Why is int comparison not just (-)?

Learning
4

This is in fact what Base.Int.compare (and therefore Core.Int.compare) does (at least, in the latest GitHub version—I think the opam version is still using Poly.compare). Apparently, it is about 20%-25% faster.

open! Core
open Core_bench.Std

let a = Sys.opaque_identity 0
let b = Sys.opaque_identity Int.min_value

let compare_int (a : int) b =
  let int_of_bool (b : bool) = (Obj.magic b : int) in
  int_of_bool (a > b) - int_of_bool (a < b)
;;

let () =
  [ Bench.Test.create ~name:"Poly.compare" (fun () -> Poly.compare a b)
  ; Bench.Test.create ~name:"Int.compare" (fun () -> compare_int a b)
  ]
  |> Bench.make_command
  |> Command.run
;;

Output:

Estimated testing time 20s (2 benchmarks x 10s). Change using '-quota'.
┌──────────────┬──────────┬────────────┐
│ Name         │ Time/Run │ Percentage │
├──────────────┼──────────┼────────────┤
│ Poly.compare │   3.57ns │    100.00% │
│ Int.compare  │   2.69ns │     75.33% │
└──────────────┴──────────┴────────────┘
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