Re: libstdc++: Speed up push_back

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 21 Nov 2023 at 12:50, Jan Hubicka <hubicka@ucw.cz> wrote:
>
> > > +           // RAII type to destroy initialized elements.
> >
> > There's only one initialized element, not "elements".
> >
> > > +           struct _Guard_elts
> > > +           {
> > > +             pointer _M_first, _M_last;  // Elements to destroy
> >
> > We only need to store one pointer here, call it _M_p.
> >
> > > +             _Tp_alloc_type& _M_alloc;
> > > +
> > > +             _GLIBCXX20_CONSTEXPR
> > > +             _Guard_elts(pointer __elt, _Tp_alloc_type& __a)
> > > +             : _M_first(__elt), _M_last(__elt + 1), _M_alloc(__a)
> > > +             { }
> > > +
> > > +             _GLIBCXX20_CONSTEXPR
> > > +             ~_Guard_elts()
> > > +             { std::_Destroy(_M_first, _M_last, _M_alloc); }
> >
> > This should be either:
> >
> >   std::_Destroy(_M_p, _M_p+1, _M_alloc);
> >
> > or avoid the loop that happens in that _Destroy function:
> >
> >   _Alloc_traits::destroy(_M_alloc, _M_p);
> >
> > > +
> > > +           private:
> > > +             _Guard_elts(const _Guard_elts&);
> > > +           };
> > > +
> > > +           // Guard the new element so it will be destroyed if anything throws.
> > > +           _Guard_elts __guard_elts(__new_start + __elems, _M_impl);
> > > +
> > > +           __new_finish = std::__uninitialized_move_if_noexcept_a(
> > > +                            __old_start, __old_finish,
> > > +                            __new_start, _M_get_Tp_allocator());
> > > +
> > > +           ++__new_finish;
> > > +           // Guard everything before the new element too.
> > > +           __guard_elts._M_first = __new_start;
> >
> > This seems redundant, we're not doing any more insertions now, and so
> > this store is dead.
>
> I am attaching patch with this check removed.  However I still think
> Guard_elts needs to stay to be able to destroy the old ellements
> >
> > > +
> > > +           // New storage has been fully initialized, destroy the old elements.
> > > +           __guard_elts._M_first = __old_start;
> > > +           __guard_elts._M_last = __old_finish;
> ... here
>
> Does it look better? (I am not really that confidend with libstdc++).

Yes, looks good, thanks.


>
> I also was wondering if we do have some data on what libstdc++ functions
> are perofrmance critical in practice.  Given that the push_back can be
> sped up very noticeably, I wonder if we don't have problems elsewhere?

We don't have that data, no.

It's possible that we could do similar things for insert(iterator pos,
...) for the case where pos==end(), i.e. inserting multiple elements
at the end.
>
> Thank you,
> Honza
>
> diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h
> index 5e18f6eedce..973f4d7e2e9 100644
> --- a/libstdc++-v3/include/bits/stl_vector.h
> +++ b/libstdc++-v3/include/bits/stl_vector.h
> @@ -1288,7 +1288,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>             _GLIBCXX_ASAN_ANNOTATE_GREW(1);
>           }
>         else
> -         _M_realloc_insert(end(), __x);
> +         _M_realloc_append(__x);
>        }
>
>  #if __cplusplus >= 201103L
> @@ -1822,6 +1822,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>
>        void
>        _M_realloc_insert(iterator __position, const value_type& __x);
> +
> +      void
> +      _M_realloc_append(const value_type& __x);
>  #else
>        // A value_type object constructed with _Alloc_traits::construct()
>        // and destroyed with _Alloc_traits::destroy().
> @@ -1871,6 +1874,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>         void
>         _M_realloc_insert(iterator __position, _Args&&... __args);
>
> +      template<typename... _Args>
> +       _GLIBCXX20_CONSTEXPR
> +       void
> +       _M_realloc_append(_Args&&... __args);
> +
>        // Either move-construct at the end, or forward to _M_insert_aux.
>        _GLIBCXX20_CONSTEXPR
>        iterator
> diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
> index 80631d1e2a1..0ccef7911b3 100644
> --- a/libstdc++-v3/include/bits/vector.tcc
> +++ b/libstdc++-v3/include/bits/vector.tcc
> @@ -120,7 +120,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>             _GLIBCXX_ASAN_ANNOTATE_GREW(1);
>           }
>         else
> -         _M_realloc_insert(end(), std::forward<_Args>(__args)...);
> +         _M_realloc_append(std::forward<_Args>(__args)...);
>  #if __cplusplus > 201402L
>         return back();
>  #endif
> @@ -459,6 +459,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>  #endif
>      {
>        const size_type __len = _M_check_len(1u, "vector::_M_realloc_insert");
> +      if (__len <= 0)
> +       __builtin_unreachable ();
>        pointer __old_start = this->_M_impl._M_start;
>        pointer __old_finish = this->_M_impl._M_finish;
>        const size_type __elems_before = __position - begin();
> @@ -571,6 +573,127 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>        this->_M_impl._M_end_of_storage = __new_start + __len;
>      }
>
> +#if __cplusplus >= 201103L
> +  template<typename _Tp, typename _Alloc>
> +    template<typename... _Args>
> +      _GLIBCXX20_CONSTEXPR
> +      void
> +      vector<_Tp, _Alloc>::
> +      _M_realloc_append(_Args&&... __args)
> +#else
> +  template<typename _Tp, typename _Alloc>
> +    void
> +    vector<_Tp, _Alloc>::
> +    _M_realloc_append(const _Tp& __x)
> +#endif
> +    {
> +      const size_type __len = _M_check_len(1u, "vector::_M_realloc_append");
> +      if (__len <= 0)
> +       __builtin_unreachable ();
> +      pointer __old_start = this->_M_impl._M_start;
> +      pointer __old_finish = this->_M_impl._M_finish;
> +      const size_type __elems = end() - begin();
> +      pointer __new_start(this->_M_allocate(__len));
> +      pointer __new_finish(__new_start);
> +
> +      // RAII guard for allocated storage.
> +      struct _Guard
> +      {
> +       pointer _M_storage;         // Storage to deallocate
> +       size_type _M_len;
> +       _Tp_alloc_type& _M_alloc;
> +
> +       _GLIBCXX20_CONSTEXPR
> +       _Guard(pointer __s, size_type __l, _Tp_alloc_type& __a)
> +       : _M_storage(__s), _M_len(__l), _M_alloc(__a)
> +       { }
> +
> +       _GLIBCXX20_CONSTEXPR
> +       ~_Guard()
> +       {
> +         if (_M_storage)
> +           __gnu_cxx::__alloc_traits<_Tp_alloc_type>::
> +             deallocate(_M_alloc, _M_storage, _M_len);
> +       }
> +
> +      private:
> +       _Guard(const _Guard&);
> +      };
> +
> +      {
> +       _Guard __guard(__new_start, __len, _M_impl);
> +
> +       // The order of the three operations is dictated by the C++11
> +       // case, where the moves could alter a new element belonging
> +       // to the existing vector.  This is an issue only for callers
> +       // taking the element by lvalue ref (see last bullet of C++11
> +       // [res.on.arguments]).
> +
> +       // If this throws, the existing elements are unchanged.
> +#if __cplusplus >= 201103L
> +       _Alloc_traits::construct(this->_M_impl,
> +                                std::__to_address(__new_start + __elems),
> +                                std::forward<_Args>(__args)...);
> +#else
> +       _Alloc_traits::construct(this->_M_impl,
> +                                __new_start + __elems,
> +                                __x);
> +#endif
> +
> +#if __cplusplus >= 201103L
> +       if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
> +         {
> +           // Relocation cannot throw.
> +           __new_finish = _S_relocate(__old_start, __old_finish,
> +                                      __new_start, _M_get_Tp_allocator());
> +           ++__new_finish;
> +         }
> +       else
> +#endif
> +         {
> +           // RAII type to destroy initialized elements.
> +           struct _Guard_elts
> +           {
> +             pointer _M_first, _M_last;  // Elements to destroy
> +             _Tp_alloc_type& _M_alloc;
> +
> +             _GLIBCXX20_CONSTEXPR
> +             _Guard_elts(pointer __elt, _Tp_alloc_type& __a)
> +             : _M_first(__elt), _M_last(__elt + 1), _M_alloc(__a)
> +             { }
> +
> +             _GLIBCXX20_CONSTEXPR
> +             ~_Guard_elts()
> +             { std::_Destroy(_M_first, _M_last, _M_alloc); }
> +
> +           private:
> +             _Guard_elts(const _Guard_elts&);
> +           };
> +
> +           // Guard the new element so it will be destroyed if anything throws.
> +           _Guard_elts __guard_elts(__new_start + __elems, _M_impl);
> +
> +           __new_finish = std::__uninitialized_move_if_noexcept_a(
> +                            __old_start, __old_finish,
> +                            __new_start, _M_get_Tp_allocator());
> +
> +           ++__new_finish;
> +
> +           // New storage has been fully initialized, destroy the old elements.
> +           __guard_elts._M_first = __old_start;
> +           __guard_elts._M_last = __old_finish;
> +         }
> +       __guard._M_storage = __old_start;
> +       __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
> +      }
> +      // deallocate should be called before assignments to _M_impl,
> +      // to avoid call-clobbering
> +
> +      this->_M_impl._M_start = __new_start;
> +      this->_M_impl._M_finish = __new_finish;
> +      this->_M_impl._M_end_of_storage = __new_start + __len;
> +    }
> +
>    template<typename _Tp, typename _Alloc>
>      _GLIBCXX20_CONSTEXPR
>      void
>