregister self
+ *   phaser.arriveAndDeregister();
+ * }}</pre>
+ *
+ * <p>One way to cause a set of threads to repeatedly perform actions
+ * for a given number of iterations is to override {@code onAdvance}:
+ *
+ *  <pre> {@code
+ * void startTasks(List<Runnable> tasks, final int iterations) {
+ *   final Phaser phaser = new Phaser() {
+ *     protected boolean onAdvance(int phase, int registeredParties) {
+ *       return phase >= iterations || registeredParties == 0;
+ *     }
+ *   };
+ *   phaser.register();
+ *   for (final Runnable task : tasks) {
+ *     phaser.register();
+ *     new Thread() {
+ *       public void run() {
+ *         do {
+ *           task.run();
+ *           phaser.arriveAndAwaitAdvance();
+ *         } while (!phaser.isTerminated());
+ *       }
+ *     }.start();
+ *   }
+ *   phaser.arriveAndDeregister(); // deregister self, don't wait
+ * }}</pre>
+ *
+ * If the main task must later await termination, it
+ * may re-register and then execute a similar loop:
+ *  <pre> {@code
+ *   // ...
+ *   phaser.register();
+ *   while (!phaser.isTerminated())
+ *     phaser.arriveAndAwaitAdvance();}</pre>
+ *
+ * <p>Related constructions may be used to await particular phase numbers
+ * in contexts where you are sure that the phase will never wrap around
+ * {@code Integer.MAX_VALUE}. For example:
+ *
+ *  <pre> {@code
+ * void awaitPhase(Phaser phaser, int phase) {
+ *   int p = phaser.register(); // assumes caller not already registered
+ *   while (p < phase) {
+ *     if (phaser.isTerminated())
+ *       // ... deal with unexpected termination
+ *     else
+ *       p = phaser.arriveAndAwaitAdvance();
+ *   }
+ *   phaser.arriveAndDeregister();
+ * }}</pre>
+ *
+ *
+ * <p>To create a set of {@code n} tasks using a tree of phasers, you
+ * could use code of the following form, assuming a Task class with a
+ * constructor accepting a {@code Phaser} that it registers with upon
+ * construction. After invocation of {@code build(new Task[n], 0, n,
+ * new Phaser())}, these tasks could then be started, for example by
+ * submitting to a pool:
+ *
+ *  <pre> {@code
+ * void build(Task[] tasks, int lo, int hi, Phaser ph) {
+ *   if (hi - lo > TASKS_PER_PHASER) {
+ *     for (int i = lo; i < hi; i += TASKS_PER_PHASER) {
+ *       int j = Math.min(i + TASKS_PER_PHASER, hi);
+ *       build(tasks, i, j, new Phaser(ph));
+ *     }
+ *   } else {
+ *     for (int i = lo; i < hi; ++i)
+ *       tasks[i] = new Task(ph);
+ *       // assumes new Task(ph) performs ph.register()
+ *   }
+ * }}</pre>
+ *
+ * The best value of {@code TASKS_PER_PHASER} depends mainly on
+ * expected synchronization rates. A value as low as four may
+ * be appropriate for extremely small per-phase task bodies (thus
+ * high rates), or up to hundreds for extremely large ones.
+ *
+ * <p><b>Implementation notes</b>: This implementation restricts the
+ * maximum number of parties to 65535. Attempts to register additional
+ * parties result in {@code IllegalStateException}. However, you can and
+ * should create tiered phasers to accommodate arbitrarily large sets
+ * of participants.
+ *
+ * @since 1.7
+ * @author Doug Lea
+ */
+public class Phaser {
+    /*
+     * This class implements an extension of X10 "clocks".  Thanks to
+     * Vijay Saraswat for the idea, and to Vivek Sarkar for
+     * enhancements to extend functionality.
+     */
+
+    /**
+     * Primary state representation, holding four bit-fields:
+     *
+     * unarrived  -- the number of parties yet to hit barrier (bits  0-15)
+     * parties    -- the number of parties to wait            (bits 16-31)
+     * phase      -- the generation of the barrier            (bits 32-62)
+     * terminated -- set if barrier is terminated             (bit  63 / sign)
+     *
+     * Except that a phaser with no registered parties is
+     * distinguished by the otherwise illegal state of having zero
+     * parties and one unarrived parties (encoded as EMPTY below).
+     *
+     * To efficiently maintain atomicity, these values are packed into
+     * a single (atomic) long. Good performance relies on keeping
+     * state decoding and encoding simple, and keeping race windows
+     * short.
+     *
+     * All state updates are performed via CAS except initial
+     * registration of a sub-phaser (i.e., one with a non-null
+     * parent).  In this (relatively rare) case, we use built-in
+     * synchronization to lock while first registering with its
+     * parent.
+     *
+     * The phase of a subphaser is allowed to lag that of its
+     * ancestors until it is actually accessed -- see method
+     * reconcileState.
+     */
+    private volatile long state;
+
+    private static final int  MAX_PARTIES     = 0xffff;
+    private static final int  MAX_PHASE       = Integer.MAX_VALUE;
+    private static final int  PARTIES_SHIFT   = 16;
+    private static final int  PHASE_SHIFT     = 32;
+    private static final int  UNARRIVED_MASK  = 0xffff;      // to mask ints
+    private static final long PARTIES_MASK    = 0xffff0000L; // to mask longs
+    private static final long COUNTS_MASK     = 0xffffffffL;
+    private static final long TERMINATION_BIT = 1L << 63;
+
+    // some special values
+    private static final int  ONE_ARRIVAL     = 1;
+    private static final int  ONE_PARTY       = 1 << PARTIES_SHIFT;
+    private static final int  ONE_DEREGISTER  = ONE_ARRIVAL|ONE_PARTY;
+    private static final int  EMPTY           = 1;
+
+    // The following unpacking methods are usually manually inlined
+
+    private static int unarrivedOf(long s) {
+        int counts = (int)s;
+        return (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
+    }
+
+    private static int partiesOf(long s) {
+        return (int)s >>> PARTIES_SHIFT;
+    }
+
+    private static int phaseOf(long s) {
+        return (int)(s >>> PHASE_SHIFT);
+    }
+
+    private static int arrivedOf(long s) {
+        int counts = (int)s;
+        return (counts == EMPTY) ? 0 :
+            (counts >>> PARTIES_SHIFT) - (counts & UNARRIVED_MASK);
+    }
+
+    /**
+     * The parent of this phaser, or null if none
+     */
+    private final Phaser parent;
+
+    /**
+     * The root of phaser tree. Equals this if not in a tree.
+     */
+    private final Phaser root;
+
+    /**
+     * Heads of Treiber stacks for waiting threads. To eliminate
+     * contention when releasing some threads while adding others, we
+     * use two of them, alternating across even and odd phases.
+     * Subphasers share queues with root to speed up releases.
+     */
+    private final AtomicReference<QNode> evenQ;
+    private final AtomicReference<QNode> oddQ;
+
+    private AtomicReference<QNode> queueFor(int phase) {
+        return ((phase & 1) == 0) ? evenQ : oddQ;
+    }
+
+    /**
+     * Returns message string for bounds exceptions on arrival.
+     */
+    private String badArrive(long s) {
+        return "Attempted arrival of unregistered party for " +
+            stateToString(s);
+    }
+
+    /**
+     * Returns message string for bounds exceptions on registration.
+     */
+    private String badRegister(long s) {
+        return "Attempt to register more than " +
+            MAX_PARTIES + " parties for " + stateToString(s);
+    }
+
+    /**
+     * Main implementation for methods arrive and arriveAndDeregister.
+     * Manually tuned to speed up and minimize race windows for the
+     * common case of just decrementing unarrived field.
+     *
+     * @param adjust value to subtract from state;
+     *               ONE_ARRIVAL for arrive,
+     *               ONE_DEREGISTER for arriveAndDeregister
+     */
+    private int doArrive(int adjust) {
+        final Phaser root = this.root;
+        for (;;) {
+            long s = (root == this) ? state : reconcileState();
+            int phase = (int)(s >>> PHASE_SHIFT);
+            if (phase < 0)
+                return phase;
+            int counts = (int)s;
+            int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
+            if (unarrived <= 0)
+                throw new IllegalStateException(badArrive(s));
+            if (UNSAFE.compareAndSwapLong(this, stateOffset, s, s-=adjust)) {
+                if (unarrived == 1) {
+                    long n = s & PARTIES_MASK;  // base of next state
+                    int nextUnarrived = (int)n >>> PARTIES_SHIFT;
+                    if (root == this) {
+                        if (onAdvance(phase, nextUnarrived))
+                            n |= TERMINATION_BIT;
+                        else if (nextUnarrived == 0)
+                            n |= EMPTY;
+                        else
+                            n |= nextUnarrived;
+                        int nextPhase = (phase + 1) & MAX_PHASE;
+                        n |= (long)nextPhase << PHASE_SHIFT;
+                        UNSAFE.compareAndSwapLong(this, stateOffset, s, n);
+                        releaseWaiters(phase);
+                    }
+                    else if (nextUnarrived == 0) { // propagate deregistration
+                        phase = parent.doArrive(ONE_DEREGISTER);
+                        UNSAFE.compareAndSwapLong(this, stateOffset,
+                                                  s, s | EMPTY);
+                    }
+                    else
+                        phase = parent.doArrive(ONE_ARRIVAL);
+                }
+                return phase;
+            }
+        }
+    }
+
+    /**
+     * Implementation of register, bulkRegister
+     *
+     * @param registrations number to add to both parties and
+     * unarrived fields. Must be greater than zero.
+     */
+    private int doRegister(int registrations) {
+        // adjustment to state
+        long adjust = ((long)registrations << PARTIES_SHIFT) | registrations;
+        final Phaser parent = this.parent;
+        int phase;
+        for (;;) {
+            long s = (parent == null) ? state : reconcileState();
+            int counts = (int)s;
+            int parties = counts >>> PARTIES_SHIFT;
+            int unarrived = counts & UNARRIVED_MASK;
+            if (registrations > MAX_PARTIES - parties)
+                throw new IllegalStateException(badRegister(s));
+            phase = (int)(s >>> PHASE_SHIFT);
+            if (phase < 0)
+                break;
+            if (counts != EMPTY) {                  // not 1st registration
+                if (parent == null || reconcileState() == s) {
+                    if (unarrived == 0)             // wait out advance
+                        root.internalAwaitAdvance(phase, null);
+                    else if (UNSAFE.compareAndSwapLong(this, stateOffset,
+                                                       s, s + adjust))
+                        break;
+                }
+            }
+            else if (parent == null) {              // 1st root registration
+                long next = ((long)phase << PHASE_SHIFT) | adjust;
+                if (UNSAFE.compareAndSwapLong(this, stateOffset, s, next))
+                    break;
+            }
+            else {
+                synchronized (this) {               // 1st sub registration
+                    if (state == s) {               // recheck under lock
+                        phase = parent.doRegister(1);
+                        if (phase < 0)
+                            break;
+                        // finish registration whenever parent registration
+                        // succeeded, even when racing with termination,
+                        // since these are part of the same "transaction".
+                        while (!UNSAFE.compareAndSwapLong
+                               (this, stateOffset, s,
+                                ((long)phase << PHASE_SHIFT) | adjust)) {
+                            s = state;
+                            phase = (int)(root.state >>> PHASE_SHIFT);
+                            // assert (int)s == EMPTY;
+                        }
+                        break;
+                    }
+                }
+            }
+        }
+        return phase;
+    }
+
+    /**
+     * Resolves lagged phase propagation from root if necessary.
+     * Reconciliation normally occurs when root has advanced but
+     * subphasers have not yet done so, in which case they must finish
+     * their own advance by setting unarrived to parties (or if
+     * parties is zero, resetting to unregistered EMPTY state).
+     *
+     * @return reconciled state
+     */
+    private long reconcileState() {
+        final Phaser root = this.root;
+        long s = state;
+        if (root != this) {
+            int phase, p;
+            // CAS to root phase with current parties, tripping unarrived
+            while ((phase = (int)(root.state >>> PHASE_SHIFT)) !=
+                   (int)(s >>> PHASE_SHIFT) &&
+                   !UNSAFE.compareAndSwapLong
+                   (this, stateOffset, s,
+                    s = (((long)phase << PHASE_SHIFT) |
+                         ((phase < 0) ? (s & COUNTS_MASK) :
+                          (((p = (int)s >>> PARTIES_SHIFT) == 0) ? EMPTY :
+                           ((s & PARTIES_MASK) | p))))))
+                s = state;
+        }
+        return s;
+    }
+
+    /**
+     * Creates a new phaser with no initially registered parties, no
+     * parent, and initial phase number 0. Any thread using this
+     * phaser will need to first register for it.
+     */
+    public Phaser() {
+        this(null, 0);
+    }
+
+    /**
+     * Creates a new phaser with the given number of registered
+     * unarrived parties, no parent, and initial phase number 0.
+     *
+     * @param parties the number of parties required to advance to the
+     * next phase
+     * @throws IllegalArgumentException if parties less than zero
+     * or greater than the maximum number of parties supported
+     */
+    public Phaser(int parties) {
+        this(null, parties);
+    }
+
+    /**
+     * Equivalent to {@link #Phaser(Phaser, int) Phaser(parent, 0)}.
+     *
+     * @param parent the parent phaser
+     */
+    public Phaser(Phaser parent) {
+        this(parent, 0);
+    }
+
+    /**
+     * Creates a new phaser with the given parent and number of
+     * registered unarrived parties.  When the given parent is non-null
+     * and the given number of parties is greater than zero, this
+     * child phaser is registered with its parent.
+     *
+     * @param parent the parent phaser
+     * @param parties the number of parties required to advance to the
+     * next phase
+     * @throws IllegalArgumentException if parties less than zero
+     * or greater than the maximum number of parties supported
+     */
+    public Phaser(Phaser parent, int parties) {
+        if (parties >>> PARTIES_SHIFT != 0)
+            throw new IllegalArgumentException("Illegal number of parties");
+        int phase = 0;
+        this.parent = parent;
+        if (parent != null) {
+            final Phaser root = parent.root;
+            this.root = root;
+            this.evenQ = root.evenQ;
+            this.oddQ = root.oddQ;
+            if (parties != 0)
+                phase = parent.doRegister(1);
+        }
+        else {
+            this.root = this;
+            this.evenQ = new AtomicReference<QNode>();
+            this.oddQ = new AtomicReference<QNode>();
+        }
+        this.state = (parties == 0) ? (long)EMPTY :
+            ((long)phase << PHASE_SHIFT) |
+            ((long)parties << PARTIES_SHIFT) |
+            ((long)parties);
+    }
+
+    /**
+     * Adds a new unarrived party to this phaser.  If an ongoing
+     * invocation of {@link #onAdvance} is in progress, this method
+     * may await its completion before returning.  If this phaser has
+     * a parent, and this phaser previously had no registered parties,
+     * this child phaser is also registered with its parent. If
+     * this phaser is terminated, the attempt to register has
+     * no effect, and a negative value is returned.
+     *
+     * @return the arrival phase number to which this registration
+     * applied.  If this value is negative, then this phaser has
+     * terminated, in which case registration has no effect.
+     * @throws IllegalStateException if attempting to register more
+     * than the maximum supported number of parties
+     */
+    public int register() {
+        return doRegister(1);
+    }
+
+    /**
+     * Adds the given number of new unarrived parties to this phaser.
+     * If an ongoing invocation of {@link #onAdvance} is in progress,
+     * this method may await its completion before returning.  If this
+     * phaser has a parent, and the given number of parties is greater
+     * than zero, and this phaser previously had no registered
+     * parties, this child phaser is also registered with its parent.
+     * If this phaser is terminated, the attempt to register has no
+     * effect, and a negative value is returned.
+     *
+     * @param parties the number of additional parties required to
+     * advance to the next phase
+     * @return the arrival phase number to which this registration
+     * applied.  If this value is negative, then this phaser has
+     * terminated, in which case registration has no effect.
+     * @throws IllegalStateException if attempting to register more
+     * than the maximum supported number of parties
+     * @throws IllegalArgumentException if {@code parties < 0}
+     */
+    public int bulkRegister(int parties) {
+        if (parties < 0)
+            throw new IllegalArgumentException();
+        if (parties == 0)
+            return getPhase();
+        return doRegister(parties);
+    }
+
+    /**
+     * Arrives at this phaser, without waiting for others to arrive.
+     *
+     * <p>It is a usage error for an unregistered party to invoke this
+     * method.  However, this error may result in an {@code
+     * IllegalStateException} only upon some subsequent operation on
+     * this phaser, if ever.
+     *
+     * @return the arrival phase number, or a negative value if terminated
+     * @throws IllegalStateException if not terminated and the number
+     * of unarrived parties would become negative
+     */
+    public int arrive() {
+        return doArrive(ONE_ARRIVAL);
+    }
+
+    /**
+     * Arrives at this phaser and deregisters from it without waiting
+     * for others to arrive. Deregistration reduces the number of
+     * parties required to advance in future phases.  If this phaser
+     * has a parent, and deregistration causes this phaser to have
+     * zero parties, this phaser is also deregistered from its parent.
+     *
+     * <p>It is a usage error for an unregistered party to invoke this
+     * method.  However, this error may result in an {@code
+     * IllegalStateException} only upon some subsequent operation on
+     * this phaser, if ever.
+     *
+     * @return the arrival phase number, or a negative value if terminated
+     * @throws IllegalStateException if not terminated and the number
+     * of registered or unarrived parties would become negative
+     */
+    public int arriveAndDeregister() {
+        return doArrive(ONE_DEREGISTER);
+    }
+
+    /**
+     * Arrives at this phaser and awaits others. Equivalent in effect
+     * to {@code awaitAdvance(arrive())}.  If you need to await with
+     * interruption or timeout, you can arrange this with an analogous
+     * construction using one of the other forms of the {@code
+     * awaitAdvance} method.  If instead you need to deregister upon
+     * arrival, use {@code awaitAdvance(arriveAndDeregister())}.
+     *
+     * <p>It is a usage error for an unregistered party to invoke this
+     * method.  However, this error may result in an {@code
+     * IllegalStateException} only upon some subsequent operation on
+     * this phaser, if ever.
+     *
+     * @return the arrival phase number, or the (negative)
+     * {@linkplain #getPhase() current phase} if terminated
+     * @throws IllegalStateException if not terminated and the number
+     * of unarrived parties would become negative
+     */
+    public int arriveAndAwaitAdvance() {
+        // Specialization of doArrive+awaitAdvance eliminating some reads/paths
+        final Phaser root = this.root;
+        for (;;) {
+            long s = (root == this) ? state : reconcileState();
+            int phase = (int)(s >>> PHASE_SHIFT);
+            if (phase < 0)
+                return phase;
+            int counts = (int)s;
+            int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
+            if (unarrived <= 0)
+                throw new IllegalStateException(badArrive(s));
+            if (UNSAFE.compareAndSwapLong(this, stateOffset, s,
+                                          s -= ONE_ARRIVAL)) {
+                if (unarrived > 1)
+                    return root.internalAwaitAdvance(phase, null);
+                if (root != this)
+                    return parent.arriveAndAwaitAdvance();
+                long n = s & PARTIES_MASK;  // base of next state
+                int nextUnarrived = (int)n >>> PARTIES_SHIFT;
+                if (onAdvance(phase, nextUnarrived))
+                    n |= TERMINATION_BIT;
+                else if (nextUnarrived == 0)
+                    n |= EMPTY;
+                else
+                    n |= nextUnarrived;
+                int nextPhase = (phase + 1) & MAX_PHASE;
+                n |= (long)nextPhase << PHASE_SHIFT;
+                if (!UNSAFE.compareAndSwapLong(this, stateOffset, s, n))
+                    return (int)(state >>> PHASE_SHIFT); // terminated
+                releaseWaiters(phase);
+                return nextPhase;
+            }
+        }
+    }
+
+    /**
+     * Awaits the phase of this phaser to advance from the given phase
+     * value, returning immediately if the current phase is not equal
+     * to the given phase value or this phaser is terminated.
+     *
+     * @param phase an arrival phase number, or negative value if
+     * terminated; this argument is normally the value returned by a
+     * previous call to {@code arrive} or {@code arriveAndDeregister}.
+     * @return the next arrival phase number, or the argument if it is
+     * negative, or the (negative) {@linkplain #getPhase() current phase}
+     * if terminated
+     */
+    public int awaitAdvance(int phase) {
+        final Phaser root = this.root;
+        long s = (root == this) ? state : reconcileState();
+        int p = (int)(s >>> PHASE_SHIFT);
+        if (phase < 0)
+            return phase;
+        if (p == phase)
+            return root.internalAwaitAdvance(phase, null);
+        return p;
+    }
+
+    /**
+     * Awaits the phase of this phaser to advance from the given phase
+     * value, throwing {@code InterruptedException} if interrupted
+     * while waiting, or returning immediately if the current phase is
+     * not equal to the given phase value or this phaser is
+     * terminated.
+     *
+     * @param phase an arrival phase number, or negative value if
+     * terminated; this argument is normally the value returned by a
+     * previous call to {@code arrive} or {@code arriveAndDeregister}.
+     * @return the next arrival phase number, or the argument if it is
+     * negative, or the (negative) {@linkplain #getPhase() current phase}
+     * if terminated
+     * @throws InterruptedException if thread interrupted while waiting
+     */
+    public int awaitAdvanceInterruptibly(int phase)
+        throws InterruptedException {
+        final Phaser root = this.root;
+        long s = (root == this) ? state : reconcileState();
+        int p = (int)(s >>> PHASE_SHIFT);
+        if (phase < 0)
+            return phase;
+        if (p == phase) {
+            QNode node = new QNode(this, phase, true, false, 0L);
+            p = root.internalAwaitAdvance(phase, node);
+            if (node.wasInterrupted)
+                throw new InterruptedException();
+        }
+        return p;
+    }
+
+    /**
+     * Awaits the phase of this phaser to advance from the given phase
+     * value or the given timeout to elapse, throwing {@code
+     * InterruptedException} if interrupted while waiting, or
+     * returning immediately if the current phase is not equal to the
+     * given phase value or this phaser is terminated.
+     *
+     * @param phase an arrival phase number, or negative value if
+     * terminated; this argument is normally the value returned by a
+     * previous call to {@code arrive} or {@code arriveAndDeregister}.
+     * @param timeout how long to wait before giving up, in units of
+     *        {@code unit}
+     * @param unit a {@code TimeUnit} determining how to interpret the
+     *        {@code timeout} parameter
+     * @return the next arrival phase number, or the argument if it is
+     * negative, or the (negative) {@linkplain #getPhase() current phase}
+     * if terminated
+     * @throws InterruptedException if thread interrupted while waiting
+     * @throws TimeoutException if timed out while waiting
+     */
+    public int awaitAdvanceInterruptibly(int phase,
+                                         long timeout, TimeUnit unit)
+        throws InterruptedException, TimeoutException {
+        long nanos = unit.toNanos(timeout);
+        final Phaser root = this.root;
+        long s = (root == this) ? state : reconcileState();
+        int p = (int)(s >>> PHASE_SHIFT);
+        if (phase < 0)
+            return phase;
+        if (p == phase) {
+            QNode node = new QNode(this, phase, true, true, nanos);
+            p = root.internalAwaitAdvance(phase, node);
+            if (node.wasInterrupted)
+                throw new InterruptedException();
+            else if (p == phase)
+                throw new TimeoutException();
+        }
+        return p;
+    }
+
+    /**
+     * Forces this phaser to enter termination state.  Counts of
+     * registered parties are unaffected.  If this phaser is a member
+     * of a tiered set of phasers, then all of the phasers in the set
+     * are terminated.  If this phaser is already terminated, this
+     * method has no effect.  This method may be useful for
+     * coordinating recovery after one or more tasks encounter
+     * unexpected exceptions.
+     */
+    public void forceTermination() {
+        // Only need to change root state
+        final Phaser root = this.root;
+        long s;
+        while ((s = root.state) >= 0) {
+            if (UNSAFE.compareAndSwapLong(root, stateOffset,
+                                          s, s | TERMINATION_BIT)) {
+                // signal all threads
+                releaseWaiters(0); // Waiters on evenQ
+                releaseWaiters(1); // Waiters on oddQ
+                return;
+            }
+        }
+    }
+
+    /**
+     * Returns the current phase number. The maximum phase number is
+     * {@code Integer.MAX_VALUE}, after which it restarts at
+     * zero. Upon termination, the phase number is negative,
+     * in which case the prevailing phase prior to termination
+     * may be obtained via {@code getPhase() + Integer.MIN_VALUE}.
+     *
+     * @return the phase number, or a negative value if terminated
+     */
+    public final int getPhase() {
+        return (int)(root.state >>> PHASE_SHIFT);
+    }
+
+    /**
+     * Returns the number of parties registered at this phaser.
+     *
+     * @return the number of parties
+     */
+    public int getRegisteredParties() {
+        return partiesOf(state);
+    }
+
+    /**
+     * Returns the number of registered parties that have arrived at
+     * the current phase of this phaser. If this phaser has terminated,
+     * the returned value is meaningless and arbitrary.
+     *
+     * @return the number of arrived parties
+     */
+    public int getArrivedParties() {
+        return arrivedOf(reconcileState());
+    }
+
+    /**
+     * Returns the number of registered parties that have not yet
+     * arrived at the current phase of this phaser. If this phaser has
+     * terminated, the returned value is meaningless and arbitrary.
+     *
+     * @return the number of unarrived parties
+     */
+    public int getUnarrivedParties() {
+        return unarrivedOf(reconcileState());
+    }
+
+    /**
+     * Returns the parent of this phaser, or {@code null} if none.
+     *
+     * @return the parent of this phaser, or {@code null} if none
+     */
+    public Phaser getParent() {
+        return parent;
+    }
+
+    /**
+     * Returns the root ancestor of this phaser, which is the same as
+     * this phaser if it has no parent.
+     *
+     * @return the root ancestor of this phaser
+     */
+    public Phaser getRoot() {
+        return root;
+    }
+
+    /**
+     * Returns {@code true} if this phaser has been terminated.
+     *
+     * @return {@code true} if this phaser has been terminated
+     */
+    public boolean isTerminated() {
+        return root.state < 0L;
+    }
+
+    /**
+     * Overridable method to perform an action upon impending phase
+     * advance, and to control termination. This method is invoked
+     * upon arrival of the party advancing this phaser (when all other
+     * waiting parties are dormant).  If this method returns {@code
+     * true}, this phaser will be set to a final termination state
+     * upon advance, and subsequent calls to {@link #isTerminated}
+     * will return true. Any (unchecked) Exception or Error thrown by
+     * an invocation of this method is propagated to the party
+     * attempting to advance this phaser, in which case no advance
+     * occurs.
+     *
+     * <p>The arguments to this method provide the state of the phaser
+     * prevailing for the current transition.  The effects of invoking
+     * arrival, registration, and waiting methods on this phaser from
+     * within {@code onAdvance} are unspecified and should not be
+     * relied on.
+     *
+     * <p>If this phaser is a member of a tiered set of phasers, then
+     * {@code onAdvance} is invoked only for its root phaser on each
+     * advance.
+     *
+     * <p>To support the most common use cases, the default
+     * implementation of this method returns {@code true} when the
+     * number of registered parties has become zero as the result of a
+     * party invoking {@code arriveAndDeregister}.  You can disable
+     * this behavior, thus enabling continuation upon future
+     * registrations, by overriding this method to always return
+     * {@code false}:
+     *
+     * <pre> {@code
+     * Phaser phaser = new Phaser() {
+     *   protected boolean onAdvance(int phase, int parties) { return false; }
+     * }}</pre>
+     *
+     * @param phase the current phase number on entry to this method,
+     * before this phaser is advanced
+     * @param registeredParties the current number of registered parties
+     * @return {@code true} if this phaser should terminate
+     */
+    protected boolean onAdvance(int phase, int registeredParties) {
+        return registeredParties == 0;
+    }
+
+    /**
+     * Returns a string identifying this phaser, as well as its
+     * state.  The state, in brackets, includes the String {@code
+     * "phase = "} followed by the phase number, {@code "parties = "}
+     * followed by the number of registered parties, and {@code
+     * "arrived = "} followed by the number of arrived parties.
+     *
+     * @return a string identifying this phaser, as well as its state
+     */
+    public String toString() {
+        return stateToString(reconcileState());
+    }
+
+    /**
+     * Implementation of toString and string-based error messages
+     */
+    private String stateToString(long s) {
+        return super.toString() +
+            "[phase = " + phaseOf(s) +
+            " parties = " + partiesOf(s) +
+            " arrived = " + arrivedOf(s) + "]";
+    }
+
+    // Waiting mechanics
+
+    /**
+     * Removes and signals threads from queue for phase.
+     */
+    private void releaseWaiters(int phase) {
+        QNode q;   // first element of queue
+        Thread t;  // its thread
+        AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;
+        while ((q = head.get()) != null &&
+               q.phase != (int)(root.state >>> PHASE_SHIFT)) {
+            if (head.compareAndSet(q, q.next) &&
+                (t = q.thread) != null) {
+                q.thread = null;
+                LockSupport.unpark(t);
+            }
+        }
+    }
+
+    /**
+     * Variant of releaseWaiters that additionally tries to remove any
+     * nodes no longer waiting for advance due to timeout or
+     * interrupt. Currently, nodes are removed only if they are at
+     * head of queue, which suffices to reduce memory footprint in
+     * most usages.
+     *
+     * @return current phase on exit
+     */
+    private int abortWait(int phase) {
+        AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;
+        for (;;) {
+            Thread t;
+            QNode q = head.get();
+            int p = (int)(root.state >>> PHASE_SHIFT);
+            if (q == null || ((t = q.thread) != null && q.phase == p))
+                return p;
+            if (head.compareAndSet(q, q.next) && t != null) {
+                q.thread = null;
+                LockSupport.unpark(t);
+            }
+        }
+    }
+
+    /** The number of CPUs, for spin control */
+    private static final int NCPU = Runtime.getRuntime().availableProcessors();
+
+    /**
+     * The number of times to spin before blocking while waiting for
+     * advance, per arrival while waiting. On multiprocessors, fully
+     * blocking and waking up a large number of threads all at once is
+     * usually a very slow process, so we use rechargeable spins to
+     * avoid it when threads regularly arrive: When a thread in
+     * internalAwaitAdvance notices another arrival before blocking,
+     * and there appear to be enough CPUs available, it spins
+     * SPINS_PER_ARRIVAL more times before blocking. The value trades
+     * off good-citizenship vs big unnecessary slowdowns.
+     */
+    static final int SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8;
+
+    /**
+     * Possibly blocks and waits for phase to advance unless aborted.
+     * Call only on root phaser.
+     *
+     * @param phase current phase
+     * @param node if non-null, the wait node to track interrupt and timeout;
+     * if null, denotes noninterruptible wait
+     * @return current phase
+     */
+    private int internalAwaitAdvance(int phase, QNode node) {
+        // assert root == this;
+        releaseWaiters(phase-1);          // ensure old queue clean
+        boolean queued = false;           // true when node is enqueued
+        int lastUnarrived = 0;            // to increase spins upon change
+        int spins = SPINS_PER_ARRIVAL;
+        long s;
+        int p;
+        while ((p = (int)((s = state) >>> PHASE_SHIFT)) == phase) {
+            if (node == null) {           // spinning in noninterruptible mode
+                int unarrived = (int)s & UNARRIVED_MASK;
+                if (unarrived != lastUnarrived &&
+                    (lastUnarrived = unarrived) < NCPU)
+                    spins += SPINS_PER_ARRIVAL;
+                boolean interrupted = Thread.interrupted();
+                if (interrupted || --spins < 0) { // need node to record intr
+                    node = new QNode(this, phase, false, false, 0L);
+                    node.wasInterrupted = interrupted;
+                }
+            }
+            else if (node.isReleasable()) // done or aborted
+                break;
+            else if (!queued) {           // push onto queue
+                AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;
+                QNode q = node.next = head.get();
+                if ((q == null || q.phase == phase) &&
+                    (int)(state >>> PHASE_SHIFT) == phase) // avoid stale enq
+                    queued = head.compareAndSet(q, node);
+            }
+            else {
+                try {
+                    ForkJoinPool.managedBlock(node);
+                } catch (InterruptedException ie) {
+                    node.wasInterrupted = true;
+                }
+            }
+        }
+
+        if (node != null) {
+            if (node.thread != null)
+                node.thread = null;       // avoid need for unpark()
+            if (node.wasInterrupted && !node.interruptible)
+                Thread.currentThread().interrupt();
+            if (p == phase && (p = (int)(state >>> PHASE_SHIFT)) == phase)
+                return abortWait(phase); // possibly clean up on abort
+        }
+        releaseWaiters(phase);
+        return p;
+    }
+
+    /**
+     * Wait nodes for Treiber stack representing wait queue
+     */
+    static final class QNode implements ForkJoinPool.ManagedBlocker {
+        final Phaser phaser;
+        final int phase;
+        final boolean interruptible;
+        final boolean timed;
+        boolean wasInterrupted;
+        long nanos;
+        long lastTime;
+        volatile Thread thread; // nulled to cancel wait
+        QNode next;
+
+        QNode(Phaser phaser, int phase, boolean interruptible,
+              boolean timed, long nanos) {
+            this.phaser = phaser;
+            this.phase = phase;
+            this.interruptible = interruptible;
+            this.nanos = nanos;
+            this.timed = timed;
+            this.lastTime = timed ? System.nanoTime() : 0L;
+            thread = Thread.currentThread();
+        }
+
+        public boolean isReleasable() {
+            if (thread == null)
+                return true;
+            if (phaser.getPhase() != phase) {
+                thread = null;
+                return true;
+            }
+            if (Thread.interrupted())
+                wasInterrupted = true;
+            if (wasInterrupted && interruptible) {
+                thread = null;
+                return true;
+            }
+            if (timed) {
+                if (nanos > 0L) {
+                    long now = System.nanoTime();
+                    nanos -= now - lastTime;
+                    lastTime = now;
+                }
+                if (nanos <= 0L) {
+                    thread = null;
+                    return true;
+                }
+            }
+            return false;
+        }
+
+        public boolean block() {
+            if (isReleasable())
+                return true;
+            else if (!timed)
+                LockSupport.park(this);
+            else if (nanos > 0)
+                LockSupport.parkNanos(this, nanos);
+            return isReleasable();
+        }
+    }
+
+    // Unsafe mechanics
+
+    private static final sun.misc.Unsafe UNSAFE;
+    private static final long stateOffset;
+    static {
+        try {
+            UNSAFE = getUnsafe();
+            Class<?> k = Phaser.class;
+            stateOffset = UNSAFE.objectFieldOffset
+                (k.getDeclaredField("state"));
+        } 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 tryReflectionInstead) {}
+        try {
+            return java.security.AccessController.doPrivileged
+            (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
+                public sun.misc.Unsafe run() throws Exception {
+                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
+                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
+                        f.setAccessible(true);
+                        Object x = f.get(null);
+                        if (k.isInstance(x))
+                            return k.cast(x);
+                    }
+                    throw new NoSuchFieldError("the Unsafe");
+                }});
+        } catch (java.security.PrivilegedActionException e) {
+            throw new RuntimeException("Could not initialize intrinsics",
+                                       e.getCause());
+        }
+    }
+}
diff --git a/src/main/java/jsr166y/RecursiveAction.java b/src/main/java/jsr166y/RecursiveAction.java
new file mode 100644
index 0000000..071ae28
--- /dev/null
+++ b/src/main/java/jsr166y/RecursiveAction.java
@@ -0,0 +1,164 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+/**
+ * A recursive resultless {@link ForkJoinTask}.  This class
+ * establishes conventions to parameterize resultless actions as
+ * {@code Void} {@code ForkJoinTask}s. Because {@code null} is the
+ * only valid value of type {@code Void}, methods such as {@code join}
+ * always return {@code null} upon completion.
+ *
+ * <p><b>Sample Usages.</b> Here is a simple but complete ForkJoin
+ * sort that sorts a given {@code long[]} array:
+ *
+ *  <pre> {@code
+ * static class SortTask extends RecursiveAction {
+ *   final long[] array; final int lo, hi;
+ *   SortTask(long[] array, int lo, int hi) {
+ *     this.array = array; this.lo = lo; this.hi = hi;
+ *   }
+ *   SortTask(long[] array) { this(array, 0, array.length); }
+ *   protected void compute() {
+ *     if (hi - lo < THRESHOLD)
+ *       sortSequentially(lo, hi);
+ *     else {
+ *       int mid = (lo + hi) >>> 1;
+ *       invokeAll(new SortTask(array, lo, mid),
+ *                 new SortTask(array, mid, hi));
+ *       merge(lo, mid, hi);
+ *     }
+ *   }
+ *   // implementation details follow:
+ *   static final int THRESHOLD = 1000;
+ *   void sortSequentially(int lo, int hi) {
+ *     Arrays.sort(array, lo, hi);
+ *   }
+ *   void merge(int lo, int mid, int hi) {
+ *     long[] buf = Arrays.copyOfRange(array, lo, mid);
+ *     for (int i = 0, j = lo, k = mid; i < buf.length; j++)
+ *       array[j] = (k == hi || buf[i] < array[k]) ?
+ *         buf[i++] : array[k++];
+ *   }
+ * }}</pre>
+ *
+ * You could then sort {@code anArray} by creating {@code new
+ * SortTask(anArray)} and invoking it in a ForkJoinPool.  As a more
+ * concrete simple example, the following task increments each element
+ * of an array:
+ *  <pre> {@code
+ * class IncrementTask extends RecursiveAction {
+ *   final long[] array; final int lo, hi;
+ *   IncrementTask(long[] array, int lo, int hi) {
+ *     this.array = array; this.lo = lo; this.hi = hi;
+ *   }
+ *   protected void compute() {
+ *     if (hi - lo < THRESHOLD) {
+ *       for (int i = lo; i < hi; ++i)
+ *         array[i]++;
+ *     }
+ *     else {
+ *       int mid = (lo + hi) >>> 1;
+ *       invokeAll(new IncrementTask(array, lo, mid),
+ *                 new IncrementTask(array, mid, hi));
+ *     }
+ *   }
+ * }}</pre>
+ *
+ * <p>The following example illustrates some refinements and idioms
+ * that may lead to better performance: RecursiveActions need not be
+ * fully recursive, so long as they maintain the basic
+ * divide-and-conquer approach. Here is a class that sums the squares
+ * of each element of a double array, by subdividing out only the
+ * right-hand-sides of repeated divisions by two, and keeping track of
+ * them with a chain of {@code next} references. It uses a dynamic
+ * threshold based on method {@code getSurplusQueuedTaskCount}, but
+ * counterbalances potential excess partitioning by directly
+ * performing leaf actions on unstolen tasks rather than further
+ * subdividing.
+ *
+ *  <pre> {@code
+ * double sumOfSquares(ForkJoinPool pool, double[] array) {
+ *   int n = array.length;
+ *   Applyer a = new Applyer(array, 0, n, null);
+ *   pool.invoke(a);
+ *   return a.result;
+ * }
+ *
+ * class Applyer extends RecursiveAction {
+ *   final double[] array;
+ *   final int lo, hi;
+ *   double result;
+ *   Applyer next; // keeps track of right-hand-side tasks
+ *   Applyer(double[] array, int lo, int hi, Applyer next) {
+ *     this.array = array; this.lo = lo; this.hi = hi;
+ *     this.next = next;
+ *   }
+ *
+ *   double atLeaf(int l, int h) {
+ *     double sum = 0;
+ *     for (int i = l; i < h; ++i) // perform leftmost base step
+ *       sum += array[i] * array[i];
+ *     return sum;
+ *   }
+ *
+ *   protected void compute() {
+ *     int l = lo;
+ *     int h = hi;
+ *     Applyer right = null;
+ *     while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) {
+ *       int mid = (l + h) >>> 1;
+ *       right = new Applyer(array, mid, h, right);
+ *       right.fork();
+ *       h = mid;
+ *     }
+ *     double sum = atLeaf(l, h);
+ *     while (right != null) {
+ *       if (right.tryUnfork()) // directly calculate if not stolen
+ *         sum += right.atLeaf(right.lo, right.hi);
+ *       else {
+ *         right.join();
+ *         sum += right.result;
+ *       }
+ *       right = right.next;
+ *     }
+ *     result = sum;
+ *   }
+ * }}</pre>
+ *
+ * @since 1.7
+ * @author Doug Lea
+ */
+public abstract class RecursiveAction extends ForkJoinTask<Void> {
+    private static final long serialVersionUID = 5232453952276485070L;
+
+    /**
+     * The main computation performed by this task.
+     */
+    protected abstract void compute();
+
+    /**
+     * Always returns {@code null}.
+     *
+     * @return {@code null} always
+     */
+    public final Void getRawResult() { return null; }
+
+    /**
+     * Requires null completion value.
+     */
+    protected final void setRawResult(Void mustBeNull) { }
+
+    /**
+     * Implements execution conventions for RecursiveActions.
+     */
+    protected final boolean exec() {
+        compute();
+        return true;
+    }
+
+}
diff --git a/src/main/java/jsr166y/RecursiveTask.java b/src/main/java/jsr166y/RecursiveTask.java
new file mode 100644
index 0000000..0192966
--- /dev/null
+++ b/src/main/java/jsr166y/RecursiveTask.java
@@ -0,0 +1,68 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+/**
+ * A recursive result-bearing {@link ForkJoinTask}.
+ *
+ * <p>For a classic example, here is a task computing Fibonacci numbers:
+ *
+ *  <pre> {@code
+ * class Fibonacci extends RecursiveTask<Integer> {
+ *   final int n;
+ *   Fibonacci(int n) { this.n = n; }
+ *   protected Integer compute() {
+ *     if (n <= 1)
+ *       return n;
+ *     Fibonacci f1 = new Fibonacci(n - 1);
+ *     f1.fork();
+ *     Fibonacci f2 = new Fibonacci(n - 2);
+ *     return f2.compute() + f1.join();
+ *   }
+ * }}</pre>
+ *
+ * However, besides being a dumb way to compute Fibonacci functions
+ * (there is a simple fast linear algorithm that you'd use in
+ * practice), this is likely to perform poorly because the smallest
+ * subtasks are too small to be worthwhile splitting up. Instead, as
+ * is the case for nearly all fork/join applications, you'd pick some
+ * minimum granularity size (for example 10 here) for which you always
+ * sequentially solve rather than subdividing.
+ *
+ * @since 1.7
+ * @author Doug Lea
+ */
+public abstract class RecursiveTask<V> extends ForkJoinTask<V> {
+    private static final long serialVersionUID = 5232453952276485270L;
+
+    /**
+     * The result of the computation.
+     */
+    V result;
+
+    /**
+     * The main computation performed by this task.
+     */
+    protected abstract V compute();
+
+    public final V getRawResult() {
+        return result;
+    }
+
+    protected final void setRawResult(V value) {
+        result = value;
+    }
+
+    /**
+     * Implements execution conventions for RecursiveTask.
+     */
+    protected final boolean exec() {
+        result = compute();
+        return true;
+    }
+
+}
diff --git a/src/main/java/jsr166y/ThreadLocalRandom.java b/src/main/java/jsr166y/ThreadLocalRandom.java
new file mode 100644
index 0000000..fe6fc8e
--- /dev/null
+++ b/src/main/java/jsr166y/ThreadLocalRandom.java
@@ -0,0 +1,197 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+
+import java.util.Random;
+
+/**
+ * A random number generator isolated to the current thread.  Like the
+ * global {@link java.util.Random} generator used by the {@link
+ * java.lang.Math} class, a {@code ThreadLocalRandom} is initialized
+ * with an internally generated seed that may not otherwise be
+ * modified. When applicable, use of {@code ThreadLocalRandom} rather
+ * than shared {@code Random} objects in concurrent programs will
+ * typically encounter much less overhead and contention.  Use of
+ * {@code ThreadLocalRandom} is particularly appropriate when multiple
+ * tasks (for example, each a {@link ForkJoinTask}) use random numbers
+ * in parallel in thread pools.
+ *
+ * <p>Usages of this class should typically be of the form:
+ * {@code ThreadLocalRandom.current().nextX(...)} (where
+ * {@code X} is {@code Int}, {@code Long}, etc).
+ * When all usages are of this form, it is never possible to
+ * accidently share a {@code ThreadLocalRandom} across multiple threads.
+ *
+ * <p>This class also provides additional commonly used bounded random
+ * generation methods.
+ *
+ * @since 1.7
+ * @author Doug Lea
+ */
+public class ThreadLocalRandom extends Random {
+    // same constants as Random, but must be redeclared because private
+    private static final long multiplier = 0x5DEECE66DL;
+    private static final long addend = 0xBL;
+    private static final long mask = (1L << 48) - 1;
+
+    /**
+     * The random seed. We can't use super.seed.
+     */
+    private long rnd;
+
+    /**
+     * Initialization flag to permit calls to setSeed to succeed only
+     * while executing the Random constructor.  We can't allow others
+     * since it would cause setting seed in one part of a program to
+     * unintentionally impact other usages by the thread.
+     */
+    boolean initialized;
+
+    // Padding to help avoid memory contention among seed updates in
+    // different TLRs in the common case that they are located near
+    // each other.
+    private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
+
+    /**
+     * The actual ThreadLocal
+     */
+    private static final ThreadLocal<ThreadLocalRandom> localRandom =
+        new ThreadLocal<ThreadLocalRandom>() {
+            protected ThreadLocalRandom initialValue() {
+                return new ThreadLocalRandom();
+            }
+    };
+
+
+    /**
+     * Constructor called only by localRandom.initialValue.
+     */
+    ThreadLocalRandom() {
+        super();
+        initialized = true;
+    }
+
+    /**
+     * Returns the current thread's {@code ThreadLocalRandom}.
+     *
+     * @return the current thread's {@code ThreadLocalRandom}
+     */
+    public static ThreadLocalRandom current() {
+        return localRandom.get();
+    }
+
+    /**
+     * Throws {@code UnsupportedOperationException}.  Setting seeds in
+     * this generator is not supported.
+     *
+     * @throws UnsupportedOperationException always
+     */
+    public void setSeed(long seed) {
+        if (initialized)
+            throw new UnsupportedOperationException();
+        rnd = (seed ^ multiplier) & mask;
+    }
+
+    protected int next(int bits) {
+        rnd = (rnd * multiplier + addend) & mask;
+        return (int) (rnd >>> (48-bits));
+    }
+
+    /**
+     * Returns a pseudorandom, uniformly distributed value between the
+     * given least value (inclusive) and bound (exclusive).
+     *
+     * @param least the least value returned
+     * @param bound the upper bound (exclusive)
+     * @return the next value
+     * @throws IllegalArgumentException if least greater than or equal
+     * to bound
+     */
+    public int nextInt(int least, int bound) {
+        if (least >= bound)
+            throw new IllegalArgumentException();
+        return nextInt(bound - least) + least;
+    }
+
+    /**
+     * Returns a pseudorandom, uniformly distributed value
+     * between 0 (inclusive) and the specified value (exclusive).
+     *
+     * @param n the bound on the random number to be returned.  Must be
+     *        positive.
+     * @return the next value
+     * @throws IllegalArgumentException if n is not positive
+     */
+    public long nextLong(long n) {
+        if (n <= 0)
+            throw new IllegalArgumentException("n must be positive");
+        // Divide n by two until small enough for nextInt. On each
+        // iteration (at most 31 of them but usually much less),
+        // randomly choose both whether to include high bit in result
+        // (offset) and whether to continue with the lower vs upper
+        // half (which makes a difference only if odd).
+        long offset = 0;
+        while (n >= Integer.MAX_VALUE) {
+            int bits = next(2);
+            long half = n >>> 1;
+            long nextn = ((bits & 2) == 0) ? half : n - half;
+            if ((bits & 1) == 0)
+                offset += n - nextn;
+            n = nextn;
+        }
+        return offset + nextInt((int) n);
+    }
+
+    /**
+     * Returns a pseudorandom, uniformly distributed value between the
+     * given least value (inclusive) and bound (exclusive).
+     *
+     * @param least the least value returned
+     * @param bound the upper bound (exclusive)
+     * @return the next value
+     * @throws IllegalArgumentException if least greater than or equal
+     * to bound
+     */
+    public long nextLong(long least, long bound) {
+        if (least >= bound)
+            throw new IllegalArgumentException();
+        return nextLong(bound - least) + least;
+    }
+
+    /**
+     * Returns a pseudorandom, uniformly distributed {@code double} value
+     * between 0 (inclusive) and the specified value (exclusive).
+     *
+     * @param n the bound on the random number to be returned.  Must be
+     *        positive.
+     * @return the next value
+     * @throws IllegalArgumentException if n is not positive
+     */
+    public double nextDouble(double n) {
+        if (n <= 0)
+            throw new IllegalArgumentException("n must be positive");
+        return nextDouble() * n;
+    }
+
+    /**
+     * Returns a pseudorandom, uniformly distributed value between the
+     * given least value (inclusive) and bound (exclusive).
+     *
+     * @param least the least value returned
+     * @param bound the upper bound (exclusive)
+     * @return the next value
+     * @throws IllegalArgumentException if least greater than or equal
+     * to bound
+     */
+    public double nextDouble(double least, double bound) {
+        if (least >= bound)
+            throw new IllegalArgumentException();
+        return nextDouble() * (bound - least) + least;
+    }
+
+    private static final long serialVersionUID = -5851777807851030925L;
+}
diff --git a/src/main/java/jsr166y/TransferQueue.java b/src/main/java/jsr166y/TransferQueue.java
new file mode 100644
index 0000000..3aeb69e
--- /dev/null
+++ b/src/main/java/jsr166y/TransferQueue.java
@@ -0,0 +1,133 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166y;
+import java.util.concurrent.*;
+
+/**
+ * A {@link BlockingQueue} in which producers may wait for consumers
+ * to receive elements.  A {@code TransferQueue} may be useful for
+ * example in message passing applications in which producers
+ * sometimes (using method {@link #transfer}) await receipt of
+ * elements by consumers invoking {@code take} or {@code poll}, while
+ * at other times enqueue elements (via method {@code put}) without
+ * waiting for receipt.
+ * {@linkplain #tryTransfer(Object) Non-blocking} and
+ * {@linkplain #tryTransfer(Object,long,TimeUnit) time-out} versions of
+ * {@code tryTransfer} are also available.
+ * A {@code TransferQueue} may also be queried, via {@link
+ * #hasWaitingConsumer}, whether there are any threads waiting for
+ * items, which is a converse analogy to a {@code peek} operation.
+ *
+ * <p>Like other blocking queues, a {@code TransferQueue} may be
+ * capacity bounded.  If so, an attempted transfer operation may
+ * initially block waiting for available space, and/or subsequently
+ * block waiting for reception by a consumer.  Note that in a queue
+ * with zero capacity, such as {@link SynchronousQueue}, {@code put}
+ * and {@code transfer} are effectively synonymous.
+ *
+ * <p>This interface is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.7
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public interface TransferQueue<E> extends BlockingQueue<E> {
+    /**
+     * Transfers the element to a waiting consumer immediately, if possible.
+     *
+     * <p>More precisely, transfers the specified element immediately
+     * if there exists a consumer already waiting to receive it (in
+     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
+     * otherwise returning {@code false} without enqueuing the element.
+     *
+     * @param e the element to transfer
+     * @return {@code true} if the element was transferred, else
+     *         {@code false}
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this queue
+     */
+    boolean tryTransfer(E e);
+
+    /**
+     * Transfers the element to a consumer, waiting if necessary to do so.
+     *
+     * <p>More precisely, transfers the specified element immediately
+     * if there exists a consumer already waiting to receive it (in
+     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
+     * else waits until the element is received by a consumer.
+     *
+     * @param e the element to transfer
+     * @throws InterruptedException if interrupted while waiting,
+     *         in which case the element is not left enqueued
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this queue
+     */
+    void transfer(E e) throws InterruptedException;
+
+    /**
+     * Transfers the element to a consumer if it is possible to do so
+     * before the timeout elapses.
+     *
+     * <p>More precisely, transfers the specified element immediately
+     * if there exists a consumer already waiting to receive it (in
+     * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
+     * else waits until the element is received by a consumer,
+     * returning {@code false} if the specified wait time elapses
+     * before the element can be transferred.
+     *
+     * @param e the element to transfer
+     * @param timeout how long to wait before giving up, in units of
+     *        {@code unit}
+     * @param unit a {@code TimeUnit} determining how to interpret the
+     *        {@code timeout} parameter
+     * @return {@code true} if successful, or {@code false} if
+     *         the specified waiting time elapses before completion,
+     *         in which case the element is not left enqueued
+     * @throws InterruptedException if interrupted while waiting,
+     *         in which case the element is not left enqueued
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this queue
+     */
+    boolean tryTransfer(E e, long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Returns {@code true} if there is at least one consumer waiting
+     * to receive an element via {@link #take} or
+     * timed {@link #poll(long,TimeUnit) poll}.
+     * The return value represents a momentary state of affairs.
+     *
+     * @return {@code true} if there is at least one waiting consumer
+     */
+    boolean hasWaitingConsumer();
+
+    /**
+     * Returns an estimate of the number of consumers waiting to
+     * receive elements via {@link #take} or timed
+     * {@link #poll(long,TimeUnit) poll}.  The return value is an
+     * approximation of a momentary state of affairs, that may be
+     * inaccurate if consumers have completed or given up waiting.
+     * The value may be useful for monitoring and heuristics, but
+     * not for synchronization control.  Implementations of this
+     * method are likely to be noticeably slower than those for
+     * {@link #hasWaitingConsumer}.
+     *
+     * @return the number of consumers waiting to receive elements
+     */
+    int getWaitingConsumerCount();
+}
diff --git a/src/main/java/jsr166y/package-info.java b/src/main/java/jsr166y/package-info.java
new file mode 100644
index 0000000..9802803
--- /dev/null
+++ b/src/main/java/jsr166y/package-info.java
@@ -0,0 +1,28 @@
+/*
+ * 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/
+ */
+
+
+/**
+ * Preview versions of classes targeted for Java 7.  Includes a
+ * fine-grained parallel computation framework: ForkJoinTasks and
+ * their related support classes provide a very efficient basis for
+ * obtaining platform-independent parallel speed-ups of
+ * computation-intensive operations.  They are not a full substitute
+ * for the kinds of arbitrary processing supported by Executors or
+ * Threads. However, when applicable, they typically provide
+ * significantly greater performance on multiprocessor platforms.
+ *
+ * <p>Candidates for fork/join processing mainly include those that
+ * can be expressed using parallel divide-and-conquer techniques: To
+ * solve a problem, break it in two (or more) parts, and then solve
+ * those parts in parallel, continuing on in this way until the
+ * problem is too small to be broken up, so is solved directly.  The
+ * underlying <em>work-stealing</em> framework makes subtasks
+ * available to other threads (normally one per CPU), that help
+ * complete the tasks.  In general, the most efficient ForkJoinTasks
+ * are those that directly implement this algorithmic design pattern.
+ */
+package jsr166y;