RFR: 8274286: Skip null for make_referent_alive in referenceProcessor [v3]

[Albert Mingkun Yang]

On Mon, 4 Oct 2021 12:01:29 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Simple change of avoiding a virtual call for null referent. With this change, two methods of processing the discovered list, `process_discovered_list_work` and `preclean_discovered_reflist`, share the same structure.
>> 
>> Test: hotspot_gc
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   comment

Thanks for the review.

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PR: https://git.openjdk.java.net/jdk/pull/5682