RFR: 8254562: ZGC: Remove ZMarkRootsTask [v2]

[Erik Österlund]

On Mon, 12 Oct 2020 08:34:23 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> After introducing concurrent stack scanning, we don't need to mark through any roots during the mark start pause.
>> Remove the code.
>> Note that the old code passed in `false` to `_roots(false /* visit_jvmti_weak_export */)`. This has the effect that no
>> roots are visited by the iterator: void ZRootsIterator::oops_do(ZRootsIteratorClosure* cl) {
>>   ZStatTimer timer(ZSubPhasePauseRoots);
>>   if (_visit_jvmti_weak_export) {
>>     _jvmti_weak_export.oops_do(cl);
>>   }
>> }
>
> Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Also remove ZRelocateRootTask and ZRootsIterator

I agree with Per that we can start by removing the marking stuff first. I think Coleen is just about to move the JVMTI
tag map oops into OopStorage, and then we can nuke the relocation code too.

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PR: https://git.openjdk.java.net/jdk/pull/601