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

[Coleen Phillimore]

On Tue, 10 Sep 2024 12:03:59 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> src/hotspot/share/oops/compressedKlass.cpp line 214:
>> 
>>> 212:     ss.print("Class space size (%zu) exceeds the maximum possible size (%zu)",
>>> 213:               len, max_encoding_range_size());
>>> 214:     vm_exit_during_initialization(ss.base());
>> 
>> Why does this exit and not turn off compressed klass pointers and compact object headers?
>
> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1751840556