RFR: 8236641: Improve Set.of(...).iterator() warmup characteristics

[Florian Weimer]

* Claes Redestad:

> we're doing plenty of iterations over Set.of() instances during
> bootstrap, which makes these operations show up prominently in
> startup profiles. This patch refactors a few hot methods to get a
> measureable startup improvement without regressing on targeted
> microbenchmarks.
>
> Bug:    https://bugs.openjdk.java.net/browse/JDK-8236641
> Webrev: http://cr.openjdk.java.net/~redestad/8236641/open.00/
>
> (The patch is baselined against 8236850)

Would it be possible to replace

  idx = SALT % elements.length;

  idx = (SALT % (table.length >> 1)) << 1;

with this?

  idx = (int) ((elements.length * (long) SALT) >>> 32);

  idx = (int) (((table.length >> 1) * (long) SALT) >>> 31);

A long multiplication followed by a shift should be faster than an int
modulo.

See this thread for some background:

<https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-November/040011.html>

Thanks,
Florian