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From: "amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/108487] [10/11/12/13 Regression] ~20-30x slowdown in
 populating std::vector from std::ranges::iota_view
Date: Sat, 21 Jan 2023 15:38:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108487

Alexander Monakov <amonakov at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|tree-optimization           |libstdc++

--- Comment #3 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
libstdc++ uses a less efficient specialization of _M_range_initialize.

With 10.2 headers, we use

      template<typename _ForwardIterator>
 void
 _M_range_initialize(_ForwardIterator __first, _ForwardIterator __last,
       std::forward_iterator_tag)
 {
   const size_type __n =3D std::distance(__first, __last);
   this->_M_impl._M_start
     =3D this->_M_allocate(_S_check_init_len(__n, _M_get_Tp_allocator()));
   this->_M_impl._M_end_of_storage =3D this->_M_impl._M_start + __n;
   this->_M_impl._M_finish =3D
     std::__uninitialized_copy_a(__first, __last,
     this->_M_impl._M_start,
     _M_get_Tp_allocator());
 }

but with 10.4 headers, we use

      template<typename _InputIterator>
 void
 _M_range_initialize(_InputIterator __first, _InputIterator __last,
       std::input_iterator_tag)
 {
   try {
     for (; __first !=3D __last; ++__first)

       emplace_back(*__first);

   } catch(...) {
     clear();
     throw;
   }
 }=