[master] RFR: Implement Shenandoah support [v22]

[Zhengyu Gu]

On Wed, 20 Apr 2022 10:16:06 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahObjectUtils.inline.hpp line 40:
>> 
>>> 38: // code is supposed to observe from-space objects.
>>> 39: #ifdef _LP64
>>> 40: markWord ShenandoahObjectUtils::stable_mark(oop obj) {
>> 
>> There is a lot of duplication with `ObjectSynchronizer::stable_mark()`. I think you can just  call `objectSynchronizer::stable_mark()` after resolve forward pointer.
>
> No, unfortunately not. We need to resolve inside the loop in order to catch concurrently evacuated objects. I considered the alternative to put some kind of GC hook into the loop in ObjectSynchronizer::stable_mark() but decided against it in order to keep shared code cleaner.

Okay.

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PR: https://git.openjdk.java.net/lilliput/pull/32