RFR: Move ShHeap::used increment out of locked allocation path

[Roman Kennke]

Am 06.04.2018 um 17:31 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/alloc-move-used/webrev.01/
> 
> Slowly moving more things out of the locked allocation path, to make contention less painful, and to
> shrink locked region to ultimately eliminate it. This moves the ShHeap::used() handling to lock-free
> path.
> 
> Thanks,
> -Aleksey
> 

Good!

I believe there's Atomic::sub() for stuff like Atomic::add(-bytes, &_used).

Roman