RFR: 8234974: Shenandoah: Do concurrent roots even when no evacuation is necessary

[Roman Kennke]

Sorry, I mixed up this change with the subsequent concurrent roots
cleaning here. Let's step back and get this part in shape first. Wait
for next webrev, ok?

Thanks,
Roman


> Second thought: I am not sure what problem you are trying to solve here,
> let's talk tomorrow.
> 
> Thanks,
> 
> -Zhengyu
> 
> On 12/11/19 5:12 PM, Roman Kennke wrote:
>> After discussing off-line with Zhengyu, I am proposing those changes:
>>
>> - Reinstate returning object for GC threads (apparently needed b/c of
>> bug in JVMTI)
>> - Always heal nmethods in barrier. We can only get there during evac
>> (see below)
>> - ShenandoahEvacUpdateOopStorageRootsClosure renamed to
>> ShenandoahEvacUpdateCleanupRootsClosure and let it also handle cleanup.
>> - Changed order of cleanup and unload: first do cleanup, then do unload.
>> - only call prepare_unloading() when we have a cset. This arms nmethods.
>> We only ever want to get into nmethod-barriers when we have a cset/are
>> doing evacuation.
>>
>> Incremental:
>> http://cr.openjdk.java.net/~rkennke/JDK-8234974/webrev.01.diff/
>> Full:
>> http://cr.openjdk.java.net/~rkennke/JDK-8234974/webrev.01/
>>
>> Good now?
>>
>> Roman
>>
>>> Shenandoah can short-cut a cycle when the collection set remains empty,
>>> and doesn't dive into concurrent evacuation and updating refs phases
>>> then. However, this currently also precludes concurrent roots processing
>>> and concurrent class unloading. This is only a minor nuisance now
>>> (effectively skipping class unloading for short-cut-cycles), but amounts
>>> to a real bug when we're going to do weak-roots-cleaning concurrently
>>> too.
>>>
>>> Bug:
>>> https://bugs.openjdk.java.net/browse/JDK-8234974
>>> Webrev:
>>> http://cr.openjdk.java.net/~rkennke/JDK-8234974/webrev.00/
>>>
>>> Testing: hotspot_gc_shenandoah
>>>
>>> Can I please get a review?
>>>
>>> Roman
>>>
>>
>