On Thu, 29 Aug 2024 15:45:17 GMT, Coleen Phillimore <cole...@openjdk.org> wrote:
>> If UseCompressedClassPointers is off, we don't have a compressed class
>> space. If its on, Klass from CDS and from class space are compressable. With
>> your patch, interfaces will live in normal metaspace, not int class space,
>> so those are excluded now.
>>
>> TBH, I am not really sure what this code here does, but I assume it tries to
>> reduce the size of a JFR recording by using a compressed identifier for X if
>> X can be expressed by such. Maybe a JFR person should look at this.
>
> 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;
};
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PR Review Comment: https://git.openjdk.org/jdk/pull/19157#discussion_r1736775534
