RFR: 8235905: ZGC: Rework how ZRootsIterator visits threads

[Stefan Karlsson]

Looks good.

StefanK

On 2019-12-13 12:47, Per Liden wrote:
> In preparation for concurrent thread stack scanning, this patch reworks 
> how we visit threads during roots scanning, so that the part visiting 
> Java threads can be easily moved to a concurrent phase at a later stage.
> 
> The new ZJavaThreadIterator is also a performance optimization over 
> Threads::possibly_parallel_threads_do(), since it will do fewer atomic 
> operations (by a factor of ParallelGCThreads).
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8235905
> Webrev: http://cr.openjdk.java.net/~pliden/8235905/webrev.0
> 
> /Per