[MLton] 64-bit pointers

[Stephen Weeks]

> > These schemes vary in the number of bits to represent a pointer in an
> > object, the time to load a pointer from memory into a register, the
> > amount of addressable memory, and the object alignment.
...
> > 	bits	time	mem(G)	align
...
> > G	64	fast	 4G	8

> Why is the object alignment in scheme G necessarily 8 and not 4?

I think it was simply generalizing over the current situation, where
pointers are 4 bytes and objects are 4-byte aligned.  I don't see any
necessary reason for doing it.  But it probably won't cost much, since
headers must be the same size as pointers, so objects will already be
bigger.