RFR: 8235905: ZGC: Rework how ZRootsIterator visits threads

[Per Liden]

Thanks Stefan!

/Per

On 2019-12-13 13:24, Stefan Karlsson wrote:
> 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