RFR: 8048180: Eager reclaim of humongous objects with references [v3]

[Thomas Schatzl]

> Hi all,
> 
>   please review this change to G1 that enables eager reclaim of humongous objects with references, either objArrays or just really large objects.
> 
> It works the same as other humongous object's eager reclaim, with the caveat that I disabled it during concurrent mark to avoid problems with SATB. There is fairly extensive documentation about why I think this works, and how in the comment in `g1YoungCollector.cpp`.
> 
> The test case exhaustively tests all combinations of when a particular type of object should be able to be reclaimed depending on when it is allocated (only for `typeArray` and `objArray` - huge objects work the same as the latter).
> 
> I added a diagnostic flag `G1EagerReclaimWithRefs` to allow disabling this particular functionality for diagnostic purposes, but I think it's not really necessary, and I'm open to remove it.
> 
> The following screenshot shows pause times before (purple) and after (brown) of one of the [Optaplanner](https://www.optaplanner.org/blog/2021/09/15/HowMuchFasterIsJava17.html) benchmarks as a result of this change.
>  
> <img width="702" height="485" alt="Screenshot 2025-09-26 at 13 59 05" src="https://github.com/user-attachments/assets/995e9e27-f861-44bd-8ced-3bd952e314e8" />
> 
> Testing: new test cases, gha, tier1-5
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  * more documentation work

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27525/files
  - new: https://git.openjdk.org/jdk/pull/27525/files/44c62fcb..7bf219a8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27525&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27525&range=01-02

  Stats: 57 lines in 1 file changed: 24 ins; 20 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/27525.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27525/head:pull/27525

PR: https://git.openjdk.org/jdk/pull/27525