RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v9]

[Coleen Phillimore]

On Wed, 18 Sep 2024 12:56:16 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> src/hotspot/share/oops/oop.inline.hpp line 90:
>> 
>>> 88:   } else {
>>> 89:     return markWord::prototype();
>>> 90:   }
>> 
>> Could this be unconditional since prototoype_header is initialized for all Klasses?
>
> yes, but there is ongoing effort (at Oracle) to get rid of ```Klass::_prototype_header``` altogether. Let's wait for that and see how it looks then.

Yes, I saw that patch.  I'm not sure I like the idea of cpu dependent code also doing the encoding.  There were some C2 changes related to it that I didn't understand if that scheme required them.  I don't see the down side to having the prototype header pre-encoded in the markWord.  Seems simpler.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1765893566