RFR: 8282381: Parallel: Remove unnecessary PCReferenceProcessor

[Kim Barrett]

On Fri, 25 Feb 2022 07:14:21 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple change of removing a redundant class.
> 
> Test: tier1-5

I think this is probably okay, but I'd like to see some performance testing.

The is_unmarked check before calling the generic reference processor's
discover_reference has been present for a long time; it's there in the initial
mercurial version.  The corresponding is_alive_closure for the reference
processor is there too.

It's hard to know whether this was just an unintentional duplication of work,
or if someone thought (or measured) that doing the is_unmarked check early was
beneficial.

Pre-checking for marking is consistent with what's done for collections that
use header marking. It looks like a different way to remove the sometimes
duplicated work would be to not provide the is_alive_non_header closure.
That's what this change should be compared to for performance.

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PR: https://git.openjdk.java.net/jdk/pull/7618