On Thu, Sep 8, 2022 at 2:05 PM Jonathan Wakely <jwakely....@gmail.com> wrote:
>
>
>
> 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.
Happy to make any changes to the patch that the group thinks are necessary!
Will
>
>
>>
>> > + __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
>>
>>
