VM_Address ref_address;
  public VM_Address getNextReturnAddressAddress() {    VM_Address ref_address;    if (!jniSavedReturnAddr.isZero()) {      ref_address = jniSavedReturnAddr;      jniSavedReturnAddr = VM_Address.zero();      return ref_address;    }    return VM_Address.zero();  } //- implements VM_GCMapIterator