RFR: 8280915: Better parallelization for AbstractSpliterator and IteratorSpliterator when size is unknown [v2]

[Tagir F.Valeev]

> See the bug description for details.
> 
> I propose a simple solution. Let's allow ArraySpliterator to be non-SIZED and report artificial estimatedSize(), much bigger than the real one. This will allow AbstractSpliterator and IteratorSpliterator to produce prefix whose size is comparable to Long.MAX_VALUE (say, starting with Long.MAX_VALUE/2), and this will enable further splitting of the prefix. This change will drastically improve parallel streaming for affected streams of size <= 1024 and significantly improve for streams of size 1025..20000. The cost is higher-grained splitting for huge streams of unknown size. This might add a minor overhead for such scenarios which, I believe, is completely tolerable.
> 
> No public API changes are necessary, sequential processing should not be affected, except an extra field in ArraySpliterator which increases a footprint by 8 bytes.
> 
> I added a simple test to ensure that at least two threads are actually used when parallelizing Stream.iterate source. More testing ideas are welcome.

Tagir F. Valeev has updated the pull request incrementally with one additional commit since the last revision:

  Stabilize test

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7279/files
  - new: https://git.openjdk.java.net/jdk/pull/7279/files/489e95b2..2713d5be

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7279&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7279&range=00-01

  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7279.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7279/head:pull/7279

PR: https://git.openjdk.java.net/jdk/pull/7279