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

[Roman Kennke]

On Thu, 19 Sep 2024 05:03:42 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> 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.
>
> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1766697849