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

[Will Hawkins <hawkinsw@obs.cr>]

On Tue, Aug 23, 2022 at 12:33 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>
> 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

Thank you.

>
>
> > 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.

Thank you for the feedback. I sent out a v2 of the patch. Again, I
hope that I followed the proper procedure by having my mailer put the
patch in reply to my previous message.

Thank you again!
Will

>
> >      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
> >
>