RFR: 8259647: Add support for JFR event ObjectCountAfterGC to Shenandoah

[Roman Kennke]

On Thu, 4 Feb 2021 19:19:35 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> > That certainly is bad news for pause times. Do you think it'd be feasible to "piggyback" the object count calculation on concurrent marking? Might address https://bugs.openjdk.java.net/browse/JDK-8258431 also.
> 
> It dose not just count number of objects, but number of objects by type, much more than liveness counting. Just add a branch in hot marking loop, I can foresee negative impact on performance.

Yes, as I suggested earlier, I'd only turn it on when requested by JFR, and otherwise leave it off. It definitely will impact performance. That means another set of mark loops that we need to generate at compile-time.

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PR: https://git.openjdk.java.net/jdk/pull/2386