Index: include/lld/Core/ArchiveLibraryFile.h
===================================================================
--- include/lld/Core/ArchiveLibraryFile.h
+++ include/lld/Core/ArchiveLibraryFile.h
@@ -12,6 +12,8 @@
 
 #include "lld/Core/File.h"
 
+#include <set>
+
 namespace lld {
 
 ///
@@ -36,6 +38,12 @@
   virtual error_code
   parseAllMembers(std::vector<std::unique_ptr<File>> &result) const = 0;
 
+  /// Returns a set of all defined symbols in the archive, i.e. all
+  /// resolvable symbol using this file.
+  virtual std::set<StringRef> getDefinedSymbols() const {
+    return std::set<StringRef>();
+  }
+
 protected:
   /// only subclasses of ArchiveLibraryFile can be instantiated
   ArchiveLibraryFile(StringRef path) : File(path, kindArchiveLibrary) {}
Index: include/lld/Core/InputGraph.h
===================================================================
--- include/lld/Core/InputGraph.h
+++ include/lld/Core/InputGraph.h
@@ -19,6 +19,7 @@
 #include "lld/Core/File.h"
 #include "llvm/Option/ArgList.h"
 
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/ErrorOr.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/raw_ostream.h"
@@ -67,6 +68,11 @@
   /// whether it should iterate over again or terminate or not.
   void notifyProgress();
 
+  /// Adds an observer of getNextFile(). Each time a new file is about to be
+  /// returned from getNextFile(), registered observers are called with the file
+  /// being returned.
+  void registerObserver(llvm::function_ref<void(File *)> fn);
+
   /// \brief Adds a node into the InputGraph
   void addInputElement(std::unique_ptr<InputElement>);
 
@@ -93,6 +99,7 @@
   // Index of the next element to be processed
   uint32_t _nextElementIndex;
   InputElement *_currentInputElement;
+  std::vector<llvm::function_ref<void(File *)>> _observers;
 
 private:
   ErrorOr<InputElement *> getNextInputElement();
Index: lib/Core/InputGraph.cpp
===================================================================
--- lib/Core/InputGraph.cpp
+++ lib/Core/InputGraph.cpp
@@ -21,9 +21,12 @@
   // it will succeed. If not, try to get the next file in the input graph.
   for (;;) {
     if (_currentInputElement) {
-      ErrorOr<File &> nextFile = _currentInputElement->getNextFile();
-      if (nextFile.getError() != InputGraphError::no_more_files)
-        return std::move(nextFile);
+      ErrorOr<File &> next = _currentInputElement->getNextFile();
+      if (next.getError() != InputGraphError::no_more_files) {
+        for (llvm::function_ref<void(File *)> observer : _observers)
+          observer(&next.get());
+        return std::move(next);
+      }
     }
 
     ErrorOr<InputElement *> elt = getNextInputElement();
@@ -35,6 +38,10 @@
 
 void InputGraph::notifyProgress() { _currentInputElement->notifyProgress(); }
 
+void InputGraph::registerObserver(llvm::function_ref<void(File *)> fn) {
+  _observers.push_back(fn);
+}
+
 void InputGraph::addInputElement(std::unique_ptr<InputElement> ie) {
   _inputArgs.push_back(std::move(ie));
 }
Index: lib/ReaderWriter/FileArchive.cpp
===================================================================
--- lib/ReaderWriter/FileArchive.cpp
+++ lib/ReaderWriter/FileArchive.cpp
@@ -97,6 +97,16 @@
     return _absoluteAtoms;
   }
 
+  /// Returns a set of all defined symbols in the archive.
+  std::set<StringRef> getDefinedSymbols() const override {
+    std::set<StringRef> ret;
+    for (const auto &e : _symbolMemberMap) {
+      StringRef sym = e.first;
+      ret.insert(sym);
+    }
+    return ret;
+  }
+
 protected:
   error_code
   instantiateMember(Archive::child_iterator member,
Index: unittests/DriverTests/InputGraphTest.cpp
===================================================================
--- unittests/DriverTests/InputGraphTest.cpp
+++ unittests/DriverTests/InputGraphTest.cpp
@@ -191,3 +191,20 @@
   EXPECT_EQ("file6", getNext());
   expectEnd();
 }
+
+TEST_F(InputGraphTest, Observer) {
+  std::vector<std::string> files;
+  _graph->registerObserver([&](File *file) {
+    files.push_back(file->path());
+  });
+
+  _graph->addInputElement(createFile1("file1"));
+  _graph->addInputElement(createFile1("file2"));
+  EXPECT_EQ("file1", getNext());
+  EXPECT_EQ("file2", getNext());
+  expectEnd();
+
+  EXPECT_EQ(2U, files.size());
+  EXPECT_EQ("file1", files[0]);
+  EXPECT_EQ("file2", files[1]);
+}