RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v21]

[Patricio Chilano Mateo]

On Wed, 30 Oct 2024 01:52:30 GMT, Dean Long <dlong at openjdk.org> wrote:

>> Patricio Chilano Mateo has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - SmallRegisterMap::instance() fix + comment typo
>>  - Add comment in call_VM_preemptable
>
> src/hotspot/share/runtime/continuation.hpp line 50:
> 
>> 48: class JavaThread;
>> 49: 
>> 50: // should match Continuation.PreemptStatus() in Continuation.java
> 
> As far as I can tell, these enum values still don't match the Java values.  If they need to match, then maybe there should be asserts that check that.

`PreemptStatus` is meant to be used with `tryPreempt()` which is not implemented yet, i.e. there is no method yet that maps between these values and the PreemptStatus enum. The closest is `Continuation.pinnedReason` which we do use. So if you want I can remove the reference to PreemptStatus and use pinnedReason instead.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1823509538