RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v8]

[Thomas Schatzl]

On Mon, 9 Sep 2024 11:53:13 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Try to avoid lea in loadNklass (aarch64)
>>  - Fix release build error
>
> src/hotspot/share/oops/klass.hpp line 169:
> 
>> 167:                                 // contention that may happen when a nearby object is modified.
>> 168:   AccessFlags _access_flags;    // Access flags. The class/interface distinction is stored here.
>> 169:                                 // Some flags created by the JVM, not in the class file itself,
> 
> Suggestion:
> 
>   markWord _prototype_header;   // Used to initialize objects' header with compact headers.
> 
> 
> Maybe some comment why this is an instance member.

>@tschatzl I just found your comment here, and I'm not sure what you mean, tbh. The prototype_header is a member of Klass because with compact headers, it encodes that Klass in the prototype header. Note that there is planned follow-up work to remove that field and encode the Klass* on the allocation path. https://bugs.openjdk.org/browse/JDK-8341703

You explained what I had wanted to see here - why do we need a per-klass prototype header, because the markWord contains it ;)
Given that it is going away, I retract this comment and the request can be resolved.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1800983876