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

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>

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index cd244191df41692b57e9ccc820060b057d74c500..9c2b57f5a1d7b145c62186d92c980f50d7e23424 100644 (file)
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -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

diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 710c2dfe0308918c6cf18ac18c0e28b54af9badd..56114f120ba726abc0f7c52f12823d0ff26d9269 100644 (file)
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -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