RFR (round 1), JDK-8214259: Implementation: JEP 189: Shenandoah: A Low-Pause Garbage Collector

[Roman Kennke]

Hi Kim,

>> Sections to review (at this point) are the following:
>>
>>  *) shared-gc
>>     - This is mostly boilerplate that is common to any GC
>>     - referenceProcessor.cpp has a little change to make one assert not fail (next to CMS and G1)
> 
> Ick.  But I don’t have a better suggestion that doesn’t involve adding a new API
> to CollectedHeap for use by this assertion, which seems a bit excessive if there
> are no other uses.

Yeah.
I guess we could add a config _discovery_is_concurrent or similar in RP,
and check that. Or maybe one of _discovery_is_mt or _discovery_is_atomic
already covers that? I couldn't immediately tell/100% understand their
semantics. Seems worthy to look at after this?

>>     - taskqueue.hpp has some small adjustments to enable subclassing
> 
> Why this change instead of JDK-8204947?  As the description from that RFE says:
> "The ShenandoahTaskTerminator from the Shenandoah project is a much better implementation of a task terminator.”

Yeah, see Zhengyu's comment. Let's ignore those changes for this review
(for now), expect our impl ported to taskqueue.hpp/cpp soon.

Roman