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Re: std::list optimizations


On 28/07/17 18:42 +0200, François Dumont wrote:
Hi

Completing execution of the testsuite revealed a bug. So here is the correct version of this patch.

François

On 21/07/2017 19:14, François Dumont wrote:
Hi

Here is a proposal for 2 optimizations in the std::list implementation.

Optimization on the move constructor taking an allocator for always equal allocators. Compare to the version in my previous std::list patch I am now doing it at std::list level rather than at _List_base level. This way we won't instantiate the insert call and we won't check for empty list when the allocator always compare equal.

2nd optimization, I replace the _S_distance method by the std::distance algo which benefit from the nice [begin(), end()) range optimization when cxx11 abi is being used.

Note that I am proposing the 2 change in 1 patch to save some review time but I can commit those separately.

Tested under x86_64 Linux normal mode.


   * include/bits/stl_list.h
   (_List_base<>::_S_distance): Remove.
   (_List_impl(_List_impl&&, _Node_alloc_type&&)): New.
   (_List_base(_List_base&&, _Node_alloc_type&&)): Use latter.
   (_List_base(_Node_alloc_type&&)): New.
   (_List_base<>::_M_distance, _List_base<>::_M_node_count): Move...
(list<>::_M_distance, list<>::_M_node_count): ...here. Replace calls to
   _S_distance with calls to std::distance.
   (list(list&&, const allocator_type&, true_type)): New.
   (list(list&&, const allocator_type&, false_type)): New.
   (list(list&&, const allocator_type&)): Adapt to call latters.

Ok to commit ?

François



diff --git a/libstdc++-v3/include/bits/stl_list.h b/libstdc++-v3/include/bits/stl_list.h
index cef94f7..eb71a5e 100644
--- a/libstdc++-v3/include/bits/stl_list.h
+++ b/libstdc++-v3/include/bits/stl_list.h
@@ -364,19 +364,6 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
	rebind<_List_node<_Tp> >::other _Node_alloc_type;
      typedef __gnu_cxx::__alloc_traits<_Node_alloc_type> _Node_alloc_traits;

-      static size_t
-      _S_distance(const __detail::_List_node_base* __first,
-		  const __detail::_List_node_base* __last)
-      {
-	size_t __n = 0;
-	while (__first != __last)
-	  {
-	    __first = __first->_M_next;
-	    ++__n;
-	  }
-	return __n;
-      }
-

Removing this function will break code that expects it to exist in an
explicit instantiation. The function could be left there, but unused.

      struct _List_impl
      : public _Node_alloc_type
      {
@@ -393,6 +380,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
#if __cplusplus >= 201103L
	_List_impl(_List_impl&&) = default;

+	_List_impl(_List_impl&& __x, _Node_alloc_type&& __a)
+	: _Node_alloc_type(std::move(__a)), _M_node(std::move(__x._M_node))
+	{ }
+

This is fine.

	_List_impl(_Node_alloc_type&& __a) noexcept
	: _Node_alloc_type(std::move(__a))
	{ }
@@ -409,28 +400,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
      void _M_inc_size(size_t __n) { _M_impl._M_node._M_size += __n; }

      void _M_dec_size(size_t __n) { _M_impl._M_node._M_size -= __n; }
-
-      size_t
-      _M_distance(const __detail::_List_node_base* __first,
-		  const __detail::_List_node_base* __last) const
-      { return _S_distance(__first, __last); }
-
-      // return the stored size
-      size_t _M_node_count() const { return _M_get_size(); }
#else
      // dummy implementations used when the size is not stored
      size_t _M_get_size() const { return 0; }
      void _M_set_size(size_t) { }
      void _M_inc_size(size_t) { }
      void _M_dec_size(size_t) { }
-      size_t _M_distance(const void*, const void*) const { return 0; }
-
-      // count the number of nodes
-      size_t _M_node_count() const
-      {
-	return _S_distance(_M_impl._M_node._M_next,
-			   std::__addressof(_M_impl._M_node));
-      }
#endif

Same problem again: these will no longer be defined by explicit
instantiations.


      typename _Node_alloc_traits::pointer
@@ -466,12 +441,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
      _List_base(_List_base&&) = default;

      _List_base(_List_base&& __x, _Node_alloc_type&& __a)
+      : _M_impl(std::move(__x._M_impl), std::move(__a))
+      { }
+
+      _List_base(_Node_alloc_type&& __a)
      : _M_impl(std::move(__a))
-      {
-	if (__x._M_get_Node_allocator() == _M_get_Node_allocator())
-	  _M_move_nodes(std::move(__x));
-	// else caller must move individual elements.
-      }
+      { }


I like this change in principle, but it alters the behaviour of an
existing constructor. Existing code might use the constructor and get
broken by this.

You can avoid that by leaving the existing constructor alone and
adding two new ones for new code to use. Reordering the parameters
will make the new one distinct from the old one:

     // Used when is_always_equal
     _List_base(_Node_alloc_type&& __a, _List_base&& __x))
     : _M_impl(std::move(__x._M_impl), std::move(__a))
     { }

     // Used when !is_always_equal
     _List_base(_Node_alloc_type&& __a)
     : _M_impl(std::move(__a))
     { }

Or:

     // Used when is_always_equal
     _List_base(_List_base&& __x, _Node_alloc_type&& __a, true_type)
     : _M_impl(std::move(__x._M_impl), std::move(__a))
     { }

     // Used when !is_always_equal
     _List_base(_Node_alloc_type&& __a)
     : _M_impl(std::move(__a))
     { }

Either of these forms will add new constructors for the new semantics.

      void
      _M_move_nodes(_List_base&& __x)
@@ -616,6 +591,25 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
	}
#endif

+#if _GLIBCXX_USE_CXX11_ABI
+      size_t
+      _M_distance(const_iterator __first, const_iterator __last) const
+      { return std::distance(__first, __last); }
+
+      // return the stored size
+      size_t
+      _M_node_count() const
+      { return this->_M_get_size(); }
+#else
+      // dummy implementations used when the size is not stored
+      size_t _M_distance(const_iterator, const_iterator) const { return 0; }
+
+      // count the number of nodes
+      size_t
+      _M_node_count() const
+      { return std::distance(begin(), end()); }
+#endif

Adding these is fine. New code will use them. Old code relying on
explicit instantiations will still find the definitions of the old
functions and use them.

_M_distance could be static though, neither version uses the 'this'
pointer, so it would be called _S_distance.


+
    public:
      // [23.2.2.1] construct/copy/destroy
      // (assign() and get_allocator() are also listed in this section)
@@ -718,15 +712,27 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
      : _Base(_Node_alloc_type(__a))
      { _M_initialize_dispatch(__x.begin(), __x.end(), __false_type()); }

-      list(list&& __x, const allocator_type& __a)
-      noexcept(_Node_alloc_traits::_S_always_equal())
+    private:
+      list(list&& __x, const allocator_type& __a, true_type) noexcept
      : _Base(std::move(__x), _Node_alloc_type(__a))

Either reorder the arguments, or add true_type{} to the end, to call
the new constructor suggested above.

+      { }
+
+      list(list&& __x, const allocator_type& __a, false_type)
+      : _Base(_Node_alloc_type(__a))
      {
-	// If __x is not empty it means its allocator is not equal to __a,
-	// so we need to move from each element individually.
-	insert(begin(), std::__make_move_if_noexcept_iterator(__x.begin()),
-			std::__make_move_if_noexcept_iterator(__x.end()));
+	if (__x._M_get_Node_allocator() == this->_M_get_Node_allocator())
+	  this->_M_move_nodes(std::move(__x));
+	else
+	  insert(begin(), std::__make_move_if_noexcept_iterator(__x.begin()),
+			  std::__make_move_if_noexcept_iterator(__x.end()));
      }
+
+    public:
+      list(list&& __x, const allocator_type& __a)
+      noexcept(_Node_alloc_traits::_S_always_equal())
+      : list(std::move(__x), __a,
+	     typename _Node_alloc_traits::is_always_equal{})
+      { }
#endif

      /**
@@ -1000,7 +1006,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
      /**  Returns the number of elements in the %list.  */
      size_type
      size() const _GLIBCXX_NOEXCEPT
-      { return this->_M_node_count(); }
+      { return _M_node_count(); }

      /**  Returns the size() of the largest possible %list.  */
      size_type
@@ -1578,7 +1584,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
	    if (this != std::__addressof(__x))
	      _M_check_equal_allocators(__x);

-	    size_t __n = this->_M_distance(__first._M_node, __last._M_node);
+	    size_t __n = _M_distance(__first, __last);
	    this->_M_inc_size(__n);
	    __x._M_dec_size(__n);


If _M_distance becomes static this would call _S_distance instead.

Do those suggestions make sense? The idea is to ensure that a given
function signature continues to have the same effects. To introduce
new effects, use a new signature.


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