Re: [PATCH] Add type-generic clz/ctz/clrsb/ffs/parity/popcount builtins [PR111309]

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Nov 10, 2023 at 10:44:12AM +0100, Jakub Jelinek wrote:
> Because the builtins are just something matching in behavior to existing
> builtins which can be used for those macros, not exact implementation of
> those.

BTW, the new builtins also allow implementation of generic signed_type_for
and unsigned_type_for macros in C (together with __builtin_classify_type),
I think _Generic over known standard and extended types plus for _BitInt
(__builtin_classify_type (__typeof (x)) == 18) something like in the
following source:

void bar (_BitInt(193) *, unsigned _BitInt(193) *);

void
foo (void)
{
  unsigned _BitInt(193) a = 0uwb;
  _BitInt(__builtin_popcountg ((__typeof (a)) -1)) b = 0wb;
  bar (&b, &a);
}

void
baz (void)
{
  _BitInt(193) a = 0wb;
  unsigned _BitInt(__builtin_clrsbg ((__typeof (a)) - 1) + 1) b = 0uwb;
  bar (&a, &b);
}

One needs to use __builtin_popcountg on all ones for unsigned types and
1 + __builtin_clrsbg on all ones for signed types, but otherwise it seems to
work fine.  Of course, for signed_type_for one would need to decide what
to do with unsigned _BitInt(1) type which doesn't have signed counterpart,
but that can be dealt in _Generic.

	Jakub