diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -157,6 +157,8 @@
 - A new builtin type trait ``__is_trivially_equaltiy_comparable`` has been added,
   which checks whether comparing two instances of a type is equivalent to
   ``memcmp(&lhs, &rhs, sizeof(T)) == 0``.
+- Clang now ignores null directives outside of the include guard when deciding
+  whether a file can be enabled for the multiple-include optimization.
 
 New Compiler Flags
 ------------------
diff --git a/clang/include/clang/Lex/MultipleIncludeOpt.h b/clang/include/clang/Lex/MultipleIncludeOpt.h
--- a/clang/include/clang/Lex/MultipleIncludeOpt.h
+++ b/clang/include/clang/Lex/MultipleIncludeOpt.h
@@ -108,6 +108,12 @@
     ImmediatelyAfterTopLevelIfndef = false;
   }
 
+  /// SetReadToken - Set whether the value of 'ReadAnyTokens'.  Called to
+  /// override when encountering tokens outside of the include guard that have
+  /// no effect if the file in question is is included multiple times (e.g. the
+  /// null directive).
+  void SetReadToken(bool Value) { ReadAnyTokens = Value; }
+
   /// ExpandedMacro - When a macro is expanded with this lexer as the current
   /// buffer, this method is called to disable the MIOpt if needed.
   void ExpandedMacro() { DidMacroExpansion = true; }
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -1177,6 +1177,10 @@
 
   switch (Result.getKind()) {
   case tok::eod:
+    // Ignore the null directive with regards to the multiple-include
+    // optimization, i.e. allow the null directive to appear outside of the
+    // include guard and still enable the multiple-include optimization.
+    CurPPLexer->MIOpt.SetReadToken(ReadAnyTokensBeforeDirective);
     return;   // null directive.
   case tok::code_completion:
     setCodeCompletionReached();
diff --git a/clang/test/Preprocessor/multiple-inclusion-opt.h b/clang/test/Preprocessor/multiple-inclusion-opt.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Preprocessor/multiple-inclusion-opt.h
@@ -0,0 +1,18 @@
+# // null directive and comments before include guard
+
+#ifndef MULTIPLE_INCLUSION_OPT
+
+int foo();
+
+// The position of the define should not matter
+#define MULTIPLE_INCLUSION_OPT
+
+int bar();
+
+#endif
+
+#
+#
+/* Two null directives
+   and a multiline comment
+   after the #endif */
diff --git a/clang/test/Preprocessor/multiple-inclusion-opt.cpp b/clang/test/Preprocessor/multiple-inclusion-opt.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Preprocessor/multiple-inclusion-opt.cpp
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -E -P -H %s 2>&1 | grep "multiple-inclusion-opt.h" | count 1
+
+#include "multiple-inclusion-opt.h"
+#include "multiple-inclusion-opt.h"
+#include "multiple-inclusion-opt.h"
+#include "multiple-inclusion-opt.h"
+#include "multiple-inclusion-opt.h"