RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v11]

[Coleen Phillimore]

On Fri, 20 Sep 2024 17:34:09 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Yes. Some background:
>> 
>> - wastage can only occur for larger Klass* alignments (aka class space arena alignment property), so only for +COH (note to self, maybe assert)
>> - wastage is, by definition, not aligned to the required Klass* alignment, so it cannot be reused. Yes, its probably also too small
>> 
>> Yes, I will write a better comment.
>
> Yes, this definitely needs a comment why since this is how we allocate small chunks of wasted because of hyper-aligning Klasses in class space.   Line 111 is somewhat surprising though.  I didn't expect there to be wastage from allocating to non-class-metaspace.
> 
> The unnerving bit of this is that CompressedKlassPointers::is_encodable() is true for memory allocated here.

I think this should also assert or be condionalized on UseCompactObjectHeaders.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1768972448