RFR (sh/jdk8): Cleanup weak JNI refs when not doing reference processing

[Roman Kennke]

System dict and string table are done while doing unloading in 
parallelCleaning.hpp. Not sure about JVMTI. JNI weakroots popped up 
because they are hidden in reference-processing.

Roman

> Hi Roman,
>
> On 12/13/19 8:34 AM, Roman Kennke wrote:
>> We may run into troubles in jdk8 when doing class-unloading but not 
>> reference-processing: in JDK8, ref-processing also handles JNI 
>> weak-refs (it doesn't really belong there, which is why it has been 
>> moved out in later releases). This means that when we have a cycle 
>> that doesn't do ref-processing, we may skip cleaning JNI refs and end 
>> up with dangling refs there. This manifests easily when running 
>> IntelliJ.
>>
>> Fix is to clean JNI weak-refs explicitely when skipping ref-processing.
>>
>
> Looks like we missed more than just JNI weak root, but whole 
> weak_roots_work() equivalency in sh/jdk11u, which includes jvmti and 
> system dictionary roots, etc.
>
>
> -Zhengyu
>
>
>> Fix:
>> http://cr.openjdk.java.net/~rkennke/shjdk8-cleanup-weakjni/webrev.00/
>>
>> Testing: hotspot_gc_shenandoah
>> IntelliJ with default and traversal mode (failed before/passes now)
>>
>> Good?
>>
>> Thanks,
>> Roman
>>
>