PR libstdc++/86127 avoid unnecessary allocator conversions

There is no need to use an allocator of the correct value_type when
calling allocator_traits::construct and allocator_traits::destroy. The
existing node allocator can be used, instead of constructing a new
allocator object every time.

There's also no benefit to using __gnu_cxx::__alloc_traits instead of
std::allocator_traits to get the pointer and const_pointer types.
std::forward_list is only available for C++11 and later, when
std::allocator_traits is available too.

Backport from mainline
2018-06-13  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/86127
	* include/bits/forward_list.h (_Fwd_list_base::_Tp_alloc_type): Remove
	unused typedef.
	(_Fwd_list_base::_M_create_node, _Fwd_list_base::_M_erase_after):
	Use node allocator to create and destroy elements.
	(forward_list::_Tp_alloc_type): Remove unused typedef.
	(forward_list::_Alloc_traits): Use allocator_traits instead of
	__gnu_cxx::__alloc_traits.
        * include/bits/forward_list.tcc (_Fwd_list_base::_M_erase_after):
	Use node allocator to create and destroy elements.

git-svn-id: https://gcc.gnu.org/svn/gcc/branches/gcc-7-branch@262411 138bc75d-0d04-0410-961f-82ee72b054a4

3 files changed, 22 insertions, 11 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index e174d8d24d1..772c62fce40 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,6 +1,20 @@
 2018-07-04  Jonathan Wakely  <jwakely@redhat.com>
 
 	Backport from mainline
+	2018-06-13  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/86127
+	* include/bits/forward_list.h (_Fwd_list_base::_Tp_alloc_type): Remove
+	unused typedef.
+	(_Fwd_list_base::_M_create_node, _Fwd_list_base::_M_erase_after):
+	Use node allocator to create and destroy elements.
+	(forward_list::_Tp_alloc_type): Remove unused typedef.
+	(forward_list::_Alloc_traits): Use allocator_traits instead of
+	__gnu_cxx::__alloc_traits.
+	* include/bits/forward_list.tcc (_Fwd_list_base::_M_erase_after):
+	Use node allocator to create and destroy elements.
+
+	Backport from mainline
 	2018-05-29  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/std/variant (__erased_dtor): Qualify call to __get.
diff --git a/libstdc++-v3/include/bits/forward_list.h b/libstdc++-v3/include/bits/forward_list.h
index c37bf01345a..338e7b3dc6f 100644
--- a/libstdc++-v3/include/bits/forward_list.h
+++ b/libstdc++-v3/include/bits/forward_list.h
@@ -274,7 +274,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
     struct _Fwd_list_base
     {
     protected:
-      typedef __alloc_rebind<_Alloc, _Tp> 		  _Tp_alloc_type;
       typedef __alloc_rebind<_Alloc, _Fwd_list_node<_Tp>> _Node_alloc_type;
       typedef __gnu_cxx::__alloc_traits<_Node_alloc_type> _Node_alloc_traits;
 
@@ -345,11 +344,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
           _Node* __node = this->_M_get_node();
           __try
             {
-	      _Tp_alloc_type __a(_M_get_Node_allocator());
-	      typedef allocator_traits<_Tp_alloc_type> _Alloc_traits;
 	      ::new ((void*)__node) _Node;
-	      _Alloc_traits::construct(__a, __node->_M_valptr(),
-				       std::forward<_Args>(__args)...);
+	      _Node_alloc_traits::construct(_M_get_Node_allocator(),
+					    __node->_M_valptr(),
+					    std::forward<_Args>(__args)...);
             }
           __catch(...)
             {
@@ -412,10 +410,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       typedef _Fwd_list_base<_Tp, _Alloc>                  _Base;
       typedef _Fwd_list_node<_Tp>                          _Node;
       typedef _Fwd_list_node_base                          _Node_base;
-      typedef typename _Base::_Tp_alloc_type               _Tp_alloc_type;
       typedef typename _Base::_Node_alloc_type             _Node_alloc_type;
       typedef typename _Base::_Node_alloc_traits           _Node_alloc_traits;
-      typedef __gnu_cxx::__alloc_traits<_Tp_alloc_type>    _Alloc_traits;
+      typedef allocator_traits<__alloc_rebind<_Alloc, _Tp>>    _Alloc_traits;
 
     public:
       // types:
diff --git a/libstdc++-v3/include/bits/forward_list.tcc b/libstdc++-v3/include/bits/forward_list.tcc
index b823b09e1af..c852aa1b716 100644
--- a/libstdc++-v3/include/bits/forward_list.tcc
+++ b/libstdc++-v3/include/bits/forward_list.tcc
@@ -69,8 +69,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
     {
       _Node* __curr = static_cast<_Node*>(__pos->_M_next);
       __pos->_M_next = __curr->_M_next;
-      _Tp_alloc_type __a(_M_get_Node_allocator());
-      allocator_traits<_Tp_alloc_type>::destroy(__a, __curr->_M_valptr());
+      _Node_alloc_traits::destroy(_M_get_Node_allocator(),
+				  __curr->_M_valptr());
       __curr->~_Node();
       _M_put_node(__curr);
       return __pos->_M_next;
@@ -87,8 +87,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
         {
           _Node* __temp = __curr;
           __curr = static_cast<_Node*>(__curr->_M_next);
-	  _Tp_alloc_type __a(_M_get_Node_allocator());
-	  allocator_traits<_Tp_alloc_type>::destroy(__a, __temp->_M_valptr());
+	  _Node_alloc_traits::destroy(_M_get_Node_allocator(),
+				      __temp->_M_valptr());
 	  __temp->~_Node();
           _M_put_node(__temp);
         }