TODO for 5.2 v3

[Niels Möller]

Torbjorn Granlund <tg at gmplib.org> writes:

> The volatile qualifier makes sure the number of memory reads and memory
> writes remain unchanged from the source code.

Question is, when is it useful for our purposes? First example,
mpn_sec_add_1:

  mp_limb_t
  mpn_sec_add_1 (mp_limb_t *rp, mp_limb_t *ap, mp_size_t n, mp_limb_t b,
  	         mp_ptr scratch)
  {
    scratch[0] = b;
    MPN_ZERO (scratch + 1, n-1);
    return mpn_add_n (rp, ap, scratch, n);
  }

volatile probably makes no difference here, as far as I see (except
possibly if there's some global optimization which inlines mpn_add_n).
But maybe we should still declare some or all of the parameters (rp, ap,
scratch) as pointing to volatile data?

Or would that give type conversion warnings when calling mpn_add_n, with
parameters which aren't volatile-declared? I'm not entirely sure what
volatile means, if it's only giving the compiler some constraints for
the *implementation* of a function, or if volatile in a function
*prototype* also affects type checking or code generation in callers in
some way.

Second example,

  void
  mpn_cnd_swap (mp_limb_t cnd,
  	        mp_limb_t *ap, mp_limb_t *bp, mp_size_t n)
  {
    mp_limb_t mask = - (mp_limb_t) (cnd != 0);
    mp_size_t i;
    for (i = 0; i < n; i++)
      {
        mp_limb_t a, b, t;
        a = ap[i];
        b = bp[i];
        t = (a ^ b) & mask;
        ap[i] = a ^ t;
        bp[i] = b ^ t;
      }
  }

Here a compiler might be tempted to do an initial branch on cnd != 0,
and skip the loop if cnd is false. Using volatile for ap and bp gives it
less reason to do so, but I guess even with volatile it may still
generate code equivalent to

  if (cnd)
    for (i = 0; i < n; i++)
      {
        mp_limb_t a, b, t;
        a = ap[i];
        b = bp[i];
        ap[i] = b;
        bp[i] = a;
      }
  else
    for (i = 0; i < n; i++)
      {
        mp_limb_t a, b, t;
        a = ap[i];
        b = bp[i];
        ap[i] = a;
        bp[i] = b;
      }

which leaks through the cache.

So is it useful or not to volatile-declare ap and bp here?

Regards,
/Niels

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