RFR(S) JDK-8178350 - klassVtable and klassItable should be ValueObj

[Ioi Lam]

Hi, please review this JVM start-up enhancement:

https://bugs.openjdk.java.net/browse/JDK-8178350
http://cr.openjdk.java.net/~iklam/jdk10/8178350-klassvtable-as-valueobj.v01/

Klass::vtable() is called frequently inside 
InstanceKlass::link_class_impl(). We have a start-up benchmark that 
loads about 4000 classes. Klass::vtable() is called 32458 times and 
costs about 2~3% of a total elapsed time of 720 ms.

Because klassVtable and klassItable are very simple, fixed sized 
structures, it's much better to allocate them on the stack:

OLD:

class klassVtable : ResourceObj {...}
klassVtable* Klass::vtable() const {
   return new klassVtable(const_cast<Klass*>(this), start_of_vtable(), 
vtable_length() / vtableEntry::size());
}

NEW:

class klassVtable VALUE_OBJ_CLASS_SPEC {...}
klassVtable Klass::vtable() const {
   return klassVtable(const_cast<Klass*>(this), start_of_vtable(), 
vtable_length() / vtableEntry::size());
}

(>> David) I am not sure whether to make klassVtable a StackObj or 
ValueObj. Since it's returned by a function call, it seems ValueObj is 
better??

I also removed some ResourceMarks that are obviously not necessary. I 
left some others unchanged, like here in link_class_impl():

  623         ResourceMark rm(THREAD);
  624         vtable().initialize_vtable(true, CHECK_false);
  625         itable().initialize_itable(true, CHECK_false);

because some code inside initialize_vtable() would do a resource 
allocation without a ResourceMark. I don't want to touch that for now, 
as it's unclear to me whether the allocated object(s) should be scoped, 
or should be returned to a higher scope. I filed JDK-8178712 to track this.

Thanks
- Ioi