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

static int unsafe_sizeOf(O o) {
  Class c = _getClass(o);
  if (o instanceof O[])
    ret unsafe_sizeOf_uncached(o, c); // can't cache
  Int l = unsafe_sizeOf_cache.get(c);
  if (l == null)
    unsafe_sizeOf_cache.put(c, l = unsafe_sizeOf_uncached(o, c));
  ret l;
}
  
static int unsafe_sizeOf_uncached(O o, Class c) {
  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;
  
  if (o instanceof O[]) {
    int n = l((O[]) o);
    maxOffset = lowestValue(unsafe_allFieldOffsets(Pair)) + 4 + java_pointerSize()*(n-1);
  } else // TODO: add primitive arrays
    for (Field f : instanceFields) {
      long offset = theUnsafe().objectFieldOffset(f);
      if (offset > maxOffset) maxOffset = offset; 
    }
  ret (((int) maxOffset/WORD) + 1)*WORD; 
}