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
