RFR: 8027545: Improve object array chunking test in G1's copy_to_survivor_space

[Kim Barrett]

This is rework of initial change from before the transition to git.
The initial RFR email is attached below.

The primary change is to limit the number of partial array tasks in
the queues for any given array.  The original change just split up an
array into N tasks that were all enqueued at once.  But for a large
array this could be a lot of tasks, leading to significant and
unnecessary queue expansion.  Instead we now limit the number of tasks
for a given array based on the number of workers, and only gradually
add tasks up to that limit.  This gives other threads an opportunity
to steal such tasks, while still keeping queue growth under control.

Most of the calculation for this is handled by a new helper class, so
this can later be shared with ParallelGC.

The dispatch on array klass type for has also been changed.  It now
affirmatively breaks Project Valhalla, rather than quietly doing
something that probably isn't what is actually wanted.  I'll discuss
with them so there is a plan for dealing with it when they take this
update.

Ran tier1-6 in mach5 and some local stress tests.

Performance testing was unchanged from previous, except I wasn't able
to reproduce the small specjbb2015 critical-jops improvement
previously seen on one platform.  My suspicion is that improvement was
a statistical abberation.

--- Initial RFR email ---

RFR: 8158045: Improve large object handling during evacuation
RFR: 8027761: Investigate fast-path for scanning only objects with references during gc
RFR: 8027545: Improve object array chunking test in G1's copy_to_survivor_space

Please review this change to type dispatching and handling in G1's
evacuation copying, in order to improve the hot paths and improve array
handling. This change addresses several closely co-located enhancement
requests; it seemed difficult to split them up in a sensible way.

do_copy_to_survivor_space now gets the klass of the object being copied
once, up front, for use in multiple places. This avoids fetching (including
re-decoding when compressed) the klass multiple times. This addresses part
of JDK-8027545.

Moved check for and handling of string deduplication later, only applying it
after the special array cases have been dealt with, since strings are not
arrays.  (They are header objects pointing to an array of character values.)

Special case typeArray, doing nothing other than the copy, since they
contain no oops that need to be processed.  This addresses JDK-8027761.

Changed handling of objArray, pushing all of the partial array tasks up
front, rather than processing the current chunk after pushing a single task
for the remaining work.  This addresses JDK-8158045.

As part of these, cached some more frequently accessed values in
G1ParScanThreadState member variables. This addresses part of part of
JDK-8027545.

While both the old and new code will work for Project Valhalla, the handling
of arrays should be updated for that project, which introduces new array
types.

Deleted a lingering reference to G1ParScanPartialArrayClosure that was
deleted long ago (JDK-8035330, JDK 9).

CR:
https://bugs.openjdk.java.net/browse/JDK-8158045
https://bugs.openjdk.java.net/browse/JDK-8027761
https://bugs.openjdk.java.net/browse/JDK-8027545

Webrev:
https://cr.openjdk.java.net/~kbarrett/8158045/open.00/

Testing:
tier1-3

performance testing - seems to be at worst performance neutral, with a
statistically significant 3% improvement in specjbb2015 critical-jops seen
on one platform.

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

Commit messages:
 - limit partial array tasks, add length_addr() rather than breaking abstraction
 - Initial RFR for partial_arrays

Changes: https://git.openjdk.java.net/jdk/pull/90/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=90&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8027545
  Stats: 357 lines in 7 files changed: 298 ins; 29 del; 30 mod
  Patch: https://git.openjdk.java.net/jdk/pull/90.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/90/head:pull/90

PR: https://git.openjdk.java.net/jdk/pull/90