Re: libstdc++: Speed up push_back

[Jonathan Wakely <jwakely@redhat.com>]

On Fri, 24 Nov 2023 at 20:07, Jan Hubicka <hubicka@ucw.cz> wrote:
> The vector.tcc change was regtested on x86_64-linux, OK?
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/vector.tcc (reserve): Copy _M_start and _M_finish
>         to local variables to allow propagation across call to
>         allocator.
>
> diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
> index 0ccef7911b3..0a9db29c1c7 100644
> --- a/libstdc++-v3/include/bits/vector.tcc
> +++ b/libstdc++-v3/include/bits/vector.tcc
> @@ -72,27 +72,30 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>        if (this->capacity() < __n)
>         {
>           const size_type __old_size = size();
> +         // Make local copies of these members because the compiler thinks
> +         // the allocator can alter them if 'this' is globally reachable.
> +         pointer __old_start = this->_M_impl._M_start;
> +         pointer __old_finish = this->_M_impl._M_finish;
>           pointer __tmp;
>  #if __cplusplus >= 201103L
>           if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
>             {
>               __tmp = this->_M_allocate(__n);
> -             _S_relocate(this->_M_impl._M_start, this->_M_impl._M_finish,
> +             _S_relocate(__old_start, __old_finish,
>                           __tmp, _M_get_Tp_allocator());

Please move "__tmp," to the first line, as it will fit there now that
the line got shorter:

          _S_relocate(__old_start, __old_finish, __tmp,
              _M_get_Tp_allocator());


>             }
>           else
>  #endif
>             {
>               __tmp = _M_allocate_and_copy(__n,
> -               _GLIBCXX_MAKE_MOVE_IF_NOEXCEPT_ITERATOR(this->_M_impl._M_start),
> -               _GLIBCXX_MAKE_MOVE_IF_NOEXCEPT_ITERATOR(this->_M_impl._M_finish));
> -             std::_Destroy(this->_M_impl._M_start, this->_M_impl._M_finish,
> +               _GLIBCXX_MAKE_MOVE_IF_NOEXCEPT_ITERATOR(__old_start),
> +               _GLIBCXX_MAKE_MOVE_IF_NOEXCEPT_ITERATOR(__old_finish));
> +             std::_Destroy(__old_start, __old_finish,
>                             _M_get_Tp_allocator());

The _Destroy call will fit on one line now:

          std::_Destroy(__old_start, __old_finish, _M_get_Tp_allocator());



>             }
>           _GLIBCXX_ASAN_ANNOTATE_REINIT;
> -         _M_deallocate(this->_M_impl._M_start,
> -                       this->_M_impl._M_end_of_storage
> -                       - this->_M_impl._M_start);
> +         _M_deallocate(__old_start,
> +                       this->_M_impl._M_end_of_storage - __old_finish);

This should be __old_start.

I think what you have here will show Asan and/or valgrind errors, as
the wrong length will be passed to operator delete.


>           this->_M_impl._M_start = __tmp;
>           this->_M_impl._M_finish = __tmp + __old_size;
>           this->_M_impl._M_end_of_storage = this->_M_impl._M_start + __n;
>