RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v8]

[Thomas Stuefe]

On Tue, 10 Sep 2024 12:19:32 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> This is tricky. We are already deep in initialization and have done a couple of decisions based on +UseCompressedClassPointers (e.g. CDS setup). I *think* we could still go with -UseCCP, but I wonder whether this is wise. 
>> 
>> Note that this error is not new. In the old code, we simply asserted. That left us with UB in release builds, which remains unresolved. I simply made the error explicit in release too.
>
> Ok, in this case, that's fine if we already asserted.  A fatal error is better.

Actually, a lot of the old code had dusty side corners that were UB. Making narrowKlass smaller than 32bit exposed a lot of them, and a lot of the changes in and around CompressedKlassPointers are about cleanly making explicit what before had been implicit or just broken (e.g. a clear distinction between encoding range and Klass range, and a clear handling of narrowKlass bit width as a runtime value).

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1758522844