Partial implementation of N3665. This paper was not voted into the C++1y draft. However I was looking at it and with some experimentation realized that I could partially implement it, and at the same time offer a performance optimization to cout. I simply added an xsputn override to the cout filebuf. The override does nothing special at all if there is a non-trivial codecvt installed. However if the codecvt returns true for always_noconv(), then this function can dump an entire string to fwrite, instead of doing it a character at a time under overflow(). This just makes sense. I stopped short of a full implementation of N3665 because in order to do so, xsputn would have to allocate a buffer when always_noconv() returned false, and I don't want to go to that expense.

llvm-svn: 188077

libcxx/include/__std_stream

@@ -233,6 +233,7 @@ public:
 
 protected:
     virtual int_type overflow (int_type __c = traits_type::eof());
+    virtual streamsize xsputn(const char_type* __s, streamsize __n);
     virtual int sync();
     virtual void imbue(const locale& __loc);
 
@@ -308,6 +309,19 @@ __stdoutbuf<_CharT>::overflow(int_type __c)
     return traits_type::not_eof(__c);
 }
 
+template <class _CharT>
+streamsize
+__stdoutbuf<_CharT>::xsputn(const char_type* __s, streamsize __n)
+{
+    if (__always_noconv_)
+        return fwrite(__s, sizeof(char_type), __n, __file_);
+    streamsize __i = 0;
+    for (; __i < __n; ++__i, ++__s)
+        if (overflow(traits_type::to_int_type(*__s)) == traits_type::eof())
+            break;
+    return __i;
+}
+
 template <class _CharT>
 int
 __stdoutbuf<_CharT>::sync()