RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v12]

[Richard Reingruber]

On Sat, 26 Oct 2024 01:42:17 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:
>> 
>>  - Restore use of atPointA in test StopThreadTest.java
>>  - remove interruptible check from conditional in Object::wait
>
> src/hotspot/cpu/aarch64/interp_masm_aarch64.cpp line 1567:
> 
>> 1565: 
>> 1566:   // In case of preemption, this is where we will resume once we finally acquire the monitor.
>> 1567:   bind(resume_pc);
> 
> If the idea is that we return directly to `resume_pc`, because of `last_Java_pc`(), then why do we poll `preempt_alternate_return_offset` above?

The address at `preempt_alternate_return_offset` is how to continue immediately after the call was preempted. It's where the vthread frames are popped off the carrier stack.

At `resume_pc` execution continues when the vthread becomes runnable again. Before its frames were thawed and copied to its carriers stack.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1817702986