RFR: 8203339: Add oopDesc::field_offset()

[Stefan Karlsson]

Hi all,

Please review this patch to add a oopDesc::field_offset() convenience 
function.

http://cr.openjdk.java.net/~stefank/8203339/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8203339

We use it in ZGC to extract the offset to oop fields in some of our 
OopClosures. The OopClosures provide the interior pointers to oop 
fields, and together with the object pointer the offset can be calculated.

See the following ZGC use-case:

http://hg.openjdk.java.net/zgc/zgc/file/3a52c8361e20/src/hotspot/share/gc/z/zHeapIterator.cpp#l87

class ZHeapIteratorPushOopClosure : public ExtendedOopClosure {
private:
   ZHeapIterator* const _iter;
   const oop            _base;

public:
   ZHeapIteratorPushOopClosure(ZHeapIterator* iter, oop base) :
       _iter(iter),
       _base(base) {}

   void do_oop_nv(oop* p) {
     const oop obj = HeapAccess<ON_UNKNOWN_OOP_REF>::oop_load_at(_base, 
_base->field_offset(p));
     _iter->push(obj);
   }

Thanks,
StefanK