A recent thread on this mailing list made me remember that this proposal is still open.

I've updated it just to add a missing std qualification.

François

On 08/06/21 5:21 pm, Jonathan Wakely wrote:
I haven't forgotten this one, I just need to double-check that we
don't create another problem like std::rotate in 9.1

I'll try to finish the review tomorrow.

J.


On 27/05/21 07:04 +0200, François Dumont via Libstdc++ wrote:
Following latest fixes in std::inplace_merge and std::stable_sort you propose Jonathan to enhance branching in the first.

Here is a proposal based on yours to do so in both algos.

    libstdc++: Enhance branching in std::inplace_merge and std::stable_sort

    libstdc++-v3/ChangeLog:

            * include/bits/stl_algo.h
            (__merge_adaptive): Adapt to merge only when buffer is large enough..             (__merge_adaptive_resize): New, adapt merge when buffer is too small.
            (__inplace_merge): Adapt, use latter.
            (__stable_sort_adaptive): Adapt to sort only when buffer is large enough.             (__stable_sort_adaptive_resize): New, adapt sort when buffer is too small.
            (__stable_sort): Adapt, use latter.

Tested under Linux x64.

Ok to commit ?

François


diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index a18bb000d0c..02ae40c1fb4 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -2401,28 +2401,42 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    }

  /// This is a helper function for the merge routines.
-  template<typename _BidirectionalIterator, typename _Distance, +  template<typename _BidirectionalIterator, typename _Distance,
       typename _Pointer, typename _Compare>
    void
    __merge_adaptive(_BidirectionalIterator __first,
             _BidirectionalIterator __middle,
             _BidirectionalIterator __last,
             _Distance __len1, _Distance __len2,
-             _Pointer __buffer, _Distance __buffer_size,
-             _Compare __comp)
+             _Pointer __buffer, _Compare __comp)
    {
-      if (__len1 <= __len2 && __len1 <= __buffer_size)
+      if (__len1 <= __len2)
    {
      _Pointer __buffer_end = _GLIBCXX_MOVE3(__first, __middle, __buffer);       std::__move_merge_adaptive(__buffer, __buffer_end, __middle, __last,
                     __first, __comp);
    }
-      else if (__len2 <= __buffer_size)
+      else
    {
      _Pointer __buffer_end = _GLIBCXX_MOVE3(__middle, __last, __buffer);
      std::__move_merge_adaptive_backward(__first, __middle, __buffer,
                          __buffer_end, __last, __comp);
    }
+    }
+
+  template<typename _BidirectionalIterator, typename _Distance,
+       typename _Pointer, typename _Compare>
+    void
+    __merge_adaptive_resize(_BidirectionalIterator __first,
+                _BidirectionalIterator __middle,
+                _BidirectionalIterator __last,
+                _Distance __len1, _Distance __len2,
+                _Pointer __buffer, _Distance __buffer_size,
+                _Compare __comp)
+    {
+      if (__len1 <= __buffer_size || __len2 <= __buffer_size)
+    std::__merge_adaptive(__first, __middle, __last,
+                  __len1, __len2, __buffer, __comp);
      else
    {
      _BidirectionalIterator __first_cut = __first;
@@ -2450,14 +2464,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

      _BidirectionalIterator __new_middle
        = std::__rotate_adaptive(__first_cut, __middle, __second_cut,
-                     __len1 - __len11, __len22, __buffer,
-                     __buffer_size);
-      std::__merge_adaptive(__first, __first_cut, __new_middle, __len11,
-                __len22, __buffer, __buffer_size, __comp);
-      std::__merge_adaptive(__new_middle, __second_cut, __last,
-                __len1 - __len11,
-                __len2 - __len22, __buffer,
-                __buffer_size, __comp);
+                     __len1 - __len11, __len22,
+                     __buffer, __buffer_size);
+      std::__merge_adaptive_resize(__first, __first_cut, __new_middle,
+                       __len11, __len22,
+                       __buffer, __buffer_size, __comp);
+      std::__merge_adaptive_resize(__new_middle, __second_cut, __last,
+                       __len1 - __len11, __len2 - __len22,
+                       __buffer, __buffer_size, __comp);
    }
    }

@@ -2535,11 +2549,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
      // [first,middle) and [middle,last).
      _TmpBuf __buf(__first, std::min(__len1, __len2));

-      if (__buf.begin() == 0)
+      if (__builtin_expect(__buf.size() == __buf.requested_size(), true))
+    std::__merge_adaptive
+      (__first, __middle, __last, __len1, __len2, __buf.begin(), __comp);
+      else if (__builtin_expect(__buf.begin() == 0, false))
    std::__merge_without_buffer
      (__first, __middle, __last, __len1, __len2, __comp);
      else
-    std::__merge_adaptive
+    std::__merge_adaptive_resize
      (__first, __middle, __last, __len1, __len2, __buf.begin(),
       _DistanceType(__buf.size()), __comp);
    }
@@ -2720,34 +2737,45 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    }
    }

-  template<typename _RandomAccessIterator, typename _Pointer,
-       typename _Distance, typename _Compare>
+  template<typename _RandomAccessIterator, typename _Pointer, typename _Compare>
    void
    __stable_sort_adaptive(_RandomAccessIterator __first,
+               _RandomAccessIterator __middle,
               _RandomAccessIterator __last,
-               _Pointer __buffer, _Distance __buffer_size,
-               _Compare __comp)
+               _Pointer __buffer, _Compare __comp)
+    {
+      std::__merge_sort_with_buffer(__first, __middle, __buffer, __comp); +      std::__merge_sort_with_buffer(__middle, __last, __buffer, __comp);
+
+      std::__merge_adaptive(__first, __middle, __last,
+                __middle - __first, __last - __middle,
+                __buffer, __comp);
+    }
+
+  template<typename _RandomAccessIterator, typename _Pointer,
+       typename _Distance, typename _Compare>
+    void
+    __stable_sort_adaptive_resize(_RandomAccessIterator __first,
+                  _RandomAccessIterator __last,
+                  _Pointer __buffer, _Distance __buffer_size,
+                  _Compare __comp)
    {
      const _Distance __len = (__last - __first + 1) / 2;
      const _RandomAccessIterator __middle = __first + __len;
      if (__len > __buffer_size)
    {
-      std::__stable_sort_adaptive(__first, __middle, __buffer,
-                      __buffer_size, __comp);
-      std::__stable_sort_adaptive(__middle, __last, __buffer,
-                      __buffer_size, __comp);
+      std::__stable_sort_adaptive_resize(__first, __middle, __buffer,
+                         __buffer_size, __comp);
+      std::__stable_sort_adaptive_resize(__middle, __last, __buffer,
+                         __buffer_size, __comp);
+      std::__merge_adaptive_resize(__first, __middle, __last,
+                       _Distance(__middle - __first),
+                       _Distance(__last - __middle),
+                       __buffer, __buffer_size,
+                       __comp);
    }
      else
-    {
-      std::__merge_sort_with_buffer(__first, __middle, __buffer, __comp); -      std::__merge_sort_with_buffer(__middle, __last, __buffer, __comp);
-    }
-
-      std::__merge_adaptive(__first, __middle, __last,
-                _Distance(__middle - __first),
-                _Distance(__last - __middle),
-                __buffer, __buffer_size,
-                __comp);
+    __stable_sort_adaptive(__first, __middle, __last, __buffer, __comp);
    }

  /// This is a helper function for the stable sorting routines.
@@ -5017,11 +5045,14 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
      // so the buffer only needs to fit half the range at once.
      _TmpBuf __buf(__first, (__last - __first + 1) / 2);

-      if (__buf.begin() == 0)
+      if (__builtin_expect(__buf.requested_size() == __buf.size(), true)) +    std::__stable_sort_adaptive(__first, __first + __buf.size(), __last,
+                    __buf.begin(), __comp);
+      else if (__builtin_expect(__buf.begin() == 0, false))
    std::__inplace_stable_sort(__first, __last, __comp);
      else
-    std::__stable_sort_adaptive(__first, __last, __buf.begin(),
-                    _DistanceType(__buf.size()), __comp);
+    std::__stable_sort_adaptive_resize(__first, __last, __buf.begin(),
+                       _DistanceType(__buf.size()), __comp);
    }

  /**


diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index bc611a95ef4..2ebc3b12713 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -2384,28 +2384,42 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   /// This is a helper function for the merge routines.
-  template<typename _BidirectionalIterator, typename _Distance, 
+  template<typename _BidirectionalIterator, typename _Distance,
 	   typename _Pointer, typename _Compare>
     void
     __merge_adaptive(_BidirectionalIterator __first,
 		     _BidirectionalIterator __middle,
 		     _BidirectionalIterator __last,
 		     _Distance __len1, _Distance __len2,
-		     _Pointer __buffer, _Distance __buffer_size,
-		     _Compare __comp)
+		     _Pointer __buffer, _Compare __comp)
     {
-      if (__len1 <= __len2 && __len1 <= __buffer_size)
+      if (__len1 <= __len2)
 	{
 	  _Pointer __buffer_end = _GLIBCXX_MOVE3(__first, __middle, __buffer);
 	  std::__move_merge_adaptive(__buffer, __buffer_end, __middle, __last,
 				     __first, __comp);
 	}
-      else if (__len2 <= __buffer_size)
+      else
 	{
 	  _Pointer __buffer_end = _GLIBCXX_MOVE3(__middle, __last, __buffer);
 	  std::__move_merge_adaptive_backward(__first, __middle, __buffer,
 					      __buffer_end, __last, __comp);
 	}
+    }
+
+  template<typename _BidirectionalIterator, typename _Distance,
+	   typename _Pointer, typename _Compare>
+    void
+    __merge_adaptive_resize(_BidirectionalIterator __first,
+			    _BidirectionalIterator __middle,
+			    _BidirectionalIterator __last,
+			    _Distance __len1, _Distance __len2,
+			    _Pointer __buffer, _Distance __buffer_size,
+			    _Compare __comp)
+    {
+      if (__len1 <= __buffer_size || __len2 <= __buffer_size)
+	std::__merge_adaptive(__first, __middle, __last,
+			      __len1, __len2, __buffer, __comp);
       else
 	{
 	  _BidirectionalIterator __first_cut = __first;
@@ -2433,14 +2447,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 	  _BidirectionalIterator __new_middle
 	    = std::__rotate_adaptive(__first_cut, __middle, __second_cut,
-				     __len1 - __len11, __len22, __buffer,
-				     __buffer_size);
-	  std::__merge_adaptive(__first, __first_cut, __new_middle, __len11,
-				__len22, __buffer, __buffer_size, __comp);
-	  std::__merge_adaptive(__new_middle, __second_cut, __last,
-				__len1 - __len11,
-				__len2 - __len22, __buffer,
-				__buffer_size, __comp);
+				     __len1 - __len11, __len22,
+				     __buffer, __buffer_size);
+	  std::__merge_adaptive_resize(__first, __first_cut, __new_middle,
+				       __len11, __len22,
+				       __buffer, __buffer_size, __comp);
+	  std::__merge_adaptive_resize(__new_middle, __second_cut, __last,
+				       __len1 - __len11, __len2 - __len22,
+				       __buffer, __buffer_size, __comp);
 	}
     }
 
@@ -2518,11 +2532,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // [first,middle) and [middle,last).
       _TmpBuf __buf(__first, std::min(__len1, __len2));
 
-      if (__buf.begin() == 0)
+      if (__builtin_expect(__buf.size() == __buf.requested_size(), true))
+	std::__merge_adaptive
+	  (__first, __middle, __last, __len1, __len2, __buf.begin(), __comp);
+      else if (__builtin_expect(__buf.begin() == 0, false))
 	std::__merge_without_buffer
 	  (__first, __middle, __last, __len1, __len2, __comp);
       else
-	std::__merge_adaptive
+	std::__merge_adaptive_resize
 	  (__first, __middle, __last, __len1, __len2, __buf.begin(),
 	   _DistanceType(__buf.size()), __comp);
     }
@@ -2703,34 +2720,46 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	}
     }
 
-  template<typename _RandomAccessIterator, typename _Pointer,
-	   typename _Distance, typename _Compare>
+  template<typename _RandomAccessIterator, typename _Pointer, typename _Compare>
     void
     __stable_sort_adaptive(_RandomAccessIterator __first,
+			   _RandomAccessIterator __middle,
 			   _RandomAccessIterator __last,
-			   _Pointer __buffer, _Distance __buffer_size,
-			   _Compare __comp)
+			   _Pointer __buffer, _Compare __comp)
+    {
+      std::__merge_sort_with_buffer(__first, __middle, __buffer, __comp);
+      std::__merge_sort_with_buffer(__middle, __last, __buffer, __comp);
+
+      std::__merge_adaptive(__first, __middle, __last,
+			    __middle - __first, __last - __middle,
+			    __buffer, __comp);
+    }
+
+  template<typename _RandomAccessIterator, typename _Pointer,
+	   typename _Distance, typename _Compare>
+    void
+    __stable_sort_adaptive_resize(_RandomAccessIterator __first,
+				  _RandomAccessIterator __last,
+				  _Pointer __buffer, _Distance __buffer_size,
+				  _Compare __comp)
     {
       const _Distance __len = (__last - __first + 1) / 2;
       const _RandomAccessIterator __middle = __first + __len;
       if (__len > __buffer_size)
 	{
-	  std::__stable_sort_adaptive(__first, __middle, __buffer,
-				      __buffer_size, __comp);
-	  std::__stable_sort_adaptive(__middle, __last, __buffer,
-				      __buffer_size, __comp);
+	  std::__stable_sort_adaptive_resize(__first, __middle, __buffer,
+					     __buffer_size, __comp);
+	  std::__stable_sort_adaptive_resize(__middle, __last, __buffer,
+					     __buffer_size, __comp);
+	  std::__merge_adaptive_resize(__first, __middle, __last,
+				       _Distance(__middle - __first),
+				       _Distance(__last - __middle),
+				       __buffer, __buffer_size,
+				       __comp);
 	}
       else
-	{
-	  std::__merge_sort_with_buffer(__first, __middle, __buffer, __comp);
-	  std::__merge_sort_with_buffer(__middle, __last, __buffer, __comp);
-	}
-
-      std::__merge_adaptive(__first, __middle, __last,
-			    _Distance(__middle - __first),
-			    _Distance(__last - __middle),
-			    __buffer, __buffer_size,
-			    __comp);
+	std::__stable_sort_adaptive(__first, __middle, __last,
+				    __buffer, __comp);
     }
 
   /// This is a helper function for the stable sorting routines.
@@ -4995,11 +5024,14 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
       // so the buffer only needs to fit half the range at once.
       _TmpBuf __buf(__first, (__last - __first + 1) / 2);
 
-      if (__buf.begin() == 0)
+      if (__builtin_expect(__buf.requested_size() == __buf.size(), true))
+	std::__stable_sort_adaptive(__first, __first + __buf.size(), __last,
+				    __buf.begin(), __comp);
+      else if (__builtin_expect(__buf.begin() == 0, false))
 	std::__inplace_stable_sort(__first, __last, __comp);
       else
-	std::__stable_sort_adaptive(__first, __last, __buf.begin(),
-				    _DistanceType(__buf.size()), __comp);
+	std::__stable_sort_adaptive_resize(__first, __last, __buf.begin(),
+					   _DistanceType(__buf.size()), __comp);
     }
 
   /**

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