[PATCH] libstdc++: Refactor implementation of operator+ for std::string

[Will Hawkins <whh8b@obs.cr>]

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 <whh8b@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(),
+				     __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>
     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
-- 
2.34.1