ObjectReference tgt, int metaDataA, int metaDataB,
ObjectReference tgt, Offset metaDataA, int metaDataB,
  public void writeBarrier(ObjectReference src, Address slot,                           ObjectReference tgt, int metaDataA, int metaDataB,                           int mode) {    // Either: write barriers are used and this is overridden, or     //         write barriers are not used and this is never called    if (Assert.VERIFY_ASSERTIONS) Assert._assert(false);  }