RFR: 8278341: Liveness check for global scope is not as fast as it could be

[Aleksey Shipilev]

On Tue, 7 Dec 2021 13:12:13 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> When doing some unrelated performance measurements, I realized that segments backed by global scope were still paying a relatively high cost for liveness checks - that's because GlobalScopeImpl extends from SharedScopeImpl, and does not override the `isAlive` method. This means that when checking for liveness, we will still do (in some cases - e.g. when calling `checkValidStateSlow`) a volatile VH get on the scope state - which is useless in this case.
> 
> This simple patch adds the missing override.

Looks good!

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6744