The Access API has a function HeapAccess<>::oop_load_at(obj, offset). When using OopClosure you are handed interior object pointers, pointing to the oop fields. I propose that we add oopDesc::field_offset, so that the offset can be found with obj->field_offset(oop_addr)

We currently have to usages in ZGC:
http://hg.openjdk.java.net/zgc/zgc/file/3a52c8361e20/src/hotspot/share/gc/z/zOopClosures.cpp#l49

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