Optint - Efficient integer types on 64-bit architectures

This library provides two new integer types, Optint.t and Int63.t, which guarantee efficient representation on 64-bit architectures and provide a best-effort boxed representation on 32-bit architectures.

Goal

The standard Int32.t and Int64.t types provided by the standard library have the same heap-allocated representation on all architectures. This consistent representation has costs in both memory and run-time performance.

On 64-bit architectures, it's often more efficient to use the native int directly. This library provides types to do exactly this:

• Optint.t: an integer containing at least 32 bits. On 64-bit, this is an immediate integer; on 32-bit, it is a boxed 32-bit value. The overflow behaviour is platform-dependent.

• Int63.t: an integer containing exactly 63 bits. On 64-bit, this is an immediate integer; on 32-bit, it is a boxed 64-bit integer that is wrapped to provide 63-bit two's complement semantics. The two implementations are observationally equivalent, modulo use of Marshal and Obj.

In summary:

Integer type	32-bit representation	64-bit representation	Semantics
Stdlib.Int.t	31-bit immediate ✅	63-bit immediate ✅	Always immediate
Stdlib.Nativeint.t	64-bit boxed ❌	32-bit boxed ❌	Exactly word size
Stdlib.Int32.t	32-bit boxed ❌	32-bit boxed ❌	Exactly 32 bits
Stdlib.Int64.t	64-bit boxed ❌	64-bit boxed ❌	Exactly 64 bits
Optint.t (new)	32-bit boxed ❌	63-bit immediate ✅	At least 32 bits
Int63.t (new)	64-bit boxed ❌	63-bit immediate ✅	Exactly 63 bits

These new types are safe and well-tested, but their architecture-dependent implementation makes them unsuitable for use with the Marshal module. Use the provided encode and decode functions instead.