RFR: 8235905: ZGC: Rework how ZRootsIterator visits threads

Per Liden per.liden at oracle.com
Fri Dec 13 11:47:42 UTC 2019

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


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