static Map<Class, Int> unsafe_sizeOf_cache = weakHashMap();

static long unsafe_sizeOf(O o) {
  if (isArray(o))
    if (o cast O[])
      ret inMemorySizeOfObjectArray(o.length);
    else
      ret inMemorySizeOfPrimitiveArray(o);
      
  Class c = _getClass(o);
  Int l = unsafe_sizeOf_cache.get(c);
  if (l == null)
    unsafe_sizeOf_cache.put(c, l = unsafe_sizeOf_uncached(c));
  ret l;
}
  
static int unsafe_sizeOf_uncached(Class src) {
  int WORD = javaDataModelWordSize();
  int MIN_SIZE = 16;

  new L<Field> instanceFields;
  while (src != null && src != O.class && empty(instanceFields)) {
    for (Field f : src.getDeclaredFields())
      if ((f.getModifiers() & Modifier.STATIC) == 0)
        instanceFields.add(f);
    src = src.getSuperclass();
  }
  
  long maxOffset = MIN_SIZE-1;
  
  for (Field f : instanceFields) {
     long offset = unsafe_objectFieldOffset(f);
     if (offset > maxOffset) maxOffset = offset; 
  }
  ret (((int) maxOffset/WORD) + 1)*WORD;
}

from https://stackoverflow.com/questions/9368764/calculate-size-of-object-in-java (modified)