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libstdc++: Refactor implementation of operator+ for std::string

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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.

After consultation with Jonathan, it seemed like a good idea to create a
single function that performed one-allocation string concatenation that
could be used by various different version of operator+. This patch adds
such a function and calls it from the relevant implementations.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (__str_cat): Add common function
	that performs single-allocation string concatenation.
	(operator+): Use __str_cat.
	* include/bits/basic_string.tcc (operator+): Move to .h and
	define inline using __str_cat.

Signed-off-by: Will Hawkins <whh8b@obs.cr>
This commit is contained in:
Will Hawkins 2022-10-19 20:05:59 -04:00 • committed by Jonathan Wakely
parent 564b111846
commit c93baa93df
2 changed files with 49 additions and 58 deletions
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66
libstdc++-v3/include/bits/basic_string.h
View file

@ -3485,6 +3485,24 @@ _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.
@ -3494,13 +3512,14 @@ _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(),
__lhs.get_allocator());
}
/**
@ -3511,9 +3530,16 @@ _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>
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.
@ -3523,8 +3549,14 @@ _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.
@ -3538,11 +3570,12 @@ _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.
@ -3554,11 +3587,10 @@ _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

41
libstdc++-v3/include/bits/basic_string.tcc
View file

@ -605,47 +605,6 @@ _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