3 files changed, 19 insertions, 3 deletions
diff --git a/clang/include/clang/Lex/HeaderSearch.h b/clang/include/clang/Lex/HeaderSearch.h
index 446a3810461..c46c8ce6ef0 100644
--- a/clang/include/clang/Lex/HeaderSearch.h
+++ b/clang/include/clang/Lex/HeaderSearch.h
@@ -492,7 +492,10 @@ public:
   ///
   /// \param Modules Will be filled with the set of known, top-level modules.
   void collectAllModules(SmallVectorImpl<Module *> &Modules);
-                         
+
+  /// \brief Load all known, top-level system modules.
+  void loadTopLevelSystemModules();
+
 private:
   /// \brief Retrieve a module with the given name, which may be part of the
   /// given framework.
diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
index b8556dde0b2..8a99ed29883 100644
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -1146,6 +1146,20 @@ void HeaderSearch::collectAllModules(SmallVectorImpl<Module *> &Modules) {
   }
 }
 
+void HeaderSearch::loadTopLevelSystemModules() {
+  // Load module maps for each of the header search directories.
+  for (unsigned Idx = 0, N = SearchDirs.size(); Idx != N; ++Idx) {
+    // We only care about normal system header directories.
+    if (!SearchDirs[Idx].isNormalDir() ||
+        SearchDirs[Idx].getDirCharacteristic() != SrcMgr::C_System) {
+      continue;
+    }
+
+    // Try to load a module map file for the search directory.
+    loadModuleMapFile(SearchDirs[Idx].getDir());
+  }
+}
+
 void HeaderSearch::loadSubdirectoryModuleMaps(DirectoryLookup &SearchDir) {
   if (SearchDir.haveSearchedAllModuleMaps())
     return;
diff --git a/clang/lib/Lex/ModuleMap.cpp b/clang/lib/Lex/ModuleMap.cpp
index de234853fbf..62b1fc73c80 100644
--- a/clang/lib/Lex/ModuleMap.cpp
+++ b/clang/lib/Lex/ModuleMap.cpp
@@ -183,8 +183,7 @@ Module *ModuleMap::findModuleForHeader(const FileEntry *File) {
   // specific module (e.g., in /usr/include).
   if (File->getDir() == BuiltinIncludeDir &&
       isBuiltinHeader(llvm::sys::path::filename(File->getName()))) {
-    SmallVector<Module *, 4> AllModules;
-    HeaderInfo.collectAllModules(AllModules);
+    HeaderInfo.loadTopLevelSystemModules();
 
     // Check again.
     Known = Headers.find(File);