RFR 8226310: Shenandoah: Concurrent evacuation of CLDG

[Roman Kennke]

Hi Zhengyu,

> Please review this patch that moves evacuation of CLDG to concurrent phase.
> 
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8226310
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8226310/webrev.00/
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)

Nice.

+  ShenandoahClassLoaderDataRoots<true /*concurrent*/, false /*single
threaded*/> _cld_roots;

Why is it only single-threaded? Does it matter?

Do you think this is back-portable?

The question came up in IRC if we should start pushing this to sh/jdk
for the start, let it bake there a little and at some point push it to
jdk/jdk in one change. It doesn't have to be now, and I guess it depends
upon how risky it is.

Roman