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

[Thomas Stuefe]

On Wed, 23 Oct 2024 18:14:50 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

> 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.

It's a bit late. We are close to pushing. While it should be harmless to drop below alignment to below cache line size, this would be a change affecting all platforms and would require all tests repeated. 

PPC/s390 are not targeted by the JEP. There had never been a discussion I am aware of that these platforms have to be clean with +COH. While it's nice that the changes had been contributed, I don't think that test errors on these platforms should hold up pushing this RFE. Therefore, if needed, we should just omit +COH part of the test for PPC/S390.

But then, what exactly is the error? If it's just the test assuming that cache line size is log 6, then the test should be fixed for ppc, not hotspot.

> 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.

?? The comment is correct. We try to avoid hyper alignment, hence we drop the shift to - if possible - log 2 cache line size. If it's equal to log 2 cache line size, we succeeded.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1814598543