RFR: 8338526: Don't store abstract and interface Klasses in class metaspace [v4]

[Markus Grönlund]

On Thu, 29 Aug 2024 17:46:11 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> It's fine to have each Klass* report whether it can be compressed. If not, it will be represented using the non-compressed version, which will be a bit more bloated, but no problems.
>
> narrowKlass is the result of encoding Klass* with CompressedKlassPointers::encode() which is relative to the compressed base, so if UseCompressedClassPointers is false then the encoding to narrowKlass from some other (CDS?) base isn't valid.
> using_class_space() above doesn't look wrong for 32 bits.  It should return false.
> 
> With this patch, interface and abstract classes cannot be encoded and decoded to yield a valid compressed narrowKlass, since they're now allocated in the non-class metaspace.  Yes, this now a function of which space the Klass resides in.  We do crash for these classes in JFR without this change.

I mean its wrong from JFRs perspective not to handle 32-bit outside of the call to using_class_space(), because that call will always be false for 32-bit, although the Klass* will still fit in 4-bytes.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19157#discussion_r1736834488