On Thu, 29 Aug 2024 17:26:57 GMT, Markus Grönlund <mgron...@openjdk.org> wrote:
>> 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.
In summary, we are agnostic about which space the Klass* is located in; we only
care if a valid means exists to perform an encode() and decode() operation to
compress the Klass* (for 64-bit to be clear).
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PR Review Comment: https://git.openjdk.org/jdk/pull/19157#discussion_r1736786419
