The SBCL thing makes me wonder if there are any modern lisps that can be restricted to a 4GB (32-bit) address space, while still using 64-bit values and registers...
Then, cons cells would take up only one machine word.
If everything you point to is aligned on an eight-byte boundary, and if you don't use type tags, then a 32-bit pointer can point to anything in a 35-bit address space. So you can get up to 32 GB.
Then, cons cells would take up only one machine word.