RFR: 8186670: Implement _onSpinWait() intrinsic for AArch64 [v13]

[Evgeny Astigeevich]

On Thu, 21 Oct 2021 14:37:18 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> @theRealAph, done
>
> Looks good. I'm not entirely sure whether this test is truly representative of the real-world cases that people have seen, but if we find out more we can always add another JMH test.

This test is too artificial. Going through my records I've found I have a microbenchmark for `java.util.concurrent. SynchronousQueue` which shows good improvements on jdk11. `SynchronousQueue` uses `onSpinWait`. Since jdk17 `SynchronousQueue` has not been using `onSpinWait` any more (See https://bugs.openjdk.java.net/browse/JDK-8267502). Maybe I can come up with a microbenchmark based on `SynchronousQueue` [code](https://github.com/openjdk/jdk11u-dev/blob/master/src/java.base/share/classes/java/util/concurrent/SynchronousQueue.java#L412):

        SNode awaitFulfill(SNode s, boolean timed, long nanos) {
            /*
             * When a node/thread is about to block, it sets its waiter
             * field and then rechecks state at least one more time
             * before actually parking, thus covering race vs
             * fulfiller noticing that waiter is non-null so should be
             * woken.
             *
             * When invoked by nodes that appear at the point of call
             * to be at the head of the stack, calls to park are
             * preceded by spins to avoid blocking when producers and
             * consumers are arriving very close in time.  This can
             * happen enough to bother only on multiprocessors.
             *
             * The order of checks for returning out of main loop
             * reflects fact that interrupts have precedence over
             * normal returns, which have precedence over
             * timeouts. (So, on timeout, one last check for match is
             * done before giving up.) Except that calls from untimed
             * SynchronousQueue.{poll/offer} don't check interrupts
             * and don't wait at all, so are trapped in transfer
             * method rather than calling awaitFulfill.
             */
            final long deadline = timed ? System.nanoTime() + nanos : 0L;
            Thread w = Thread.currentThread();
            int spins = shouldSpin(s)
                ? (timed ? MAX_TIMED_SPINS : MAX_UNTIMED_SPINS)
                : 0;
            for (;;) {
                if (w.isInterrupted())
                    s.tryCancel();
                SNode m = s.match;
                if (m != null)
                    return m;
                if (timed) {
                    nanos = deadline - System.nanoTime();
                    if (nanos <= 0L) {
                        s.tryCancel();
                        continue;
                    }
                }
                if (spins > 0) {
                    Thread.onSpinWait();
                    spins = shouldSpin(s) ? (spins - 1) : 0;
                }
                else if (s.waiter == null)
                    s.waiter = w; // establish waiter so can park next iter
                else if (!timed)
                    LockSupport.park(this);
                else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
                    LockSupport.parkNanos(this, nanos);
            }
        }


I've created https://bugs.openjdk.java.net/browse/JDK-8275728 to write such a microbenchmark.

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PR: https://git.openjdk.java.net/jdk/pull/5562