RFR(S) 8223951: Shenandoah: Only need to update thread roots during final update refs

[Aleksey Shipilev]

On 5/16/19 4:25 PM, Zhengyu Gu wrote:
> After switching to strong to-space invariant and pre-evacuating all root, thread roots are only
> places that may contain to-space oops after updating references during normal concurrent cycles, we
> only need to update them during final updaterefs pause, which should reduce pause time.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8223951
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8223951/webrev.00/

Looks fine to me.

Aside: I wonder how would be get the thread root with from-space ptr if we don't have safepoints
between loads and the LRBs that give us to-space ptrs after potential evac.

-Aleksey