[PATCH] Simplify branching in algos
François Dumont
frs.dumont@gmail.com
Sun Nov 21 20:34:52 GMT 2021
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);
>> }
>>
>> /**
>
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