Index: include/clang/Lex/Preprocessor.h
===================================================================
--- include/clang/Lex/Preprocessor.h
+++ include/clang/Lex/Preprocessor.h
@@ -522,10 +522,13 @@
 
   /// \brief Information about a submodule's preprocessor state.
   struct SubmoduleState {
+    SubmoduleState() : NumCompiledHeaders(0) {}
     /// The macros for the submodule.
     MacroMap Macros;
     /// The set of modules that are visible within the submodule.
     VisibleModuleSet VisibleModules;
+    /// Number of module headers compiled so far.
+    unsigned NumCompiledHeaders;
     // FIXME: CounterValue?
     // FIXME: PragmaPushMacroInfo?
   };
Index: lib/Lex/PPLexerChange.cpp
===================================================================
--- lib/Lex/PPLexerChange.cpp
+++ lib/Lex/PPLexerChange.cpp
@@ -404,6 +404,8 @@
 
     bool LeavingSubmodule = CurSubmodule && CurLexer;
     if (LeavingSubmodule) {
+      ++CurSubmoduleState->NumCompiledHeaders;
+
       // Notify the parser that we've left the module.
       const char *EndPos = getCurLexerEndPos();
       Result.startToken();
@@ -701,42 +703,47 @@
 
     // This module may have exported a new macro. If so, create a ModuleMacro
     // representing that fact.
-    bool ExplicitlyPublic = false;
-    for (auto *MD = Macro.second.getLatest(); MD != OldMD;
-         MD = MD->getPrevious()) {
-      assert(MD && "broken macro directive chain");
-
-      // Stop on macros defined in other submodules we #included along the way.
-      // There's no point doing this if we're tracking local submodule
-      // visibility, since there can be no such directives in our list.
-      if (!getLangOpts().ModulesLocalVisibility) {
-        Module *Mod = getModuleContainingLocation(MD->getLocation());
-        if (Mod != LeavingMod)
-          break;
-      }
+    // We do this only if when the last module header is compiled, to avoid
+    // repeating updates of macro set.
+    if (CurSubmoduleState->NumCompiledHeaders >=
+        LeavingMod->Headers[Module::HK_Normal].size()) {
+      bool ExplicitlyPublic = false;
+      for (auto *MD = Macro.second.getLatest(); MD != OldMD;
+           MD = MD->getPrevious()) {
+        assert(MD && "broken macro directive chain");
+
+        // Stop on macros defined in other submodules we #included along the
+        // way. There's no point doing this if we're tracking local submodule
+        // visibility, since there can be no such directives in our list.
+        if (!getLangOpts().ModulesLocalVisibility) {
+          Module *Mod = getModuleContainingLocation(MD->getLocation());
+          if (Mod != LeavingMod)
+            break;
+        }
 
-      if (auto *VisMD = dyn_cast<VisibilityMacroDirective>(MD)) {
-        // The latest visibility directive for a name in a submodule affects
-        // all the directives that come before it.
-        if (VisMD->isPublic())
-          ExplicitlyPublic = true;
-        else if (!ExplicitlyPublic)
-          // Private with no following public directive: not exported.
+        if (auto *VisMD = dyn_cast<VisibilityMacroDirective>(MD)) {
+          // The latest visibility directive for a name in a submodule affects
+          // all the directives that come before it.
+          if (VisMD->isPublic())
+            ExplicitlyPublic = true;
+          else if (!ExplicitlyPublic)
+            // Private with no following public directive: not exported.
+            break;
+        } else {
+          MacroInfo *Def = nullptr;
+          if (DefMacroDirective *DefMD = dyn_cast<DefMacroDirective>(MD))
+            Def = DefMD->getInfo();
+
+          // FIXME: Issue a warning if multiple headers for the same submodule
+          // define a macro, rather than silently ignoring all but the first.
+          bool IsNew;
+          // Don't bother creating a module macro if it would represent a #undef
+          // that doesn't override anything.
+          if (Def || !Macro.second.getOverriddenMacros().empty())
+            addModuleMacro(LeavingMod, II, Def,
+                           Macro.second.getOverriddenMacros(), IsNew);
           break;
-      } else {
-        MacroInfo *Def = nullptr;
-        if (DefMacroDirective *DefMD = dyn_cast<DefMacroDirective>(MD))
-          Def = DefMD->getInfo();
-
-        // FIXME: Issue a warning if multiple headers for the same submodule
-        // define a macro, rather than silently ignoring all but the first.
-        bool IsNew;
-        // Don't bother creating a module macro if it would represent a #undef
-        // that doesn't override anything.
-        if (Def || !Macro.second.getOverriddenMacros().empty())
-          addModuleMacro(LeavingMod, II, Def,
-                         Macro.second.getOverriddenMacros(), IsNew);
-        break;
+        }
       }
     }
   }