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

[Markus Grönlund]

On Thu, 29 Aug 2024 17:24:47 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

>> With UseCompressedClassPointers off, I think Metaspace::is_in_shared_metaspace() would still return true but I don't think he compression base is the bottom of the CDS archive.  I asked Markus to have a look.
>
> The code supports the JfrTraceID load barrier that enqueues tagged Klass*. It selects a more compact representation (a single word, instead of two words), if a Klass* can be compressed (i.e. there exists a compress class scheme in place (CompressedKlassPointers::encode(const_cast<Klass*>(klass)); AND the traceid (u8) value is low enough to be represented by only 4 bytes).
> 
> struct JfrEpochQueueKlassElement {
>   traceid id;
>   const Klass* klass;
> };
> 
> struct JfrEpochQueueNarrowKlassElement {
>   u4 id;
>   narrowKlass compressed_klass;
> };

// Return TRUE only if UseCompressedClassPointers is True.
  static bool using_class_space() {
    return NOT_LP64(false) LP64_ONLY(UseCompressedClassPointers);
  }
  
  I see now that was wrong for 32-bit.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19157#discussion_r1736779802