Re: [PATCH WIP 2/2] libstdc++: Optimize ranges::distance for segmented iterators [PR123211]

Hi,

On Tue, 14 Jul 2026, Yuao Ma wrote:

> Hi!
> 
> On Tue, Jul 14, 2026 at 12:30 PM Patrick Palka <[email protected]> wrote:
> >
> > For segmented iterators, ranges::distance is equivalent to the sum of
> > ranges::distance of each of its segments.
> >
> >         PR libstdc++/123211
> >
> > libstdc++-v3/ChangeLog:
> >
> >         * include/bits/ranges_base.h (__distance_fn::operator()): For
> >         the non-sized-sentinel overload,
> > ---
> >  libstdc++-v3/include/bits/ranges_base.h | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
> > index ce1c40fc17f6..7f169feaa6b1 100644
> > --- a/libstdc++-v3/include/bits/ranges_base.h
> > +++ b/libstdc++-v3/include/bits/ranges_base.h
> > @@ -994,6 +994,16 @@ namespace ranges
> >        constexpr iter_difference_t<_It>
> >        operator()[[nodiscard]](_It __first, _Sent __last) const
> >        {
> > +       if constexpr (__segmented_iterator<_It> && same_as<_It, _Sent>)
> > +         {
> > +           iter_difference_t<_It> __n = 0;
> > +           std::__for_each_segment(__first, __last, [this, &__n](auto __first, auto __last) {
> > +             __n += iter_difference_t<_It>(this->operator()(__first, __last));
> > +             return __last;
> > +           });
> > +           return __n;
> > +         }
> > +
> >         iter_difference_t<_It> __n = 0;
> >         while (__first != __last)
> >           {
> > --
> > 2.55.0.141.g55526a1826
> >
> 
> Great to see this optimization into libstdc++! Beyond
> ranges::distance, I think there are more algorithms we can optimize
> for segmented iterators. I helped implement some of these for libc++,
> and I'd love to contribute similar optimizations to libstdc++ too!

That'd be great!  I don't have any plans to implement more optimizations
myself but would happy to review.