[PATCH 4/8] libstdc++: Directly implement ranges::stable_sort [PR100795]
Jonathan Wakely
jwakely@redhat.com
Fri Jun 27 13:21:36 GMT 2025
On 26/06/25 22:25 -0400, Patrick Palka wrote:
> PR libstdc++/100795
>
>libstdc++-v3/ChangeLog:
>
> * include/bits/ranges_algo.h (__detail::__move_merge): New,
> based on the stl_algo.h implementation.
> (__detail::__merge_sort_loop): Likewise.
> (__detail::__chunk_insertion_sort): Likewise.
> (__detail::__merge_sort_with_buffer): Likewise.
> (__detail::__stable_sort_adaptive): Likewise.
> (__detail::__stable_sort_adaptive_resize): Likewise.
> (__detail::__inplace_stable_sort): Likewise.
> (__stable_sort_fn::operator()): Reimplement in terms of the above.
> * testsuite/25_algorithms/stable_sort/constrained.cc:
>---
> libstdc++-v3/include/bits/ranges_algo.h | 207 +++++++++++++++++-
> .../25_algorithms/stable_sort/constrained.cc | 30 +++
> 2 files changed, 233 insertions(+), 4 deletions(-)
>
>diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
>index b0357600adbc..7dfd4e7ed64c 100644
>--- a/libstdc++-v3/include/bits/ranges_algo.h
>+++ b/libstdc++-v3/include/bits/ranges_algo.h
>@@ -2388,6 +2388,170 @@ namespace ranges
>
> inline constexpr __sort_fn sort{};
>
>+ namespace __detail
>+ {
>+ /// This is a helper function for the __merge_sort_loop routines.
>+ template<typename _Iter, typename _Out, typename _Comp>
>+ _Out
>+ __move_merge(_Iter __first1, _Iter __last1,
>+ _Iter __first2, _Iter __last2,
>+ _Out __result, _Comp __comp)
>+ {
>+ while (__first1 != __last1 && __first2 != __last2)
>+ {
>+ if (__comp(*__first2, *__first1))
>+ {
>+ *__result = ranges::iter_move(__first2);
>+ ++__first2;
>+ }
>+ else
>+ {
>+ *__result = ranges::iter_move(__first1);
>+ ++__first1;
>+ }
>+ ++__result;
>+ }
>+ return ranges::move(__first2, __last2,
>+ ranges::move(__first1, __last1, __result).out).out;
>+ }
>+
>+ template<typename _Iter, typename _Out, typename _Distance, typename _Comp>
>+ void
>+ __merge_sort_loop(_Iter __first, _Iter __last, _Out __result,
>+ _Distance __step_size, _Comp __comp)
>+ {
>+ const _Distance __two_step = 2 * __step_size;
>+
>+ while (__last - __first >= __two_step)
>+ {
>+ __result = __detail::__move_merge(__first, __first + __step_size,
>+ __first + __step_size,
>+ __first + __two_step,
>+ __result, __comp);
>+ __first += __two_step;
>+ }
>+ __step_size = ranges::min(_Distance(__last - __first), __step_size);
>+
>+ __detail::__move_merge(__first, __first + __step_size,
>+ __first + __step_size, __last, __result, __comp);
>+ }
>+
>+ template<typename _Iter, typename _Distance, typename _Compare>
>+ constexpr void
>+ __chunk_insertion_sort(_Iter __first, _Iter __last,
>+ _Distance __chunk_size, _Compare __comp)
>+ {
>+ while (__last - __first >= __chunk_size)
>+ {
>+ __detail::__insertion_sort(__first, __first + __chunk_size, __comp);
>+ __first += __chunk_size;
>+ }
>+ __detail::__insertion_sort(__first, __last, __comp);
>+ }
>+
>+ template<typename _Iter, typename _Pointer, typename _Comp>
>+ void
>+ __merge_sort_with_buffer(_Iter __first, _Iter __last,
>+ _Pointer __buffer, _Comp __comp)
>+ {
>+ using _Distance = iter_difference_t<_Iter>;
>+
>+ const _Distance __len = __last - __first;
>+ const _Pointer __buffer_last = __buffer + ptrdiff_t(__len);
>+
>+ constexpr int __chunk_size = 7;
>+ _Distance __step_size = __chunk_size;
>+ __detail::__chunk_insertion_sort(__first, __last, __step_size, __comp);
>+
>+ while (__step_size < __len)
>+ {
>+ __detail::__merge_sort_loop(__first, __last, __buffer,
>+ __step_size, __comp);
>+ __step_size *= 2;
>+ __detail::__merge_sort_loop(__buffer, __buffer_last, __first,
>+ ptrdiff_t(__step_size), __comp);
>+ __step_size *= 2;
>+ }
>+ }
>+
>+ template<typename _Iter, typename _Pointer, typename _Comp>
>+ void
>+ __merge_adaptive(_Iter __first, _Iter __middle, _Iter __last,
>+ iter_difference_t<_Iter> __len1,
>+ iter_difference_t<_Iter> __len2,
>+ _Pointer __buffer, _Comp __comp); // defined near inplace_merge
>+
>+ template<typename _Iter, typename _Distance, typename _Pointer, typename _Comp>
>+ void
>+ __merge_adaptive_resize(_Iter __first, _Iter __middle, _Iter __last,
>+ _Distance __len1, _Distance __len2,
>+ _Pointer __buffer, _Distance __buffer_size,
>+ _Comp __comp); // defined near inplace_merge
>+
>+ template<typename _Iter, typename _Distance, typename _Comp>
>+ constexpr void
>+ __merge_without_buffer(_Iter __first, _Iter __middle, _Iter __last,
>+ _Distance __len1, _Distance __len2,
>+ _Comp __comp); // defined near inplace_merge
>+
>+ template<typename _Iter, typename _Pointer, typename _Comp>
>+ void
>+ __stable_sort_adaptive(_Iter __first, _Iter __middle, _Iter __last,
>+ _Pointer __buffer, _Comp __comp)
>+ {
>+ __detail::__merge_sort_with_buffer(__first, __middle, __buffer, __comp);
>+ __detail::__merge_sort_with_buffer(__middle, __last, __buffer, __comp);
>+
>+ __detail::__merge_adaptive(__first, __middle, __last,
>+ __middle - __first, __last - __middle,
>+ __buffer, __comp);
>+ }
>+
>+ template<typename _Iter, typename _Pointer, typename _Distance, typename _Comp>
>+ void
>+ __stable_sort_adaptive_resize(_Iter __first, _Iter __last,
>+ _Pointer __buffer, _Distance __buffer_size,
>+ _Comp __comp)
>+ {
>+ const _Distance __len = (__last - __first + 1) / 2;
>+ const _Iter __middle = __first + __len;
>+ if (__len > __buffer_size)
>+ {
>+ __detail::__stable_sort_adaptive_resize(__first, __middle, __buffer,
>+ __buffer_size, __comp);
>+ __detail::__stable_sort_adaptive_resize(__middle, __last, __buffer,
>+ __buffer_size, __comp);
>+ __detail::__merge_adaptive_resize(__first, __middle, __last,
>+ _Distance(__middle - __first),
>+ _Distance(__last - __middle),
>+ __buffer, __buffer_size,
>+ __comp);
>+ }
>+ else
>+ __detail::__stable_sort_adaptive(__first, __middle, __last,
>+ __buffer, __comp);
>+ }
>+
>+ template<typename _Iter, typename _Comp>
>+ _GLIBCXX26_CONSTEXPR
I think this could be unconditionally constexpr. It will never be used
during constant evaluation before C++26, but we can avoid the ugly
macro. Or do you think the macro serves as documentation for when we
expect to be constexpr? ... I suppose the 26_CONSTEXPR macro it makes
it clear that it could potentially use 'if consteval' in the body for
any constexpr paths, whereas it couldn't do that if it was used in
C++20.
>+ void
>+ __inplace_stable_sort(_Iter __first, _Iter __last, _Comp __comp)
>+ {
>+ if (__last - __first < 15)
>+ {
>+ __detail::__insertion_sort(__first, __last, __comp);
>+ return;
>+ }
>+ _Iter __middle = __first + (__last - __first) / 2;
>+ __detail::__inplace_stable_sort(__first, __middle, __comp);
>+ __detail::__inplace_stable_sort(__middle, __last, __comp);
>+ __detail::__merge_without_buffer(__first, __middle, __last,
>+ __middle - __first,
>+ __last - __middle,
>+ __comp);
>+ }
>+ } // namespace __detail
>+
> struct __stable_sort_fn
> {
> template<random_access_iterator _Iter, sentinel_for<_Iter> _Sent,
>@@ -2398,10 +2562,45 @@ namespace ranges
> operator()(_Iter __first, _Sent __last,
> _Comp __comp = {}, _Proj __proj = {}) const
> {
>- auto __lasti = ranges::next(__first, __last);
>- std::stable_sort(std::move(__first), __lasti,
>- __detail::__make_comp_proj(__comp, __proj));
>- return __lasti;
>+ if constexpr (!same_as<_Iter, _Sent>)
>+ return (*this)(__first, ranges::next(__first, __last),
>+ std::move(__comp), std::move(__proj));
>+ else
>+ {
>+ using _DistanceType = iter_difference_t<_Iter>;
>+
>+ if (__first == __last)
>+ return __last;
>+
>+#if _GLIBCXX_HOSTED
>+# if __glibcxx_constexpr_algorithms >= 202306L // >= C++26
>+ if consteval {
>+ __detail::__inplace_stable_sort(__first, __last, __comp);
>+ return __last;
>+ }
>+# endif
>+
>+ typedef _Temporary_buffer<_Iter, iter_value_t<_Iter>> _TmpBuf;
>+ // __stable_sort_adaptive sorts the range in two halves,
>+ // so the buffer only needs to fit half the range at once.
>+ _TmpBuf __buf(__first, ptrdiff_t((__last - __first + 1) / 2));
>+
>+ auto __comp_proj = __detail::__make_comp_proj(__comp, __proj);
>+ if (__buf._M_requested_size() == __buf.size()) [[likely]]
>+ __detail::__stable_sort_adaptive(__first,
>+ __first + _DistanceType(__buf.size()),
>+ __last, __buf.begin(), __comp_proj);
>+ else if (__buf.begin()) [[unlikely]]
>+ __detail::__inplace_stable_sort(__first, __last, __comp_proj);
>+ else
>+ __detail::__stable_sort_adaptive_resize(__first, __last, __buf.begin(),
>+ _DistanceType(__buf.size()),
>+ __comp_proj);
>+#else
>+ __detail::__inplace_stable_sort(__first, __last, __comp_proj);
>+#endif
>+ return __last;
>+ }
> }
>
> template<random_access_range _Range,
>diff --git a/libstdc++-v3/testsuite/25_algorithms/stable_sort/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/stable_sort/constrained.cc
>index 0bd438fa5f29..55043442f4be 100644
>--- a/libstdc++-v3/testsuite/25_algorithms/stable_sort/constrained.cc
>+++ b/libstdc++-v3/testsuite/25_algorithms/stable_sort/constrained.cc
>@@ -20,6 +20,7 @@
>
> #include <algorithm>
> #include <random>
>+#include <ranges>
> #include <vector>
> #include <testsuite_hooks.h>
> #include <testsuite_iterators.h>
>@@ -62,8 +63,37 @@ test01()
> }
> }
>
>+void
>+test02()
>+{
>+ // PR libstdc++/100795 - ranges::stable_sort should not use std::stable_sort
>+ // directly
>+#if __SIZEOF_INT128__
>+ auto v = std::views::iota(__int128(0), __int128(20));
>+#else
>+ auto v = std::views::iota(0ll, 20ll);
>+#endif
>+
>+ int storage[20] = {2,5,4,3,1,6,7,9,10,8,11,14,12,13,15,16,18,0,19,17};
>+ auto w = v | std::views::transform([&](auto i) -> int& { return storage[i]; });
>+ using type = decltype(w);
>+ using cat = std::iterator_traits<std::ranges::iterator_t<type>>::iterator_category;
>+ static_assert( std::same_as<cat, std::output_iterator_tag> );
>+ static_assert( std::ranges::random_access_range<type> );
>+
>+ ranges::stable_sort(w);
>+ VERIFY( ranges::equal(w, v) );
>+
>+ ranges::stable_sort(w, std::ranges::greater{});
>+ VERIFY( ranges::equal(w, v | std::views::reverse) );
>+
>+ ranges::stable_sort(w, std::ranges::greater{}, std::negate{});
>+ VERIFY( ranges::equal(w, v) );
>+}
>+
> int
> main()
> {
> test01();
>+ test02();
> }
>--
>2.50.0.131.gcf6f63ea6b
>
>
More information about the Libstdc++
mailing list