Index: lib/Support/raw_ostream.cpp
===================================================================
--- lib/Support/raw_ostream.cpp
+++ lib/Support/raw_ostream.cpp
@@ -613,10 +613,17 @@
   assert(FD >= 0 && "File already closed.");
   pos += Size;
 
+#ifdef _WIN64
+  // Windows _write() requires unsigned 32 bit input for 'count',
+  // however to follow POSIX a value above unsigned max should not be used.
+  // Win64 size_t is 64 bit so int 32 max is used below.
+  size_t MaxWriteSize = _I32_MAX;
+#else
   // The maximum write size is limited to SSIZE_MAX because a write
   // greater than SSIZE_MAX is implementation-defined in POSIX.
   // Since SSIZE_MAX is not portable, we use SIZE_MAX >> 1 instead.
   size_t MaxWriteSize = SIZE_MAX >> 1;
+#endif
 
 #if defined(__linux__)
   // It is observed that Linux returns EINVAL for a very large write (>2G).