But assuming that the other bit in those 4 bytes is required and there isn't some other place available where you could sneak it in, those ~4 bytes would still be allocated (for that one other bit) and only take less memory under some compression scheme.
And chances are that even if you did get by without that bit (or found another place for it) you'd in many cases spend far more memory on workarounds for stuff where the creator was too stingy for implementing "IHashable" but you actually need it
Well, in Virgil, a class without a superclass is a new root class, with no assumed methods. To use an object of that class as a key in a hashtable, one provides a hash function to the hashtable constructor. (The hash function can be a class method, or a top-level function; with generics, a hashtable key could as well be any type, including primitives). Of course it depends on how your write your code, but most of my objects do not end up as keys in a hashtable, so don't need a hash function nor storage for an identity hashcode.
So the smallest object header for a Virgil object is 4 bytes: it's just a type ID that is used for dynamic casts and as an index into a table for GC scanning. Arrays have two 4 byte header fields: the type ID and a 32-bit length. That's considerably more memory efficient than 2 or 3 64-bit header words for Java objects and arrays, respectively.
And chances are that even if you did get by without that bit (or found another place for it) you'd in many cases spend far more memory on workarounds for stuff where the creator was too stingy for implementing "IHashable" but you actually need it