On Fri, 12 Sept 2025 at 21:57, Patrick Palka <[email protected]> wrote:
>
> On Fri, 12 Sep 2025, Patrick Palka wrote:
>
> > On Fri, 12 Sep 2025, Jonathan Wakely wrote:
> >
> > > On Fri, 12 Sept 2025 at 18:39, Patrick Palka <[email protected]> wrote:
> > > >
> > > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> > > > Patch generated with -w due to otherwise noisy whitespace changes.
> > > >
> > > > -- >8 --
> > > >
> > > > The ranges::shuffle optimization, copied from std::shuffle, has a
> > > > two-at-a-time PRNG optimization that considers the range of the
> > > > PRNG vs the size of the range. But in C++20 a sentinel is not
> > > > necessarily sized so we can't unconditionally do __last - __first.
> > > >
> > > > We could instead use ranges::size, but that'd take linear time for a
> > > > non-sized sentinel which makes the optimization less clear of a win.
> > > > So this patch instead makes us only consider this optimization for
> > > > arguments that model sized_sentinel_for or sized_range.
> > > >
> > > > PR libstdc++/121917
> > > >
> > > > libstdc++-v3/ChangeLog:
> > > >
> > > > * include/bits/ranges_algo.h (__shuffle_fn::_S_impl): Factor
> > > > out from main operator() overload. Add optional size parameter,
> > > > and only consider the two-at-a-time PRNG optimization if the
> > > > size is given.
> > > > (__shuffle_fn::operator()): Adjust to call _S_impl directly,
> > > > computing the range size for sized_sentinel_for or sized_range
> > > > arguments.
> > > > * testsuite/25_algorithms/shuffle/constrained.cc:
> > > > ---
> > > > libstdc++-v3/include/bits/ranges_algo.h | 35 ++++++++++++++-----
> > > > .../25_algorithms/shuffle/constrained.cc | 18 ++++++++++
> > > > 2 files changed, 44 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/libstdc++-v3/include/bits/ranges_algo.h
> > > > b/libstdc++-v3/include/bits/ranges_algo.h
> > > > index 6e1e06cb2d0f..855ab1395149 100644
> > > > --- a/libstdc++-v3/include/bits/ranges_algo.h
> > > > +++ b/libstdc++-v3/include/bits/ranges_algo.h
> > > > @@ -1945,12 +1945,10 @@ namespace ranges
> > > >
> > > > struct __shuffle_fn
> > > > {
> > > > - template<random_access_iterator _Iter, sentinel_for<_Iter> _Sent,
> > > > - typename _Gen>
> > > > - requires permutable<_Iter>
> > > > - && uniform_random_bit_generator<remove_reference_t<_Gen>>
> > > > - _Iter
> > > > - operator()(_Iter __first, _Sent __last, _Gen&& __g) const
> > > > + template<typename _Iter, typename _Sent, typename _Gen>
> > > > + static _Iter
> > > > + _S_impl(_Iter __first, _Sent __last, _Gen&& __g,
> > > > + iter_difference_t<_Iter> __n)
> > >
> > > This _S_impl should be private, however ...
> > >
> > > > {
> > > > // FIXME: Correctly handle integer-class difference types.
> > > > if (__first == __last)
> > > > @@ -1964,8 +1962,10 @@ namespace ranges
> > > > using __uc_type
> > > > = common_type_t<typename
> > > > remove_reference_t<_Gen>::result_type, __ud_type>;
> > > >
> > > > + if (__n != -1)
> > >
> > > Having this as a runtime check for != 1 seems a little inelegant when
> > > we've already determined statically whether we want to consider the
> > > optimization.
> > >
> > > We could leave the main implementation in the operator()(Iter, Sent,
> > > Gen&&) overload and just add:
> > >
> > > if constexpr (sized_sentinel_for<Sent, Iter>)
> > >
> > > here instead of 'if ( n != 1)'
> > >
> > > Then in the operator()(R&&, Gen&&) overload do:
> > >
> > > if constexpr (sized_range<_Range>)
> > > {
> > > auto __first = ranges::begin(__r);
> > > return (*this)(__first, __first + ranges::size(__r),
> > > std::forward<_Gen>(__g));
> > > }
> > > else
> > > return (*this)(ranges::begin(__r), ranges::end(__r),
> > > std::forward<_Gen>(__g));
> > >
> > > This way we don't need to pass n to _S_impl, we just ensure that we
> > > pass a sized sentinel instead, and that enables the optimization.
> >
> > Sounds good, like so?
> >
> > -- >8 --
> >
> > Subject: [PATCH] libstdc++: Fix ranges::shuffle for non-sized range
> > [PR121917]
> >
> > The ranges::shuffle optimization, copied from std::shuffle, has a
> > two-at-a-time PRNG optimization that considers the range of the
> > PRNG vs the size of the range. But in C++20 a sentinel is not
> > necessarily sized so we can't unconditionally do __last - __first.
> >
> > We could instead use ranges::size, but that'd take linear time for a
> > non-sized sentinel which makes the optimization less clear of a win.
> > So this patch instead makes us only consider this optimization for
> > sized ranges.
> >
> > PR libstdc++/121917
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/bits/ranges_algo.h (__shuffle_fn::operator()): Only
> > consider the two-at-a-time PRNG optimization if the range is
> > sized.
> > * testsuite/25_algorithms/shuffle/constrained.cc (test03): New
> > test.
>
> Whoops, forgot to --amend the commit. This is the correct diff:
>
> -- >8 --
>
> Subject: [PATCH] libstdc++: Fix ranges::shuffle for non-sized range [PR121917]
>
> ---
> libstdc++-v3/include/bits/ranges_algo.h | 11 ++++++++++-
> .../25_algorithms/shuffle/constrained.cc | 18 ++++++++++++++++++
> 2 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/bits/ranges_algo.h
> b/libstdc++-v3/include/bits/ranges_algo.h
> index 6e1e06cb2d0f..f886edbb952c 100644
> --- a/libstdc++-v3/include/bits/ranges_algo.h
> +++ b/libstdc++-v3/include/bits/ranges_algo.h
> @@ -1964,9 +1964,10 @@ namespace ranges
> using __uc_type
> = common_type_t<typename remove_reference_t<_Gen>::result_type,
> __ud_type>;
>
> + if constexpr (sized_sentinel_for<_Sent, _Iter>)
> + {
> const __uc_type __urngrange = __g.max() - __g.min();
> const __uc_type __urange = __uc_type(__last - __first);
> -
> if (__urngrange / __urange >= __urange)
> // I.e. (__urngrange >= __urange * __urange) but without wrap
> issues.
> {
> @@ -1999,6 +2000,7 @@ namespace ranges
>
> return __i;
> }
> + }
>
> __distr_type __d;
>
> @@ -2015,6 +2017,13 @@ namespace ranges
> borrowed_iterator_t<_Range>
> operator()(_Range&& __r, _Gen&& __g) const
> {
> + if constexpr (sized_range<_Range>
> + && !sized_sentinel_for<sentinel_t<_Range>,
> + iterator_t<_Range>>)
> + return (*this)(ranges::begin(__r),
> + ranges::begin(__r) + ranges::size(__r),
Oops, the addition needs to use
range_difference_t<Range>(ranges::size(__r)), or just
ranges::distance(__r). We know ranges::distance will be O(1) because
it's a sized_range.
> + std::forward<_Gen>(__g));
> + else
> return (*this)(ranges::begin(__r), ranges::end(__r),
> std::forward<_Gen>(__g));
> }
> diff --git a/libstdc++-v3/testsuite/25_algorithms/shuffle/constrained.cc
> b/libstdc++-v3/testsuite/25_algorithms/shuffle/constrained.cc
> index 70c6bdfc3d9e..4d633561508b 100644
> --- a/libstdc++-v3/testsuite/25_algorithms/shuffle/constrained.cc
> +++ b/libstdc++-v3/testsuite/25_algorithms/shuffle/constrained.cc
> @@ -86,9 +86,27 @@ test02()
> ranges::shuffle(w, g);
> }
>
> +struct non_default_sentinel_t { };
> +
> +template<std::input_or_output_iterator I>
> +bool operator==(const I& i, non_default_sentinel_t)
> +{ return i == std::default_sentinel; }
> +
> +void
> +test03()
> +{
> + // PR libstdc++/121917 - ranges::shuffle incorrectly requires its arguments
> + // to model sized_sentinel_for
> + int a[2]{};
> + std::counted_iterator iter(a, 2);
> + std::default_random_engine e;
> + std::ranges::shuffle(iter, non_default_sentinel_t{}, e);
> +}
> +
> int
> main()
> {
> test01();
> test02();
> + test03();
> }
> --
> 2.51.0.193.g4975ec3473
>
