diff --git a/clang/tools/clang-extdef-mapping/ClangExtDefMapGen.cpp b/clang/tools/clang-extdef-mapping/ClangExtDefMapGen.cpp
--- a/clang/tools/clang-extdef-mapping/ClangExtDefMapGen.cpp
+++ b/clang/tools/clang-extdef-mapping/ClangExtDefMapGen.cpp
@@ -1,4 +1,4 @@
-//===- ClangExtDefMapGen.cpp -----------------------------------------------===//
+//===- ClangExtDefMapGen.cpp ---------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -13,10 +13,12 @@
 
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/CrossTU/CrossTranslationUnit.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/FrontendActions.h"
+#include "clang/Frontend/TextDiagnosticPrinter.h"
 #include "clang/Tooling/CommonOptionsParser.h"
 #include "clang/Tooling/Tooling.h"
 #include "llvm/Support/CommandLine.h"
@@ -29,12 +31,16 @@
 using namespace clang::cross_tu;
 using namespace clang::tooling;
 
-static cl::OptionCategory ClangExtDefMapGenCategory("clang-extdefmapgen options");
+static cl::OptionCategory
+    ClangExtDefMapGenCategory("clang-extdefmapgen options");
 
 class MapExtDefNamesConsumer : public ASTConsumer {
 public:
-  MapExtDefNamesConsumer(ASTContext &Context)
-      : Ctx(Context), SM(Context.getSourceManager()) {}
+  MapExtDefNamesConsumer(ASTContext &Context,
+                         StringRef astFilePath = StringRef())
+      : Ctx(Context), SM(Context.getSourceManager()) {
+    CurrentFileName = astFilePath.str();
+  }
 
   ~MapExtDefNamesConsumer() {
     // Flush results to standard output.
@@ -111,6 +117,46 @@
 
 static cl::extrahelp CommonHelp(CommonOptionsParser::HelpMessage);
 
+void HandleAST(StringRef astPath) {
+
+  CompilerInstance CI;
+
+  IntrusiveRefCntPtr<DiagnosticOptions> DiagOpts = new DiagnosticOptions();
+  TextDiagnosticPrinter *DiagClient =
+      new TextDiagnosticPrinter(llvm::errs(), &*DiagOpts);
+  IntrusiveRefCntPtr<DiagnosticIDs> DiagID(new DiagnosticIDs());
+  IntrusiveRefCntPtr<DiagnosticsEngine> Diags(
+      new DiagnosticsEngine(DiagID, &*DiagOpts, DiagClient));
+
+  std::unique_ptr<ASTUnit> unit = ASTUnit::LoadFromASTFile(
+      astPath.str(), CI.getPCHContainerOperations()->getRawReader(),
+      ASTUnit::LoadASTOnly, Diags, CI.getFileSystemOpts());
+
+  FileManager fm(CI.getFileSystemOpts());
+  SmallString<128> absPath(astPath);
+  fm.makeAbsolutePath(absPath);
+
+  std::unique_ptr<MapExtDefNamesConsumer> consumer =
+      std::make_unique<MapExtDefNamesConsumer>(unit->getASTContext(), absPath);
+  consumer->HandleTranslationUnit(unit->getASTContext());
+}
+
+void HandleFiles(ArrayRef<std::string> sourceFiles,
+                 CompilationDatabase &compilations) {
+  std::vector<std::string> sourceToBeParsed;
+  for (StringRef src : sourceFiles) {
+    if (src.endswith(".ast"))
+      HandleAST(src);
+    else
+      sourceToBeParsed.push_back(src.str());
+  }
+
+  if (!sourceToBeParsed.empty()) {
+    ClangTool Tool(compilations, sourceToBeParsed);
+    Tool.run(newFrontendActionFactory<MapExtDefNamesAction>().get());
+  }
+}
+
 int main(int argc, const char **argv) {
   // Print a stack trace if we signal out.
   sys::PrintStackTraceOnErrorSignal(argv[0], false);
@@ -127,8 +173,6 @@
   }
   CommonOptionsParser &OptionsParser = ExpectedParser.get();
 
-  ClangTool Tool(OptionsParser.getCompilations(),
-                 OptionsParser.getSourcePathList());
-
-  return Tool.run(newFrontendActionFactory<MapExtDefNamesAction>().get());
+  HandleFiles(OptionsParser.getSourcePathList(),
+              OptionsParser.getCompilations());
 }