static int unsafe_sizeOf(O o){
  int NR_BITS = Integer.valueOf(System.getProperty("sun.arch.data.model"));
  int BYTE = 8;
  int WORD = NR_BITS/BYTE;
  int MIN_SIZE = 16;
  
  Class src = _getClass(o);
  
  new L<Field> instanceFields;
  do {
      for (Field f : src.getDeclaredFields()) {
        if((f.getModifiers() & Modifier.STATIC) == 0)
          instanceFields.add(f);
      }
      src = src.getSuperclass();
  } while(instanceFields.isEmpty());
  
  long maxOffset = MIN_SIZE;
  
  if (o instanceof O[]) {
    int n = l((O[]) o);
    print("n=" + n);
    maxOffset = lowestValue(unsafe_allFieldOffsets(Pair)) + 4 + 4*(n-1); // TODO: without compressed oops etc.
  } else
    for (Field f : instanceFields) {
      long offset = theUnsafe().objectFieldOffset(f);
      if (offset > maxOffset) maxOffset = offset; 
    }
  ret (((int) maxOffset/WORD) + 1)*WORD; 
}