[gcc r13-2175] libstdc++: Optimize operator+(string/char*, char*/string) equally

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:0b7c9254998b3fb2c39f6b86b5b196f415530205

commit r13-2175-g0b7c9254998b3fb2c39f6b86b5b196f415530205
Author: Will Hawkins <whh8b@obs.cr>
Date:   Wed Aug 24 02:16:48 2022 -0400

    libstdc++: Optimize operator+(string/char*, char*/string) equally
    
    Until now operator+(char*, const string&) and operator+(const string&,
    char*) had different performance characteristics. The former required a
    single memory allocation and the latter required two. This patch makes
    the performance equal.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h (operator+(const string&, const char*)):
            Remove naive implementation.
            * include/bits/basic_string.tcc (operator+(const string&, const char*)):
            Add single-allocation implementation.
    
    Signed-off-by: Will Hawkins <whh8b@obs.cr>

Diff:
---
 libstdc++-v3/include/bits/basic_string.h   |  9 ++-------
 libstdc++-v3/include/bits/basic_string.tcc | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index b04fba95678..fa6738925bb 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -3521,14 +3521,9 @@ _GLIBCXX_END_NAMESPACE_CXX11
    */
   template<typename _CharT, typename _Traits, typename _Alloc>
     _GLIBCXX20_CONSTEXPR
-    inline basic_string<_CharT, _Traits, _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>
     operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
-	      const _CharT* __rhs)
-    {
-      basic_string<_CharT, _Traits, _Alloc> __str(__lhs);
-      __str.append(__rhs);
-      return __str;
-    }
+	      const _CharT* __rhs);
 
   /**
    *  @brief  Concatenate string and character.
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 4563c61429a..95ba8e503e9 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -640,6 +640,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __str;
     }
 
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    _GLIBCXX20_CONSTEXPR
+    basic_string<_CharT, _Traits, _Alloc>
+    operator+(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
+	      const _CharT* __rhs)
+    {
+      __glibcxx_requires_string(__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;
+      const __size_type __len = _Traits::length(__rhs);
+      __string_type __str(_Alloc_traits::_S_select_on_copy(
+          __lhs.get_allocator()));
+      __str.reserve(__len + __lhs.size());
+      __str.append(__lhs);
+      __str.append(__rhs, __len);
+      return __str;
+    }
+
   template<typename _CharT, typename _Traits, typename _Alloc>
     _GLIBCXX_STRING_CONSTEXPR
     typename basic_string<_CharT, _Traits, _Alloc>::size_type