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

[Stefan Karlsson]

On Thu, 19 Sep 2024 11:54:50 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> We already have a cpu dependent code for both C1 and the interpreter. Adding cpu dependent code to C2 doesn't make it significantly worse. My latest patch also refactors the code so that C1, interpreter, and C2 all calls into shared functions in the macro assembler.
>
> 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 are currently running it through functional testing.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1766934571