3 files changed, 31 insertions, 1 deletions
diff --git a/clang/lib/Basic/VirtualFileSystem.cpp b/clang/lib/Basic/VirtualFileSystem.cpp
index 881e1663953..539cbb74097 100644
--- a/clang/lib/Basic/VirtualFileSystem.cpp
+++ b/clang/lib/Basic/VirtualFileSystem.cpp
@@ -761,7 +761,7 @@ error_code VFSFromYAML::openFileForRead(const Twine &Path,
   if (!F) // FIXME: errc::not_a_file?
     return error_code(errc::invalid_argument, system_category());
 
-  return ExternalFS->openFileForRead(Path, Result);
+  return ExternalFS->openFileForRead(F->getExternalContentsPath(), Result);
 }
 
 IntrusiveRefCntPtr<FileSystem>
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 4ef5cfef7c0..ad2d37cc297 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1020,6 +1020,10 @@ static void ParseHeaderSearchArgs(HeaderSearchOptions &Opts, ArgList &Args) {
        I != E; ++I)
     Opts.AddSystemHeaderPrefix((*I)->getValue(),
                                (*I)->getOption().matches(OPT_isystem_prefix));
+
+  for (arg_iterator I = Args.filtered_begin(OPT_ivfsoverlay),
+       E = Args.filtered_end(); I != E; ++I)
+    Opts.AddVFSOverlayFile((*I)->getValue());
 }
 
 void CompilerInvocation::setLangDefaults(LangOptions &Opts, InputKind IK,
diff --git a/clang/lib/Frontend/FrontendAction.cpp b/clang/lib/Frontend/FrontendAction.cpp
index 72377d84572..6cfa12168db 100644
--- a/clang/lib/Frontend/FrontendAction.cpp
+++ b/clang/lib/Frontend/FrontendAction.cpp
@@ -211,6 +211,32 @@ bool FrontendAction::BeginSourceFile(CompilerInstance &CI,
     return true;
   }
 
+  if (!CI.getHeaderSearchOpts().VFSOverlayFiles.empty()) {
+    IntrusiveRefCntPtr<vfs::OverlayFileSystem>
+        Overlay(new vfs::OverlayFileSystem(vfs::getRealFileSystem()));
+    // earlier vfs files are on the bottom
+    const std::vector<std::string> &Files =
+        CI.getHeaderSearchOpts().VFSOverlayFiles;
+    for (std::vector<std::string>::const_iterator I = Files.begin(),
+                                                  E = Files.end();
+         I != E; ++I) {
+      OwningPtr<llvm::MemoryBuffer> Buffer;
+      if (llvm::errc::success != llvm::MemoryBuffer::getFile(*I, Buffer)) {
+        CI.getDiagnostics().Report(diag::err_missing_vfs_overlay_file) << *I;
+        goto failure;
+      }
+
+      IntrusiveRefCntPtr<vfs::FileSystem> FS =
+          vfs::getVFSFromYAML(Buffer.take(), /*DiagHandler*/0);
+      if (!FS.getPtr()) {
+        CI.getDiagnostics().Report(diag::err_invalid_vfs_overlay) << *I;
+        goto failure;
+      }
+      Overlay->pushOverlay(FS);
+    }
+    CI.setVirtualFileSystem(Overlay);
+  }
+
   // Set up the file and source managers, if needed.
   if (!CI.hasFileManager())
     CI.createFileManager();