[15] RFR 8236878: Use atomic instruction to update StringDedupTable's entries and entries_removed counters

[Zhengyu Gu]

Hi,

Please review this small change that uses atomic operations to update 
StringDedupTable's entries and entries_removed counter.

This is *not* a correctness fix or performance enhancement, but for 
Shenandoah GC to move StringDedupTable cleanup task into concurrent 
phase, while holding StringDedupTable_lock.

Bug: https://bugs.openjdk.java.net/browse/JDK-8236878
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8236878/webrev.00/index.html

Test:
   hotspot_gc (fastdebug and release) on x86_64 Linux
   Submit test in progress.

Thanks,

-Zhengyu