[PATCH] for STDCXX-491 - string::push_back() slow

[mark.g.brown]

Hi all,

The attached simple patch speeds up push_back() nearly six times
relative to stdcxx 4.1.3 and makes it more than twice faster that
gcc's.

$ time ./push_back-stdcxx-patched 500000000

real    0m2.800s
user    0m1.676s
sys     0m1.084s

$ time ./push_back-stdcxx-4.1.3 500000000

real    0m11.206s
user    0m10.017s
sys     0m1.184s

$ time ./push_back-gcc 500000000

real    0m4.555s
user    0m3.840s
sys     0m0.716s


--Mark

Index: /home/mbrown/stdcxx/include/string
===================================================================
--- /home/mbrown/stdcxx/include/string	(revision 558531)
+++ /home/mbrown/stdcxx/include/string	(working copy)
@@ -340,9 +340,7 @@
     }
 
     // lwg issue 7
-    void push_back (value_type __c) {
-        append (size_type (1), __c);
-    }
+    void push_back (value_type);
 
     basic_string& assign (const basic_string &__str) {
         return *this = __str;
@@ -1088,6 +1086,22 @@
 
 template <class _CharT, class _Traits , class _Allocator>
 inline void basic_string<_CharT, _Traits, _Allocator>::
+push_back (value_type __c)
+{
+    const size_type __size = size () + 1;
+
+    if (   capacity () < __size
+        || size_type (1) < size_type (_C_pref ()->_C_get_ref ()))
+        append (1, __c);
+    else {
+        traits_type::assign (_C_data [size ()], __c);
+        _C_pref ()->_C_size._C_size = __size;
+    }
+}
+
+
+template <class _CharT, class _Traits , class _Allocator>
+inline void basic_string<_CharT, _Traits, _Allocator>::
 reserve (size_type __cap)
 {
     _RWSTD_REQUIRES (__cap <= max_size (),