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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@188077 91177308-0d34-0410-b5e6-96231b3b80d8

include/__std_stream

diff --git a/include/__std_stream b/include/__std_stream
index cff4331..5403ada 100644
--- a/include/__std_stream
+++ b/include/__std_stream
@@ -233,6 +233,7 @@
 
 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);
 
@@ -309,6 +310,19 @@
 }
 
 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()
 {