RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v21]

[Thomas Stuefe]

On Thu, 19 Sep 2024 11:43:12 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> src/hotspot/share/oops/compressedKlass.cpp line 231:
>> 
>>> 229:     // The reason is that we want to avoid, if possible, shifts larger than
>>> 230:     // a cacheline size.
>>> 231:     _base = addr;
>> 
>> Why is this important?
>
> It lessens the cache effects of Klass hyperaligning.

Note that if we go with my KLUT proposal for post-Lilliput (the GC oop iteration improvements), this will not matter anymore and can be simplified to a fixed shift of 10.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1766688756