RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v23]

[Coleen Phillimore]

On Fri, 20 Sep 2024 18:11:43 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Merge remote-tracking branch 'lilliput/JEP-450-temporary-fix-branch-2' into JDK-8305895-v4
>>  - review feedback
>
> src/hotspot/share/memory/metaspace/metablock.hpp line 74:
> 
>> 72: #define METABLOCKFORMATARGS(__block__)  p2i((__block__).base()), (__block__).word_size()
>> 73: 
>> 74: } // namespace metaspace
> 
> I am wondering if some of these metaspace changes, that is, the addition of MetaBlock could be upstreamed ahead of the CompactObjectHeaders.  Some is refactoring so that you can use the wastage to allocate into class-arena but a lot of this seems neutral to compact object headers, and would reduce this patch and allow different people to focus on just this.

For the record, I am fine with these metaspace changes going in with this PR if the timing for that is better.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1773607587