RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v11]

[Thomas Stuefe]

On Wed, 11 Sep 2024 12:25:37 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix FullGCForwarding initialization
>
> src/hotspot/share/memory/classLoaderMetaspace.hpp line 81:
> 
>> 79:   metaspace::MetaspaceArena* class_space_arena() const       { return _class_space_arena; }
>> 80: 
>> 81:   bool have_class_space_arena() const { return _class_space_arena != nullptr; }
> 
> This is unnecessary. Instead of having this and having to remember to check for nullness each time, just change the `_class_space_arena` to point to the same arena as the `_non_class_space_arena` does when we run with `-XX:-UseCompressedClassPointers`

I'd prefer not to. 

This logic (when -UCCP class space arena is NULL, with the implicit assumption that both are different entities) has been in there forever, and changing that is out of scope for and unrelated to this PR. I am not sure what will break if I change this but don't want to chase risk test errors at this point (one example, reporting would have to be adapted to recognize that both arenas are the same, and there are plenty of tests that would also need to be fixd).

This can be done in a follow-up RFE if necessary.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1762917467