more on nativeint interface

[Norman Ramsey <nr@eecs.harvard.edu>]

It would be useful to have
  val width : int   (* number of bits in an integer of type nativeint *)
so that, for example, I could do sign extension by suitable shifting.

Meanwhile, I could play the following sorts of games:

  module N = Nativeint
  let width =
    if N.shift_right_logical N.minus_one 32 = N.zero then 32 else 64

  let sx k n = N.shift_right (N.shift_left n (width-k)) (width-k)
    (* sign-extend the least significant k bits of integer n *)

  let sx13 = sx 13

Unfortunately, the code that the ocamlopt produces for this function
(on x86) is pretty bad.

I can get significantly better code by using this source:

  let width=32
  let sx13b n = N.shift_right (N.shift_left n (width-13)) (width-13)

But of course this code isn't portable.


Is there anything I can do to get the compiler to do a better job with
nativeint? 


Norman