RFR: 8215985: ZGC: Simplify reference processing in light of JDK-8175797

[Kim Barrett]

> On Jan 2, 2019, at 7:09 AM, Erik Österlund <erik.osterlund at oracle.com> wrote:
> 
> Hi Kim,
> 
> Looks good to me.

Thanks!

> Thanks,
> /Erik
> 
> On 2019-01-02 08:31, Kim Barrett wrote:
>> Please review this change to ZGC reference processing to take
>> advantage of the changes made by JDK-8175797, which made
>> Reference.enqueue clear the referent and made Reference.enqueue
>> unsupported by FinalReference.
>> As a result of JDK-8175797, the special treatment of the 'next' field
>> by the GC is no longer needed for most reference type; only
>> FinalReference needs special handling of that field.  This means
>> (except for FinalReference, e.g. REF_FINAL) the 'next' field no longer
>> needs to be examined by discovery, and does not need to be self-looped
>> by notification (eliminating a CAS to deal with possible concurrent
>> enqueue).
>> In discover_reference, the order of tests was changed because the
>> inactive final reference case should be relatively rare.  Doing the
>> "strongly alive or null" test first should be a better order, with the
>> remaining tests being for somewhat uncommon special cases that would
>> pass that discoverability test.
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8215985
>> Webrev:
>> http://cr.openjdk.java.net/~kbarrett/8215985/open.00/
>> Testing:
>> mach5 tier{1-4}, hs-tier{5-8}-gc.