RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v6]

[Roman Kennke]

On Mon, 26 Aug 2024 21:52:58 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix bit counts in GCForwarding
>
> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/oops/Oop.java line 169:
> 
>> 167:         } else {
>> 168:           visitor.doMetadata(klass, true);
>> 169:         }
> 
> Why is there no `visitor.doMetadata()` call for the compact object header case?

There is no dedicated klass field anymore, the Klass* is encoded in the mark, and we would need to extract it. What is the purpose of the visitors? Do they need to see the klass/compressedKlass, or is it sufficient to visit the mark-word (which we already do, but as CInt).

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750386024