RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v3]

[Roman Kennke]

On Fri, 30 Aug 2024 18:10:44 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> FWIW, the ParallelGC does something very similar to what you propose, except that it walks bitmaps instead of paring the space to find the self-forwarded objects. It then has a check inside object_iterate to make sure that it doesn't expose the dead objects (in eden and the from space) to heap dumpers and histogram printers.
>> 
>> Because of the the code above, the SerialGC clears away the information about what objects are dead in eden and the from space, so heap dumpers and histogram printers will include these dead objects. We might want to fix that as a future RFE.
>
>> If we get a promotion failure in the young gen, we are leaving the dead objects marked as forwarded. 
> 
> True; need to do sth like `obj->init_mark();` for the non-self-forwarded case. The postcondition is that no forwarded objs in eden/from.

ParallelGC actually doesn't use bitmaps, it pushes all forwarded objs to preserved-marks-table, and uses that to find forwarded objects, which is why we can't remove the preserved-marks table in ParallelGC (IOW, after this patch, the preserved-marks-stuff in Parallel scavenger is *only* used to find forwarded objects. We might want to think about more efficient solutions for this).

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750199051