RFR: 8205922: Add reference iteration mode that skips visiting the referents

[Erik Osterlund]

Hi Stefan,

Looks good.

Thanks,
/Erik

> On 27 Jun 2018, at 15:10, Stefan Karlsson <stefan.karlsson at oracle.com> wrote:
> 
> Hi all,
> 
> Please review this patch to allow us to skip visiting the j.l.Reference::referent during oop_iteration.
> 
> http://cr.openjdk.java.net/~stefank/8205922/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8205922
> 
> One use-case for this is when ZGC does verification at the end of the Mark End pause. At that point we have complete marking information roots and fields are good, but the processing of j.l.Reference::referents are deferred to the concurrent phase after this pause. In this case we would like to skip applying the verification closure to the referents.
> 
> Thanks,
> StefanK
>