[15] RFR 8242602: Shenandoah: allow earlier recycle of trashed regions during concurrent root processing

[Zhengyu Gu]

Please review this patch that allows early recycling of trashed regions 
during concurrent root processing.

We need to preserve immediate garbage regions during weak root 
processing, since they may contain those dead oops and we need to be 
able to identify them via is_alive closure. After that, we can safely 
recycle them.


Bug: https://bugs.openjdk.java.net/browse/JDK-8242602
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8242602/webrev.00/

Test:
   hotspot_gc_shenandoah
   tools/javac

Thanks,

-Zhengyu