RFR: Implement ShenandoahHeap::requires_barriers() [v2]

[Ron Pressler]

On Fri, 12 Feb 2021 20:59:09 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> This add proper implementation of requires_barriers() for ShenandoahHeap, and adds some comments to CollectedHeap about it.
>> 
>> Testing (with a few other fixes in the pipeline):
>>  - [x] java/lang/Continuation (-XX:+UseShenandoahGC -XX:ShenandoahGCMode=aggressive)
>>  - [x] java/lang/Thread/virtual  (-XX:+UseShenandoahGC -XX:ShenandoahGCMode=aggressive)
>
> Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:
> 
>  - Merge remote-tracking branch 'upstream/fibers' into shenandoah-req-barriers
>  - Implement ShenandoahHeap::requires_barriers()

Marked as reviewed by rpressler (Lead).

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

PR: https://git.openjdk.java.net/loom/pull/29