RFR: 8289739: Add G1 specific GC breakpoints for testing

[Albert Mingkun Yang]

On Tue, 5 Jul 2022 11:35:19 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   can I have reviews for this change that adds a few G1 specific GC breakpoints for future testing [JDK-8289740](https://bugs.openjdk.org/browse/JDK-8289740).
> 
> Testing: gha, local testing
> 
> Thanks,
>   Thomas

It's unclear to me why "BEFORE REBUILD COMPLETED" is placed inside `phase_cleanup`; a more natural place to me is at the end of `G1ConcurrentMarkThread::phase_rebuild_remembered_sets`. (The same goes for the other pair of breakpoints.)

Sort of a preexisting issue but this change builds on top of it: "cleanup" can mean both the "cleanup pause" (`phase_cleanup`) or the "concurrent bitmap clearing" (`phase_clear_bitmap_for_next_mark`). IMO, it's better to get rid of such overloading.

(Not specific to this change. I also find the breakpoints naming pattern, `before-X-started & after-X-completed`, rather odd -- since they are break**points**, names reflecting a particular point in time, instead of a period, would have been more accurate, sth like `at-X-start & at-X-end`.)

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PR: https://git.openjdk.org/jdk/pull/9376