[PATCH] libstdc++: Optimize operator+(string/char*, string/char*) equally

[whh8b]

From: Will Hawkins <wh...@obs.cr>

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.

libstdc++-v3/ChangeLog:
        * libstdc++-v3/include/bits/basic_string.h (operator+(string, char*)):
        Remove naive implementation.
        * libstdc++-v3/include/bits/basic_string.tcc (operator+(string, char*)):
        Add single-allocation implementation.
---
 libstdc++-v3/include/bits/basic_string.h   |  7 +------
 libstdc++-v3/include/bits/basic_string.tcc | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h 
b/libstdc++-v3/include/bits/basic_string.h
index b04fba95678..bc048fc0689 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -3523,12 +3523,7 @@ _GLIBCXX_END_NAMESPACE_CXX11
     _GLIBCXX20_CONSTEXPR
     inline 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
-- 
2.34.1