RFR(XS): Heap memory usage counting not longer needs to be atomic

[Zhengyu Gu]

What's your concern for the naked read? seeing partially updated value? 
which
>> can not happen for x64.
> Yes. And it can happen even on x64 that our unlucky counter update loop would
> hoist it. Let's at least keep it "volatile"?

Ok, restored the barrier for read.

http://cr.openjdk.java.net/~zgu/shenandoah/memory_counting/webrev.01/

Thanks,

-Zhengyu


> Thanks,
> -Aleksey
>