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.
+ __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
