On Wed, 17 Dec 2025 at 20:26, Jakub Jelinek <[email protected]> wrote:
>
> On Wed, Dec 17, 2025 at 07:50:16PM +0000, Jonathan Wakely wrote:
> > --- a/libstdc++-v3/include/bits/random.tcc
> > +++ b/libstdc++-v3/include/bits/random.tcc
> > @@ -3522,10 +3522,9 @@ namespace __detail
> > // r = 2; // Redundant, we only support radix 2.
> > using _Rng = decltype(_Urbg::max());
> > const _Rng __rng_range_less_1 = _Urbg::max() - _Urbg::min();
> > - const _UInt __uint_range_less_1 = ~_UInt(0);
> > // R = _UInt(__rng_range_less_1) + 1; // May wrap to 0.
> > const auto __log2_R = __builtin_popcountg(__rng_range_less_1);
> > - const auto __log2_uint_max =
> > __builtin_popcountg(__uint_range_less_1);
> > + const auto __log2_uint_max = __builtin_popcountg(~_UInt(0));
>
> I don't see how this can work. __builtin_popcountg requires some integral
> unsigned type, unsigned __int128 is fine, but __rand_uint128 struct is not.
Hmm, good point. I wonder how it's compiling.
> So, if you want to use something like popcount, then you need some
> function template which for normal unsigned integrals will return
> __builtin_popcountg on the argument and for struct __rand_uint128 will
> return __builtin_popcountg(_M_hi) + __builtin_popcountg(_M_lo);
Do we need popcount at all? Isn't it just sizeof(_UInt) * CHAR_BIT?
>
> > @@ -3586,25 +3585,25 @@ namespace __detail
> > _RealT
> > __generate_canonical_any(_Urbg& __urng)
> > {
> > - static_assert(__d < __builtin_popcountg(~_UInt(0)));
>
> And if the above is changed, then this static_assert could stay.
>
> Jakub
>
