diff --git a/clang/lib/Frontend/FrontendActions.cpp b/clang/lib/Frontend/FrontendActions.cpp
index 40008bf..d0d8307 100644
--- a/clang/lib/Frontend/FrontendActions.cpp
+++ b/clang/lib/Frontend/FrontendActions.cpp
@@ -242,6 +242,76 @@
   return CI.createDefaultOutputFile(/*Binary=*/true, InFile, "pcm");
 }
 
+bool GenerateHeaderModuleAction::PrepareToExecuteAction(
+    CompilerInstance &CI) {
+  if (!CI.getLangOpts().Modules && !CI.getLangOpts().ModulesTS) {
+    CI.getDiagnostics().Report(diag::err_header_module_requires_modules);
+    return false;
+  }
+
+  auto &Inputs = CI.getFrontendOpts().Inputs;
+  if (Inputs.empty())
+    return GenerateModuleAction::BeginInvocation(CI);
+
+  auto Kind = Inputs[0].getKind();
+
+  // Convert the header file inputs into a single module input buffer.
+  SmallString<256> HeaderContents;
+  ModuleHeaders.reserve(Inputs.size());
+  for (const FrontendInputFile &FIF : Inputs) {
+    // FIXME: We should support re-compiling from an AST file.
+    if (FIF.getKind().getFormat() != InputKind::Source || !FIF.isFile()) {
+      CI.getDiagnostics().Report(diag::err_module_header_file_not_found)
+          << (FIF.isFile() ? FIF.getFile()
+                           : FIF.getBuffer()->getBufferIdentifier());
+      return true;
+    }
+
+    HeaderContents += "#include \"";
+    HeaderContents += FIF.getFile();
+    HeaderContents += "\"\n";
+    ModuleHeaders.push_back(FIF.getFile());
+  }
+  Buffer = llvm::MemoryBuffer::getMemBufferCopy(
+      HeaderContents, Module::getModuleInputBufferName());
+
+  // Set that buffer up as our "real" input.
+  Inputs.clear();
+  Inputs.push_back(FrontendInputFile(Buffer.get(), Kind, /*IsSystem*/false));
+
+  return GenerateModuleAction::PrepareToExecuteAction(CI);
+}
+
+bool GenerateHeaderModuleAction::BeginSourceFileAction(
+    CompilerInstance &CI) {
+  CI.getLangOpts().setCompilingModule(LangOptions::CMK_HeaderModule);
+
+  // Synthesize a Module object for the given headers.
+  auto &HS = CI.getPreprocessor().getHeaderSearchInfo();
+  SmallVector<Module::Header, 16> Headers;
+  for (StringRef Name : ModuleHeaders) {
+    const DirectoryLookup *CurDir = nullptr;
+    const FileEntry *FE = HS.LookupFile(
+        Name, SourceLocation(), /*Angled*/ false, nullptr, CurDir,
+        None, nullptr, nullptr, nullptr, nullptr, nullptr);
+    if (!FE) {
+      CI.getDiagnostics().Report(diag::err_module_header_file_not_found)
+        << Name;
+      continue;
+    }
+    Headers.push_back({Name, FE});
+  }
+  HS.getModuleMap().createHeaderModule(CI.getLangOpts().CurrentModule, Headers);
+
+  return GenerateModuleAction::BeginSourceFileAction(CI);
+}
+
+std::unique_ptr<raw_pwrite_stream>
+GenerateHeaderModuleAction::CreateOutputFile(CompilerInstance &CI,
+                                             StringRef InFile) {
+  return CI.createDefaultOutputFile(/*Binary=*/true, InFile, "pcm");
+}
+
 SyntaxOnlyAction::~SyntaxOnlyAction() {
 }