RFR: 8305425: Thread.isAlive0 doesn't need to call into the VM [v6]

[ExE Boss]

On Wed, 5 Apr 2023 02:30:15 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 two additional commits since the last revision:
> 
>  - Switch from using synchronized to using a volatile eetop field
>  - Added Shipilev's test (with a small addition)

src/java.base/share/classes/java/lang/Thread.java line 1846:

> 1844:     /**
> 1845:      * Returns true if this thread is alive.
> 1846:      * This method is non-final so it can be overridden.

Suggestion:

     * This method is non-final so it can be overridden by VirtualThread.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13287#discussion_r1157995444