RFR: 8372250: Merge PtrQueue into SATBMarkQueue [v2]

[Kim Barrett]

On Thu, 11 Dec 2025 00:36:47 GMT, Ben Taylor <btaylor at openjdk.org> wrote:

>> This change merges `PtrQueue` into its only child class, `SATBMarkQueue`. The corresponding `PtrQueueSet` is merged with `SATBMarkQueueSet`, and `G1SATBMarkQueueSet` and `ShenandoahSATBMarkQueueSet` continue to inherit from that class.
>> 
>> There are few code modifications - most methods were moved directly from `PtrQueue` to `SATBMarkQueue`. The one meaningful change is to  preserve `SATBMarkQueue`'s call to `filter(queue);` before `flush_queue`. 
>> 
>> This change passes all tier1 jtreg tests with both Shenandoah and G1 in local testing.
>
> Ben Taylor has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address PR feedback from kimbarrett

commit 148aa4f didn't address this comment:
> [appropriate place is not commentable in github diff]
> SATBMarkQueueSet::enqueue_completed_buffer should no longer be virtual. (This
> would have been caught if it had been changed to "override" once that was
> permitted, but nobody got around to that.)

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PR Comment: https://git.openjdk.org/jdk/pull/28755#issuecomment-3639591608