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

[Zhengyu Gu]

Hi Roman,

On 7/30/19 4:31 PM, Zhengyu Gu wrote:
> 
> 
> On 7/30/19 1:26 PM, Zhengyu Gu wrote:
>>
>>
>> On 7/30/19 12:50 PM, Roman Kennke wrote:
>>> 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?

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
>>>