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.
---
libstdc++-v3/include/bits/ranges_algo.h | 8 ++++++++
.../25_algorithms/shuffle/constrained.cc | 18 ++++++++++++++++++
2 files changed, 26 insertions(+)
diff --git a/libstdc++-v3/include/bits/ranges_algo.h
b/libstdc++-v3/include/bits/ranges_algo.h
index 6e1e06cb2d0f..911ef90b971a 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -1967,6 +1967,7 @@ namespace ranges
const __uc_type __urngrange = __g.max() - __g.min();
const __uc_type __urange = __uc_type(__last - __first);
+ if constexpr (sized_sentinel_for<__Sent, _Iter>)
if (__urngrange / __urange >= __urange)
// I.e. (__urngrange >= __urange * __urange) but without wrap
issues.
{
@@ -2015,6 +2016,13 @@ 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),
+ 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
