Re: [PATCH] libstdc++: Optimize operator+(string/char*, string/char*) equally

[Jonathan Wakely <jwakely@redhat.com>]

On Mon, 22 Aug 2022 at 19:15, Will Hawkins wrote:
>
> Until now operator+(char*, string) and operator+(string, char*) had
> different performance characteristics. The former required a single
> memory allocation and the latter required two. This patch makes the
> performance equal.

If you don't have a GCC copyright assignment on file with the FSF then
please follow the procedure described at https://gcc.gnu.org/dco.html


> libstdc++-v3/ChangeLog:
>         * libstdc++-v3/include/bits/basic_string.h (operator+(string, char*)):
>         Remove naive implementation.
>         * libstdc++-v3/include/bits/basic_string.tcc (operator+(string, char*)):
>         Add single-allocation implementation.
> ---
>  libstdc++-v3/include/bits/basic_string.h   |  7 +------
>  libstdc++-v3/include/bits/basic_string.tcc | 21 +++++++++++++++++++++
>  2 files changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> index b04fba95678..bc048fc0689 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -3523,12 +3523,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
>      _GLIBCXX20_CONSTEXPR
>      inline basic_string<_CharT, _Traits, _Alloc>

Please remove the 'inline' specifier here, since you're moving the
definition into the non-inline .tcc file.

There's a separate discussion to be had about whether these operator+
overloads *should* be inline. But for the purposes of this change, we
want these two operator+ overloads to be consistent, and so they
should both be non-inline.

>      operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
> -             const _CharT* __rhs)
> -    {
> -      basic_string<_CharT, _Traits, _Alloc> __str(__lhs);
> -      __str.append(__rhs);
> -      return __str;
> -    }
> +             const _CharT* __rhs);
>
>    /**
>     *  @brief  Concatenate string and character.
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> index 4563c61429a..95ba8e503e9 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -640,6 +640,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        return __str;
>      }
>
> +  template<typename _CharT, typename _Traits, typename _Alloc>
> +    _GLIBCXX20_CONSTEXPR
> +    basic_string<_CharT, _Traits, _Alloc>
> +    operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
> +             const _CharT* __rhs)
> +    {
> +      __glibcxx_requires_string(__rhs);
> +      typedef basic_string<_CharT, _Traits, _Alloc> __string_type;
> +      typedef typename __string_type::size_type          __size_type;
> +      typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
> +       rebind<_CharT>::other _Char_alloc_type;
> +      typedef __gnu_cxx::__alloc_traits<_Char_alloc_type> _Alloc_traits;
> +      const __size_type __len = _Traits::length(__rhs);
> +      __string_type __str(_Alloc_traits::_S_select_on_copy(
> +          __lhs.get_allocator()));
> +      __str.reserve(__len + __lhs.size());
> +      __str.append(__lhs);
> +      __str.append(__rhs, __len);
> +      return __str;
> +    }
> +
>    template<typename _CharT, typename _Traits, typename _Alloc>
>      _GLIBCXX_STRING_CONSTEXPR
>      typename basic_string<_CharT, _Traits, _Alloc>::size_type
> --
> 2.34.1
>