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

[Coleen Phillimore]

On Thu, 19 Sep 2024 14:22:51 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> We haven't decided whether or not we will git rid of ```Klass::_prototype_header``` before intergrating this PR, or not. @stefank could point you to a WIP branch, if that's helpful.
>
> This is my current work-in-progress code:
> https://github.com/stefank/jdk/compare/pull/20677...stefank:jdk:lilliput_remove_prototype_header_wip_2
> 
> I've made some large rewrites and I'm currently running it through functional testing.

The refactoring is better in this last version with encode_and_store_compact_object_header, although some comments around the c2 version would be good.  Still don't know what the c2 version does.  Someone else should review that.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1769075714