RFR(S) 228777: Shenandoah: Cleanup STW weak root processing

[Roman Kennke]

>>>> I am not sure about this. If final-mark could not finish to
>>>> pre-evacuate
>>>> all roots we might have from-space refs there. Or does the OOM-protocol
>>>> ensure that any ref should be updated correctly? We need to be careful
>>>> here. Better safe than sorry.
>>
>> Actually, you are right. It does not work with traversal. I wonder how
>> the tests passed early.
>>
>> I would like to withdraw this RFR.
> 
> It is not true. Traversal fixes forwarding before processes weak roots,
> so this patch should work.
> 
> The failed tests were due to broken builds, they are passed on clean
> builds, also specjvm tests (normal and traversal mode). Therefore, I
> would like re-open this RFR. This invariant is important for
> implementing concurrent weak root processing (JDK-8228818).
> 
> If you still have concerns, let's push it to shenandoah/jdk, Okay?

Yeah, let's push it to sh/jdk and if it's ok, take it upstream to jdk/jdk.

Thanks,
Roman


> Thanks,
> 
> -Zhengyu
> 
>>
>> Thanks,
>>
>> -Zhengyu
>>
>>
>>
>>>
>>> If pre-evacuation fails, it runs degenerated GC cycle and roots are
>>> updated in final_updaterefs().
>>>
>>> We do have no-forwarded assertion in place, should catch the problem
>>> if otherwise.
>>>
>>> Thanks,
>>>
>>> -Zhengyu
>>>
>>>>
>>>> Roman
>>>>
>>>>
>>>>> Shenandoah pre-evacuates or updates weak roots at STW pauses.
>>>>> Therefore,
>>>>> weak roots should not have forwarded references during mark phase.
>>>>>
>>>>> Current has_forwarded_objects() branch does not break things, but does
>>>>> unncessary works, let's prune it.
>>>>>
>>>>>
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8228777
>>>>> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8228777/webrev.00/
>>>>>
>>>>> Test:
>>>>>    hotspot_gc_shenandoah (fastdebug and release)
>>>>>
>>>>> Thanks,
>>>>>
>>>>> -Zhengyu
>>>>