RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v6]

[Chris Plummer]

On Mon, 9 Sep 2024 16:51:35 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> There is no dedicated klass field anymore, the Klass* is encoded in the mark, and we would need to extract it. What is the purpose of the visitors? Do they need to see the klass/compressedKlass, or is it sufficient to visit the mark-word (which we already do, but as CInt).
>
> I've been looking into this. It's a bit hard to follow but I think you do need to do something more here. Can you run ClhsdbInspect.java and send me the output. In specific I need to know if the log includes the following:
> 
> 
> hsdb> + inspect 0x00000007cff154b8
> instance of Oop for java/util/concurrent/locks/AbstractQueuedSynchronizer$ConditionObject @ 0x00000007cff154b8 (size = 24)
> _mark: 1
> _metadata._compressed_klass: InstanceKlass for java/util/concurrent/locks/AbstractQueuedSynchronizer$ConditionObject
> firstWaiter: Oop for java/util/concurrent/locks/AbstractQueuedSynchronizer$ConditionNode @ 0x00000007cfff5f80
> lastWaiter: Oop for java/util/concurrent/locks/AbstractQueuedSynchronizer$ConditionNode @ 0x00000007cfff5f80
> this$0: Oop for java/util/concurrent/locks/ReentrantLock$NonfairSync @ 0x00000007cff15498

I pulled your changes and I see one slight difference in the output. The following line is missing:

`_metadata._compressed_klass: InstanceKlass for java/util/concurrent/locks/AbstractQueuedSynchronizer$ConditionObject`

I realize that there is no `_metadata._compressed_klass` when you have compact headers, and that the Klass* is encoded in the `_mark` word, which is now looks something like this in the output:

_mark: 16294762323640321

So you can say that the Klass* is embedded in the _mark work, but this isn't of much help to SA users. I think what is expected is that the visitor is passed a MetadataField object that when getValue() is called on it, the Klass mirror is returned. Maybe we need a new CompactKlassField type like we current have a NarrowKlassField field type, and it will do the decoding of the _mark work into a Klass. The current getKlass() is related to this.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750743693