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);
>>     }
>>
>>   /**
>