Re: libstdc++: Speed up push_back

[Jan Hubicka <hubicka@ucw.cz>]

> > +           // 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 removed this one.
> 
> > +
> > +           // New storage has been fully initialized, destroy the old elements.
> > +           __guard_elts._M_first = __old_start;
> > +           __guard_elts._M_last = __old_finish;

However here I think I need __guard_elts supporting destruction of many
elements in case the vector has moved to new location....
So I do not quite  see how to simplify the code as suggested above
except that the constructor can be simplified to not require first and
last argument since we always initialize it for 1 destruction but later
we may update it.

Thanks,
Honza