RFR: 8319955: Improve dependencies removal during class unloading

[Thomas Schatzl]

On Mon, 13 Nov 2023 18:52:47 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   please review this performance improvement to dependency removal during class unloading: instead of moving the dependencies item by item via a `cmpxchg`, batch this operation.
> 
> This makes that part of class unloading ~7x faster in my measurements (class unloading stress test, loading/unloading 6k classes).
> 
> Testing: tier1-7
> 
> Thanks,
>   Thomas

Concurrent additions to the purge list (during (concurrent) class unloading) should be fine due to the `cmpxchg` used, i.e. callers of `DependencyContext::release()` while the first part of class unloading (i.e. CLD unlinking) is in progress (i.e. `!delete_on_release()`).

This is as before afaict.

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PR Comment: https://git.openjdk.org/jdk/pull/16639#issuecomment-1809952061