[15] RFR 8239354: Shenandoah: minor enhancements to traversal GC

[Roman Kennke]

Hi Zhengyu,

This looks good to me, thank you!

Roman


> 1) Added assertion to catch evacuation after completion of heap
> traversal. This should help catch the bug demonstrated in sh-jdk11 w/o
> JDK-8237396.
> 
> 2) Retire TLAB/GCLAB after completion of heap traversal. Current code
> retires TLAB/GCLAB at the beginning final traversal, but STW traversal
> still uses GCLAB to evacuate remaining objects.
> 
> 3) Added comments regarding why need to retire TLAB/GCLAB, even we don't
> need heap to be parsable.
> 
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8239354
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8239354/webrev.00/index.html
> 
> Test:
>   hotspot_gc_shenandoah
> 
> Thanks,
> 
> -Zhengyu
>