[PATCH] PR libstdc++/86127 avoid unnecessary allocator conversions

[Jonathan Wakely]

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.

        PR libstdc++/86127
        * include/bits/forward_list.h (_Fwd_list_base::_Tp_alloc_type): Remove
        unused typedef.
        (_Fwd_list_base::_Node_alloc_traits): Use allocator_traits instead of
        __gnu_cxx::__alloc_traits.
        (_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.

Tested powerpc64le-linux, committed to trunk.



commit 777e063c0bb527a72539169ea25f06d7ad3de536
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Wed Jun 13 13:54:47 2018 +0100

    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.
    
            PR libstdc++/86127
            * include/bits/forward_list.h (_Fwd_list_base::_Tp_alloc_type): 
Remove
            unused typedef.
            (_Fwd_list_base::_Node_alloc_traits): Use allocator_traits instead 
of
            __gnu_cxx::__alloc_traits.
            (_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.

diff --git a/libstdc++-v3/include/bits/forward_list.h 
b/libstdc++-v3/include/bits/forward_list.h
index b40256bd104..8c4c074e454 100644
--- a/libstdc++-v3/include/bits/forward_list.h
+++ b/libstdc++-v3/include/bits/forward_list.h
@@ -289,7 +289,6 @@ _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;
 
@@ -363,11 +362,10 @@ _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(...)
            {
@@ -437,10 +435,9 @@ _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 3e12cd531d4..b41fbbb52f2 100644
--- a/libstdc++-v3/include/bits/forward_list.tcc
+++ b/libstdc++-v3/include/bits/forward_list.tcc
@@ -65,8 +65,8 @@ _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;
@@ -83,8 +83,8 @@ _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);
        }