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

[Martin Doerr]

On Tue, 22 Oct 2024 13:53:03 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> I will do some benchmarks
>
> I did SpecJBB runs with shift of 6, 8 and 10, respectively, which amounts to Klass alignment of 64, 256 and 1K. Benchmark scores did not show a significant pattern. I did not measure CPU stats though. 
> 
> But I still think a dynamically calculated shift makes sense, and I hesitate to change this code at this point. I therefore would like to move this question to followup RFEs if necessary.

This code causes test errors in `CompressedClassPointersEncodingScheme.java` on s390 and PPC64. It forces the shift to `log_cacheline` which is 7 on PPC64 and 9 on s390. The test passes when we remove "s > log_cacheline && " from the condition below.
In addition, it doesn't fit to the comment which claims we should avoid shifts larger than the cacheline size. This enforces shifts to be larger (or equal to) than the cacheline size.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1813304646