8056249: Improve CompletableFuture resource usage

Reviewed-by: psandoz, chegar, martin

src/share/classes/java/util/concurrent/CompletableFuture.java

@@ -50,7 +50,6 @@ import java.util.concurrent.TimeoutException;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.CompletionException;
 import java.util.concurrent.CompletionStage;
-import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.locks.LockSupport;
 
 /**
@@ -77,9 +76,9 @@ import java.util.concurrent.locks.LockSupport;
  * <li>All <em>async</em> methods without an explicit Executor
  * argument are performed using the {@link ForkJoinPool#commonPool()}
  * (unless it does not support a parallelism level of at least two, in
- * which case, a new Thread is used). To simplify monitoring,
- * debugging, and tracking, all generated asynchronous tasks are
- * instances of the marker interface {@link
+ * which case, a new Thread is created to run each task).  To simplify
+ * monitoring, debugging, and tracking, all generated asynchronous
+ * tasks are instances of the marker interface {@link
  * AsynchronousCompletionTask}. </li>
  *
  * <li>All CompletionStage methods are implemented independently of
@@ -113,301 +112,273 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
     /*
      * Overview:
      *
-     * 1. Non-nullness of field result (set via CAS) indicates done.
-     * An AltResult is used to box null as a result, as well as to
-     * hold exceptions.  Using a single field makes completion fast
-     * and simple to detect and trigger, at the expense of a lot of
-     * encoding and decoding that infiltrates many methods. One minor
-     * simplification relies on the (static) NIL (to box null results)
-     * being the only AltResult with a null exception field, so we
-     * don't usually need explicit comparisons with NIL. The CF
-     * exception propagation mechanics surrounding decoding rely on
-     * unchecked casts of decoded results really being unchecked,
-     * where user type errors are caught at point of use, as is
-     * currently the case in Java. These are highlighted by using
-     * SuppressWarnings-annotated temporaries.
+     * A CompletableFuture may have dependent completion actions,
+     * collected in a linked stack. It atomically completes by CASing
+     * a result field, and then pops off and runs those actions. This
+     * applies across normal vs exceptional outcomes, sync vs async
+     * actions, binary triggers, and various forms of completions.
      *
-     * 2. Waiters are held in a Treiber stack similar to the one used
-     * in FutureTask, Phaser, and SynchronousQueue. See their
-     * internal documentation for algorithmic details.
+     * Non-nullness of field result (set via CAS) indicates done.  An
+     * AltResult is used to box null as a result, as well as to hold
+     * exceptions.  Using a single field makes completion simple to
+     * detect and trigger.  Encoding and decoding is straightforward
+     * but adds to the sprawl of trapping and associating exceptions
+     * with targets.  Minor simplifications rely on (static) NIL (to
+     * box null results) being the only AltResult with a null
+     * exception field, so we don't usually need explicit comparisons.
+     * Even though some of the generics casts are unchecked (see
+     * SuppressWarnings annotations), they are placed to be
+     * appropriate even if checked.
      *
-     * 3. Completions are also kept in a list/stack, and pulled off
-     * and run when completion is triggered. (We could even use the
-     * same stack as for waiters, but would give up the potential
-     * parallelism obtained because woken waiters help release/run
-     * others -- see method postComplete).  Because post-processing
-     * may race with direct calls, class Completion opportunistically
-     * extends AtomicInteger so callers can claim the action via
-     * compareAndSet(0, 1).  The Completion.run methods are all
-     * written a boringly similar uniform way (that sometimes includes
-     * unnecessary-looking checks, kept to maintain uniformity).
-     * There are enough dimensions upon which they differ that
-     * attempts to factor commonalities while maintaining efficiency
-     * require more lines of code than they would save.
+     * Dependent actions are represented by Completion objects linked
+     * as Treiber stacks headed by field "stack". There are Completion
+     * classes for each kind of action, grouped into single-input
+     * (UniCompletion), two-input (BiCompletion), projected
+     * (BiCompletions using either (not both) of two inputs), shared
+     * (CoCompletion, used by the second of two sources), zero-input
+     * source actions, and Signallers that unblock waiters. Class
+     * Completion extends ForkJoinTask to enable async execution
+     * (adding no space overhead because we exploit its "tag" methods
+     * to maintain claims). It is also declared as Runnable to allow
+     * usage with arbitrary executors.
      *
-     * 4. The exported then/and/or methods do support a bit of
-     * factoring (see doThenApply etc). They must cope with the
-     * intrinsic races surrounding addition of a dependent action
-     * versus performing the action directly because the task is
-     * already complete.  For example, a CF may not be complete upon
-     * entry, so a dependent completion is added, but by the time it
-     * is added, the target CF is complete, so must be directly
-     * executed. This is all done while avoiding unnecessary object
-     * construction in safe-bypass cases.
+     * Support for each kind of CompletionStage relies on a separate
+     * class, along with two CompletableFuture methods:
+     *
+     * * A Completion class with name X corresponding to function,
+     *   prefaced with "Uni", "Bi", or "Or". Each class contains
+     *   fields for source(s), actions, and dependent. They are
+     *   boringly similar, differing from others only with respect to
+     *   underlying functional forms. We do this so that users don't
+     *   encounter layers of adaptors in common usages. We also
+     *   include "Relay" classes/methods that don't correspond to user
+     *   methods; they copy results from one stage to another.
+     *
+     * * Boolean CompletableFuture method x(...) (for example
+     *   uniApply) takes all of the arguments needed to check that an
+     *   action is triggerable, and then either runs the action or
+     *   arranges its async execution by executing its Completion
+     *   argument, if present. The method returns true if known to be
+     *   complete.
+     *
+     * * Completion method tryFire(int mode) invokes the associated x
+     *   method with its held arguments, and on success cleans up.
+     *   The mode argument allows tryFire to be called twice (SYNC,
+     *   then ASYNC); the first to screen and trap exceptions while
+     *   arranging to execute, and the second when called from a
+     *   task. (A few classes are not used async so take slightly
+     *   different forms.)  The claim() callback suppresses function
+     *   invocation if already claimed by another thread.
+     *
+     * * CompletableFuture method xStage(...) is called from a public
+     *   stage method of CompletableFuture x. It screens user
+     *   arguments and invokes and/or creates the stage object.  If
+     *   not async and x is already complete, the action is run
+     *   immediately.  Otherwise a Completion c is created, pushed to
+     *   x's stack (unless done), and started or triggered via
+     *   c.tryFire.  This also covers races possible if x completes
+     *   while pushing.  Classes with two inputs (for example BiApply)
+     *   deal with races across both while pushing actions.  The
+     *   second completion is a CoCompletion pointing to the first,
+     *   shared so that at most one performs the action.  The
+     *   multiple-arity methods allOf and anyOf do this pairwise to
+     *   form trees of completions.
+     *
+     * Note that the generic type parameters of methods vary according
+     * to whether "this" is a source, dependent, or completion.
+     *
+     * Method postComplete is called upon completion unless the target
+     * is guaranteed not to be observable (i.e., not yet returned or
+     * linked). Multiple threads can call postComplete, which
+     * atomically pops each dependent action, and tries to trigger it
+     * via method tryFire, in NESTED mode.  Triggering can propagate
+     * recursively, so NESTED mode returns its completed dependent (if
+     * one exists) for further processing by its caller (see method
+     * postFire).
+     *
+     * Blocking methods get() and join() rely on Signaller Completions
+     * that wake up waiting threads.  The mechanics are similar to
+     * Treiber stack wait-nodes used in FutureTask, Phaser, and
+     * SynchronousQueue. See their internal documentation for
+     * algorithmic details.
+     *
+     * Without precautions, CompletableFutures would be prone to
+     * garbage accumulation as chains of Completions build up, each
+     * pointing back to its sources. So we null out fields as soon as
+     * possible (see especially method Completion.detach). The
+     * screening checks needed anyway harmlessly ignore null arguments
+     * that may have been obtained during races with threads nulling
+     * out fields.  We also try to unlink fired Completions from
+     * stacks that might never be popped (see method postFire).
+     * Completion fields need not be declared as final or volatile
+     * because they are only visible to other threads upon safe
+     * publication.
      */
 
-    // preliminaries
+    volatile Object result;       // Either the result or boxed AltResult
+    volatile Completion stack;    // Top of Treiber stack of dependent actions
+
+    final boolean internalComplete(Object r) { // CAS from null to r
+        return UNSAFE.compareAndSwapObject(this, RESULT, null, r);
+    }
+
+    final boolean casStack(Completion cmp, Completion val) {
+        return UNSAFE.compareAndSwapObject(this, STACK, cmp, val);
+    }
+
+    /** Returns true if successfully pushed c onto stack. */
+    final boolean tryPushStack(Completion c) {
+        Completion h = stack;
+        lazySetNext(c, h);
+        return UNSAFE.compareAndSwapObject(this, STACK, h, c);
+    }
+
+    /** Unconditionally pushes c onto stack, retrying if necessary. */
+    final void pushStack(Completion c) {
+        do {} while (!tryPushStack(c));
+    }
+
+    /* ------------- Encoding and decoding outcomes -------------- */
 
-    static final class AltResult {
-        final Throwable ex; // null only for NIL
-        AltResult(Throwable ex) { this.ex = ex; }
+    static final class AltResult { // See above
+        final Throwable ex;        // null only for NIL
+        AltResult(Throwable x) { this.ex = x; }
     }
 
+    /** The encoding of the null value. */
     static final AltResult NIL = new AltResult(null);
 
-    // Fields
+    /** Completes with the null value, unless already completed. */
+    final boolean completeNull() {
+        return UNSAFE.compareAndSwapObject(this, RESULT, null,
+                                           NIL);
+    }
 
-    volatile Object result;    // Either the result or boxed AltResult
-    volatile WaitNode waiters; // Treiber stack of threads blocked on get()
-    volatile CompletionNode completions; // list (Treiber stack) of completions
+    /** Returns the encoding of the given non-exceptional value. */
+    final Object encodeValue(T t) {
+        return (t == null) ? NIL : t;
+    }
 
-    // Basic utilities for triggering and processing completions
+    /** Completes with a non-exceptional result, unless already completed. */
+    final boolean completeValue(T t) {
+        return UNSAFE.compareAndSwapObject(this, RESULT, null,
+                                           (t == null) ? NIL : t);
+    }
 
     /**
-     * Removes and signals all waiting threads and runs all completions.
+     * Returns the encoding of the given (non-null) exception as a
+     * wrapped CompletionException unless it is one already.
      */
-    final void postComplete() {
-        WaitNode q; Thread t;
-        while ((q = waiters) != null) {
-            if (UNSAFE.compareAndSwapObject(this, WAITERS, q, q.next) &&
-                (t = q.thread) != null) {
-                q.thread = null;
-                LockSupport.unpark(t);
-            }
-        }
+    static AltResult encodeThrowable(Throwable x) {
+        return new AltResult((x instanceof CompletionException) ? x :
+                             new CompletionException(x));
+    }
 
-        CompletionNode h; Completion c;
-        while ((h = completions) != null) {
-            if (UNSAFE.compareAndSwapObject(this, COMPLETIONS, h, h.next) &&
-                (c = h.completion) != null)
-                c.run();
-        }
+    /** Completes with an exceptional result, unless already completed. */
+    final boolean completeThrowable(Throwable x) {
+        return UNSAFE.compareAndSwapObject(this, RESULT, null,
+                                           encodeThrowable(x));
     }
 
     /**
-     * Triggers completion with the encoding of the given arguments:
-     * if the exception is non-null, encodes it as a wrapped
-     * CompletionException unless it is one already.  Otherwise uses
-     * the given result, boxed as NIL if null.
+     * Returns the encoding of the given (non-null) exception as a
+     * wrapped CompletionException unless it is one already.  May
+     * return the given Object r (which must have been the result of a
+     * source future) if it is equivalent, i.e. if this is a simple
+     * relay of an existing CompletionException.
      */
-    final void internalComplete(T v, Throwable ex) {
-        if (result == null)
-            UNSAFE.compareAndSwapObject
-                (this, RESULT, null,
-                 (ex == null) ? (v == null) ? NIL : v :
-                 new AltResult((ex instanceof CompletionException) ? ex :
-                               new CompletionException(ex)));
-        postComplete(); // help out even if not triggered
+    static Object encodeThrowable(Throwable x, Object r) {
+        if (!(x instanceof CompletionException))
+            x = new CompletionException(x);
+        else if (r instanceof AltResult && x == ((AltResult)r).ex)
+            return r;
+        return new AltResult(x);
     }
 
     /**
-     * If triggered, helps release and/or process completions.
+     * Completes with the given (non-null) exceptional result as a
+     * wrapped CompletionException unless it is one already, unless
+     * already completed.  May complete with the given Object r
+     * (which must have been the result of a source future) if it is
+     * equivalent, i.e. if this is a simple propagation of an
+     * existing CompletionException.
      */
-    final void helpPostComplete() {
-        if (result != null)
-            postComplete();
+    final boolean completeThrowable(Throwable x, Object r) {
+        return UNSAFE.compareAndSwapObject(this, RESULT, null,
+                                           encodeThrowable(x, r));
     }
 
-    /* ------------- waiting for completions -------------- */
-
-    /** Number of processors, for spin control */
-    static final int NCPU = Runtime.getRuntime().availableProcessors();
-
     /**
-     * Heuristic spin value for waitingGet() before blocking on
-     * multiprocessors
+     * Returns the encoding of the given arguments: if the exception
+     * is non-null, encodes as AltResult.  Otherwise uses the given
+     * value, boxed as NIL if null.
      */
-    static final int SPINS = (NCPU > 1) ? 1 << 8 : 0;
+    Object encodeOutcome(T t, Throwable x) {
+        return (x == null) ? (t == null) ? NIL : t : encodeThrowable(x);
+    }
 
     /**
-     * Linked nodes to record waiting threads in a Treiber stack.  See
-     * other classes such as Phaser and SynchronousQueue for more
-     * detailed explanation. This class implements ManagedBlocker to
-     * avoid starvation when blocking actions pile up in
-     * ForkJoinPools.
+     * Returns the encoding of a copied outcome; if exceptional,
+     * rewraps as a CompletionException, else returns argument.
      */
-    static final class WaitNode implements ForkJoinPool.ManagedBlocker {
-        long nanos;          // wait time if timed
-        final long deadline; // non-zero if timed
-        volatile int interruptControl; // > 0: interruptible, < 0: interrupted
-        volatile Thread thread;
-        volatile WaitNode next;
-        WaitNode(boolean interruptible, long nanos, long deadline) {
-            this.thread = Thread.currentThread();
-            this.interruptControl = interruptible ? 1 : 0;
-            this.nanos = nanos;
-            this.deadline = deadline;
-        }
-        public boolean isReleasable() {
-            if (thread == null)
-                return true;
-            if (Thread.interrupted()) {
-                int i = interruptControl;
-                interruptControl = -1;
-                if (i > 0)
-                    return true;
-            }
-            if (deadline != 0L &&
-                (nanos <= 0L || (nanos = deadline - System.nanoTime()) <= 0L)) {
-                thread = null;
-                return true;
-            }
-            return false;
-        }
-        public boolean block() {
-            if (isReleasable())
-                return true;
-            else if (deadline == 0L)
-                LockSupport.park(this);
-            else if (nanos > 0L)
-                LockSupport.parkNanos(this, nanos);
-            return isReleasable();
-        }
+    static Object encodeRelay(Object r) {
+        Throwable x;
+        return (((r instanceof AltResult) &&
+                 (x = ((AltResult)r).ex) != null &&
+                 !(x instanceof CompletionException)) ?
+                new AltResult(new CompletionException(x)) : r);
     }
 
     /**
-     * Returns raw result after waiting, or null if interruptible and
-     * interrupted.
+     * Completes with r or a copy of r, unless already completed.
+     * If exceptional, r is first coerced to a CompletionException.
      */
-    private Object waitingGet(boolean interruptible) {
-        WaitNode q = null;
-        boolean queued = false;
-        int spins = SPINS;
-        for (Object r;;) {
-            if ((r = result) != null) {
-                if (q != null) { // suppress unpark
-                    q.thread = null;
-                    if (q.interruptControl < 0) {
-                        if (interruptible) {
-                            removeWaiter(q);
-                            return null;
-                        }
-                        Thread.currentThread().interrupt();
-                    }
-                }
-                postComplete(); // help release others
-                return r;
-            }
-            else if (spins > 0) {
-                int rnd = ThreadLocalRandom.nextSecondarySeed();
-                if (rnd == 0)
-                    rnd = ThreadLocalRandom.current().nextInt();
-                if (rnd >= 0)
-                    --spins;
-            }
-            else if (q == null)
-                q = new WaitNode(interruptible, 0L, 0L);
-            else if (!queued)
-                queued = UNSAFE.compareAndSwapObject(this, WAITERS,
-                                                     q.next = waiters, q);
-            else if (interruptible && q.interruptControl < 0) {
-                removeWaiter(q);
-                return null;
-            }
-            else if (q.thread != null && result == null) {
-                try {
-                    ForkJoinPool.managedBlock(q);
-                } catch (InterruptedException ex) {
-                    q.interruptControl = -1;
-                }
-            }
-        }
+    final boolean completeRelay(Object r) {
+        return UNSAFE.compareAndSwapObject(this, RESULT, null,
+                                           encodeRelay(r));
     }
 
     /**
-     * Awaits completion or aborts on interrupt or timeout.
-     *
-     * @param nanos time to wait
-     * @return raw result
+     * Reports result using Future.get conventions.
      */
-    private Object timedAwaitDone(long nanos)
-        throws InterruptedException, TimeoutException {
-        WaitNode q = null;
-        boolean queued = false;
-        for (Object r;;) {
-            if ((r = result) != null) {
-                if (q != null) {
-                    q.thread = null;
-                    if (q.interruptControl < 0) {
-                        removeWaiter(q);
-                        throw new InterruptedException();
-                    }
-                }
-                postComplete();
-                return r;
-            }
-            else if (q == null) {
-                if (nanos <= 0L)
-                    throw new TimeoutException();
-                long d = System.nanoTime() + nanos;
-                q = new WaitNode(true, nanos, d == 0L ? 1L : d); // avoid 0
-            }
-            else if (!queued)
-                queued = UNSAFE.compareAndSwapObject(this, WAITERS,
-                                                     q.next = waiters, q);
-            else if (q.interruptControl < 0) {
-                removeWaiter(q);
-                throw new InterruptedException();
-            }
-            else if (q.nanos <= 0L) {
-                if (result == null) {
-                    removeWaiter(q);
-                    throw new TimeoutException();
-                }
-            }
-            else if (q.thread != null && result == null) {
-                try {
-                    ForkJoinPool.managedBlock(q);
-                } catch (InterruptedException ex) {
-                    q.interruptControl = -1;
-                }
-            }
+    private static <T> T reportGet(Object r)
+        throws InterruptedException, ExecutionException {
+        if (r == null) // by convention below, null means interrupted
+            throw new InterruptedException();
+        if (r instanceof AltResult) {
+            Throwable x, cause;
+            if ((x = ((AltResult)r).ex) == null)
+                return null;
+            if (x instanceof CancellationException)
+                throw (CancellationException)x;
+            if ((x instanceof CompletionException) &&
+                (cause = x.getCause()) != null)
+                x = cause;
+            throw new ExecutionException(x);
         }
+        @SuppressWarnings("unchecked") T t = (T) r;
+        return t;
     }
 
     /**
-     * Tries to unlink a timed-out or interrupted wait node to avoid
-     * accumulating garbage.  Internal nodes are simply unspliced
-     * without CAS since it is harmless if they are traversed anyway
-     * by releasers.  To avoid effects of unsplicing from already
-     * removed nodes, the list is retraversed in case of an apparent
-     * race.  This is slow when there are a lot of nodes, but we don't
-     * expect lists to be long enough to outweigh higher-overhead
-     * schemes.
+     * Decodes outcome to return result or throw unchecked exception.
      */
-    private void removeWaiter(WaitNode node) {
-        if (node != null) {
-            node.thread = null;
-            retry:
-            for (;;) {          // restart on removeWaiter race
-                for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
-                    s = q.next;
-                    if (q.thread != null)
-                        pred = q;
-                    else if (pred != null) {
-                        pred.next = s;
-                        if (pred.thread == null) // check for race
-                            continue retry;
-                    }
-                    else if (!UNSAFE.compareAndSwapObject(this, WAITERS, q, s))
-                        continue retry;
-                }
-                break;
-            }
+    private static <T> T reportJoin(Object r) {
+        if (r instanceof AltResult) {
+            Throwable x;
+            if ((x = ((AltResult)r).ex) == null)
+                return null;
+            if (x instanceof CancellationException)
+                throw (CancellationException)x;
+            if (x instanceof CompletionException)
+                throw (CompletionException)x;
+            throw new CompletionException(x);
         }
+        @SuppressWarnings("unchecked") T t = (T) r;
+        return t;
     }
 
-    /* ------------- Async tasks -------------- */
+    /* ------------- Async task preliminaries -------------- */
 
     /**
      * A marker interface identifying asynchronous tasks produced by
@@ -419,1693 +390,1394 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
     public static interface AsynchronousCompletionTask {
     }
 
-    /** Base class can act as either FJ or plain Runnable */
+    private static final boolean useCommonPool =
+        (ForkJoinPool.getCommonPoolParallelism() > 1);
+
+    /**
+     * Default executor -- ForkJoinPool.commonPool() unless it cannot
+     * support parallelism.
+     */
+    private static final Executor asyncPool = useCommonPool ?
+        ForkJoinPool.commonPool() : new ThreadPerTaskExecutor();
+
+    /** Fallback if ForkJoinPool.commonPool() cannot support parallelism */
+    static final class ThreadPerTaskExecutor implements Executor {
+        public void execute(Runnable r) { new Thread(r).start(); }
+    }
+
+    /**
+     * Null-checks user executor argument, and translates uses of
+     * commonPool to asyncPool in case parallelism disabled.
+     */
+    static Executor screenExecutor(Executor e) {
+        if (!useCommonPool && e == ForkJoinPool.commonPool())
+            return asyncPool;
+        if (e == null) throw new NullPointerException();
+        return e;
+    }
+
+    // Modes for Completion.tryFire. Signedness matters.
+    static final int SYNC   =  0;
+    static final int ASYNC  =  1;
+    static final int NESTED = -1;
+
+    /* ------------- Base Completion classes and operations -------------- */
+
     @SuppressWarnings("serial")
-    abstract static class Async extends ForkJoinTask<Void>
+    abstract static class Completion extends ForkJoinTask<Void>
         implements Runnable, AsynchronousCompletionTask {
-        public final Void getRawResult() { return null; }
-        public final void setRawResult(Void v) { }
-        public final void run() { exec(); }
+        volatile Completion next;      // Treiber stack link
+
+        /**
+         * Performs completion action if triggered, returning a
+         * dependent that may need propagation, if one exists.
+         *
+         * @param mode SYNC, ASYNC, or NESTED
+         */
+        abstract CompletableFuture<?> tryFire(int mode);
+
+        /** Returns true if possibly still triggerable. Used by cleanStack. */
+        abstract boolean isLive();
+
+        public final void run()                { tryFire(ASYNC); }
+        public final boolean exec()            { tryFire(ASYNC); return true; }
+        public final Void getRawResult()       { return null; }
+        public final void setRawResult(Void v) {}
+    }
+
+    static void lazySetNext(Completion c, Completion next) {
+        UNSAFE.putOrderedObject(c, NEXT, next);
     }
 
     /**
-     * Starts the given async task using the given executor, unless
-     * the executor is ForkJoinPool.commonPool and it has been
-     * disabled, in which case starts a new thread.
+     * Pops and tries to trigger all reachable dependents.  Call only
+     * when known to be done.
      */
-    static void execAsync(Executor e, Async r) {
-        if (e == ForkJoinPool.commonPool() &&
-            ForkJoinPool.getCommonPoolParallelism() <= 1)
-            new Thread(r).start();
-        else
-            e.execute(r);
-    }
-
-    static final class AsyncRun extends Async {
-        final Runnable fn;
-        final CompletableFuture<Void> dst;
-        AsyncRun(Runnable fn, CompletableFuture<Void> dst) {
-            this.fn = fn; this.dst = dst;
-        }
-        public final boolean exec() {
-            CompletableFuture<Void> d; Throwable ex;
-            if ((d = this.dst) != null && d.result == null) {
-                try {
-                    fn.run();
-                    ex = null;
-                } catch (Throwable rex) {
-                    ex = rex;
+    final void postComplete() {
+        /*
+         * On each step, variable f holds current dependents to pop
+         * and run.  It is extended along only one path at a time,
+         * pushing others to avoid unbounded recursion.
+         */
+        CompletableFuture<?> f = this; Completion h;
+        while ((h = f.stack) != null ||
+               (f != this && (h = (f = this).stack) != null)) {
+            CompletableFuture<?> d; Completion t;
+            if (f.casStack(h, t = h.next)) {
+                if (t != null) {
+                    if (f != this) {
+                        pushStack(h);
+                        continue;
+                    }
+                    h.next = null;    // detach
                 }
-                d.internalComplete(null, ex);
+                f = (d = h.tryFire(NESTED)) == null ? this : d;
             }
-            return true;
         }
-        private static final long serialVersionUID = 5232453952276885070L;
     }
 
-    static final class AsyncSupply<U> extends Async {
-        final Supplier<U> fn;
-        final CompletableFuture<U> dst;
-        AsyncSupply(Supplier<U> fn, CompletableFuture<U> dst) {
-            this.fn = fn; this.dst = dst;
-        }
-        public final boolean exec() {
-            CompletableFuture<U> d; U u; Throwable ex;
-            if ((d = this.dst) != null && d.result == null) {
-                try {
-                    u = fn.get();
-                    ex = null;
-                } catch (Throwable rex) {
-                    ex = rex;
-                    u = null;
+    /** Traverses stack and unlinks dead Completions. */
+    final void cleanStack() {
+        for (Completion p = null, q = stack; q != null;) {
+            Completion s = q.next;
+            if (q.isLive()) {
+                p = q;
+                q = s;
+            }
+            else if (p == null) {
+                casStack(q, s);
+                q = stack;
+            }
+            else {
+                p.next = s;
+                if (p.isLive())
+                    q = s;
+                else {
+                    p = null;  // restart
+                    q = stack;
                 }
-                d.internalComplete(u, ex);
             }
-            return true;
         }
-        private static final long serialVersionUID = 5232453952276885070L;
     }
 
-    static final class AsyncApply<T,U> extends Async {
-        final T arg;
-        final Function<? super T,? extends U> fn;
-        final CompletableFuture<U> dst;
-        AsyncApply(T arg, Function<? super T,? extends U> fn,
-                   CompletableFuture<U> dst) {
-            this.arg = arg; this.fn = fn; this.dst = dst;
-        }
-        public final boolean exec() {
-            CompletableFuture<U> d; U u; Throwable ex;
-            if ((d = this.dst) != null && d.result == null) {
-                try {
-                    u = fn.apply(arg);
-                    ex = null;
-                } catch (Throwable rex) {
-                    ex = rex;
-                    u = null;
-                }
-                d.internalComplete(u, ex);
+    /* ------------- One-input Completions -------------- */
+
+    /** A Completion with a source, dependent, and executor. */
+    @SuppressWarnings("serial")
+    abstract static class UniCompletion<T,V> extends Completion {
+        Executor executor;                 // executor to use (null if none)
+        CompletableFuture<V> dep;          // the dependent to complete
+        CompletableFuture<T> src;          // source for action
+
+        UniCompletion(Executor executor, CompletableFuture<V> dep,
+                      CompletableFuture<T> src) {
+            this.executor = executor; this.dep = dep; this.src = src;
+        }
+
+        /**
+         * Returns true if action can be run. Call only when known to
+         * be triggerable. Uses FJ tag bit to ensure that only one
+         * thread claims ownership.  If async, starts as task -- a
+         * later call to tryFire will run action.
+         */
+        final boolean claim() {
+            Executor e = executor;
+            if (compareAndSetForkJoinTaskTag((short)0, (short)1)) {
+                if (e == null)
+                    return true;
+                executor = null; // disable
+                e.execute(this);
             }
-            return true;
+            return false;
         }
-        private static final long serialVersionUID = 5232453952276885070L;
+
+        final boolean isLive() { return dep != null; }
     }
 
-    static final class AsyncCombine<T,U,V> extends Async {
-        final T arg1;
-        final U arg2;
-        final BiFunction<? super T,? super U,? extends V> fn;
-        final CompletableFuture<V> dst;
-        AsyncCombine(T arg1, U arg2,
-                     BiFunction<? super T,? super U,? extends V> fn,
-                     CompletableFuture<V> dst) {
-            this.arg1 = arg1; this.arg2 = arg2; this.fn = fn; this.dst = dst;
+    /** Pushes the given completion (if it exists) unless done. */
+    final void push(UniCompletion<?,?> c) {
+        if (c != null) {
+            while (result == null && !tryPushStack(c))
+                lazySetNext(c, null); // clear on failure
         }
-        public final boolean exec() {
-            CompletableFuture<V> d; V v; Throwable ex;
-            if ((d = this.dst) != null && d.result == null) {
-                try {
-                    v = fn.apply(arg1, arg2);
-                    ex = null;
-                } catch (Throwable rex) {
-                    ex = rex;
-                    v = null;
-                }
-                d.internalComplete(v, ex);
-            }
-            return true;
-        }
-        private static final long serialVersionUID = 5232453952276885070L;
     }
 
-    static final class AsyncAccept<T> extends Async {
-        final T arg;
-        final Consumer<? super T> fn;
-        final CompletableFuture<?> dst;
-        AsyncAccept(T arg, Consumer<? super T> fn,
-                    CompletableFuture<?> dst) {
-            this.arg = arg; this.fn = fn; this.dst = dst;
+    /**
+     * Post-processing by dependent after successful UniCompletion
+     * tryFire.  Tries to clean stack of source a, and then either runs
+     * postComplete or returns this to caller, depending on mode.
+     */
+    final CompletableFuture<T> postFire(CompletableFuture<?> a, int mode) {
+        if (a != null && a.stack != null) {
+            if (mode < 0 || a.result == null)
+                a.cleanStack();
+            else
+                a.postComplete();
         }
-        public final boolean exec() {
-            CompletableFuture<?> d; Throwable ex;
-            if ((d = this.dst) != null && d.result == null) {
-                try {
-                    fn.accept(arg);
-                    ex = null;
-                } catch (Throwable rex) {
-                    ex = rex;
-                }
-                d.internalComplete(null, ex);
-            }
-            return true;
+        if (result != null && stack != null) {
+            if (mode < 0)
+                return this;
+            else
+                postComplete();
         }
-        private static final long serialVersionUID = 5232453952276885070L;
+        return null;
     }
 
-    static final class AsyncAcceptBoth<T,U> extends Async {
-        final T arg1;
-        final U arg2;
-        final BiConsumer<? super T,? super U> fn;
-        final CompletableFuture<?> dst;
-        AsyncAcceptBoth(T arg1, U arg2,
-                        BiConsumer<? super T,? super U> fn,
-                        CompletableFuture<?> dst) {
-            this.arg1 = arg1; this.arg2 = arg2; this.fn = fn; this.dst = dst;
+    @SuppressWarnings("serial")
+    static final class UniApply<T,V> extends UniCompletion<T,V> {
+        Function<? super T,? extends V> fn;
+        UniApply(Executor executor, CompletableFuture<V> dep,
+                 CompletableFuture<T> src,
+                 Function<? super T,? extends V> fn) {
+            super(executor, dep, src); this.fn = fn;
+        }
+        final CompletableFuture<V> tryFire(int mode) {
+            CompletableFuture<V> d; CompletableFuture<T> a;
+            if ((d = dep) == null ||
+                !d.uniApply(a = src, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; fn = null;
+            return d.postFire(a, mode);
         }
-        public final boolean exec() {
-            CompletableFuture<?> d; Throwable ex;
-            if ((d = this.dst) != null && d.result == null) {
-                try {
-                    fn.accept(arg1, arg2);
-                    ex = null;
-                } catch (Throwable rex) {
-                    ex = rex;
+    }
+
+    final <S> boolean uniApply(CompletableFuture<S> a,
+                               Function<? super S,? extends T> f,
+                               UniApply<S,T> c) {
+        Object r; Throwable x;
+        if (a == null || (r = a.result) == null || f == null)
+            return false;
+        tryComplete: if (result == null) {
+            if (r instanceof AltResult) {
+                if ((x = ((AltResult)r).ex) != null) {
+                    completeThrowable(x, r);
+                    break tryComplete;
                 }
-                d.internalComplete(null, ex);
+                r = null;
+            }
+            try {
+                if (c != null && !c.claim())
+                    return false;
+                @SuppressWarnings("unchecked") S s = (S) r;
+                completeValue(f.apply(s));
+            } catch (Throwable ex) {
+                completeThrowable(ex);
             }
-            return true;
         }
-        private static final long serialVersionUID = 5232453952276885070L;
+        return true;
     }
 
-    static final class AsyncCompose<T,U> extends Async {
-        final T arg;
-        final Function<? super T, ? extends CompletionStage<U>> fn;
-        final CompletableFuture<U> dst;
-        AsyncCompose(T arg,
-                     Function<? super T, ? extends CompletionStage<U>> fn,
-                     CompletableFuture<U> dst) {
-            this.arg = arg; this.fn = fn; this.dst = dst;
+    private <V> CompletableFuture<V> uniApplyStage(
+        Executor e, Function<? super T,? extends V> f) {
+        if (f == null) throw new NullPointerException();
+        CompletableFuture<V> d =  new CompletableFuture<V>();
+        if (e != null || !d.uniApply(this, f, null)) {
+            UniApply<T,V> c = new UniApply<T,V>(e, d, this, f);
+            push(c);
+            c.tryFire(SYNC);
         }
-        public final boolean exec() {
-            CompletableFuture<U> d, fr; U u; Throwable ex;
-            if ((d = this.dst) != null && d.result == null) {
-                try {
-                    CompletionStage<U> cs = fn.apply(arg);
-                    fr = (cs == null) ? null : cs.toCompletableFuture();
-                    ex = (fr == null) ? new NullPointerException() : null;
-                } catch (Throwable rex) {
-                    ex = rex;
-                    fr = null;
-                }
-                if (ex != null)
-                    u = null;
-                else {
-                    Object r = fr.result;
-                    if (r == null)
-                        r = fr.waitingGet(false);
-                    if (r instanceof AltResult) {
-                        ex = ((AltResult)r).ex;
-                        u = null;
-                    }
-                    else {
-                        @SuppressWarnings("unchecked") U ur = (U) r;
-                        u = ur;
-                    }
+        return d;
+    }
+
+    @SuppressWarnings("serial")
+    static final class UniAccept<T> extends UniCompletion<T,Void> {
+        Consumer<? super T> fn;
+        UniAccept(Executor executor, CompletableFuture<Void> dep,
+                  CompletableFuture<T> src, Consumer<? super T> fn) {
+            super(executor, dep, src); this.fn = fn;
+        }
+        final CompletableFuture<Void> tryFire(int mode) {
+            CompletableFuture<Void> d; CompletableFuture<T> a;
+            if ((d = dep) == null ||
+                !d.uniAccept(a = src, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; fn = null;
+            return d.postFire(a, mode);
+        }
+    }
+
+    final <S> boolean uniAccept(CompletableFuture<S> a,
+                                Consumer<? super S> f, UniAccept<S> c) {
+        Object r; Throwable x;
+        if (a == null || (r = a.result) == null || f == null)
+            return false;
+        tryComplete: if (result == null) {
+            if (r instanceof AltResult) {
+                if ((x = ((AltResult)r).ex) != null) {
+                    completeThrowable(x, r);
+                    break tryComplete;
                 }
-                d.internalComplete(u, ex);
+                r = null;
             }
-            return true;
+            try {
+                if (c != null && !c.claim())
+                    return false;
+                @SuppressWarnings("unchecked") S s = (S) r;
+                f.accept(s);
+                completeNull();
+            } catch (Throwable ex) {
+                completeThrowable(ex);
+            }
+        }
+        return true;
+    }
+
+    private CompletableFuture<Void> uniAcceptStage(Executor e,
+                                                   Consumer<? super T> f) {
+        if (f == null) throw new NullPointerException();
+        CompletableFuture<Void> d = new CompletableFuture<Void>();
+        if (e != null || !d.uniAccept(this, f, null)) {
+            UniAccept<T> c = new UniAccept<T>(e, d, this, f);
+            push(c);
+            c.tryFire(SYNC);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
+        return d;
     }
 
-    static final class AsyncWhenComplete<T> extends Async {
-        final T arg1;
-        final Throwable arg2;
-        final BiConsumer<? super T,? super Throwable> fn;
-        final CompletableFuture<T> dst;
-        AsyncWhenComplete(T arg1, Throwable arg2,
-                          BiConsumer<? super T,? super Throwable> fn,
-                          CompletableFuture<T> dst) {
-            this.arg1 = arg1; this.arg2 = arg2; this.fn = fn; this.dst = dst;
+    @SuppressWarnings("serial")
+    static final class UniRun<T> extends UniCompletion<T,Void> {
+        Runnable fn;
+        UniRun(Executor executor, CompletableFuture<Void> dep,
+               CompletableFuture<T> src, Runnable fn) {
+            super(executor, dep, src); this.fn = fn;
+        }
+        final CompletableFuture<Void> tryFire(int mode) {
+            CompletableFuture<Void> d; CompletableFuture<T> a;
+            if ((d = dep) == null ||
+                !d.uniRun(a = src, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; fn = null;
+            return d.postFire(a, mode);
         }
-        public final boolean exec() {
-            CompletableFuture<T> d;
-            if ((d = this.dst) != null && d.result == null) {
-                Throwable ex = arg2;
+    }
+
+    final boolean uniRun(CompletableFuture<?> a, Runnable f, UniRun<?> c) {
+        Object r; Throwable x;
+        if (a == null || (r = a.result) == null || f == null)
+            return false;
+        if (result == null) {
+            if (r instanceof AltResult && (x = ((AltResult)r).ex) != null)
+                completeThrowable(x, r);
+            else
                 try {
-                    fn.accept(arg1, ex);
-                } catch (Throwable rex) {
-                    if (ex == null)
-                        ex = rex;
+                    if (c != null && !c.claim())
+                        return false;
+                    f.run();
+                    completeNull();
+                } catch (Throwable ex) {
+                    completeThrowable(ex);
                 }
-                d.internalComplete(arg1, ex);
-            }
-            return true;
         }
-        private static final long serialVersionUID = 5232453952276885070L;
+        return true;
     }
 
-    /* ------------- Completions -------------- */
-
-    /**
-     * Simple linked list nodes to record completions, used in
-     * basically the same way as WaitNodes. (We separate nodes from
-     * the Completions themselves mainly because for the And and Or
-     * methods, the same Completion object resides in two lists.)
-     */
-    static final class CompletionNode {
-        final Completion completion;
-        volatile CompletionNode next;
-        CompletionNode(Completion completion) { this.completion = completion; }
+    private CompletableFuture<Void> uniRunStage(Executor e, Runnable f) {
+        if (f == null) throw new NullPointerException();
+        CompletableFuture<Void> d = new CompletableFuture<Void>();
+        if (e != null || !d.uniRun(this, f, null)) {
+            UniRun<T> c = new UniRun<T>(e, d, this, f);
+            push(c);
+            c.tryFire(SYNC);
+        }
+        return d;
     }
 
-    // Opportunistically subclass AtomicInteger to use compareAndSet to claim.
     @SuppressWarnings("serial")
-    abstract static class Completion extends AtomicInteger implements Runnable {
-    }
-
-    static final class ThenApply<T,U> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final Function<? super T,? extends U> fn;
-        final CompletableFuture<U> dst;
-        final Executor executor;
-        ThenApply(CompletableFuture<? extends T> src,
-                  Function<? super T,? extends U> fn,
-                  CompletableFuture<U> dst,
-                  Executor executor) {
-            this.src = src; this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final Function<? super T,? extends U> fn;
-            final CompletableFuture<U> dst;
-            Object r; T t; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
-                }
-                Executor e = executor;
-                U u = null;
-                if (ex == null) {
-                    try {
-                        if (e != null)
-                            execAsync(e, new AsyncApply<T,U>(t, fn, dst));
-                        else
-                            u = fn.apply(t);
-                    } catch (Throwable rex) {
-                        ex = rex;
-                    }
-                }
-                if (e == null || ex != null)
-                    dst.internalComplete(u, ex);
-            }
+    static final class UniWhenComplete<T> extends UniCompletion<T,T> {
+        BiConsumer<? super T, ? super Throwable> fn;
+        UniWhenComplete(Executor executor, CompletableFuture<T> dep,
+                        CompletableFuture<T> src,
+                        BiConsumer<? super T, ? super Throwable> fn) {
+            super(executor, dep, src); this.fn = fn;
+        }
+        final CompletableFuture<T> tryFire(int mode) {
+            CompletableFuture<T> d; CompletableFuture<T> a;
+            if ((d = dep) == null ||
+                !d.uniWhenComplete(a = src, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; fn = null;
+            return d.postFire(a, mode);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class ThenAccept<T> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final Consumer<? super T> fn;
-        final CompletableFuture<?> dst;
-        final Executor executor;
-        ThenAccept(CompletableFuture<? extends T> src,
-                   Consumer<? super T> fn,
-                   CompletableFuture<?> dst,
-                   Executor executor) {
-            this.src = src; this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final Consumer<? super T> fn;
-            final CompletableFuture<?> dst;
-            Object r; T t; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                compareAndSet(0, 1)) {
+    }
+
+    final boolean uniWhenComplete(CompletableFuture<T> a,
+                                  BiConsumer<? super T,? super Throwable> f,
+                                  UniWhenComplete<T> c) {
+        Object r; T t; Throwable x = null;
+        if (a == null || (r = a.result) == null || f == null)
+            return false;
+        if (result == null) {
+            try {
+                if (c != null && !c.claim())
+                    return false;
                 if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
+                    x = ((AltResult)r).ex;
                     t = null;
-                }
-                else {
-                    ex = null;
+                } else {
                     @SuppressWarnings("unchecked") T tr = (T) r;
                     t = tr;
                 }
-                Executor e = executor;
-                if (ex == null) {
-                    try {
-                        if (e != null)
-                            execAsync(e, new AsyncAccept<T>(t, fn, dst));
-                        else
-                            fn.accept(t);
-                    } catch (Throwable rex) {
-                        ex = rex;
-                    }
+                f.accept(t, x);
+                if (x == null) {
+                    internalComplete(r);
+                    return true;
                 }
-                if (e == null || ex != null)
-                    dst.internalComplete(null, ex);
+            } catch (Throwable ex) {
+                if (x == null)
+                    x = ex;
             }
+            completeThrowable(x, r);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class ThenRun extends Completion {
-        final CompletableFuture<?> src;
-        final Runnable fn;
-        final CompletableFuture<Void> dst;
-        final Executor executor;
-        ThenRun(CompletableFuture<?> src,
-                Runnable fn,
-                CompletableFuture<Void> dst,
-                Executor executor) {
-            this.src = src; this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<?> a;
-            final Runnable fn;
-            final CompletableFuture<Void> dst;
-            Object r; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult)
-                    ex = ((AltResult)r).ex;
-                else
-                    ex = null;
-                Executor e = executor;
-                if (ex == null) {
-                    try {
-                        if (e != null)
-                            execAsync(e, new AsyncRun(fn, dst));
-                        else
-                            fn.run();
-                    } catch (Throwable rex) {
-                        ex = rex;
-                    }
-                }
-                if (e == null || ex != null)
-                    dst.internalComplete(null, ex);
-            }
+        return true;
+    }
+
+    private CompletableFuture<T> uniWhenCompleteStage(
+        Executor e, BiConsumer<? super T, ? super Throwable> f) {
+        if (f == null) throw new NullPointerException();
+        CompletableFuture<T> d = new CompletableFuture<T>();
+        if (e != null || !d.uniWhenComplete(this, f, null)) {
+            UniWhenComplete<T> c = new UniWhenComplete<T>(e, d, this, f);
+            push(c);
+            c.tryFire(SYNC);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class ThenCombine<T,U,V> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final CompletableFuture<? extends U> snd;
-        final BiFunction<? super T,? super U,? extends V> fn;
-        final CompletableFuture<V> dst;
-        final Executor executor;
-        ThenCombine(CompletableFuture<? extends T> src,
-                    CompletableFuture<? extends U> snd,
-                    BiFunction<? super T,? super U,? extends V> fn,
-                    CompletableFuture<V> dst,
-                    Executor executor) {
-            this.src = src; this.snd = snd;
-            this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final CompletableFuture<? extends U> b;
-            final BiFunction<? super T,? super U,? extends V> fn;
-            final CompletableFuture<V> dst;
-            Object r, s; T t; U u; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                (b = this.snd) != null &&
-                (s = b.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
-                }
-                if (ex != null)
-                    u = null;
-                else if (s instanceof AltResult) {
-                    ex = ((AltResult)s).ex;
-                    u = null;
-                }
-                else {
-                    @SuppressWarnings("unchecked") U us = (U) s;
-                    u = us;
-                }
-                Executor e = executor;
-                V v = null;
-                if (ex == null) {
-                    try {
-                        if (e != null)
-                            execAsync(e, new AsyncCombine<T,U,V>(t, u, fn, dst));
-                        else
-                            v = fn.apply(t, u);
-                    } catch (Throwable rex) {
-                        ex = rex;
-                    }
-                }
-                if (e == null || ex != null)
-                    dst.internalComplete(v, ex);
-            }
+        return d;
+    }
+
+    @SuppressWarnings("serial")
+    static final class UniHandle<T,V> extends UniCompletion<T,V> {
+        BiFunction<? super T, Throwable, ? extends V> fn;
+        UniHandle(Executor executor, CompletableFuture<V> dep,
+                  CompletableFuture<T> src,
+                  BiFunction<? super T, Throwable, ? extends V> fn) {
+            super(executor, dep, src); this.fn = fn;
+        }
+        final CompletableFuture<V> tryFire(int mode) {
+            CompletableFuture<V> d; CompletableFuture<T> a;
+            if ((d = dep) == null ||
+                !d.uniHandle(a = src, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; fn = null;
+            return d.postFire(a, mode);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class ThenAcceptBoth<T,U> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final CompletableFuture<? extends U> snd;
-        final BiConsumer<? super T,? super U> fn;
-        final CompletableFuture<Void> dst;
-        final Executor executor;
-        ThenAcceptBoth(CompletableFuture<? extends T> src,
-                       CompletableFuture<? extends U> snd,
-                       BiConsumer<? super T,? super U> fn,
-                       CompletableFuture<Void> dst,
-                       Executor executor) {
-            this.src = src; this.snd = snd;
-            this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final CompletableFuture<? extends U> b;
-            final BiConsumer<? super T,? super U> fn;
-            final CompletableFuture<Void> dst;
-            Object r, s; T t; U u; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                (b = this.snd) != null &&
-                (s = b.result) != null &&
-                compareAndSet(0, 1)) {
+    }
+
+    final <S> boolean uniHandle(CompletableFuture<S> a,
+                                BiFunction<? super S, Throwable, ? extends T> f,
+                                UniHandle<S,T> c) {
+        Object r; S s; Throwable x;
+        if (a == null || (r = a.result) == null || f == null)
+            return false;
+        if (result == null) {
+            try {
+                if (c != null && !c.claim())
+                    return false;
                 if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
+                    x = ((AltResult)r).ex;
+                    s = null;
+                } else {
+                    x = null;
+                    @SuppressWarnings("unchecked") S ss = (S) r;
+                    s = ss;
                 }
-                if (ex != null)
-                    u = null;
-                else if (s instanceof AltResult) {
-                    ex = ((AltResult)s).ex;
-                    u = null;
-                }
-                else {
-                    @SuppressWarnings("unchecked") U us = (U) s;
-                    u = us;
-                }
-                Executor e = executor;
-                if (ex == null) {
-                    try {
-                        if (e != null)
-                            execAsync(e, new AsyncAcceptBoth<T,U>(t, u, fn, dst));
-                        else
-                            fn.accept(t, u);
-                    } catch (Throwable rex) {
-                        ex = rex;
-                    }
-                }
-                if (e == null || ex != null)
-                    dst.internalComplete(null, ex);
+                completeValue(f.apply(s, x));
+            } catch (Throwable ex) {
+                completeThrowable(ex);
             }
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class RunAfterBoth extends Completion {
-        final CompletableFuture<?> src;
-        final CompletableFuture<?> snd;
-        final Runnable fn;
-        final CompletableFuture<Void> dst;
-        final Executor executor;
-        RunAfterBoth(CompletableFuture<?> src,
-                     CompletableFuture<?> snd,
-                     Runnable fn,
-                     CompletableFuture<Void> dst,
-                     Executor executor) {
-            this.src = src; this.snd = snd;
-            this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<?> a;
-            final CompletableFuture<?> b;
-            final Runnable fn;
-            final CompletableFuture<Void> dst;
-            Object r, s; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                (b = this.snd) != null &&
-                (s = b.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult)
-                    ex = ((AltResult)r).ex;
-                else
-                    ex = null;
-                if (ex == null && (s instanceof AltResult))
-                    ex = ((AltResult)s).ex;
-                Executor e = executor;
-                if (ex == null) {
-                    try {
-                        if (e != null)
-                            execAsync(e, new AsyncRun(fn, dst));
-                        else
-                            fn.run();
-                    } catch (Throwable rex) {
-                        ex = rex;
-                    }
-                }
-                if (e == null || ex != null)
-                    dst.internalComplete(null, ex);
-            }
+        return true;
+    }
+
+    private <V> CompletableFuture<V> uniHandleStage(
+        Executor e, BiFunction<? super T, Throwable, ? extends V> f) {
+        if (f == null) throw new NullPointerException();
+        CompletableFuture<V> d = new CompletableFuture<V>();
+        if (e != null || !d.uniHandle(this, f, null)) {
+            UniHandle<T,V> c = new UniHandle<T,V>(e, d, this, f);
+            push(c);
+            c.tryFire(SYNC);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class AndCompletion extends Completion {
-        final CompletableFuture<?> src;
-        final CompletableFuture<?> snd;
-        final CompletableFuture<Void> dst;
-        AndCompletion(CompletableFuture<?> src,
-                      CompletableFuture<?> snd,
-                      CompletableFuture<Void> dst) {
-            this.src = src; this.snd = snd; this.dst = dst;
-        }
-        public final void run() {
-            final CompletableFuture<?> a;
-            final CompletableFuture<?> b;
-            final CompletableFuture<Void> dst;
-            Object r, s; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                (b = this.snd) != null &&
-                (s = b.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult)
-                    ex = ((AltResult)r).ex;
-                else
-                    ex = null;
-                if (ex == null && (s instanceof AltResult))
-                    ex = ((AltResult)s).ex;
-                dst.internalComplete(null, ex);
-            }
+        return d;
+    }
+
+    @SuppressWarnings("serial")
+    static final class UniExceptionally<T> extends UniCompletion<T,T> {
+        Function<? super Throwable, ? extends T> fn;
+        UniExceptionally(CompletableFuture<T> dep, CompletableFuture<T> src,
+                         Function<? super Throwable, ? extends T> fn) {
+            super(null, dep, src); this.fn = fn;
+        }
+        final CompletableFuture<T> tryFire(int mode) { // never ASYNC
+            // assert mode != ASYNC;
+            CompletableFuture<T> d; CompletableFuture<T> a;
+            if ((d = dep) == null || !d.uniExceptionally(a = src, fn, this))
+                return null;
+            dep = null; src = null; fn = null;
+            return d.postFire(a, mode);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class ApplyToEither<T,U> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final CompletableFuture<? extends T> snd;
-        final Function<? super T,? extends U> fn;
-        final CompletableFuture<U> dst;
-        final Executor executor;
-        ApplyToEither(CompletableFuture<? extends T> src,
-                      CompletableFuture<? extends T> snd,
-                      Function<? super T,? extends U> fn,
-                      CompletableFuture<U> dst,
-                      Executor executor) {
-            this.src = src; this.snd = snd;
-            this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final CompletableFuture<? extends T> b;
-            final Function<? super T,? extends U> fn;
-            final CompletableFuture<U> dst;
-            Object r; T t; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (((a = this.src) != null && (r = a.result) != null) ||
-                 ((b = this.snd) != null && (r = b.result) != null)) &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
-                }
-                Executor e = executor;
-                U u = null;
-                if (ex == null) {
-                    try {
-                        if (e != null)
-                            execAsync(e, new AsyncApply<T,U>(t, fn, dst));
-                        else
-                            u = fn.apply(t);
-                    } catch (Throwable rex) {
-                        ex = rex;
-                    }
-                }
-                if (e == null || ex != null)
-                    dst.internalComplete(u, ex);
+    }
+
+    final boolean uniExceptionally(CompletableFuture<T> a,
+                                   Function<? super Throwable, ? extends T> f,
+                                   UniExceptionally<T> c) {
+        Object r; Throwable x;
+        if (a == null || (r = a.result) == null || f == null)
+            return false;
+        if (result == null) {
+            try {
+                if (r instanceof AltResult && (x = ((AltResult)r).ex) != null) {
+                    if (c != null && !c.claim())
+                        return false;
+                    completeValue(f.apply(x));
+                } else
+                    internalComplete(r);
+            } catch (Throwable ex) {
+                completeThrowable(ex);
             }
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class AcceptEither<T> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final CompletableFuture<? extends T> snd;
-        final Consumer<? super T> fn;
-        final CompletableFuture<Void> dst;
-        final Executor executor;
-        AcceptEither(CompletableFuture<? extends T> src,
-                     CompletableFuture<? extends T> snd,
-                     Consumer<? super T> fn,
-                     CompletableFuture<Void> dst,
-                     Executor executor) {
-            this.src = src; this.snd = snd;
-            this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final CompletableFuture<? extends T> b;
-            final Consumer<? super T> fn;
-            final CompletableFuture<Void> dst;
-            Object r; T t; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (((a = this.src) != null && (r = a.result) != null) ||
-                 ((b = this.snd) != null && (r = b.result) != null)) &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
-                }
-                Executor e = executor;
-                if (ex == null) {
-                    try {
-                        if (e != null)
-                            execAsync(e, new AsyncAccept<T>(t, fn, dst));
-                        else
-                            fn.accept(t);
-                    } catch (Throwable rex) {
-                        ex = rex;
-                    }
-                }
-                if (e == null || ex != null)
-                    dst.internalComplete(null, ex);
-            }
+        return true;
+    }
+
+    private CompletableFuture<T> uniExceptionallyStage(
+        Function<Throwable, ? extends T> f) {
+        if (f == null) throw new NullPointerException();
+        CompletableFuture<T> d = new CompletableFuture<T>();
+        if (!d.uniExceptionally(this, f, null)) {
+            UniExceptionally<T> c = new UniExceptionally<T>(d, this, f);
+            push(c);
+            c.tryFire(SYNC);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class RunAfterEither extends Completion {
-        final CompletableFuture<?> src;
-        final CompletableFuture<?> snd;
-        final Runnable fn;
-        final CompletableFuture<Void> dst;
-        final Executor executor;
-        RunAfterEither(CompletableFuture<?> src,
-                       CompletableFuture<?> snd,
-                       Runnable fn,
-                       CompletableFuture<Void> dst,
-                       Executor executor) {
-            this.src = src; this.snd = snd;
-            this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<?> a;
-            final CompletableFuture<?> b;
-            final Runnable fn;
-            final CompletableFuture<Void> dst;
-            Object r; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (((a = this.src) != null && (r = a.result) != null) ||
-                 ((b = this.snd) != null && (r = b.result) != null)) &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult)
-                    ex = ((AltResult)r).ex;
-                else
-                    ex = null;
-                Executor e = executor;
-                if (ex == null) {
-                    try {
-                        if (e != null)
-                            execAsync(e, new AsyncRun(fn, dst));
-                        else
-                            fn.run();
-                    } catch (Throwable rex) {
-                        ex = rex;
-                    }
-                }
-                if (e == null || ex != null)
-                    dst.internalComplete(null, ex);
-            }
+        return d;
+    }
+
+    @SuppressWarnings("serial")
+    static final class UniRelay<T> extends UniCompletion<T,T> { // for Compose
+        UniRelay(CompletableFuture<T> dep, CompletableFuture<T> src) {
+            super(null, dep, src);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class OrCompletion extends Completion {
-        final CompletableFuture<?> src;
-        final CompletableFuture<?> snd;
-        final CompletableFuture<Object> dst;
-        OrCompletion(CompletableFuture<?> src,
-                     CompletableFuture<?> snd,
-                     CompletableFuture<Object> dst) {
-            this.src = src; this.snd = snd; this.dst = dst;
-        }
-        public final void run() {
-            final CompletableFuture<?> a;
-            final CompletableFuture<?> b;
-            final CompletableFuture<Object> dst;
-            Object r, t; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (((a = this.src) != null && (r = a.result) != null) ||
-                 ((b = this.snd) != null && (r = b.result) != null)) &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    t = r;
-                }
-                dst.internalComplete(t, ex);
-            }
+        final CompletableFuture<T> tryFire(int mode) {
+            CompletableFuture<T> d; CompletableFuture<T> a;
+            if ((d = dep) == null || !d.uniRelay(a = src))
+                return null;
+            src = null; dep = null;
+            return d.postFire(a, mode);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class ExceptionCompletion<T> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final Function<? super Throwable, ? extends T> fn;
-        final CompletableFuture<T> dst;
-        ExceptionCompletion(CompletableFuture<? extends T> src,
-                            Function<? super Throwable, ? extends T> fn,
-                            CompletableFuture<T> dst) {
-            this.src = src; this.fn = fn; this.dst = dst;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final Function<? super Throwable, ? extends T> fn;
-            final CompletableFuture<T> dst;
-            Object r; T t = null; Throwable ex, dx = null;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                compareAndSet(0, 1)) {
-                if ((r instanceof AltResult) &&
-                    (ex = ((AltResult)r).ex) != null) {
-                    try {
-                        t = fn.apply(ex);
-                    } catch (Throwable rex) {
-                        dx = rex;
-                    }
-                }
-                else {
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
-                }
-                dst.internalComplete(t, dx);
-            }
+    }
+
+    final boolean uniRelay(CompletableFuture<T> a) {
+        Object r;
+        if (a == null || (r = a.result) == null)
+            return false;
+        if (result == null) // no need to claim
+            completeRelay(r);
+        return true;
+    }
+
+    @SuppressWarnings("serial")
+    static final class UniCompose<T,V> extends UniCompletion<T,V> {
+        Function<? super T, ? extends CompletionStage<V>> fn;
+        UniCompose(Executor executor, CompletableFuture<V> dep,
+                   CompletableFuture<T> src,
+                   Function<? super T, ? extends CompletionStage<V>> fn) {
+            super(executor, dep, src); this.fn = fn;
+        }
+        final CompletableFuture<V> tryFire(int mode) {
+            CompletableFuture<V> d; CompletableFuture<T> a;
+            if ((d = dep) == null ||
+                !d.uniCompose(a = src, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; fn = null;
+            return d.postFire(a, mode);
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class WhenCompleteCompletion<T> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final BiConsumer<? super T, ? super Throwable> fn;
-        final CompletableFuture<T> dst;
-        final Executor executor;
-        WhenCompleteCompletion(CompletableFuture<? extends T> src,
-                                  BiConsumer<? super T, ? super Throwable> fn,
-                                  CompletableFuture<T> dst,
-                                  Executor executor) {
-            this.src = src; this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final BiConsumer<? super T, ? super Throwable> fn;
-            final CompletableFuture<T> dst;
-            Object r; T t; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
+    }
+
+    final <S> boolean uniCompose(
+        CompletableFuture<S> a,
+        Function<? super S, ? extends CompletionStage<T>> f,
+        UniCompose<S,T> c) {
+        Object r; Throwable x;
+        if (a == null || (r = a.result) == null || f == null)
+            return false;
+        tryComplete: if (result == null) {
+            if (r instanceof AltResult) {
+                if ((x = ((AltResult)r).ex) != null) {
+                    completeThrowable(x, r);
+                    break tryComplete;
                 }
-                Executor e = executor;
-                Throwable dx = null;
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncWhenComplete<T>(t, ex, fn, dst));
-                    else
-                        fn.accept(t, ex);
-                } catch (Throwable rex) {
-                    dx = rex;
+                r = null;
+            }
+            try {
+                if (c != null && !c.claim())
+                    return false;
+                @SuppressWarnings("unchecked") S s = (S) r;
+                CompletableFuture<T> g = f.apply(s).toCompletableFuture();
+                if (g.result == null || !uniRelay(g)) {
+                    UniRelay<T> copy = new UniRelay<T>(this, g);
+                    g.push(copy);
+                    copy.tryFire(SYNC);
+                    if (result == null)
+                        return false;
+                }
+            } catch (Throwable ex) {
+                completeThrowable(ex);
+            }
+        }
+        return true;
+    }
+
+    private <V> CompletableFuture<V> uniComposeStage(
+        Executor e, Function<? super T, ? extends CompletionStage<V>> f) {
+        if (f == null) throw new NullPointerException();
+        Object r; Throwable x;
+        if (e == null && (r = result) != null) {
+            // try to return function result directly
+            if (r instanceof AltResult) {
+                if ((x = ((AltResult)r).ex) != null) {
+                    return new CompletableFuture<V>(encodeThrowable(x, r));
                 }
-                if (e == null || dx != null)
-                    dst.internalComplete(t, ex != null ? ex : dx);
+                r = null;
+            }
+            try {
+                @SuppressWarnings("unchecked") T t = (T) r;
+                return f.apply(t).toCompletableFuture();
+            } catch (Throwable ex) {
+                return new CompletableFuture<V>(encodeThrowable(ex));
             }
         }
-        private static final long serialVersionUID = 5232453952276885070L;
+        CompletableFuture<V> d = new CompletableFuture<V>();
+        UniCompose<T,V> c = new UniCompose<T,V>(e, d, this, f);
+        push(c);
+        c.tryFire(SYNC);
+        return d;
     }
 
-    static final class ThenCopy<T> extends Completion {
-        final CompletableFuture<?> src;
-        final CompletableFuture<T> dst;
-        ThenCopy(CompletableFuture<?> src,
-                 CompletableFuture<T> dst) {
-            this.src = src; this.dst = dst;
+    /* ------------- Two-input Completions -------------- */
+
+    /** A Completion for an action with two sources */
+    @SuppressWarnings("serial")
+    abstract static class BiCompletion<T,U,V> extends UniCompletion<T,V> {
+        CompletableFuture<U> snd; // second source for action
+        BiCompletion(Executor executor, CompletableFuture<V> dep,
+                     CompletableFuture<T> src, CompletableFuture<U> snd) {
+            super(executor, dep, src); this.snd = snd;
         }
-        public final void run() {
-            final CompletableFuture<?> a;
-            final CompletableFuture<T> dst;
-            Object r; T t; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
-                }
-                dst.internalComplete(t, ex);
-            }
+    }
+
+    /** A Completion delegating to a BiCompletion */
+    @SuppressWarnings("serial")
+    static final class CoCompletion extends Completion {
+        BiCompletion<?,?,?> base;
+        CoCompletion(BiCompletion<?,?,?> base) { this.base = base; }
+        final CompletableFuture<?> tryFire(int mode) {
+            BiCompletion<?,?,?> c; CompletableFuture<?> d;
+            if ((c = base) == null || (d = c.tryFire(mode)) == null)
+                return null;
+            base = null; // detach
+            return d;
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    // version of ThenCopy for CompletableFuture<Void> dst
-    static final class ThenPropagate extends Completion {
-        final CompletableFuture<?> src;
-        final CompletableFuture<Void> dst;
-        ThenPropagate(CompletableFuture<?> src,
-                      CompletableFuture<Void> dst) {
-            this.src = src; this.dst = dst;
-        }
-        public final void run() {
-            final CompletableFuture<?> a;
-            final CompletableFuture<Void> dst;
-            Object r; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult)
-                    ex = ((AltResult)r).ex;
-                else
-                    ex = null;
-                dst.internalComplete(null, ex);
-            }
+        final boolean isLive() {
+            BiCompletion<?,?,?> c;
+            return (c = base) != null && c.dep != null;
         }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class HandleCompletion<T,U> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final BiFunction<? super T, Throwable, ? extends U> fn;
-        final CompletableFuture<U> dst;
-        final Executor executor;
-        HandleCompletion(CompletableFuture<? extends T> src,
-                         BiFunction<? super T, Throwable, ? extends U> fn,
-                         CompletableFuture<U> dst,
-                          Executor executor) {
-            this.src = src; this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final BiFunction<? super T, Throwable, ? extends U> fn;
-            final CompletableFuture<U> dst;
-            Object r; T t; Throwable ex;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
-                }
-                Executor e = executor;
-                U u = null;
-                Throwable dx = null;
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncCombine<T,Throwable,U>(t, ex, fn, dst));
-                    else
-                        u = fn.apply(t, ex);
-                } catch (Throwable rex) {
-                    dx = rex;
-                }
-                if (e == null || dx != null)
-                    dst.internalComplete(u, dx);
-            }
-        }
-        private static final long serialVersionUID = 5232453952276885070L;
-    }
-
-    static final class ThenCompose<T,U> extends Completion {
-        final CompletableFuture<? extends T> src;
-        final Function<? super T, ? extends CompletionStage<U>> fn;
-        final CompletableFuture<U> dst;
-        final Executor executor;
-        ThenCompose(CompletableFuture<? extends T> src,
-                    Function<? super T, ? extends CompletionStage<U>> fn,
-                    CompletableFuture<U> dst,
-                    Executor executor) {
-            this.src = src; this.fn = fn; this.dst = dst;
-            this.executor = executor;
-        }
-        public final void run() {
-            final CompletableFuture<? extends T> a;
-            final Function<? super T, ? extends CompletionStage<U>> fn;
-            final CompletableFuture<U> dst;
-            Object r; T t; Throwable ex; Executor e;
-            if ((dst = this.dst) != null &&
-                (fn = this.fn) != null &&
-                (a = this.src) != null &&
-                (r = a.result) != null &&
-                compareAndSet(0, 1)) {
-                if (r instanceof AltResult) {
-                    ex = ((AltResult)r).ex;
-                    t = null;
-                }
-                else {
-                    ex = null;
-                    @SuppressWarnings("unchecked") T tr = (T) r;
-                    t = tr;
-                }
-                CompletableFuture<U> c = null;
-                U u = null;
-                boolean complete = false;
-                if (ex == null) {
-                    if ((e = executor) != null)
-                        execAsync(e, new AsyncCompose<T,U>(t, fn, dst));
-                    else {
-                        try {
-                            CompletionStage<U> cs = fn.apply(t);
-                            c = (cs == null) ? null : cs.toCompletableFuture();
-                            if (c == null)
-                                ex = new NullPointerException();
-                        } catch (Throwable rex) {
-                            ex = rex;
-                        }
-                    }
-                }
-                if (c != null) {
-                    ThenCopy<U> d = null;
-                    Object s;
-                    if ((s = c.result) == null) {
-                        CompletionNode p = new CompletionNode
-                            (d = new ThenCopy<U>(c, dst));
-                        while ((s = c.result) == null) {
-                            if (UNSAFE.compareAndSwapObject
-                                (c, COMPLETIONS, p.next = c.completions, p))
-                                break;
-                        }
-                    }
-                    if (s != null && (d == null || d.compareAndSet(0, 1))) {
-                        complete = true;
-                        if (s instanceof AltResult) {
-                            ex = ((AltResult)s).ex;  // no rewrap
-                            u = null;
-                        }
-                        else {
-                            @SuppressWarnings("unchecked") U us = (U) s;
-                            u = us;
-                        }
-                    }
-                }
-                if (complete || ex != null)
-                    dst.internalComplete(u, ex);
-                if (c != null)
-                    c.helpPostComplete();
+    }
+
+    /** Pushes completion to this and b unless both done. */
+    final void bipush(CompletableFuture<?> b, BiCompletion<?,?,?> c) {
+        if (c != null) {
+            Object r;
+            while ((r = result) == null && !tryPushStack(c))
+                lazySetNext(c, null); // clear on failure
+            if (b != null && b != this && b.result == null) {
+                Completion q = (r != null) ? c : new CoCompletion(c);
+                while (b.result == null && !b.tryPushStack(q))
+                    lazySetNext(q, null); // clear on failure
             }
         }
-        private static final long serialVersionUID = 5232453952276885070L;
     }
 
-    // Implementations of stage methods with (plain, async, Executor) forms
+    /** Post-processing after successful BiCompletion tryFire. */
+    final CompletableFuture<T> postFire(CompletableFuture<?> a,
+                                        CompletableFuture<?> b, int mode) {
+        if (b != null && b.stack != null) { // clean second source
+            if (mode < 0 || b.result == null)
+                b.cleanStack();
+            else
+                b.postComplete();
+        }
+        return postFire(a, mode);
+    }
 
-    private <U> CompletableFuture<U> doThenApply
-        (Function<? super T,? extends U> fn,
-         Executor e) {
-        if (fn == null) throw new NullPointerException();
-        CompletableFuture<U> dst = new CompletableFuture<U>();
-        ThenApply<T,U> d = null;
-        Object r;
-        if ((r = result) == null) {
-            CompletionNode p = new CompletionNode
-                (d = new ThenApply<T,U>(this, fn, dst, e));
-            while ((r = result) == null) {
-                if (UNSAFE.compareAndSwapObject
-                    (this, COMPLETIONS, p.next = completions, p))
-                    break;
-            }
+    @SuppressWarnings("serial")
+    static final class BiApply<T,U,V> extends BiCompletion<T,U,V> {
+        BiFunction<? super T,? super U,? extends V> fn;
+        BiApply(Executor executor, CompletableFuture<V> dep,
+                CompletableFuture<T> src, CompletableFuture<U> snd,
+                BiFunction<? super T,? super U,? extends V> fn) {
+            super(executor, dep, src, snd); this.fn = fn;
+        }
+        final CompletableFuture<V> tryFire(int mode) {
+            CompletableFuture<V> d;
+            CompletableFuture<T> a;
+            CompletableFuture<U> b;
+            if ((d = dep) == null ||
+                !d.biApply(a = src, b = snd, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; snd = null; fn = null;
+            return d.postFire(a, b, mode);
         }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            T t; Throwable ex;
+    }
+
+    final <R,S> boolean biApply(CompletableFuture<R> a,
+                                CompletableFuture<S> b,
+                                BiFunction<? super R,? super S,? extends T> f,
+                                BiApply<R,S,T> c) {
+        Object r, s; Throwable x;
+        if (a == null || (r = a.result) == null ||
+            b == null || (s = b.result) == null || f == null)
+            return false;
+        tryComplete: if (result == null) {
             if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
-            }
-            else {
-                ex = null;
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
+                if ((x = ((AltResult)r).ex) != null) {
+                    completeThrowable(x, r);
+                    break tryComplete;
+                }
+                r = null;
             }
-            U u = null;
-            if (ex == null) {
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncApply<T,U>(t, fn, dst));
-                    else
-                        u = fn.apply(t);
-                } catch (Throwable rex) {
-                    ex = rex;
+            if (s instanceof AltResult) {
+                if ((x = ((AltResult)s).ex) != null) {
+                    completeThrowable(x, s);
+                    break tryComplete;
                 }
+                s = null;
+            }
+            try {
+                if (c != null && !c.claim())
+                    return false;
+                @SuppressWarnings("unchecked") R rr = (R) r;
+                @SuppressWarnings("unchecked") S ss = (S) s;
+                completeValue(f.apply(rr, ss));
+            } catch (Throwable ex) {
+                completeThrowable(ex);
             }
-            if (e == null || ex != null)
-                dst.internalComplete(u, ex);
         }
-        helpPostComplete();
-        return dst;
+        return true;
     }
 
-    private CompletableFuture<Void> doThenAccept(Consumer<? super T> fn,
-                                                 Executor e) {
-        if (fn == null) throw new NullPointerException();
-        CompletableFuture<Void> dst = new CompletableFuture<Void>();
-        ThenAccept<T> d = null;
-        Object r;
-        if ((r = result) == null) {
-            CompletionNode p = new CompletionNode
-                (d = new ThenAccept<T>(this, fn, dst, e));
-            while ((r = result) == null) {
-                if (UNSAFE.compareAndSwapObject
-                    (this, COMPLETIONS, p.next = completions, p))
-                    break;
-            }
+    private <U,V> CompletableFuture<V> biApplyStage(
+        Executor e, CompletionStage<U> o,
+        BiFunction<? super T,? super U,? extends V> f) {
+        CompletableFuture<U> b;
+        if (f == null || (b = o.toCompletableFuture()) == null)
+            throw new NullPointerException();
+        CompletableFuture<V> d = new CompletableFuture<V>();
+        if (e != null || !d.biApply(this, b, f, null)) {
+            BiApply<T,U,V> c = new BiApply<T,U,V>(e, d, this, b, f);
+            bipush(b, c);
+            c.tryFire(SYNC);
+        }
+        return d;
+    }
+
+    @SuppressWarnings("serial")
+    static final class BiAccept<T,U> extends BiCompletion<T,U,Void> {
+        BiConsumer<? super T,? super U> fn;
+        BiAccept(Executor executor, CompletableFuture<Void> dep,
+                 CompletableFuture<T> src, CompletableFuture<U> snd,
+                 BiConsumer<? super T,? super U> fn) {
+            super(executor, dep, src, snd); this.fn = fn;
+        }
+        final CompletableFuture<Void> tryFire(int mode) {
+            CompletableFuture<Void> d;
+            CompletableFuture<T> a;
+            CompletableFuture<U> b;
+            if ((d = dep) == null ||
+                !d.biAccept(a = src, b = snd, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; snd = null; fn = null;
+            return d.postFire(a, b, mode);
         }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            T t; Throwable ex;
+    }
+
+    final <R,S> boolean biAccept(CompletableFuture<R> a,
+                                 CompletableFuture<S> b,
+                                 BiConsumer<? super R,? super S> f,
+                                 BiAccept<R,S> c) {
+        Object r, s; Throwable x;
+        if (a == null || (r = a.result) == null ||
+            b == null || (s = b.result) == null || f == null)
+            return false;
+        tryComplete: if (result == null) {
             if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
-            }
-            else {
-                ex = null;
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
+                if ((x = ((AltResult)r).ex) != null) {
+                    completeThrowable(x, r);
+                    break tryComplete;
+                }
+                r = null;
             }
-            if (ex == null) {
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncAccept<T>(t, fn, dst));
-                    else
-                        fn.accept(t);
-                } catch (Throwable rex) {
-                    ex = rex;
+            if (s instanceof AltResult) {
+                if ((x = ((AltResult)s).ex) != null) {
+                    completeThrowable(x, s);
+                    break tryComplete;
                 }
+                s = null;
+            }
+            try {
+                if (c != null && !c.claim())
+                    return false;
+                @SuppressWarnings("unchecked") R rr = (R) r;
+                @SuppressWarnings("unchecked") S ss = (S) s;
+                f.accept(rr, ss);
+                completeNull();
+            } catch (Throwable ex) {
+                completeThrowable(ex);
             }
-            if (e == null || ex != null)
-                dst.internalComplete(null, ex);
         }
-        helpPostComplete();
-        return dst;
+        return true;
     }
 
-    private CompletableFuture<Void> doThenRun(Runnable action,
-                                              Executor e) {
-        if (action == null) throw new NullPointerException();
-        CompletableFuture<Void> dst = new CompletableFuture<Void>();
-        ThenRun d = null;
-        Object r;
-        if ((r = result) == null) {
-            CompletionNode p = new CompletionNode
-                (d = new ThenRun(this, action, dst, e));
-            while ((r = result) == null) {
-                if (UNSAFE.compareAndSwapObject
-                    (this, COMPLETIONS, p.next = completions, p))
-                    break;
-            }
+    private <U> CompletableFuture<Void> biAcceptStage(
+        Executor e, CompletionStage<U> o,
+        BiConsumer<? super T,? super U> f) {
+        CompletableFuture<U> b;
+        if (f == null || (b = o.toCompletableFuture()) == null)
+            throw new NullPointerException();
+        CompletableFuture<Void> d = new CompletableFuture<Void>();
+        if (e != null || !d.biAccept(this, b, f, null)) {
+            BiAccept<T,U> c = new BiAccept<T,U>(e, d, this, b, f);
+            bipush(b, c);
+            c.tryFire(SYNC);
+        }
+        return d;
+    }
+
+    @SuppressWarnings("serial")
+    static final class BiRun<T,U> extends BiCompletion<T,U,Void> {
+        Runnable fn;
+        BiRun(Executor executor, CompletableFuture<Void> dep,
+              CompletableFuture<T> src,
+              CompletableFuture<U> snd,
+              Runnable fn) {
+            super(executor, dep, src, snd); this.fn = fn;
+        }
+        final CompletableFuture<Void> tryFire(int mode) {
+            CompletableFuture<Void> d;
+            CompletableFuture<T> a;
+            CompletableFuture<U> b;
+            if ((d = dep) == null ||
+                !d.biRun(a = src, b = snd, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; snd = null; fn = null;
+            return d.postFire(a, b, mode);
         }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            Throwable ex;
-            if (r instanceof AltResult)
-                ex = ((AltResult)r).ex;
+    }
+
+    final boolean biRun(CompletableFuture<?> a, CompletableFuture<?> b,
+                        Runnable f, BiRun<?,?> c) {
+        Object r, s; Throwable x;
+        if (a == null || (r = a.result) == null ||
+            b == null || (s = b.result) == null || f == null)
+            return false;
+        if (result == null) {
+            if (r instanceof AltResult && (x = ((AltResult)r).ex) != null)
+                completeThrowable(x, r);
+            else if (s instanceof AltResult && (x = ((AltResult)s).ex) != null)
+                completeThrowable(x, s);
             else
-                ex = null;
-            if (ex == null) {
                 try {
-                    if (e != null)
-                        execAsync(e, new AsyncRun(action, dst));
-                    else
-                        action.run();
-                } catch (Throwable rex) {
-                    ex = rex;
-                }
-            }
-            if (e == null || ex != null)
-                dst.internalComplete(null, ex);
-        }
-        helpPostComplete();
-        return dst;
-    }
-
-    private <U,V> CompletableFuture<V> doThenCombine
-        (CompletableFuture<? extends U> other,
-         BiFunction<? super T,? super U,? extends V> fn,
-         Executor e) {
-        if (other == null || fn == null) throw new NullPointerException();
-        CompletableFuture<V> dst = new CompletableFuture<V>();
-        ThenCombine<T,U,V> d = null;
-        Object r, s = null;
-        if ((r = result) == null || (s = other.result) == null) {
-            d = new ThenCombine<T,U,V>(this, other, fn, dst, e);
-            CompletionNode q = null, p = new CompletionNode(d);
-            while ((r == null && (r = result) == null) ||
-                   (s == null && (s = other.result) == null)) {
-                if (q != null) {
-                    if (s != null ||
-                        UNSAFE.compareAndSwapObject
-                        (other, COMPLETIONS, q.next = other.completions, q))
-                        break;
-                }
-                else if (r != null ||
-                         UNSAFE.compareAndSwapObject
-                         (this, COMPLETIONS, p.next = completions, p)) {
-                    if (s != null)
-                        break;
-                    q = new CompletionNode(d);
+                    if (c != null && !c.claim())
+                        return false;
+                    f.run();
+                    completeNull();
+                } catch (Throwable ex) {
+                    completeThrowable(ex);
                 }
-            }
         }
-        if (r != null && s != null && (d == null || d.compareAndSet(0, 1))) {
-            T t; U u; Throwable ex;
-            if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
-            }
-            else {
-                ex = null;
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
-            }
-            if (ex != null)
-                u = null;
-            else if (s instanceof AltResult) {
-                ex = ((AltResult)s).ex;
-                u = null;
-            }
-            else {
-                @SuppressWarnings("unchecked") U us = (U) s;
-                u = us;
-            }
-            V v = null;
-            if (ex == null) {
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncCombine<T,U,V>(t, u, fn, dst));
-                    else
-                        v = fn.apply(t, u);
-                } catch (Throwable rex) {
-                    ex = rex;
-                }
-            }
-            if (e == null || ex != null)
-                dst.internalComplete(v, ex);
-        }
-        helpPostComplete();
-        other.helpPostComplete();
-        return dst;
-    }
-
-    private <U> CompletableFuture<Void> doThenAcceptBoth
-        (CompletableFuture<? extends U> other,
-         BiConsumer<? super T,? super U> fn,
-         Executor e) {
-        if (other == null || fn == null) throw new NullPointerException();
-        CompletableFuture<Void> dst = new CompletableFuture<Void>();
-        ThenAcceptBoth<T,U> d = null;
-        Object r, s = null;
-        if ((r = result) == null || (s = other.result) == null) {
-            d = new ThenAcceptBoth<T,U>(this, other, fn, dst, e);
-            CompletionNode q = null, p = new CompletionNode(d);
-            while ((r == null && (r = result) == null) ||
-                   (s == null && (s = other.result) == null)) {
-                if (q != null) {
-                    if (s != null ||
-                        UNSAFE.compareAndSwapObject
-                        (other, COMPLETIONS, q.next = other.completions, q))
-                        break;
-                }
-                else if (r != null ||
-                         UNSAFE.compareAndSwapObject
-                         (this, COMPLETIONS, p.next = completions, p)) {
-                    if (s != null)
-                        break;
-                    q = new CompletionNode(d);
-                }
-            }
+        return true;
+    }
+
+    private CompletableFuture<Void> biRunStage(Executor e, CompletionStage<?> o,
+                                               Runnable f) {
+        CompletableFuture<?> b;
+        if (f == null || (b = o.toCompletableFuture()) == null)
+            throw new NullPointerException();
+        CompletableFuture<Void> d = new CompletableFuture<Void>();
+        if (e != null || !d.biRun(this, b, f, null)) {
+            BiRun<T,?> c = new BiRun<>(e, d, this, b, f);
+            bipush(b, c);
+            c.tryFire(SYNC);
         }
-        if (r != null && s != null && (d == null || d.compareAndSet(0, 1))) {
-            T t; U u; Throwable ex;
-            if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
-            }
-            else {
-                ex = null;
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
-            }
-            if (ex != null)
-                u = null;
-            else if (s instanceof AltResult) {
-                ex = ((AltResult)s).ex;
-                u = null;
-            }
-            else {
-                @SuppressWarnings("unchecked") U us = (U) s;
-                u = us;
-            }
-            if (ex == null) {
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncAcceptBoth<T,U>(t, u, fn, dst));
-                    else
-                        fn.accept(t, u);
-                } catch (Throwable rex) {
-                    ex = rex;
-                }
+        return d;
+    }
+
+    @SuppressWarnings("serial")
+    static final class BiRelay<T,U> extends BiCompletion<T,U,Void> { // for And
+        BiRelay(CompletableFuture<Void> dep,
+                CompletableFuture<T> src,
+                CompletableFuture<U> snd) {
+            super(null, dep, src, snd);
+        }
+        final CompletableFuture<Void> tryFire(int mode) {
+            CompletableFuture<Void> d;
+            CompletableFuture<T> a;
+            CompletableFuture<U> b;
+            if ((d = dep) == null || !d.biRelay(a = src, b = snd))
+                return null;
+            src = null; snd = null; dep = null;
+            return d.postFire(a, b, mode);
+        }
+    }
+
+    boolean biRelay(CompletableFuture<?> a, CompletableFuture<?> b) {
+        Object r, s; Throwable x;
+        if (a == null || (r = a.result) == null ||
+            b == null || (s = b.result) == null)
+            return false;
+        if (result == null) {
+            if (r instanceof AltResult && (x = ((AltResult)r).ex) != null)
+                completeThrowable(x, r);
+            else if (s instanceof AltResult && (x = ((AltResult)s).ex) != null)
+                completeThrowable(x, s);
+            else
+                completeNull();
+        }
+        return true;
+    }
+
+    /** Recursively constructs a tree of completions. */
+    static CompletableFuture<Void> andTree(CompletableFuture<?>[] cfs,
+                                           int lo, int hi) {
+        CompletableFuture<Void> d = new CompletableFuture<Void>();
+        if (lo > hi) // empty
+            d.result = NIL;
+        else {
+            CompletableFuture<?> a, b;
+            int mid = (lo + hi) >>> 1;
+            if ((a = (lo == mid ? cfs[lo] :
+                      andTree(cfs, lo, mid))) == null ||
+                (b = (lo == hi ? a : (hi == mid+1) ? cfs[hi] :
+                      andTree(cfs, mid+1, hi)))  == null)
+                throw new NullPointerException();
+            if (!d.biRelay(a, b)) {
+                BiRelay<?,?> c = new BiRelay<>(d, a, b);
+                a.bipush(b, c);
+                c.tryFire(SYNC);
             }
-            if (e == null || ex != null)
-                dst.internalComplete(null, ex);
-        }
-        helpPostComplete();
-        other.helpPostComplete();
-        return dst;
-    }
-
-    private CompletableFuture<Void> doRunAfterBoth(CompletableFuture<?> other,
-                                                   Runnable action,
-                                                   Executor e) {
-        if (other == null || action == null) throw new NullPointerException();
-        CompletableFuture<Void> dst = new CompletableFuture<Void>();
-        RunAfterBoth d = null;
-        Object r, s = null;
-        if ((r = result) == null || (s = other.result) == null) {
-            d = new RunAfterBoth(this, other, action, dst, e);
-            CompletionNode q = null, p = new CompletionNode(d);
-            while ((r == null && (r = result) == null) ||
-                   (s == null && (s = other.result) == null)) {
-                if (q != null) {
-                    if (s != null ||
-                        UNSAFE.compareAndSwapObject
-                        (other, COMPLETIONS, q.next = other.completions, q))
-                        break;
-                }
-                else if (r != null ||
-                         UNSAFE.compareAndSwapObject
-                         (this, COMPLETIONS, p.next = completions, p)) {
-                    if (s != null)
-                        break;
-                    q = new CompletionNode(d);
+        }
+        return d;
+    }
+
+    /* ------------- Projected (Ored) BiCompletions -------------- */
+
+    /** Pushes completion to this and b unless either done. */
+    final void orpush(CompletableFuture<?> b, BiCompletion<?,?,?> c) {
+        if (c != null) {
+            while ((b == null || b.result == null) && result == null) {
+                if (tryPushStack(c)) {
+                    if (b != null && b != this && b.result == null) {
+                        Completion q = new CoCompletion(c);
+                        while (result == null && b.result == null &&
+                               !b.tryPushStack(q))
+                            lazySetNext(q, null); // clear on failure
+                    }
+                    break;
                 }
+                lazySetNext(c, null); // clear on failure
             }
         }
-        if (r != null && s != null && (d == null || d.compareAndSet(0, 1))) {
-            Throwable ex;
-            if (r instanceof AltResult)
-                ex = ((AltResult)r).ex;
-            else
-                ex = null;
-            if (ex == null && (s instanceof AltResult))
-                ex = ((AltResult)s).ex;
-            if (ex == null) {
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncRun(action, dst));
-                    else
-                        action.run();
-                } catch (Throwable rex) {
-                    ex = rex;
-                }
-            }
-            if (e == null || ex != null)
-                dst.internalComplete(null, ex);
+    }
+
+    @SuppressWarnings("serial")
+    static final class OrApply<T,U extends T,V> extends BiCompletion<T,U,V> {
+        Function<? super T,? extends V> fn;
+        OrApply(Executor executor, CompletableFuture<V> dep,
+                CompletableFuture<T> src,
+                CompletableFuture<U> snd,
+                Function<? super T,? extends V> fn) {
+            super(executor, dep, src, snd); this.fn = fn;
+        }
+        final CompletableFuture<V> tryFire(int mode) {
+            CompletableFuture<V> d;
+            CompletableFuture<T> a;
+            CompletableFuture<U> b;
+            if ((d = dep) == null ||
+                !d.orApply(a = src, b = snd, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; snd = null; fn = null;
+            return d.postFire(a, b, mode);
         }
-        helpPostComplete();
-        other.helpPostComplete();
-        return dst;
     }
 
-    private <U> CompletableFuture<U> doApplyToEither
-        (CompletableFuture<? extends T> other,
-         Function<? super T, U> fn,
-         Executor e) {
-        if (other == null || fn == null) throw new NullPointerException();
-        CompletableFuture<U> dst = new CompletableFuture<U>();
-        ApplyToEither<T,U> d = null;
-        Object r;
-        if ((r = result) == null && (r = other.result) == null) {
-            d = new ApplyToEither<T,U>(this, other, fn, dst, e);
-            CompletionNode q = null, p = new CompletionNode(d);
-            while ((r = result) == null && (r = other.result) == null) {
-                if (q != null) {
-                    if (UNSAFE.compareAndSwapObject
-                        (other, COMPLETIONS, q.next = other.completions, q))
-                        break;
+    final <R,S extends R> boolean orApply(CompletableFuture<R> a,
+                                          CompletableFuture<S> b,
+                                          Function<? super R, ? extends T> f,
+                                          OrApply<R,S,T> c) {
+        Object r; Throwable x;
+        if (a == null || b == null ||
+            ((r = a.result) == null && (r = b.result) == null) || f == null)
+            return false;
+        tryComplete: if (result == null) {
+            try {
+                if (c != null && !c.claim())
+                    return false;
+                if (r instanceof AltResult) {
+                    if ((x = ((AltResult)r).ex) != null) {
+                        completeThrowable(x, r);
+                        break tryComplete;
+                    }
+                    r = null;
                 }
-                else if (UNSAFE.compareAndSwapObject
-                         (this, COMPLETIONS, p.next = completions, p))
-                    q = new CompletionNode(d);
+                @SuppressWarnings("unchecked") R rr = (R) r;
+                completeValue(f.apply(rr));
+            } catch (Throwable ex) {
+                completeThrowable(ex);
             }
         }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            T t; Throwable ex;
-            if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
-            }
-            else {
-                ex = null;
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
-            }
-            U u = null;
-            if (ex == null) {
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncApply<T,U>(t, fn, dst));
-                    else
-                        u = fn.apply(t);
-                } catch (Throwable rex) {
-                    ex = rex;
-                }
-            }
-            if (e == null || ex != null)
-                dst.internalComplete(u, ex);
+        return true;
+    }
+
+    private <U extends T,V> CompletableFuture<V> orApplyStage(
+        Executor e, CompletionStage<U> o,
+        Function<? super T, ? extends V> f) {
+        CompletableFuture<U> b;
+        if (f == null || (b = o.toCompletableFuture()) == null)
+            throw new NullPointerException();
+        CompletableFuture<V> d = new CompletableFuture<V>();
+        if (e != null || !d.orApply(this, b, f, null)) {
+            OrApply<T,U,V> c = new OrApply<T,U,V>(e, d, this, b, f);
+            orpush(b, c);
+            c.tryFire(SYNC);
         }
-        helpPostComplete();
-        other.helpPostComplete();
-        return dst;
+        return d;
     }
 
-    private CompletableFuture<Void> doAcceptEither
-        (CompletableFuture<? extends T> other,
-         Consumer<? super T> fn,
-         Executor e) {
-        if (other == null || fn == null) throw new NullPointerException();
-        CompletableFuture<Void> dst = new CompletableFuture<Void>();
-        AcceptEither<T> d = null;
-        Object r;
-        if ((r = result) == null && (r = other.result) == null) {
-            d = new AcceptEither<T>(this, other, fn, dst, e);
-            CompletionNode q = null, p = new CompletionNode(d);
-            while ((r = result) == null && (r = other.result) == null) {
-                if (q != null) {
-                    if (UNSAFE.compareAndSwapObject
-                        (other, COMPLETIONS, q.next = other.completions, q))
-                        break;
+    @SuppressWarnings("serial")
+    static final class OrAccept<T,U extends T> extends BiCompletion<T,U,Void> {
+        Consumer<? super T> fn;
+        OrAccept(Executor executor, CompletableFuture<Void> dep,
+                 CompletableFuture<T> src,
+                 CompletableFuture<U> snd,
+                 Consumer<? super T> fn) {
+            super(executor, dep, src, snd); this.fn = fn;
+        }
+        final CompletableFuture<Void> tryFire(int mode) {
+            CompletableFuture<Void> d;
+            CompletableFuture<T> a;
+            CompletableFuture<U> b;
+            if ((d = dep) == null ||
+                !d.orAccept(a = src, b = snd, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; snd = null; fn = null;
+            return d.postFire(a, b, mode);
+        }
+    }
+
+    final <R,S extends R> boolean orAccept(CompletableFuture<R> a,
+                                           CompletableFuture<S> b,
+                                           Consumer<? super R> f,
+                                           OrAccept<R,S> c) {
+        Object r; Throwable x;
+        if (a == null || b == null ||
+            ((r = a.result) == null && (r = b.result) == null) || f == null)
+            return false;
+        tryComplete: if (result == null) {
+            try {
+                if (c != null && !c.claim())
+                    return false;
+                if (r instanceof AltResult) {
+                    if ((x = ((AltResult)r).ex) != null) {
+                        completeThrowable(x, r);
+                        break tryComplete;
+                    }
+                    r = null;
                 }
-                else if (UNSAFE.compareAndSwapObject
-                         (this, COMPLETIONS, p.next = completions, p))
-                    q = new CompletionNode(d);
+                @SuppressWarnings("unchecked") R rr = (R) r;
+                f.accept(rr);
+                completeNull();
+            } catch (Throwable ex) {
+                completeThrowable(ex);
             }
         }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            T t; Throwable ex;
-            if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
-            }
-            else {
-                ex = null;
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
-            }
-            if (ex == null) {
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncAccept<T>(t, fn, dst));
-                    else
-                        fn.accept(t);
-                } catch (Throwable rex) {
-                    ex = rex;
+        return true;
+    }
+
+    private <U extends T> CompletableFuture<Void> orAcceptStage(
+        Executor e, CompletionStage<U> o, Consumer<? super T> f) {
+        CompletableFuture<U> b;
+        if (f == null || (b = o.toCompletableFuture()) == null)
+            throw new NullPointerException();
+        CompletableFuture<Void> d = new CompletableFuture<Void>();
+        if (e != null || !d.orAccept(this, b, f, null)) {
+            OrAccept<T,U> c = new OrAccept<T,U>(e, d, this, b, f);
+            orpush(b, c);
+            c.tryFire(SYNC);
+        }
+        return d;
+    }
+
+    @SuppressWarnings("serial")
+    static final class OrRun<T,U> extends BiCompletion<T,U,Void> {
+        Runnable fn;
+        OrRun(Executor executor, CompletableFuture<Void> dep,
+              CompletableFuture<T> src,
+              CompletableFuture<U> snd,
+              Runnable fn) {
+            super(executor, dep, src, snd); this.fn = fn;
+        }
+        final CompletableFuture<Void> tryFire(int mode) {
+            CompletableFuture<Void> d;
+            CompletableFuture<T> a;
+            CompletableFuture<U> b;
+            if ((d = dep) == null ||
+                !d.orRun(a = src, b = snd, fn, mode > 0 ? null : this))
+                return null;
+            dep = null; src = null; snd = null; fn = null;
+            return d.postFire(a, b, mode);
+        }
+    }
+
+    final boolean orRun(CompletableFuture<?> a, CompletableFuture<?> b,
+                        Runnable f, OrRun<?,?> c) {
+        Object r; Throwable x;
+        if (a == null || b == null ||
+            ((r = a.result) == null && (r = b.result) == null) || f == null)
+            return false;
+        if (result == null) {
+            try {
+                if (c != null && !c.claim())
+                    return false;
+                if (r instanceof AltResult && (x = ((AltResult)r).ex) != null)
+                    completeThrowable(x, r);
+                else {
+                    f.run();
+                    completeNull();
                 }
+            } catch (Throwable ex) {
+                completeThrowable(ex);
             }
-            if (e == null || ex != null)
-                dst.internalComplete(null, ex);
         }
-        helpPostComplete();
-        other.helpPostComplete();
-        return dst;
+        return true;
+    }
+
+    private CompletableFuture<Void> orRunStage(Executor e, CompletionStage<?> o,
+                                               Runnable f) {
+        CompletableFuture<?> b;
+        if (f == null || (b = o.toCompletableFuture()) == null)
+            throw new NullPointerException();
+        CompletableFuture<Void> d = new CompletableFuture<Void>();
+        if (e != null || !d.orRun(this, b, f, null)) {
+            OrRun<T,?> c = new OrRun<>(e, d, this, b, f);
+            orpush(b, c);
+            c.tryFire(SYNC);
+        }
+        return d;
     }
 
-    private CompletableFuture<Void> doRunAfterEither
-        (CompletableFuture<?> other,
-         Runnable action,
-         Executor e) {
-        if (other == null || action == null) throw new NullPointerException();
-        CompletableFuture<Void> dst = new CompletableFuture<Void>();
-        RunAfterEither d = null;
+    @SuppressWarnings("serial")
+    static final class OrRelay<T,U> extends BiCompletion<T,U,Object> { // for Or
+        OrRelay(CompletableFuture<Object> dep, CompletableFuture<T> src,
+                CompletableFuture<U> snd) {
+            super(null, dep, src, snd);
+        }
+        final CompletableFuture<Object> tryFire(int mode) {
+            CompletableFuture<Object> d;
+            CompletableFuture<T> a;
+            CompletableFuture<U> b;
+            if ((d = dep) == null || !d.orRelay(a = src, b = snd))
+                return null;
+            src = null; snd = null; dep = null;
+            return d.postFire(a, b, mode);
+        }
+    }
+
+    final boolean orRelay(CompletableFuture<?> a, CompletableFuture<?> b) {
         Object r;
-        if ((r = result) == null && (r = other.result) == null) {
-            d = new RunAfterEither(this, other, action, dst, e);
-            CompletionNode q = null, p = new CompletionNode(d);
-            while ((r = result) == null && (r = other.result) == null) {
-                if (q != null) {
-                    if (UNSAFE.compareAndSwapObject
-                        (other, COMPLETIONS, q.next = other.completions, q))
-                        break;
-                }
-                else if (UNSAFE.compareAndSwapObject
-                         (this, COMPLETIONS, p.next = completions, p))
-                    q = new CompletionNode(d);
+        if (a == null || b == null ||
+            ((r = a.result) == null && (r = b.result) == null))
+            return false;
+        if (result == null)
+            completeRelay(r);
+        return true;
+    }
+
+    /** Recursively constructs a tree of completions. */
+    static CompletableFuture<Object> orTree(CompletableFuture<?>[] cfs,
+                                            int lo, int hi) {
+        CompletableFuture<Object> d = new CompletableFuture<Object>();
+        if (lo <= hi) {
+            CompletableFuture<?> a, b;
+            int mid = (lo + hi) >>> 1;
+            if ((a = (lo == mid ? cfs[lo] :
+                      orTree(cfs, lo, mid))) == null ||
+                (b = (lo == hi ? a : (hi == mid+1) ? cfs[hi] :
+                      orTree(cfs, mid+1, hi)))  == null)
+                throw new NullPointerException();
+            if (!d.orRelay(a, b)) {
+                OrRelay<?,?> c = new OrRelay<>(d, a, b);
+                a.orpush(b, c);
+                c.tryFire(SYNC);
             }
         }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            Throwable ex;
-            if (r instanceof AltResult)
-                ex = ((AltResult)r).ex;
-            else
-                ex = null;
-            if (ex == null) {
-                try {
-                    if (e != null)
-                        execAsync(e, new AsyncRun(action, dst));
-                    else
-                        action.run();
-                } catch (Throwable rex) {
-                    ex = rex;
+        return d;
+    }
+
+    /* ------------- Zero-input Async forms -------------- */
+
+    @SuppressWarnings("serial")
+    static final class AsyncSupply<T> extends ForkJoinTask<Void>
+            implements Runnable, AsynchronousCompletionTask {
+        CompletableFuture<T> dep; Supplier<T> fn;
+        AsyncSupply(CompletableFuture<T> dep, Supplier<T> fn) {
+            this.dep = dep; this.fn = fn;
+        }
+
+        public final Void getRawResult() { return null; }
+        public final void setRawResult(Void v) {}
+        public final boolean exec() { run(); return true; }
+
+        public void run() {
+            CompletableFuture<T> d; Supplier<T> f;
+            if ((d = dep) != null && (f = fn) != null) {
+                dep = null; fn = null;
+                if (d.result == null) {
+                    try {
+                        d.completeValue(f.get());
+                    } catch (Throwable ex) {
+                        d.completeThrowable(ex);
+                    }
                 }
+                d.postComplete();
             }
-            if (e == null || ex != null)
-                dst.internalComplete(null, ex);
         }
-        helpPostComplete();
-        other.helpPostComplete();
-        return dst;
     }
 
-    private <U> CompletableFuture<U> doThenCompose
-        (Function<? super T, ? extends CompletionStage<U>> fn,
-         Executor e) {
-        if (fn == null) throw new NullPointerException();
-        CompletableFuture<U> dst = null;
-        ThenCompose<T,U> d = null;
-        Object r;
-        if ((r = result) == null) {
-            dst = new CompletableFuture<U>();
-            CompletionNode p = new CompletionNode
-                (d = new ThenCompose<T,U>(this, fn, dst, e));
-            while ((r = result) == null) {
-                if (UNSAFE.compareAndSwapObject
-                    (this, COMPLETIONS, p.next = completions, p))
-                    break;
-            }
+    static <U> CompletableFuture<U> asyncSupplyStage(Executor e,
+                                                     Supplier<U> f) {
+        if (f == null) throw new NullPointerException();
+        CompletableFuture<U> d = new CompletableFuture<U>();
+        e.execute(new AsyncSupply<U>(d, f));
+        return d;
+    }
+
+    @SuppressWarnings("serial")
+    static final class AsyncRun extends ForkJoinTask<Void>
+            implements Runnable, AsynchronousCompletionTask {
+        CompletableFuture<Void> dep; Runnable fn;
+        AsyncRun(CompletableFuture<Void> dep, Runnable fn) {
+            this.dep = dep; this.fn = fn;
         }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            T t; Throwable ex;
-            if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
-            }
-            else {
-                ex = null;
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
-            }
-            if (ex == null) {
-                if (e != null) {
-                    if (dst == null)
-                        dst = new CompletableFuture<U>();
-                    execAsync(e, new AsyncCompose<T,U>(t, fn, dst));
-                }
-                else {
+
+        public final Void getRawResult() { return null; }
+        public final void setRawResult(Void v) {}
+        public final boolean exec() { run(); return true; }
+
+        public void run() {
+            CompletableFuture<Void> d; Runnable f;
+            if ((d = dep) != null && (f = fn) != null) {
+                dep = null; fn = null;
+                if (d.result == null) {
                     try {
-                        CompletionStage<U> cs = fn.apply(t);
-                        if (cs == null ||
-                            (dst = cs.toCompletableFuture()) == null)
-                            ex = new NullPointerException();
-                    } catch (Throwable rex) {
-                        ex = rex;
+                        f.run();
+                        d.completeNull();
+                    } catch (Throwable ex) {
+                        d.completeThrowable(ex);
                     }
                 }
-            }
-            if (dst == null)
-                dst = new CompletableFuture<U>();
-            if (ex != null)
-                dst.internalComplete(null, ex);
-        }
-        helpPostComplete();
-        dst.helpPostComplete();
-        return dst;
-    }
-
-    private CompletableFuture<T> doWhenComplete
-        (BiConsumer<? super T, ? super Throwable> fn,
-         Executor e) {
-        if (fn == null) throw new NullPointerException();
-        CompletableFuture<T> dst = new CompletableFuture<T>();
-        WhenCompleteCompletion<T> d = null;
-        Object r;
-        if ((r = result) == null) {
-            CompletionNode p =
-                new CompletionNode(d = new WhenCompleteCompletion<T>
-                                   (this, fn, dst, e));
-            while ((r = result) == null) {
-                if (UNSAFE.compareAndSwapObject(this, COMPLETIONS,
-                                                p.next = completions, p))
-                    break;
+                d.postComplete();
             }
         }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            T t; Throwable ex;
-            if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
+    }
+
+    static CompletableFuture<Void> asyncRunStage(Executor e, Runnable f) {
+        if (f == null) throw new NullPointerException();
+        CompletableFuture<Void> d = new CompletableFuture<Void>();
+        e.execute(new AsyncRun(d, f));
+        return d;
+    }
+
+    /* ------------- Signallers -------------- */
+
+    /**
+     * Completion for recording and releasing a waiting thread.  This
+     * class implements ManagedBlocker to avoid starvation when
+     * blocking actions pile up in ForkJoinPools.
+     */
+    @SuppressWarnings("serial")
+    static final class Signaller extends Completion
+        implements ForkJoinPool.ManagedBlocker {
+        long nanos;                    // wait time if timed
+        final long deadline;           // non-zero if timed
+        volatile int interruptControl; // > 0: interruptible, < 0: interrupted
+        volatile Thread thread;
+
+        Signaller(boolean interruptible, long nanos, long deadline) {
+            this.thread = Thread.currentThread();
+            this.interruptControl = interruptible ? 1 : 0;
+            this.nanos = nanos;
+            this.deadline = deadline;
+        }
+        final CompletableFuture<?> tryFire(int ignore) {
+            Thread w; // no need to atomically claim
+            if ((w = thread) != null) {
+                thread = null;
+                LockSupport.unpark(w);
             }
-            else {
-                ex = null;
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
+            return null;
+        }
+        public boolean isReleasable() {
+            if (thread == null)
+                return true;
+            if (Thread.interrupted()) {
+                int i = interruptControl;
+                interruptControl = -1;
+                if (i > 0)
+                    return true;
             }
-            Throwable dx = null;
-            try {
-                if (e != null)
-                    execAsync(e, new AsyncWhenComplete<T>(t, ex, fn, dst));
-                else
-                    fn.accept(t, ex);
-            } catch (Throwable rex) {
-                dx = rex;
+            if (deadline != 0L &&
+                (nanos <= 0L || (nanos = deadline - System.nanoTime()) <= 0L)) {
+                thread = null;
+                return true;
             }
-            if (e == null || dx != null)
-                dst.internalComplete(t, ex != null ? ex : dx);
+            return false;
+        }
+        public boolean block() {
+            if (isReleasable())
+                return true;
+            else if (deadline == 0L)
+                LockSupport.park(this);
+            else if (nanos > 0L)
+                LockSupport.parkNanos(this, nanos);
+            return isReleasable();
         }
-        helpPostComplete();
-        return dst;
+        final boolean isLive() { return thread != null; }
     }
 
-    private <U> CompletableFuture<U> doHandle
-        (BiFunction<? super T, Throwable, ? extends U> fn,
-         Executor e) {
-        if (fn == null) throw new NullPointerException();
-        CompletableFuture<U> dst = new CompletableFuture<U>();
-        HandleCompletion<T,U> d = null;
+    /**
+     * Returns raw result after waiting, or null if interruptible and
+     * interrupted.
+     */
+    private Object waitingGet(boolean interruptible) {
+        Signaller q = null;
+        boolean queued = false;
+        int spins = -1;
         Object r;
-        if ((r = result) == null) {
-            CompletionNode p =
-                new CompletionNode(d = new HandleCompletion<T,U>
-                                   (this, fn, dst, e));
-            while ((r = result) == null) {
-                if (UNSAFE.compareAndSwapObject(this, COMPLETIONS,
-                                                p.next = completions, p))
-                    break;
+        while ((r = result) == null) {
+            if (spins < 0)
+                spins = (Runtime.getRuntime().availableProcessors() > 1) ?
+                    1 << 8 : 0; // Use brief spin-wait on multiprocessors
+            else if (spins > 0) {
+                if (ThreadLocalRandom.nextSecondarySeed() >= 0)
+                    --spins;
+            }
+            else if (q == null)
+                q = new Signaller(interruptible, 0L, 0L);
+            else if (!queued)
+                queued = tryPushStack(q);
+            else if (interruptible && q.interruptControl < 0) {
+                q.thread = null;
+                cleanStack();
+                return null;
+            }
+            else if (q.thread != null && result == null) {
+                try {
+                    ForkJoinPool.managedBlock(q);
+                } catch (InterruptedException ie) {
+                    q.interruptControl = -1;
+                }
             }
         }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            T t; Throwable ex;
-            if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
+        if (q != null) {
+            q.thread = null;
+            if (q.interruptControl < 0) {
+                if (interruptible)
+                    r = null; // report interruption
+                else
+                    Thread.currentThread().interrupt();
             }
-            else {
-                ex = null;
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
+        }
+        postComplete();
+        return r;
+    }
+
+    /**
+     * Returns raw result after waiting, or null if interrupted, or
+     * throws TimeoutException on timeout.
+     */
+    private Object timedGet(long nanos) throws TimeoutException {
+        if (Thread.interrupted())
+            return null;
+        if (nanos <= 0L)
+            throw new TimeoutException();
+        long d = System.nanoTime() + nanos;
+        Signaller q = new Signaller(true, nanos, d == 0L ? 1L : d); // avoid 0
+        boolean queued = false;
+        Object r;
+        // We intentionally don't spin here (as waitingGet does) because
+        // the call to nanoTime() above acts much like a spin.
+        while ((r = result) == null) {
+            if (!queued)
+                queued = tryPushStack(q);
+            else if (q.interruptControl < 0 || q.nanos <= 0L) {
+                q.thread = null;
+                cleanStack();
+                if (q.interruptControl < 0)
+                    return null;
+                throw new TimeoutException();
             }
-            U u = null;
-            Throwable dx = null;
-            try {
-                if (e != null)
-                    execAsync(e, new AsyncCombine<T,Throwable,U>(t, ex, fn, dst));
-                else {
-                    u = fn.apply(t, ex);
-                    dx = null;
+            else if (q.thread != null && result == null) {
+                try {
+                    ForkJoinPool.managedBlock(q);
+                } catch (InterruptedException ie) {
+                    q.interruptControl = -1;
                 }
-            } catch (Throwable rex) {
-                dx = rex;
-                u = null;
             }
-            if (e == null || dx != null)
-                dst.internalComplete(u, dx);
         }
-        helpPostComplete();
-        return dst;
+        if (q.interruptControl < 0)
+            r = null;
+        q.thread = null;
+        postComplete();
+        return r;
     }
 
-
-    // public methods
+    /* ------------- public methods -------------- */
 
     /**
      * Creates a new incomplete CompletableFuture.
@@ -2113,6 +1785,13 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
     public CompletableFuture() {
     }
 
+    /**
+     * Creates a new complete CompletableFuture with given encoded result.
+     */
+    private CompletableFuture(Object r) {
+        this.result = r;
+    }
+
     /**
      * Returns a new CompletableFuture that is asynchronously completed
      * by a task running in the {@link ForkJoinPool#commonPool()} with
@@ -2124,10 +1803,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * @return the new CompletableFuture
      */
     public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier) {
-        if (supplier == null) throw new NullPointerException();
-        CompletableFuture<U> f = new CompletableFuture<U>();
-        execAsync(ForkJoinPool.commonPool(), new AsyncSupply<U>(supplier, f));
-        return f;
+        return asyncSupplyStage(asyncPool, supplier);
     }
 
     /**
@@ -2143,11 +1819,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      */
     public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier,
                                                        Executor executor) {
-        if (executor == null || supplier == null)
-            throw new NullPointerException();
-        CompletableFuture<U> f = new CompletableFuture<U>();
-        execAsync(executor, new AsyncSupply<U>(supplier, f));
-        return f;
+        return asyncSupplyStage(screenExecutor(executor), supplier);
     }
 
     /**
@@ -2160,10 +1832,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * @return the new CompletableFuture
      */
     public static CompletableFuture<Void> runAsync(Runnable runnable) {
-        if (runnable == null) throw new NullPointerException();
-        CompletableFuture<Void> f = new CompletableFuture<Void>();
-        execAsync(ForkJoinPool.commonPool(), new AsyncRun(runnable, f));
-        return f;
+        return asyncRunStage(asyncPool, runnable);
     }
 
     /**
@@ -2178,11 +1847,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      */
     public static CompletableFuture<Void> runAsync(Runnable runnable,
                                                    Executor executor) {
-        if (executor == null || runnable == null)
-            throw new NullPointerException();
-        CompletableFuture<Void> f = new CompletableFuture<Void>();
-        execAsync(executor, new AsyncRun(runnable, f));
-        return f;
+        return asyncRunStage(screenExecutor(executor), runnable);
     }
 
     /**
@@ -2194,9 +1859,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * @return the completed CompletableFuture
      */
     public static <U> CompletableFuture<U> completedFuture(U value) {
-        CompletableFuture<U> f = new CompletableFuture<U>();
-        f.result = (value == null) ? NIL : value;
-        return f;
+        return new CompletableFuture<U>((value == null) ? NIL : value);
     }
 
     /**
@@ -2220,21 +1883,8 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * while waiting
      */
     public T get() throws InterruptedException, ExecutionException {
-        Object r; Throwable ex, cause;
-        if ((r = result) == null && (r = waitingGet(true)) == null)
-            throw new InterruptedException();
-        if (!(r instanceof AltResult)) {
-            @SuppressWarnings("unchecked") T tr = (T) r;
-            return tr;
-        }
-        if ((ex = ((AltResult)r).ex) == null)
-            return null;
-        if (ex instanceof CancellationException)
-            throw (CancellationException)ex;
-        if ((ex instanceof CompletionException) &&
-            (cause = ex.getCause()) != null)
-            ex = cause;
-        throw new ExecutionException(ex);
+        Object r;
+        return reportGet((r = result) == null ? waitingGet(true) : r);
     }
 
     /**
@@ -2252,24 +1902,9 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      */
     public T get(long timeout, TimeUnit unit)
         throws InterruptedException, ExecutionException, TimeoutException {
-        Object r; Throwable ex, cause;
+        Object r;
         long nanos = unit.toNanos(timeout);
-        if (Thread.interrupted())
-            throw new InterruptedException();
-        if ((r = result) == null)
-            r = timedAwaitDone(nanos);
-        if (!(r instanceof AltResult)) {
-            @SuppressWarnings("unchecked") T tr = (T) r;
-            return tr;
-        }
-        if ((ex = ((AltResult)r).ex) == null)
-            return null;
-        if (ex instanceof CancellationException)
-            throw (CancellationException)ex;
-        if ((ex instanceof CompletionException) &&
-            (cause = ex.getCause()) != null)
-            ex = cause;
-        throw new ExecutionException(ex);
+        return reportGet((r = result) == null ? timedGet(nanos) : r);
     }
 
     /**
@@ -2287,20 +1922,8 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * exceptionally or a completion computation threw an exception
      */
     public T join() {
-        Object r; Throwable ex;
-        if ((r = result) == null)
-            r = waitingGet(false);
-        if (!(r instanceof AltResult)) {
-            @SuppressWarnings("unchecked") T tr = (T) r;
-            return tr;
-        }
-        if ((ex = ((AltResult)r).ex) == null)
-            return null;
-        if (ex instanceof CancellationException)
-            throw (CancellationException)ex;
-        if (ex instanceof CompletionException)
-            throw (CompletionException)ex;
-        throw new CompletionException(ex);
+        Object r;
+        return reportJoin((r = result) == null ? waitingGet(false) : r);
     }
 
     /**
@@ -2314,20 +1937,8 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * exceptionally or a completion computation threw an exception
      */
     public T getNow(T valueIfAbsent) {
-        Object r; Throwable ex;
-        if ((r = result) == null)
-            return valueIfAbsent;
-        if (!(r instanceof AltResult)) {
-            @SuppressWarnings("unchecked") T tr = (T) r;
-            return tr;
-        }
-        if ((ex = ((AltResult)r).ex) == null)
-            return null;
-        if (ex instanceof CancellationException)
-            throw (CancellationException)ex;
-        if (ex instanceof CompletionException)
-            throw (CompletionException)ex;
-        throw new CompletionException(ex);
+        Object r;
+        return ((r = result) == null) ? valueIfAbsent : reportJoin(r);
     }
 
     /**
@@ -2339,9 +1950,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * to transition to a completed state, else {@code false}
      */
     public boolean complete(T value) {
-        boolean triggered = result == null &&
-            UNSAFE.compareAndSwapObject(this, RESULT, null,
-                                        value == null ? NIL : value);
+        boolean triggered = completeValue(value);
         postComplete();
         return triggered;
     }
@@ -2356,244 +1965,200 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      */
     public boolean completeExceptionally(Throwable ex) {
         if (ex == null) throw new NullPointerException();
-        boolean triggered = result == null &&
-            UNSAFE.compareAndSwapObject(this, RESULT, null, new AltResult(ex));
+        boolean triggered = internalComplete(new AltResult(ex));
         postComplete();
         return triggered;
     }
 
-    // CompletionStage methods
-
-    public <U> CompletableFuture<U> thenApply
-        (Function<? super T,? extends U> fn) {
-        return doThenApply(fn, null);
+    public <U> CompletableFuture<U> thenApply(
+        Function<? super T,? extends U> fn) {
+        return uniApplyStage(null, fn);
     }
 
-    public <U> CompletableFuture<U> thenApplyAsync
-        (Function<? super T,? extends U> fn) {
-        return doThenApply(fn, ForkJoinPool.commonPool());
+    public <U> CompletableFuture<U> thenApplyAsync(
+        Function<? super T,? extends U> fn) {
+        return uniApplyStage(asyncPool, fn);
     }
 
-    public <U> CompletableFuture<U> thenApplyAsync
-        (Function<? super T,? extends U> fn,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doThenApply(fn, executor);
+    public <U> CompletableFuture<U> thenApplyAsync(
+        Function<? super T,? extends U> fn, Executor executor) {
+        return uniApplyStage(screenExecutor(executor), fn);
     }
 
-    public CompletableFuture<Void> thenAccept
-        (Consumer<? super T> action) {
-        return doThenAccept(action, null);
+    public CompletableFuture<Void> thenAccept(Consumer<? super T> action) {
+        return uniAcceptStage(null, action);
     }
 
-    public CompletableFuture<Void> thenAcceptAsync
-        (Consumer<? super T> action) {
-        return doThenAccept(action, ForkJoinPool.commonPool());
+    public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action) {
+        return uniAcceptStage(asyncPool, action);
     }
 
-    public CompletableFuture<Void> thenAcceptAsync
-        (Consumer<? super T> action,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doThenAccept(action, executor);
+    public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action,
+                                                   Executor executor) {
+        return uniAcceptStage(screenExecutor(executor), action);
     }
 
-    public CompletableFuture<Void> thenRun
-        (Runnable action) {
-        return doThenRun(action, null);
+    public CompletableFuture<Void> thenRun(Runnable action) {
+        return uniRunStage(null, action);
     }
 
-    public CompletableFuture<Void> thenRunAsync
-        (Runnable action) {
-        return doThenRun(action, ForkJoinPool.commonPool());
+    public CompletableFuture<Void> thenRunAsync(Runnable action) {
+        return uniRunStage(asyncPool, action);
     }
 
-    public CompletableFuture<Void> thenRunAsync
-        (Runnable action,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doThenRun(action, executor);
+    public CompletableFuture<Void> thenRunAsync(Runnable action,
+                                                Executor executor) {
+        return uniRunStage(screenExecutor(executor), action);
     }
 
-    public <U,V> CompletableFuture<V> thenCombine
-        (CompletionStage<? extends U> other,
-         BiFunction<? super T,? super U,? extends V> fn) {
-        return doThenCombine(other.toCompletableFuture(), fn, null);
+    public <U,V> CompletableFuture<V> thenCombine(
+        CompletionStage<? extends U> other,
+        BiFunction<? super T,? super U,? extends V> fn) {
+        return biApplyStage(null, other, fn);
     }
 
-    public <U,V> CompletableFuture<V> thenCombineAsync
-        (CompletionStage<? extends U> other,
-         BiFunction<? super T,? super U,? extends V> fn) {
-        return doThenCombine(other.toCompletableFuture(), fn,
-                             ForkJoinPool.commonPool());
+    public <U,V> CompletableFuture<V> thenCombineAsync(
+        CompletionStage<? extends U> other,
+        BiFunction<? super T,? super U,? extends V> fn) {
+        return biApplyStage(asyncPool, other, fn);
     }
 
-    public <U,V> CompletableFuture<V> thenCombineAsync
-        (CompletionStage<? extends U> other,
-         BiFunction<? super T,? super U,? extends V> fn,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doThenCombine(other.toCompletableFuture(), fn, executor);
+    public <U,V> CompletableFuture<V> thenCombineAsync(
+        CompletionStage<? extends U> other,
+        BiFunction<? super T,? super U,? extends V> fn, Executor executor) {
+        return biApplyStage(screenExecutor(executor), other, fn);
     }
 
-    public <U> CompletableFuture<Void> thenAcceptBoth
-        (CompletionStage<? extends U> other,
-         BiConsumer<? super T, ? super U> action) {
-        return doThenAcceptBoth(other.toCompletableFuture(), action, null);
+    public <U> CompletableFuture<Void> thenAcceptBoth(
+        CompletionStage<? extends U> other,
+        BiConsumer<? super T, ? super U> action) {
+        return biAcceptStage(null, other, action);
     }
 
-    public <U> CompletableFuture<Void> thenAcceptBothAsync
-        (CompletionStage<? extends U> other,
-         BiConsumer<? super T, ? super U> action) {
-        return doThenAcceptBoth(other.toCompletableFuture(), action,
-                                ForkJoinPool.commonPool());
+    public <U> CompletableFuture<Void> thenAcceptBothAsync(
+        CompletionStage<? extends U> other,
+        BiConsumer<? super T, ? super U> action) {
+        return biAcceptStage(asyncPool, other, action);
     }
 
-    public <U> CompletableFuture<Void> thenAcceptBothAsync
-        (CompletionStage<? extends U> other,
-         BiConsumer<? super T, ? super U> action,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doThenAcceptBoth(other.toCompletableFuture(), action, executor);
+    public <U> CompletableFuture<Void> thenAcceptBothAsync(
+        CompletionStage<? extends U> other,
+        BiConsumer<? super T, ? super U> action, Executor executor) {
+        return biAcceptStage(screenExecutor(executor), other, action);
     }
 
-    public CompletableFuture<Void> runAfterBoth
-        (CompletionStage<?> other,
-         Runnable action) {
-        return doRunAfterBoth(other.toCompletableFuture(), action, null);
+    public CompletableFuture<Void> runAfterBoth(CompletionStage<?> other,
+                                                Runnable action) {
+        return biRunStage(null, other, action);
     }
 
-    public CompletableFuture<Void> runAfterBothAsync
-        (CompletionStage<?> other,
-         Runnable action) {
-        return doRunAfterBoth(other.toCompletableFuture(), action,
-                              ForkJoinPool.commonPool());
+    public CompletableFuture<Void> runAfterBothAsync(CompletionStage<?> other,
+                                                     Runnable action) {
+        return biRunStage(asyncPool, other, action);
     }
 
-    public CompletableFuture<Void> runAfterBothAsync
-        (CompletionStage<?> other,
-         Runnable action,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doRunAfterBoth(other.toCompletableFuture(), action, executor);
+    public CompletableFuture<Void> runAfterBothAsync(CompletionStage<?> other,
+                                                     Runnable action,
+                                                     Executor executor) {
+        return biRunStage(screenExecutor(executor), other, action);
     }
 
-
-    public <U> CompletableFuture<U> applyToEither
-        (CompletionStage<? extends T> other,
-         Function<? super T, U> fn) {
-        return doApplyToEither(other.toCompletableFuture(), fn, null);
+    public <U> CompletableFuture<U> applyToEither(
+        CompletionStage<? extends T> other, Function<? super T, U> fn) {
+        return orApplyStage(null, other, fn);
     }
 
-    public <U> CompletableFuture<U> applyToEitherAsync
-        (CompletionStage<? extends T> other,
-         Function<? super T, U> fn) {
-        return doApplyToEither(other.toCompletableFuture(), fn,
-                               ForkJoinPool.commonPool());
+    public <U> CompletableFuture<U> applyToEitherAsync(
+        CompletionStage<? extends T> other, Function<? super T, U> fn) {
+        return orApplyStage(asyncPool, other, fn);
     }
 
-    public <U> CompletableFuture<U> applyToEitherAsync
-        (CompletionStage<? extends T> other,
-         Function<? super T, U> fn,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doApplyToEither(other.toCompletableFuture(), fn, executor);
+    public <U> CompletableFuture<U> applyToEitherAsync(
+        CompletionStage<? extends T> other, Function<? super T, U> fn,
+        Executor executor) {
+        return orApplyStage(screenExecutor(executor), other, fn);
     }
 
-    public CompletableFuture<Void> acceptEither
-        (CompletionStage<? extends T> other,
-         Consumer<? super T> action) {
-        return doAcceptEither(other.toCompletableFuture(), action, null);
+    public CompletableFuture<Void> acceptEither(
+        CompletionStage<? extends T> other, Consumer<? super T> action) {
+        return orAcceptStage(null, other, action);
     }
 
-    public CompletableFuture<Void> acceptEitherAsync
-        (CompletionStage<? extends T> other,
-         Consumer<? super T> action) {
-        return doAcceptEither(other.toCompletableFuture(), action,
-                              ForkJoinPool.commonPool());
+    public CompletableFuture<Void> acceptEitherAsync(
+        CompletionStage<? extends T> other, Consumer<? super T> action) {
+        return orAcceptStage(asyncPool, other, action);
     }
 
-    public CompletableFuture<Void> acceptEitherAsync
-        (CompletionStage<? extends T> other,
-         Consumer<? super T> action,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doAcceptEither(other.toCompletableFuture(), action, executor);
+    public CompletableFuture<Void> acceptEitherAsync(
+        CompletionStage<? extends T> other, Consumer<? super T> action,
+        Executor executor) {
+        return orAcceptStage(screenExecutor(executor), other, action);
     }
 
     public CompletableFuture<Void> runAfterEither(CompletionStage<?> other,
                                                   Runnable action) {
-        return doRunAfterEither(other.toCompletableFuture(), action, null);
+        return orRunStage(null, other, action);
     }
 
-    public CompletableFuture<Void> runAfterEitherAsync
-        (CompletionStage<?> other,
-         Runnable action) {
-        return doRunAfterEither(other.toCompletableFuture(), action,
-                                ForkJoinPool.commonPool());
+    public CompletableFuture<Void> runAfterEitherAsync(CompletionStage<?> other,
+                                                       Runnable action) {
+        return orRunStage(asyncPool, other, action);
     }
 
-    public CompletableFuture<Void> runAfterEitherAsync
-        (CompletionStage<?> other,
-         Runnable action,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doRunAfterEither(other.toCompletableFuture(), action, executor);
+    public CompletableFuture<Void> runAfterEitherAsync(CompletionStage<?> other,
+                                                       Runnable action,
+                                                       Executor executor) {
+        return orRunStage(screenExecutor(executor), other, action);
     }
 
-    public <U> CompletableFuture<U> thenCompose
-        (Function<? super T, ? extends CompletionStage<U>> fn) {
-        return doThenCompose(fn, null);
+    public <U> CompletableFuture<U> thenCompose(
+        Function<? super T, ? extends CompletionStage<U>> fn) {
+        return uniComposeStage(null, fn);
     }
 
-    public <U> CompletableFuture<U> thenComposeAsync
-        (Function<? super T, ? extends CompletionStage<U>> fn) {
-        return doThenCompose(fn, ForkJoinPool.commonPool());
+    public <U> CompletableFuture<U> thenComposeAsync(
+        Function<? super T, ? extends CompletionStage<U>> fn) {
+        return uniComposeStage(asyncPool, fn);
     }
 
-    public <U> CompletableFuture<U> thenComposeAsync
-        (Function<? super T, ? extends CompletionStage<U>> fn,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doThenCompose(fn, executor);
+    public <U> CompletableFuture<U> thenComposeAsync(
+        Function<? super T, ? extends CompletionStage<U>> fn,
+        Executor executor) {
+        return uniComposeStage(screenExecutor(executor), fn);
     }
 
-    public CompletableFuture<T> whenComplete
-        (BiConsumer<? super T, ? super Throwable> action) {
-        return doWhenComplete(action, null);
+    public CompletableFuture<T> whenComplete(
+        BiConsumer<? super T, ? super Throwable> action) {
+        return uniWhenCompleteStage(null, action);
     }
 
-    public CompletableFuture<T> whenCompleteAsync
-        (BiConsumer<? super T, ? super Throwable> action) {
-        return doWhenComplete(action, ForkJoinPool.commonPool());
+    public CompletableFuture<T> whenCompleteAsync(
+        BiConsumer<? super T, ? super Throwable> action) {
+        return uniWhenCompleteStage(asyncPool, action);
     }
 
-    public CompletableFuture<T> whenCompleteAsync
-        (BiConsumer<? super T, ? super Throwable> action,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doWhenComplete(action, executor);
+    public CompletableFuture<T> whenCompleteAsync(
+        BiConsumer<? super T, ? super Throwable> action, Executor executor) {
+        return uniWhenCompleteStage(screenExecutor(executor), action);
     }
 
-    public <U> CompletableFuture<U> handle
-        (BiFunction<? super T, Throwable, ? extends U> fn) {
-        return doHandle(fn, null);
+    public <U> CompletableFuture<U> handle(
+        BiFunction<? super T, Throwable, ? extends U> fn) {
+        return uniHandleStage(null, fn);
     }
 
-    public <U> CompletableFuture<U> handleAsync
-        (BiFunction<? super T, Throwable, ? extends U> fn) {
-        return doHandle(fn, ForkJoinPool.commonPool());
+    public <U> CompletableFuture<U> handleAsync(
+        BiFunction<? super T, Throwable, ? extends U> fn) {
+        return uniHandleStage(asyncPool, fn);
     }
 
-    public <U> CompletableFuture<U> handleAsync
-        (BiFunction<? super T, Throwable, ? extends U> fn,
-         Executor executor) {
-        if (executor == null) throw new NullPointerException();
-        return doHandle(fn, executor);
+    public <U> CompletableFuture<U> handleAsync(
+        BiFunction<? super T, Throwable, ? extends U> fn, Executor executor) {
+        return uniHandleStage(screenExecutor(executor), fn);
     }
 
     /**
-     * Returns this CompletableFuture
+     * Returns this CompletableFuture.
      *
      * @return this CompletableFuture
      */
@@ -2618,52 +2183,13 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * exceptionally
      * @return the new CompletableFuture
      */
-    public CompletableFuture<T> exceptionally
-        (Function<Throwable, ? extends T> fn) {
-        if (fn == null) throw new NullPointerException();
-        CompletableFuture<T> dst = new CompletableFuture<T>();
-        ExceptionCompletion<T> d = null;
-        Object r;
-        if ((r = result) == null) {
-            CompletionNode p =
-                new CompletionNode(d = new ExceptionCompletion<T>
-                                   (this, fn, dst));
-            while ((r = result) == null) {
-                if (UNSAFE.compareAndSwapObject(this, COMPLETIONS,
-                                                p.next = completions, p))
-                    break;
-            }
-        }
-        if (r != null && (d == null || d.compareAndSet(0, 1))) {
-            T t = null; Throwable ex, dx = null;
-            if (r instanceof AltResult) {
-                if ((ex = ((AltResult)r).ex) != null) {
-                    try {
-                        t = fn.apply(ex);
-                    } catch (Throwable rex) {
-                        dx = rex;
-                    }
-                }
-            }
-            else {
-                @SuppressWarnings("unchecked") T tr = (T) r;
-                t = tr;
-            }
-            dst.internalComplete(t, dx);
-        }
-        helpPostComplete();
-        return dst;
+    public CompletableFuture<T> exceptionally(
+        Function<Throwable, ? extends T> fn) {
+        return uniExceptionallyStage(fn);
     }
 
     /* ------------- Arbitrary-arity constructions -------------- */
 
-    /*
-     * The basic plan of attack is to recursively form binary
-     * completion trees of elements. This can be overkill for small
-     * sets, but scales nicely. The And/All vs Or/Any forms use the
-     * same idea, but details differ.
-     */
-
     /**
      * Returns a new CompletableFuture that is completed when all of
      * the given CompletableFutures complete.  If any of the given
@@ -2688,82 +2214,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * {@code null}
      */
     public static CompletableFuture<Void> allOf(CompletableFuture<?>... cfs) {
-        int len = cfs.length; // Directly handle empty and singleton cases
-        if (len > 1)
-            return allTree(cfs, 0, len - 1);
-        else {
-            CompletableFuture<Void> dst = new CompletableFuture<Void>();
-            CompletableFuture<?> f;
-            if (len == 0)
-                dst.result = NIL;
-            else if ((f = cfs[0]) == null)
-                throw new NullPointerException();
-            else {
-                ThenPropagate d = null;
-                CompletionNode p = null;
-                Object r;
-                while ((r = f.result) == null) {
-                    if (d == null)
-                        d = new ThenPropagate(f, dst);
-                    else if (p == null)
-                        p = new CompletionNode(d);
-                    else if (UNSAFE.compareAndSwapObject
-                             (f, COMPLETIONS, p.next = f.completions, p))
-                        break;
-                }
-                if (r != null && (d == null || d.compareAndSet(0, 1)))
-                    dst.internalComplete(null, (r instanceof AltResult) ?
-                                         ((AltResult)r).ex : null);
-                f.helpPostComplete();
-            }
-            return dst;
-        }
-    }
-
-    /**
-     * Recursively constructs an And'ed tree of CompletableFutures.
-     * Called only when array known to have at least two elements.
-     */
-    private static CompletableFuture<Void> allTree(CompletableFuture<?>[] cfs,
-                                                   int lo, int hi) {
-        CompletableFuture<?> fst, snd;
-        int mid = (lo + hi) >>> 1;
-        if ((fst = (lo == mid   ? cfs[lo] : allTree(cfs, lo,    mid))) == null ||
-            (snd = (hi == mid+1 ? cfs[hi] : allTree(cfs, mid+1, hi))) == null)
-            throw new NullPointerException();
-        CompletableFuture<Void> dst = new CompletableFuture<Void>();
-        AndCompletion d = null;
-        CompletionNode p = null, q = null;
-        Object r = null, s = null;
-        while ((r = fst.result) == null || (s = snd.result) == null) {
-            if (d == null)
-                d = new AndCompletion(fst, snd, dst);
-            else if (p == null)
-                p = new CompletionNode(d);
-            else if (q == null) {
-                if (UNSAFE.compareAndSwapObject
-                    (fst, COMPLETIONS, p.next = fst.completions, p))
-                    q = new CompletionNode(d);
-            }
-            else if (UNSAFE.compareAndSwapObject
-                     (snd, COMPLETIONS, q.next = snd.completions, q))
-                break;
-        }
-        if ((r != null || (r = fst.result) != null) &&
-            (s != null || (s = snd.result) != null) &&
-            (d == null || d.compareAndSet(0, 1))) {
-            Throwable ex;
-            if (r instanceof AltResult)
-                ex = ((AltResult)r).ex;
-            else
-                ex = null;
-            if (ex == null && (s instanceof AltResult))
-                ex = ((AltResult)s).ex;
-            dst.internalComplete(null, ex);
-        }
-        fst.helpPostComplete();
-        snd.helpPostComplete();
-        return dst;
+        return andTree(cfs, 0, cfs.length - 1);
     }
 
     /**
@@ -2782,92 +2233,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * {@code null}
      */
     public static CompletableFuture<Object> anyOf(CompletableFuture<?>... cfs) {
-        int len = cfs.length; // Same idea as allOf
-        if (len > 1)
-            return anyTree(cfs, 0, len - 1);
-        else {
-            CompletableFuture<Object> dst = new CompletableFuture<Object>();
-            CompletableFuture<?> f;
-            if (len == 0)
-                ; // skip
-            else if ((f = cfs[0]) == null)
-                throw new NullPointerException();
-            else {
-                ThenCopy<Object> d = null;
-                CompletionNode p = null;
-                Object r;
-                while ((r = f.result) == null) {
-                    if (d == null)
-                        d = new ThenCopy<Object>(f, dst);
-                    else if (p == null)
-                        p = new CompletionNode(d);
-                    else if (UNSAFE.compareAndSwapObject
-                             (f, COMPLETIONS, p.next = f.completions, p))
-                        break;
-                }
-                if (r != null && (d == null || d.compareAndSet(0, 1))) {
-                    Throwable ex; Object t;
-                    if (r instanceof AltResult) {
-                        ex = ((AltResult)r).ex;
-                        t = null;
-                    }
-                    else {
-                        ex = null;
-                        t = r;
-                    }
-                    dst.internalComplete(t, ex);
-                }
-                f.helpPostComplete();
-            }
-            return dst;
-        }
-    }
-
-    /**
-     * Recursively constructs an Or'ed tree of CompletableFutures.
-     */
-    private static CompletableFuture<Object> anyTree(CompletableFuture<?>[] cfs,
-                                                     int lo, int hi) {
-        CompletableFuture<?> fst, snd;
-        int mid = (lo + hi) >>> 1;
-        if ((fst = (lo == mid   ? cfs[lo] : anyTree(cfs, lo,    mid))) == null ||
-            (snd = (hi == mid+1 ? cfs[hi] : anyTree(cfs, mid+1, hi))) == null)
-            throw new NullPointerException();
-        CompletableFuture<Object> dst = new CompletableFuture<Object>();
-        OrCompletion d = null;
-        CompletionNode p = null, q = null;
-        Object r;
-        while ((r = fst.result) == null && (r = snd.result) == null) {
-            if (d == null)
-                d = new OrCompletion(fst, snd, dst);
-            else if (p == null)
-                p = new CompletionNode(d);
-            else if (q == null) {
-                if (UNSAFE.compareAndSwapObject
-                    (fst, COMPLETIONS, p.next = fst.completions, p))
-                    q = new CompletionNode(d);
-            }
-            else if (UNSAFE.compareAndSwapObject
-                     (snd, COMPLETIONS, q.next = snd.completions, q))
-                break;
-        }
-        if ((r != null || (r = fst.result) != null ||
-             (r = snd.result) != null) &&
-            (d == null || d.compareAndSet(0, 1))) {
-            Throwable ex; Object t;
-            if (r instanceof AltResult) {
-                ex = ((AltResult)r).ex;
-                t = null;
-            }
-            else {
-                ex = null;
-                t = r;
-            }
-            dst.internalComplete(t, ex);
-        }
-        fst.helpPostComplete();
-        snd.helpPostComplete();
-        return dst;
+        return orTree(cfs, 0, cfs.length - 1);
     }
 
     /* ------------- Control and status methods -------------- */
@@ -2887,8 +2253,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      */
     public boolean cancel(boolean mayInterruptIfRunning) {
         boolean cancelled = (result == null) &&
-            UNSAFE.compareAndSwapObject
-            (this, RESULT, null, new AltResult(new CancellationException()));
+            internalComplete(new AltResult(new CancellationException()));
         postComplete();
         return cancelled || isCancelled();
     }
@@ -2940,11 +2305,12 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      * Forcibly causes subsequent invocations of method {@link #get()}
      * and related methods to throw the given exception, whether or
      * not already completed. This method is designed for use only in
-     * recovery actions, and even in such situations may result in
-     * ongoing dependent completions using established versus
+     * error recovery actions, and even in such situations may result
+     * in ongoing dependent completions using established versus
      * overwritten outcomes.
      *
      * @param ex the exception
+     * @throws NullPointerException if the exception is null
      */
     public void obtrudeException(Throwable ex) {
         if (ex == null) throw new NullPointerException();
@@ -2962,7 +2328,7 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
      */
     public int getNumberOfDependents() {
         int count = 0;
-        for (CompletionNode p = completions; p != null; p = p.next)
+        for (Completion p = stack; p != null; p = p.next)
             ++count;
         return count;
     }
@@ -2993,20 +2359,19 @@ public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {
     // Unsafe mechanics
     private static final sun.misc.Unsafe UNSAFE;
     private static final long RESULT;
-    private static final long WAITERS;
-    private static final long COMPLETIONS;
+    private static final long STACK;
+    private static final long NEXT;
     static {
         try {
-            UNSAFE = sun.misc.Unsafe.getUnsafe();
+            final sun.misc.Unsafe u;
+            UNSAFE = u = sun.misc.Unsafe.getUnsafe();
             Class<?> k = CompletableFuture.class;
-            RESULT = UNSAFE.objectFieldOffset
-                (k.getDeclaredField("result"));
-            WAITERS = UNSAFE.objectFieldOffset
-                (k.getDeclaredField("waiters"));
-            COMPLETIONS = UNSAFE.objectFieldOffset
-                (k.getDeclaredField("completions"));
-        } catch (Exception e) {
-            throw new Error(e);
+            RESULT = u.objectFieldOffset(k.getDeclaredField("result"));
+            STACK = u.objectFieldOffset(k.getDeclaredField("stack"));
+            NEXT = u.objectFieldOffset
+                (Completion.class.getDeclaredField("next"));
+        } catch (Exception x) {
+            throw new Error(x);
         }
     }
 }

src/share/classes/java/util/concurrent/CompletionStage.java

@@ -407,7 +407,7 @@ public interface CompletionStage<T> {
     /**
      * Returns a new CompletionStage that, when this and the other
      * given stage complete normally, executes the given action using
-     * the supplied executor
+     * the supplied executor.
      *
      * See the {@link CompletionStage} documentation for rules
      * covering exceptional completion.
@@ -569,7 +569,7 @@ public interface CompletionStage<T> {
     /**
      * Returns a new CompletionStage that, when either this or the
      * other given stage complete normally, executes the given action
-     * using supplied executor.
+     * using the supplied executor.
      *
      * See the {@link CompletionStage} documentation for rules
      * covering exceptional completion.
@@ -649,10 +649,15 @@ public interface CompletionStage<T> {
         (Function<Throwable, ? extends T> fn);
 
     /**
-     * Returns a new CompletionStage with the same result or exception
-     * as this stage, and when this stage completes, executes the
-     * given action with the result (or {@code null} if none) and the
-     * exception (or {@code null} if none) of this stage.
+     * Returns a new CompletionStage with the same result or exception as
+     * this stage, that executes the given action when this stage completes.
+     *
+     * <p>When this stage is complete, the given action is invoked with the
+     * result (or {@code null} if none) and the exception (or {@code null}
+     * if none) of this stage as arguments.  The returned stage is completed
+     * when the action returns.  If the supplied action itself encounters an
+     * exception, then the returned stage exceptionally completes with this
+     * exception unless this stage also completed exceptionally.
      *
      * @param action the action to perform
      * @return the new CompletionStage
@@ -661,12 +666,16 @@ public interface CompletionStage<T> {
         (BiConsumer<? super T, ? super Throwable> action);
 
     /**
-     * Returns a new CompletionStage with the same result or exception
-     * as this stage, and when this stage completes, executes the
-     * given action executes the given action using this stage's
-     * default asynchronous execution facility, with the result (or
-     * {@code null} if none) and the exception (or {@code null} if
-     * none) of this stage as arguments.
+     * Returns a new CompletionStage with the same result or exception as
+     * this stage, that executes the given action using this stage's
+     * default asynchronous execution facility when this stage completes.
+     *
+     * <p>When this stage is complete, the given action is invoked with the
+     * result (or {@code null} if none) and the exception (or {@code null}
+     * if none) of this stage as arguments.  The returned stage is completed
+     * when the action returns.  If the supplied action itself encounters an
+     * exception, then the returned stage exceptionally completes with this
+     * exception unless this stage also completed exceptionally.
      *
      * @param action the action to perform
      * @return the new CompletionStage
@@ -675,11 +684,16 @@ public interface CompletionStage<T> {
         (BiConsumer<? super T, ? super Throwable> action);
 
     /**
-     * Returns a new CompletionStage with the same result or exception
-     * as this stage, and when this stage completes, executes using
-     * the supplied Executor, the given action with the result (or
-     * {@code null} if none) and the exception (or {@code null} if
-     * none) of this stage as arguments.
+     * Returns a new CompletionStage with the same result or exception as
+     * this stage, that executes the given action using the supplied
+     * Executor when this stage completes.
+     *
+     * <p>When this stage is complete, the given action is invoked with the
+     * result (or {@code null} if none) and the exception (or {@code null}
+     * if none) of this stage as arguments.  The returned stage is completed
+     * when the action returns.  If the supplied action itself encounters an
+     * exception, then the returned stage exceptionally completes with this
+     * exception unless this stage also completed exceptionally.
      *
      * @param action the action to perform
      * @param executor the executor to use for asynchronous execution
@@ -693,9 +707,11 @@ public interface CompletionStage<T> {
      * Returns a new CompletionStage that, when this stage completes
      * either normally or exceptionally, is executed with this stage's
      * result and exception as arguments to the supplied function.
-     * The given function is invoked with the result (or {@code null}
-     * if none) and the exception (or {@code null} if none) of this
-     * stage when complete as arguments.
+     *
+     * <p>When this stage is complete, the given function is invoked
+     * with the result (or {@code null} if none) and the exception (or
+     * {@code null} if none) of this stage as arguments, and the
+     * function's result is used to complete the returned stage.
      *
      * @param fn the function to use to compute the value of the
      * returned CompletionStage
@@ -710,9 +726,11 @@ public interface CompletionStage<T> {
      * either normally or exceptionally, is executed using this stage's
      * default asynchronous execution facility, with this stage's
      * result and exception as arguments to the supplied function.
-     * The given function is invoked with the result (or {@code null}
-     * if none) and the exception (or {@code null} if none) of this
-     * stage when complete as arguments.
+     *
+     * <p>When this stage is complete, the given function is invoked
+     * with the result (or {@code null} if none) and the exception (or
+     * {@code null} if none) of this stage as arguments, and the
+     * function's result is used to complete the returned stage.
      *
      * @param fn the function to use to compute the value of the
      * returned CompletionStage
@@ -726,10 +744,12 @@ public interface CompletionStage<T> {
      * Returns a new CompletionStage that, when this stage completes
      * either normally or exceptionally, is executed using the
      * supplied executor, with this stage's result and exception as
-     * arguments to the supplied function.  The given function is
-     * invoked with the result (or {@code null} if none) and the
-     * exception (or {@code null} if none) of this stage when complete
-     * as arguments.
+     * arguments to the supplied function.
+     *
+     * <p>When this stage is complete, the given function is invoked
+     * with the result (or {@code null} if none) and the exception (or
+     * {@code null} if none) of this stage as arguments, and the
+     * function's result is used to complete the returned stage.
      *
      * @param fn the function to use to compute the value of the
      * returned CompletionStage