Index: clang/include/clang/Lex/MultipleIncludeOpt.h
===================================================================
--- clang/include/clang/Lex/MultipleIncludeOpt.h
+++ clang/include/clang/Lex/MultipleIncludeOpt.h
@@ -108,6 +108,12 @@
     ImmediatelyAfterTopLevelIfndef = false;
   }
 
+  /// ResetReadToken - reset whether we have read any tokens. Called 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 ResetReadToken() { ReadAnyTokens = false; }
+
   /// 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; }
Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -1177,6 +1177,12 @@
 
   switch (Result.getKind()) {
   case tok::eod:
+    // Allow the null directive to appear outside of the include guard and still
+    // keep multiple-include optimization. So if we haven't recorded any tokens
+    // yet we can reset MIOpt to forget about the null directive.
+    if (!ReadAnyTokensBeforeDirective) {
+      CurPPLexer->MIOpt.ResetReadToken();
+    }
     return;   // null directive.
   case tok::code_completion:
     setCodeCompletionReached();
Index: clang/test/Preprocessor/multiple-inclusion-opt.h
===================================================================
--- /dev/null
+++ 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 */
Index: clang/test/Preprocessor/multiple-inclusion-opt.cpp
===================================================================
--- /dev/null
+++ 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"