On Thu, 8 Sep 2022, 18:51 François Dumont via Libstdc++, <
libstd...@gcc.gnu.org> wrote:
> On 05/09/22 20:30, Will Hawkins wrote:
> > Based on Jonathan's work, here is a patch for the implementation of
> operator+
> > on std::string that makes sure we always use the best allocation
> strategy.
> >
> > I have attempted to learn from all the feedback that I got on a previous
> > submission -- I hope I did the right thing.
> >
> > Passes abi and conformance testing on x86-64 trunk.
> >
> > Sincerely,
> > Will
> >
> > -- >8 --
> >
> > Create a single function that performs one-allocation string
> concatenation
> > that can be used by various different version of operator+.
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/bits/basic_string.h:
> > Add common function that performs single-allocation string
> > concatenation. (__str_cat)
> > Use __str_cat to perform optimized operator+, where relevant.
> > * include/bits/basic_string.tcc::
> > Remove single-allocation implementation of operator+.
> >
> > Signed-off-by: Will Hawkins <wh...@obs.cr>
> > ---
> > libstdc++-v3/include/bits/basic_string.h | 66 ++++++++++++++++------
> > libstdc++-v3/include/bits/basic_string.tcc | 41 --------------
> > 2 files changed, 49 insertions(+), 58 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/bits/basic_string.h
> b/libstdc++-v3/include/bits/basic_string.h
> > index 0df64ea98ca..4078651fadb 100644
> > --- a/libstdc++-v3/include/bits/basic_string.h
> > +++ b/libstdc++-v3/include/bits/basic_string.h
> > @@ -3481,6 +3481,24 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > _GLIBCXX_END_NAMESPACE_CXX11
> > #endif
> >
> > + template<typename _Str>
> > + _GLIBCXX20_CONSTEXPR
> > + inline _Str
> > + __str_concat(typename _Str::value_type const* __lhs,
> > + typename _Str::size_type __lhs_len,
> > + typename _Str::value_type const* __rhs,
> > + typename _Str::size_type __rhs_len,
> > + typename _Str::allocator_type const& __a)
> > + {
> > + typedef typename _Str::allocator_type allocator_type;
> > + typedef __gnu_cxx::__alloc_traits<allocator_type> _Alloc_traits;
> > + _Str __str(_Alloc_traits::_S_select_on_copy(__a));
> > + __str.reserve(__lhs_len + __rhs_len);
> > + __str.append(__lhs, __lhs_len);
> > + __str.append(__rhs, __rhs_len);
> > + return __str;
> > + }
> > +
> > // operator+
> > /**
> > * @brief Concatenate two strings.
> > @@ -3490,13 +3508,14 @@ _GLIBCXX_END_NAMESPACE_CXX11
> > */
> > template<typename _CharT, typename _Traits, typename _Alloc>
> > _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
> > - basic_string<_CharT, _Traits, _Alloc>
> > + inline basic_string<_CharT, _Traits, _Alloc>
> > operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
> > const basic_string<_CharT, _Traits, _Alloc>& __rhs)
> > {
> > - basic_string<_CharT, _Traits, _Alloc> __str(__lhs);
> > - __str.append(__rhs);
> > - return __str;
> > + typedef basic_string<_CharT, _Traits, _Alloc> _Str;
> > + return std::__str_concat<_Str>(__lhs.c_str(), __lhs.size(),
> > + __rhs.c_str(), __rhs.size(),
>
> You should use data() rather than c_str() here and all other operators.
>
> It is currently the same but is more accurate in your context. Maybe one
> day it will make a difference.
>
I don't think so, that would be a major breaking change, for no benefit. I
think it's safe to assume they will always stay equivalent now.
> > + __lhs.get_allocator());
> > }
> >
> > /**
> > @@ -3507,9 +3526,16 @@ _GLIBCXX_END_NAMESPACE_CXX11
> > */
> > template<typename _CharT, typename _Traits, typename _Alloc>
> > _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
> > - basic_string<_CharT,_Traits,_Alloc>
> > + inline basic_string<_CharT,_Traits,_Alloc>
>
> Why inlining ?
>
> I guess it is done this way to limit code bloat.
>
> > operator+(const _CharT* __lhs,
> > - const basic_string<_CharT,_Traits,_Alloc>& __rhs);
> > + const basic_string<_CharT,_Traits,_Alloc>& __rhs)
> > + {
> > + __glibcxx_requires_string(__lhs);
> > + typedef basic_string<_CharT, _Traits, _Alloc> _Str;
> > + return std::__str_concat<_Str>(__lhs, _Traits::length(__lhs),
> > + __rhs.c_str(), __rhs.size(),
> > + __rhs.get_allocator());
> > + }
> >
> > /**
> > * @brief Concatenate character and string.
> > @@ -3519,8 +3545,14 @@ _GLIBCXX_END_NAMESPACE_CXX11
> > */
> > template<typename _CharT, typename _Traits, typename _Alloc>
> > _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
> > - basic_string<_CharT,_Traits,_Alloc>
> > - operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Alloc>&
> __rhs);
> > + inline basic_string<_CharT,_Traits,_Alloc>
> > + operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Alloc>&
> __rhs)
> > + {
> > + typedef basic_string<_CharT, _Traits, _Alloc> _Str;
> > + return std::__str_concat<_Str>(__builtin_addressof(__lhs), 1,
> > + __rhs.c_str(), __rhs.size(),
> > + __rhs.get_allocator());
> > + }
> >
> > /**
> > * @brief Concatenate string and C string.
> > @@ -3534,11 +3566,12 @@ _GLIBCXX_END_NAMESPACE_CXX11
> > operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
> > const _CharT* __rhs)
> > {
> > - basic_string<_CharT, _Traits, _Alloc> __str(__lhs);
> > - __str.append(__rhs);
> > - return __str;
> > + __glibcxx_requires_string(__rhs);
> > + typedef basic_string<_CharT, _Traits, _Alloc> _Str;
> > + return std::__str_concat<_Str>(__lhs.c_str(), __lhs.size(),
> > + __rhs, _Traits::length(__rhs),
> > + __lhs.get_allocator());
> > }
> > -
> > /**
> > * @brief Concatenate string and character.
> > * @param __lhs First string.
> > @@ -3550,11 +3583,10 @@ _GLIBCXX_END_NAMESPACE_CXX11
> > inline basic_string<_CharT, _Traits, _Alloc>
> > operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
> _CharT __rhs)
> > {
> > - typedef basic_string<_CharT, _Traits, _Alloc> __string_type;
> > - typedef typename __string_type::size_type
> __size_type;
> > - __string_type __str(__lhs);
> > - __str.append(__size_type(1), __rhs);
> > - return __str;
> > + typedef basic_string<_CharT, _Traits, _Alloc> _Str;
> > + return std::__str_concat<_Str>(__lhs.c_str(), __lhs.size(),
> > + __builtin_addressof(__rhs), 1,
> > + __lhs.get_allocator());
> > }
> >
> > #if __cplusplus >= 201103L
> > diff --git a/libstdc++-v3/include/bits/basic_string.tcc
> b/libstdc++-v3/include/bits/basic_string.tcc
> > index 4563c61429a..07a94d36757 100644
> > --- a/libstdc++-v3/include/bits/basic_string.tcc
> > +++ b/libstdc++-v3/include/bits/basic_string.tcc
> > @@ -599,47 +599,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > #else
> > # define _GLIBCXX_STRING_CONSTEXPR
> > #endif
> > -
> > - template<typename _CharT, typename _Traits, typename _Alloc>
> > - _GLIBCXX20_CONSTEXPR
> > - basic_string<_CharT, _Traits, _Alloc>
> > - operator+(const _CharT* __lhs,
> > - const basic_string<_CharT, _Traits, _Alloc>& __rhs)
> > - {
> > - __glibcxx_requires_string(__lhs);
> > - 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(__lhs);
> > - __string_type __str(_Alloc_traits::_S_select_on_copy(
> > - __rhs.get_allocator()));
> > - __str.reserve(__len + __rhs.size());
> > - __str.append(__lhs, __len);
> > - __str.append(__rhs);
> > - return __str;
> > - }
> > -
> > - template<typename _CharT, typename _Traits, typename _Alloc>
> > - _GLIBCXX20_CONSTEXPR
> > - basic_string<_CharT, _Traits, _Alloc>
> > - operator+(_CharT __lhs, const basic_string<_CharT, _Traits,
> _Alloc>& __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;
> > - __string_type __str(_Alloc_traits::_S_select_on_copy(
> > - __rhs.get_allocator()));
> > - const __size_type __len = __rhs.size();
> > - __str.reserve(__len + 1);
> > - __str.append(__size_type(1), __lhs);
> > - __str.append(__rhs);
> > - return __str;
> > - }
> > -
> > template<typename _CharT, typename _Traits, typename _Alloc>
> > _GLIBCXX_STRING_CONSTEXPR
> > typename basic_string<_CharT, _Traits, _Alloc>::size_type
>
>
>
