From the article: Unlike floats, ints are represented as immediates, i.e., they are not allocated on the heap. To tag ints OCaml steals the LSB bit of an int a

From the article: Unlike floats, ints are represented as immediates, i.e., they are not allocated on the heap. To tag ints OCaml steals the LSB bit of an int and sets it to 1 always. This means that we get 63 bits of precision in OCaml ints and any int value n will appear in the assembly as 2*n+1 (the LSB is always 1 and the bits of the number shifted one place in).