libstdc++: Fix suboptimal complexity of flat_map::_M_insert

Ever since r16-1742 ranges::inplace_merge is now correctly C++20 iterator
aware which allows us to idiomatically implement this helper with the
correct optimal complexity N + M log M instead of N log N.

libstdc++-v3/ChangeLog:

	* include/std/flat_map (_Flat_map_impl::_M_insert): New bool
	parameter __is_sorted defaulted to false.  Reimplement using
	views::zip and ranges::inplace_merge.
	(_Flat_map_impl::insert): In the __sorted_t overload, pass
	__is_sorted=true to _M_insert.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
index 18f06e6bdaf81fdfb078f8356b135a4a3c676349..a51c6625d54ab203d7e778a37666ded543a3d467 100644 (file)
--- a/libstdc++-v3/include/std/flat_map
+++ b/libstdc++-v3/include/std/flat_map
@@ -602,13 +602,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Iter, typename _Sent>
        _GLIBCXX26_CONSTEXPR
        void
-       _M_insert(_Iter __first, _Sent __last)
+       _M_insert(_Iter __first, _Sent __last, bool __is_sorted = false)
        {
-         // FIXME: This implementation fails its complexity requirements.
-         // We can't idiomatically implement an efficient version (as in the
-         // disabled code) until ranges::inplace_merge is fixed to dispatch
-         // on iterator concept instead of iterator category.
-#if 0
          auto __guard = _M_make_clear_guard();
          auto __n = size();
          for (; __first != __last; ++__first)
@@ -618,22 +613,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
              _M_cont.values.emplace_back(std::move(__value.second));
            }
          auto __zv = views::zip(_M_cont.keys, _M_cont.values);
-         ranges::sort(__zv.begin() + __n, __zv.end(), value_comp());
-         ranges::inplace_merge(__zv.begin(), __zv.begin() + __n, __zv.end(), value_comp());
+         if (__is_sorted)
+           _GLIBCXX_DEBUG_ASSERT(ranges::is_sorted(__zv.begin() + __n, __zv.end(),
+                                                   value_comp()));
+         else
+           ranges::sort(__zv.begin() + __n, __zv.end(), value_comp());
+         ranges::inplace_merge(__zv.begin(), __zv.begin() + __n, __zv.end(),
+                               value_comp());
          if constexpr (!_Multi)
            _M_unique();
-         __guard._M_cont = nullptr;
-#else
-         auto __guard = _M_make_clear_guard();
-         for (; __first != __last; ++__first)
-           {
-             value_type __value = *__first;
-             _M_cont.keys.emplace_back(std::move(__value.first));
-             _M_cont.values.emplace_back(std::move(__value.second));
-           }
-         _M_sort_uniq();
          __guard._M_disable();
-#endif
        }
 
     public:
@@ -647,10 +636,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        _GLIBCXX26_CONSTEXPR
        void
        insert(__sorted_t, _InputIterator __first, _InputIterator __last)
-       {
-         // FIXME: This implementation fails its complexity requirements; see above.
-         insert(std::move(__first), std::move(__last));
-       }
+       { _M_insert(std::move(__first), std::move(__last), true); }
 
       template<__detail::__container_compatible_range<value_type> _Rg>
        _GLIBCXX26_CONSTEXPR