Re: Rb_tree constructor optimization

[François Dumont <frs dot dumont at gmail dot com>]

On 08/09/2017 17:50, Jonathan Wakely wrote:
> Since we know __a == __x.get_allocator() we could just do:
>
>      _Rb_tree(_Rb_tree&& __x, _Node_allocator&&, true_type)
> noexcept(is_nothrow_move_constructible<_Rb_tree_impl<_Compare>>::value)
>      : _M_impl(std::move(__x._M_impl))
>      { }
>
> This means we don't need the new constructor.

    You want to consider that a always equal allocator is stateless and 
so that the provided allocator rvalue reference do not need to be moved. 
IMHO you can have allocator with state that do not participate in 
comparison like some monitoring info.

    I'm not confortable with that and prefer to keep current behavior 
so I propose this new patch considering all your other remarks.

    I change noexcept qualification on [multi]map/set constructors to 
just rely on _Rep_type constructor noexcept qualification to not 
duplicate it.

François


diff --git a/libstdc++-v3/include/bits/stl_map.h b/libstdc++-v3/include/bits/stl_map.h
index 0e8a98a..cdd2e7c 100644
--- a/libstdc++-v3/include/bits/stl_map.h
+++ b/libstdc++-v3/include/bits/stl_map.h
@@ -235,8 +235,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
       /// Allocator-extended move constructor.
       map(map&& __m, const allocator_type& __a)
-      noexcept(is_nothrow_copy_constructible<_Compare>::value
-	       && _Alloc_traits::_S_always_equal())
+      noexcept( noexcept(
+	_Rep_type(declval<_Rep_type>(), declval<_Pair_alloc_type>())) )
       : _M_t(std::move(__m._M_t), _Pair_alloc_type(__a)) { }
 
       /// Allocator-extended initialier-list constructor.
diff --git a/libstdc++-v3/include/bits/stl_multimap.h b/libstdc++-v3/include/bits/stl_multimap.h
index 7e3cea4..d32104d 100644
--- a/libstdc++-v3/include/bits/stl_multimap.h
+++ b/libstdc++-v3/include/bits/stl_multimap.h
@@ -232,8 +232,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
       /// Allocator-extended move constructor.
       multimap(multimap&& __m, const allocator_type& __a)
-      noexcept(is_nothrow_copy_constructible<_Compare>::value
-	       && _Alloc_traits::_S_always_equal())
+      noexcept( noexcept(
+	_Rep_type(declval<_Rep_type>(), declval<_Pair_alloc_type>())) )
       : _M_t(std::move(__m._M_t), _Pair_alloc_type(__a)) { }
 
       /// Allocator-extended initialier-list constructor.
diff --git a/libstdc++-v3/include/bits/stl_multiset.h b/libstdc++-v3/include/bits/stl_multiset.h
index 517e77e..9ab4ab7 100644
--- a/libstdc++-v3/include/bits/stl_multiset.h
+++ b/libstdc++-v3/include/bits/stl_multiset.h
@@ -244,8 +244,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
       /// Allocator-extended move constructor.
       multiset(multiset&& __m, const allocator_type& __a)
-      noexcept(is_nothrow_copy_constructible<_Compare>::value
-	       && _Alloc_traits::_S_always_equal())
+      noexcept( noexcept(
+	_Rep_type(declval<_Rep_type>(), declval<_Key_alloc_type>())) )
       : _M_t(std::move(__m._M_t), _Key_alloc_type(__a)) { }
 
       /// Allocator-extended initialier-list constructor.
diff --git a/libstdc++-v3/include/bits/stl_set.h b/libstdc++-v3/include/bits/stl_set.h
index e804a7c..6b64bcd 100644
--- a/libstdc++-v3/include/bits/stl_set.h
+++ b/libstdc++-v3/include/bits/stl_set.h
@@ -248,8 +248,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
       /// Allocator-extended move constructor.
       set(set&& __x, const allocator_type& __a)
-      noexcept(is_nothrow_copy_constructible<_Compare>::value
-	       && _Alloc_traits::_S_always_equal())
+      noexcept( noexcept(
+	_Rep_type(declval<_Rep_type>(), declval<_Key_alloc_type>())) )
       : _M_t(std::move(__x._M_t), _Key_alloc_type(__a)) { }
 
       /// Allocator-extended initialier-list constructor.
diff --git a/libstdc++-v3/include/bits/stl_tree.h b/libstdc++-v3/include/bits/stl_tree.h
index c2417f1..7aacc24 100644
--- a/libstdc++-v3/include/bits/stl_tree.h
+++ b/libstdc++-v3/include/bits/stl_tree.h
@@ -704,6 +704,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #else
 	  _Rb_tree_impl(_Rb_tree_impl&&) = default;
 
+	  _Rb_tree_impl(_Rb_tree_impl&& __x, _Node_allocator&& __a)
+	  : _Node_allocator(std::move(__a)),
+	    _Base_key_compare(std::move(__x)),
+	    _Rb_tree_header(std::move(__x))
+	  { }
+
 	  _Rb_tree_impl(const _Key_compare& __comp, _Node_allocator&& __a)
 	  : _Node_allocator(std::move(__a)), _Base_key_compare(__comp)
 	  { }
@@ -947,7 +953,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       : _Rb_tree(std::move(__x), _Node_allocator(__a))
       { }
 
-      _Rb_tree(_Rb_tree&& __x, _Node_allocator&& __a);
+    private:
+      _Rb_tree(_Rb_tree&& __x, _Node_allocator&& __a, true_type)
+      : _M_impl(std::move(__x._M_impl), std::move(__a))
+      { }
+
+      _Rb_tree(_Rb_tree&& __x, _Node_allocator&& __a, false_type)
+      : _M_impl(__x._M_impl._M_key_compare, std::move(__a))
+      {
+	if (__x._M_root() != nullptr)
+	  _M_move_data(__x, false_type{});
+      }
+
+    public:
+      _Rb_tree(_Rb_tree&& __x, _Node_allocator&& __a)
+      noexcept(is_nothrow_move_constructible<_Rb_tree_impl<_Compare>>::value
+	       && _Alloc_traits::_S_always_equal())
+      : _Rb_tree(std::move(__x), std::move(__a),
+		 typename _Alloc_traits::is_always_equal{})
+      { }
 #endif
 
       ~_Rb_tree() _GLIBCXX_NOEXCEPT
@@ -1347,22 +1371,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     private:
       // Move elements from container with equal allocator.
       void
-      _M_move_data(_Rb_tree& __x, std::true_type)
+      _M_move_data(_Rb_tree& __x, true_type)
       { _M_impl._M_move_data(__x._M_impl); }
 
       // Move elements from container with possibly non-equal allocator,
       // which might result in a copy not a move.
       void
-      _M_move_data(_Rb_tree&, std::false_type);
+      _M_move_data(_Rb_tree&, false_type);
 
       // Move assignment from container with equal allocator.
       void
-      _M_move_assign(_Rb_tree&, std::true_type);
+      _M_move_assign(_Rb_tree&, true_type);
 
       // Move assignment from container with possibly non-equal allocator,
       // which might result in a copy not a move.
       void
-      _M_move_assign(_Rb_tree&, std::false_type);
+      _M_move_assign(_Rb_tree&, false_type);
 #endif
 
 #if __cplusplus > 201402L
@@ -1590,23 +1614,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 201103L
   template<typename _Key, typename _Val, typename _KeyOfValue,
 	   typename _Compare, typename _Alloc>
-    _Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::
-    _Rb_tree(_Rb_tree&& __x, _Node_allocator&& __a)
-    : _M_impl(__x._M_impl._M_key_compare, std::move(__a))
-    {
-      using __eq = typename _Alloc_traits::is_always_equal;
-      if (__x._M_root() != nullptr)
-	_M_move_data(__x, __eq());
-    }
-
-  template<typename _Key, typename _Val, typename _KeyOfValue,
-	   typename _Compare, typename _Alloc>
     void
     _Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::
-    _M_move_data(_Rb_tree& __x, std::false_type)
+    _M_move_data(_Rb_tree& __x, false_type)
     {
       if (_M_get_Node_allocator() == __x._M_get_Node_allocator())
-	_M_move_data(__x, std::true_type());
+	_M_move_data(__x, true_type());
       else
 	{
 	  _Alloc_node __an(*this);
@@ -1628,7 +1641,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
       clear();
       if (__x._M_root() != nullptr)
-	_M_move_data(__x, std::true_type());
+	_M_move_data(__x, true_type());
       std::__alloc_on_move(_M_get_Node_allocator(),
 			   __x._M_get_Node_allocator());
     }