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

[Alan Bateman]

On Mon, 3 Apr 2023 07:17:59 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

src/java.base/share/classes/java/lang/Thread.java line 231:

> 229:     /* Reserved for exclusive use by the JVM. Cannot be moved to FieldHolder
> 230:        as it needs to be set by the VM before executing the constructor that
> 231:        will set FieldHolder.

"will set FieldHolder" is a bit confusing. It creates the FieldHolder and sets the field "holder" so I think the comment could be a bit clearer.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13287#discussion_r1155601934