RFR: 8305425: Thread.isAlive0 doesn't need to call into the VM [v8]

[Alan Bateman]

On Wed, 5 Apr 2023 08:37:26 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> We have the strange situation where calling `t.isAlive()` on a `java.lang.Thread` `t`, will call into the VM (via `alive()` then `isAlive0()`) where the VM then examines the `eetop` field of `t` to extract its `JavaThread` pointer and compare it to null. We can simply read `eetop` directly in `Thread.alive()`:
>> 
>> boolean alive() {
>>   return eetop != 0;
>> } 
>> 
>> I also updated a comment in relation to `eetop`.
>> 
>> Testing: tiers 1-3
>> 
>> Thanks
>
> David Holmes has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Test tweaks

Yes, I suspect having isAlive be synchronized would have had side-effects and maybe deadlock in some cases. Volatile-read on the libs side + OrderAccess::release in the VM at termination time looks okay.

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Marked as reviewed by alanb (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13287#pullrequestreview-1372446213