RFR: 8210064: ZGC: Introduce ZConcurrentRootsIterator for scanning a subset of strong IN_NATIVE roots concurrently

[Kim Barrett]

> On Aug 30, 2018, at 4:46 AM, Erik Österlund <erik.osterlund at oracle.com> wrote:
> 
> Hi,
> 
> We now have enough load barriers to support scanning of CLDs and JNI handles concurrently. I propose to do that and move these root sets out from ZRootsIterator, and hence the GC pause. They will be scanned during concurrent marking (and heap iteration), but no longer during relocation.
> 
> I still perform ClassLoaderDataGraph::clear_claimed_marks() in the pause because it seems cheap. But it can be moved out of the pause when Coleen gets her new cool CLDG lock in.
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8210064/webrev.00/
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8210064
> 
> Tested through hs-tier1-3.
> 
> Thanks,
> /Erik

Looks good.