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

[Per Liden]

Looks good!

/Per

On 2018-06-27 15:10, Stefan Karlsson 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
>