RFR: 8276887: G1: Move precleaning to Concurrent Mark From Roots subphase [v2]

[Albert Mingkun Yang]

On Wed, 10 Nov 2021 13:40:09 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Simple change of moving the "Preclean" subphase into "Mark from roots" subphase so that precleaning becomes parallel automatically. Evaluation using a contrived java program shows that multiple GC threads do precleaning. More detailed testing results are available in the JBS ticket.
>> 
>> Test: hotspot_gc
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   worker_id

In the initial patch, the case `ConcGCThreads=1 && ParallelGCThreads>1` is not properly handle. The latest commit fixes that by calling `get_discovered_list` multiple times when the discovery is single-threaded.

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PR: https://git.openjdk.java.net/jdk/pull/6327