RFR: 8249004: Reduce ThreadsListHandle overhead in relation to direct handshakes [v6]

[David Holmes]

On Fri, 29 Oct 2021 22:35:50 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> The `is_exiting` check seems unnecessary as the handshake code will not handshake with an exiting thread. The nested TLH was unnecessary too AFAICS.
>
> Ummmm... The purpose of the new `is_exiting()` check and the baseline's
> `ThreadsListHandle::includes()` check is to avoid making this call:
> 
>   return this->handshake_state()->suspend();
> 
> The call we are avoiding is the one that makes the synchronous
> SuspendThreadHandshake request (for threads other than self)
> so by detecting the exiting thread early, we are avoiding entering
> the handshake machinery.

But why do we care about that? The low-level mechanism handles exiting threads so there is no need for the higher-level code to do that. The current logic gives the appearance that we must filter out exiting threads at this level because the lower-level code does not handle them.

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PR: https://git.openjdk.java.net/jdk/pull/4677