This is a libstdc++ bug and patch, not the C++ front end. So I'm
adding the libstdc++ list to CC.
On Fri, Jan 19, 2018 at 3:02 AM, chang jc <r97922...@gmail.com> wrote:
> Current std::inplace_merge() suffers from performance issue by inefficient
> logic under limited memory,
>
> It leads to performance downgrade.
>
> Please help to review it.
>
> Index: include/bits/stl_algo.h
> ===================================================================
> --- include/bits/stl_algo.h (revision 256871)
> +++ include/bits/stl_algo.h (working copy)
> @@ -2437,7 +2437,7 @@
> _BidirectionalIterator __second_cut = __middle;
> _Distance __len11 = 0;
> _Distance __len22 = 0;
> - if (__len1 > __len2)
> + if (__len1 < __len2)
> {
> __len11 = __len1 / 2;
> std::advance(__first_cut, __len11);
> @@ -2539,9 +2539,15 @@
> const _DistanceType __len1 = std::distance(__first, __middle);
> const _DistanceType __len2 = std::distance(__middle, __last);
>
> +
> typedef _Temporary_buffer<_BidirectionalIterator, _ValueType> _TmpBuf;
> - _TmpBuf __buf(__first, __last);
> -
> + _BidirectionalIterator __start, __end;
> + if (__len1 < __len2) {
> + __start = __first; __end = __middle;
> + } else {
> + __start = __middle; __end = __last;
> + }
> + _TmpBuf __buf(__start, ___end);
> if (__buf.begin() == 0)
> std::__merge_without_buffer
> (__first, __middle, __last, __len1, __len2, __comp);
> Index: include/bits/stl_tempbuf.h
> ===================================================================
> --- include/bits/stl_tempbuf.h (revision 256871)
> +++ include/bits/stl_tempbuf.h (working copy)
> @@ -95,7 +95,7 @@
> std::nothrow));
> if (__tmp != 0)
> return std::pair<_Tp*, ptrdiff_t>(__tmp, __len);
> - __len /= 2;
> + __len = (__len + 1) / 2;
> }
> return std::pair<_Tp*, ptrdiff_t>(static_cast<_Tp*>(0), 0);
> }
>
>
>
>
> Thanks
