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 <[email protected]>
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 GlisseIndex: 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
+}
