ResolvedMethodName::vmholder unused?

[coleen.phillimore at oracle.com]

The vmholder field is the owner (mirror) of the Method* that is stored 
in the ResolvedMethodName.  If all references to the mirror go away, 
then the Method* memory in metaspace will be reclaimed. Unfortunately 
for a special case, the clazz field of MemberName is not the holder of 
the method.  Unless this has changed.

   java_lang_invoke_MemberName::set_clazz  (mname_oop, 
m_klass->java_mirror());

m_klass for an interface for vtable_call is java.lang.Object so would 
not keep the Method* from being reclaimed.

     // Add a reference to the loader (actually mirror because anonymous 
classes will not have
     // distinct loaders) to ensure the metadata is kept alive.
     // This mirror may be different than the one in clazz field.
     new_resolved_method->obj_field_put(_vmholder_offset, 
m->method_holder()->java_mirror());
     resolved_method = ResolvedMethodTable::add_method(Handle(THREAD, 
new_resolved_method));


Thanks,
Coleen

On 5/7/18 8:45 PM, Vladimir Ivanov wrote:
> Ioi,
>
> Though the field is never accessed directly, 
> ResolvedMethodName::vmholder is still tracked by GC and keeps the 
> metadata it accompanies (RMN::vmtarget) alive until RMN is reachable.
>
> Best regards,
> Vladimir Ivanov
>
> On 5/7/18 17:29, Ioi Lam wrote:
>> I don't see anywhere in HotSpot that uses 
>> java_lang_invoke_ResolvedMethodName::_vmholder_offset, which is 
>> declared here:
>>
>> http://hg.openjdk.java.net/jdk/jdk/file/7444101401b2/src/hotspot/share/classfile/javaClasses.hpp#l1057 
>>
>> http://hg.openjdk.java.net/jdk/jdk/file/9608f7f41c4e/src/java.base/share/classes/java/lang/invoke/MemberName.java#l75 
>>
>>
>> I tried commenting out the initialization of this field and was able 
>> to run a simple Lambda test.
>>
>> diff -r 9255cb73f048 src/hotspot/share/classfile/javaClasses.cpp
>> --- a/src/hotspot/share/classfile/javaClasses.cpp    Mon May 07 
>> 15:29:31 2018 -0700
>> +++ b/src/hotspot/share/classfile/javaClasses.cpp    Mon May 07 
>> 17:27:27 2018 -0700
>> @@ -3808,7 +3808,7 @@
>>       // Add a reference to the loader (actually mirror because 
>> anonymous classes will not have
>>       // distinct loaders) to ensure the metadata is kept alive.
>>       // This mirror may be different than the one in clazz field.
>> -    new_resolved_method->obj_field_put(_vmholder_offset, 
>> m->method_holder()->java_mirror());
>> +    //new_resolved_method->obj_field_put(_vmholder_offset, 
>> m->method_holder()->java_mirror());
>>       resolved_method = 
>> ResolvedMethodTable::add_method(Handle(THREAD, new_resolved_method));
>>     }
>>
>>
>> Any plans to use vmholder in the future? Or, is this used by any 
>> non-HotSpot VM?
>>
>> If no one uses it, I'll file an RFE to remove it, so we can save a 
>> pointer per MemberName.
>>
>> Thanks
>> - Ioi
>>
>>
>>
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