[libstdc++/61347] std::distance(list.first(),list.end()) in O(1)

[Marc Glisse <marc.glisse@inria.fr>]

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Hello,

this patch makes std::distance(list.first(),list.end()) a constant time 
operation when optimizing, with no penalty for other calls. We could do 
the test always (no __builtin_constant_p) but then it will add a small 
runtime penalty for other calls, someone else can take responsibility for 
that. I could protect the whole overload with #ifdef __OPTIMIZE__ (at -O0 
the compiler does not remove the test ++end==first as dead code), but I 
assume it is better to minimize differences. There are other ways to 
specialize distance, but overloading __distance seems to have the least 
drawbacks (most others involve a new extra file). From an optimization 
POV, it would be a bit better to avoid the while loop and instead call a 
separate function that does it (say the regular __distance), it would make 
the function smaller and thus easier to inline, but it is simpler this way 
and works.

We could do something similar for std::set, but C++ will not let us find 
the address of _Rb_tree_impl::_M_node_count from that of 
_Rb_tree_impl::_M_header, except when _Key_compare is pod, which luckily 
is an easily available information. Avoiding this complication is why I 
insisted on wrapping the size of std::list in a _List_node<size_t> for 
gcc-5, which may look a bit strange at first sight.

2015-04-13  Marc Glisse  <marc.glisse@inria.fr>

 	PR libstdc++/61347
 	* include/bits/stl_iterator_base_funcs.h (distance): Do not
 	qualify the call to __distance.
 	* include/bits/stl_list.h (__distance): New overloads for
 	_List_iterator and _List_const_iterator.
 	* testsuite/23_containers/list/61347.cc: New testcase.

-- 
Marc Glisse

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Index: include/bits/stl_iterator_base_funcs.h
===================================================================
--- include/bits/stl_iterator_base_funcs.h	(revision 222041)
+++ include/bits/stl_iterator_base_funcs.h	(working copy)
@@ -107,22 +107,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  n may be negative.
    *
    *  For random access iterators, this uses their @c + and @c - operations
    *  and are constant time.  For other %iterator classes they are linear time.
   */
   template<typename _InputIterator>
     inline typename iterator_traits<_InputIterator>::difference_type
     distance(_InputIterator __first, _InputIterator __last)
     {
       // concept requirements -- taken care of in __distance
-      return std::__distance(__first, __last,
-			     std::__iterator_category(__first));
+      return __distance(__first, __last, std::__iterator_category(__first));
     }
 
   template<typename _InputIterator, typename _Distance>
     inline void
     __advance(_InputIterator& __i, _Distance __n, input_iterator_tag)
     {
       // concept requirements
       __glibcxx_function_requires(_InputIteratorConcept<_InputIterator>)
       _GLIBCXX_DEBUG_ASSERT(__n >= 0);
       while (__n--)
Index: include/bits/stl_list.h
===================================================================
--- include/bits/stl_list.h	(revision 222041)
+++ include/bits/stl_list.h	(working copy)
@@ -1861,13 +1861,64 @@ _GLIBCXX_END_NAMESPACE_CXX11
     operator>=(const list<_Tp, _Alloc>& __x, const list<_Tp, _Alloc>& __y)
     { return !(__x < __y); }
 
   /// See std::list::swap().
   template<typename _Tp, typename _Alloc>
     inline void
     swap(list<_Tp, _Alloc>& __x, list<_Tp, _Alloc>& __y)
     { __x.swap(__y); }
 
 _GLIBCXX_END_NAMESPACE_CONTAINER
+
+#if _GLIBCXX_USE_CXX11_ABI
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  // Detect when distance is used to compute the size of the whole list.
+  template<typename _Tp>
+    inline ptrdiff_t
+    __distance(_GLIBCXX_STD_C::_List_iterator<_Tp> __first,
+	       _GLIBCXX_STD_C::_List_iterator<_Tp> __last,
+               input_iterator_tag)
+    {
+      typedef _GLIBCXX_STD_C::_List_node<size_t> _Sentinel;
+      _GLIBCXX_STD_C::_List_iterator<_Tp> __beyond = __last;
+      ++__beyond;
+      bool __whole = __first == __beyond;
+      if (__builtin_constant_p (__whole) && __whole)
+	return static_cast<const _Sentinel*>(__last._M_node)->_M_data;
+
+      ptrdiff_t __n = 0;
+      while (__first != __last)
+	{
+	  ++__first;
+	  ++__n;
+	}
+      return __n;
+    }
+
+  template<typename _Tp>
+    inline ptrdiff_t
+    __distance(_GLIBCXX_STD_C::_List_const_iterator<_Tp> __first,
+	       _GLIBCXX_STD_C::_List_const_iterator<_Tp> __last,
+               input_iterator_tag)
+    {
+      typedef _GLIBCXX_STD_C::_List_node<size_t> _Sentinel;
+      _GLIBCXX_STD_C::_List_const_iterator<_Tp> __beyond = __last;
+      ++__beyond;
+      bool __whole = __first == __beyond;
+      if (__builtin_constant_p (__whole) && __whole)
+	return static_cast<const _Sentinel*>(__last._M_node)->_M_data;
+
+      ptrdiff_t __n = 0;
+      while (__first != __last)
+	{
+	  ++__first;
+	  ++__n;
+	}
+      return __n;
+    }
+
+_GLIBCXX_END_NAMESPACE_VERSION
+#endif
 } // namespace std
 
 #endif /* _STL_LIST_H */
Index: testsuite/23_containers/list/61347.cc
===================================================================
--- testsuite/23_containers/list/61347.cc	(revision 0)
+++ testsuite/23_containers/list/61347.cc	(working copy)
@@ -0,0 +1,49 @@
+// { dg-options "-std=gnu++11 -O2 -D_GLIBCXX_USE_CXX11_ABI" }
+
+// Copyright (C) 2015 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include <list>
+#include <iterator>
+#include <testsuite_hooks.h>
+
+__attribute__((__noinline__, __noclone__))
+void testm(std::list<short>& l)
+{
+  bool b = std::distance(l.begin(), l.end()) == l.size();
+  VERIFY( __builtin_constant_p(b) );
+  VERIFY( b );
+}
+
+__attribute__((__noinline__, __noclone__))
+void testc(const std::list<short>& l)
+{
+  bool b = std::distance(l.begin(), l.end()) == l.size();
+  VERIFY( __builtin_constant_p(b) );
+  VERIFY( b );
+}
+
+int main()
+{
+  bool test __attribute__((unused)) = true;
+
+#if _GLIBCXX_USE_DUAL_ABI
+  std::list<short> l;
+  testm(l);
+  testc(l);
+#endif
+}