Re: [PATCH] libstdc++: extraneous begin in cartesian_product_view::end [PR107572]

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 7 Mar 2023 at 20:49, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Tue, 7 Mar 2023, Patrick Palka wrote:
>
> > ranges::begin() isn't guaranteed to be equality-preserving for
> > non-forward ranges, so in cartesian_product_view::end we need to be
> > careful about calling begin() on the first range (which could be
> > non-forward) in the (non-degenerate) case where __empty_tail is false.
> >
> > Since we're already using a variadic lambda to compute __empty_tail, we
> > might as well use that same lambda to build up the tuple of iterators
> > instead of doing it via __tuple_transform.
> >
> > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> >
> >       PR libstdc++/107572
> >
> > libstdc++-v3/ChangeLog:
> >
> >       * include/std/ranges (cartesian_product_view::end): When
> >       building the tuple of iterators, avoid calling ranges::begin on
> >       the first range if __empty_tail is false.
> >       * testsuite/std/ranges/cartesian_product/1.cc (test07): New test.
> > ---
> >  libstdc++-v3/include/std/ranges               | 36 +++++++++++++------
> >  .../std/ranges/cartesian_product/1.cc         | 22 ++++++++++++
> >  2 files changed, 48 insertions(+), 10 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> > index e0cac15a64f..0de7bdef504 100644
> > --- a/libstdc++-v3/include/std/ranges
> > +++ b/libstdc++-v3/include/std/ranges
> > @@ -8078,26 +8078,42 @@ namespace views::__adaptor
> >      end() requires ((!__detail::__simple_view<_First> || ... || !__detail::__simple_view<_Vs>)
> >                   && __detail::__cartesian_product_is_common<_First, _Vs...>)
> >      {
> > -      bool __empty_tail = [this]<size_t... _Is>(index_sequence<_Is...>) {
> > -     return (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
> > +      auto __it = [this]<size_t... _Is>(index_sequence<_Is...>) {
> > +     bool __empty_tail = (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
> > +     auto& __first = std::get<0>(_M_bases);
> > +     auto __first_it = __empty_tail
> > +       ? ranges::begin(__first)
> > +       : __detail::__cartesian_common_arg_end(__first);
> > +     // N.B. When implementing P2165R4 this should be changed to always return tuple.
> > +     if constexpr (sizeof...(_Is) == 1)
> > +       return std::make_pair(std::move(__first_it),
> > +                              ranges::begin(std::get<1 + _Is>(_M_bases))...);
> > +     else
> > +       return std::make_tuple(std::move(__first_it),
> > +                              ranges::begin(std::get<1 + _Is>(_M_bases))...);
>
> On second thought, it might be better to use __tuple_or_pair_t here
> instead of manually determining whether to use a pair or tuple, so that
> we don't forget to adjust this site when implementing P2165R4 (which
> removes __tuple_or_pair_t):

Good idea.

OK for trunk.

>
> -- >8 --
>
> Subject: [PATCH] libstdc++: extraneous begin in cartesian_product_view::end
>  [PR107572]
>
> ranges::begin() isn't guaranteed to be equality-preserving for
> non-forward ranges, so in cartesian_product_view::end we need to avoid
> calling begin() on the first range (which could be non-forward) in the
> (non-degenerate) case where __empty_tail is false.
>
> Since we're already using a variadic lambda to compute __empty_tail, we
> might as well use that same lambda to build up the tuple of iterators
> instead of doing it separately via __tuple_transform.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
>         PR libstdc++/107572
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges (cartesian_product_view::end): When
>         building the tuple of iterators, avoid calling ranges::begin on
>         the first range if __empty_tail is false.
>         * testsuite/std/ranges/cartesian_product/1.cc (test07): New test.
> ---
>  libstdc++-v3/include/std/ranges               | 28 ++++++++++++-------
>  .../std/ranges/cartesian_product/1.cc         | 25 +++++++++++++++++
>  2 files changed, 43 insertions(+), 10 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index e0cac15a64f..d2ab79179ca 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -8078,26 +8078,34 @@ namespace views::__adaptor
>      end() requires ((!__detail::__simple_view<_First> || ... || !__detail::__simple_view<_Vs>)
>                     && __detail::__cartesian_product_is_common<_First, _Vs...>)
>      {
> -      bool __empty_tail = [this]<size_t... _Is>(index_sequence<_Is...>) {
> -       return (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
> +      auto __it = [this]<size_t... _Is>(index_sequence<_Is...>) {
> +       using _Ret = __detail::__tuple_or_pair_t<iterator_t<_First>,
> +                                                iterator_t<_Vs>...>;
> +       bool __empty_tail = (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
> +       auto& __first = std::get<0>(_M_bases);
> +       return _Ret{(__empty_tail
> +                    ? ranges::begin(__first)
> +                    : __detail::__cartesian_common_arg_end(__first)),
> +                   ranges::begin(std::get<1 + _Is>(_M_bases))...};
>        }(make_index_sequence<sizeof...(_Vs)>{});
>
> -      auto __it = __detail::__tuple_transform(ranges::begin, _M_bases);
> -      if (!__empty_tail)
> -       std::get<0>(__it) = __detail::__cartesian_common_arg_end(std::get<0>(_M_bases));
>        return _Iterator<false>{*this, std::move(__it)};
>      }
>
>      constexpr _Iterator<true>
>      end() const requires __detail::__cartesian_product_is_common<const _First, const _Vs...>
>      {
> -      bool __empty_tail = [this]<size_t... _Is>(index_sequence<_Is...>) {
> -       return (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
> +      auto __it = [this]<size_t... _Is>(index_sequence<_Is...>) {
> +       using _Ret = __detail::__tuple_or_pair_t<iterator_t<const _First>,
> +                                                iterator_t<const _Vs>...>;
> +       bool __empty_tail = (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
> +       auto& __first = std::get<0>(_M_bases);
> +       return _Ret{(__empty_tail
> +                    ? ranges::begin(__first)
> +                    : __detail::__cartesian_common_arg_end(__first)),
> +                   ranges::begin(std::get<1 + _Is>(_M_bases))...};
>        }(make_index_sequence<sizeof...(_Vs)>{});
>
> -      auto __it = __detail::__tuple_transform(ranges::begin, _M_bases);
> -      if (!__empty_tail)
> -       std::get<0>(__it) = __detail::__cartesian_common_arg_end(std::get<0>(_M_bases));
>        return _Iterator<true>{*this, std::move(__it)};
>      }
>
> diff --git a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
> index 1ec4422e6f3..8069eb0ab3c 100644
> --- a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
> @@ -3,6 +3,7 @@
>
>  #include <ranges>
>  #include <algorithm>
> +#include <sstream>
>  #include <testsuite_hooks.h>
>  #include <testsuite_iterators.h>
>
> @@ -178,6 +179,29 @@ test06()
>    return true;
>  }
>
> +void
> +test07()
> +{
> +  // PR libstdc++/107572
> +  static std::istringstream ints("0 1 2 3 4");
> +  struct istream_range
> +  {
> +    auto begin() { return std::istream_iterator<int>{ints}; }
> +    auto end() { return std::istream_iterator<int>{}; }
> +    using iterator_concept = std::input_iterator_tag;
> +  };
> +  static_assert(!ranges::forward_range<istream_range>
> +               && ranges::common_range<istream_range>);
> +  istream_range r;
> +  int i = 0;
> +  for (auto [v] : views::cartesian_product(r))
> +    {
> +      VERIFY( v == i );
> +      ++i;
> +    };
> +  VERIFY( i == 5 );
> +}
> +
>  int
>  main()
>  {
> @@ -187,4 +211,5 @@ main()
>    test04();
>    test05();
>    static_assert(test06());
> +  test07();
>  }
> --
> 2.40.0.rc0.57.g454dfcbddf
>