It is absolutely idiotic to have 64-bit pointers when I compile a program that uses less than 4 gigabytes of RAM. When such pointer values appear inside a struct, they not only waste half the memory, they effectively throw away half of the cache. – Donald Knuth (2008)

see also

• What is gained and lost with 63-bit integers? - OCaml uses a special 63-bit representation. How does it affect OCaml?