[Bug libstdc++/108487] [10/11/12/13 Regression] ~20-30x slowdown in populating std::vector from std::ranges::iota_view

[amonakov at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108487

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 = std::distance(__first, __last);
   this->_M_impl._M_start
     = this->_M_allocate(_S_check_init_len(__n, _M_get_Tp_allocator()));
   this->_M_impl._M_end_of_storage = this->_M_impl._M_start + __n;
   this->_M_impl._M_finish =
     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 != __last; ++__first)

       emplace_back(*__first);

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