diff -r bbd1da3f538f -r f3108e56b502 src/os/windows/vm/os_windows.cpp
--- a/src/os/windows/vm/os_windows.cpp	Wed Aug 01 04:19:22 2018 -0400
+++ b/src/os/windows/vm/os_windows.cpp	Thu Aug 02 03:54:51 2018 -0700
@@ -1837,6 +1837,42 @@
   st->cr();
 }
 
+
+int os::vsnprintf(char* buf, size_t len, const char* fmt, va_list args) {
+#if _MSC_VER >= 1900
+  // Starting with Visual Studio 2015, vsnprint is C99 compliant.
+  int result = ::vsnprintf(buf, len, fmt, args);
+  // If an encoding error occurred (result < 0) then it's not clear
+  // whether the buffer is NUL terminated, so ensure it is.
+  if ((result < 0) && (len > 0)) {
+    buf[len - 1] = '\0';
+  }
+  return result;
+#else
+  // Before Visual Studio 2015, vsnprintf is not C99 compliant, so use
+  // _vsnprintf, whose behavior seems to be *mostly* consistent across
+  // versions.  However, when len == 0, avoid _vsnprintf too, and just
+  // go straight to _vscprintf.  The output is going to be truncated in
+  // that case, except in the unusual case of empty output.  More
+  // importantly, the documentation for various versions of Visual Studio
+  // are inconsistent about the behavior of _vsnprintf when len == 0,
+  // including it possibly being an error.
+  int result = -1;
+  if (len > 0) {
+    result = _vsnprintf(buf, len, fmt, args);
+    // If output (including NUL terminator) is truncated, the buffer
+    // won't be NUL terminated.  Add the trailing NUL specified by C99.
+    if ((result < 0) || (result >= (int) len)) {
+      buf[len - 1] = '\0';
+    }
+  }
+  if (result < 0) {
+    result = _vscprintf(fmt, args);
+  }
+  return result;
+#endif // _MSC_VER dispatch
+}
+
 void os::print_signal_handlers(outputStream* st, char* buf, size_t buflen) {
   // do nothing
 }