Improve std::rotate usages

[François Dumont <frs dot dumont at gmail dot com>]

Hi

    std::rotate already returns the expected iterator so there is no 
need for calls to std::advance/std::distance.

Tested under Linux x86_64, ok to commit ?

François

diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index e10a692..9c1b2d4 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -1245,11 +1245,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	     _ForwardIterator __last,
 	     forward_iterator_tag)
     {
-      if (__first == __middle)
-	return __last;
-      else if (__last  == __middle)
-	return __first;
-
       _ForwardIterator __first2 = __middle;
       do
 	{
@@ -1290,11 +1285,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __glibcxx_function_requires(_Mutable_BidirectionalIteratorConcept<
 				  _BidirectionalIterator>)
 
-      if (__first == __middle)
-	return __last;
-      else if (__last  == __middle)
-	return __first;
-
       std::__reverse(__first,  __middle, bidirectional_iterator_tag());
       std::__reverse(__middle, __last,   bidirectional_iterator_tag());
 
@@ -1328,11 +1318,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __glibcxx_function_requires(_Mutable_RandomAccessIteratorConcept<
 				  _RandomAccessIterator>)
 
-      if (__first == __middle)
-	return __last;
-      else if (__last  == __middle)
-	return __first;
-
       typedef typename iterator_traits<_RandomAccessIterator>::difference_type
 	_Distance;
       typedef typename iterator_traits<_RandomAccessIterator>::value_type
@@ -1434,6 +1419,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __glibcxx_requires_valid_range(__first, __middle);
       __glibcxx_requires_valid_range(__middle, __last);
 
+      if (__first == __middle)
+	return __last;
+      else if (__last  == __middle)
+	return __first;
+
       return std::__rotate(__first, __middle, __last,
 			   std::__iterator_category(__first));
     }
@@ -1595,9 +1585,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 					   __right_len,
 					   __buffer, __buffer_size);
 
-      std::rotate(__left_split, __middle, __right_split);
-      std::advance(__left_split, std::distance(__middle, __right_split));
-      return __left_split;
+      return std::rotate(__left_split, __middle, __right_split);
     }
 
   template<typename _ForwardIterator, typename _Predicate>
@@ -2396,11 +2384,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return __last;
 	}
       else
-	{
-	  std::rotate(__first, __middle, __last);
-	  std::advance(__first, std::distance(__middle, __last));
-	  return __first;
-	}
+	return std::rotate(__first, __middle, __last);
     }
 
   /// This is a helper function for the merge routines.
@@ -2507,9 +2491,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  __len11 = std::distance(__first, __first_cut);
 	}
 
-      std::rotate(__first_cut, __middle, __second_cut);
-      _BidirectionalIterator __new_middle = __first_cut;
-      std::advance(__new_middle, std::distance(__middle, __second_cut));
+      _BidirectionalIterator __new_middle
+	= std::rotate(__first_cut, __middle, __second_cut);
       std::__merge_without_buffer(__first, __first_cut, __new_middle,
 				  __len11, __len22, __comp);
       std::__merge_without_buffer(__new_middle, __second_cut, __last,