diff --git a/include/clang/Driver/CC1Options.td b/include/clang/Driver/CC1Options.td
index d485303..77fa82a 100644
--- a/include/clang/Driver/CC1Options.td
+++ b/include/clang/Driver/CC1Options.td
@@ -289,6 +289,8 @@
   HelpText<"Use with -ast-dump or -ast-print to dump/print only AST declaration"
            " nodes having a certain substring in a qualified name. Use"
            " -ast-list to list all filterable declaration node names.">;
+def generate_module_index : Flag<["-"], "generate-module-index">,
+  HelpText<"Automatically generate or update the global module index">;
 
 let Group = Action_Group in {
 
diff --git a/include/clang/Frontend/CompilerInstance.h b/include/clang/Frontend/CompilerInstance.h
index 492a3c5..d6b3afa 100644
--- a/include/clang/Frontend/CompilerInstance.h
+++ b/include/clang/Frontend/CompilerInstance.h
@@ -112,7 +112,11 @@
   /// \brief The result of the last module import.
   ///
   ModuleLoadResult LastModuleImportResult;
-  
+
+  /// \brief Whether we should (re)build the global module index once we
+  /// have finished with this translation unit.
+  bool BuildGlobalModuleIndex;
+
   /// \brief Holds information about the output file.
   ///
   /// If TempFilename is not empty we must rename it to Filename at the end.
@@ -186,6 +190,15 @@
   /// setInvocation - Replace the current invocation.
   void setInvocation(CompilerInvocation *Value);
 
+  /// \brief Indicates whether we should (re)build the global module index.
+  bool getBuildGlobalModuleIndex() const { return BuildGlobalModuleIndex; }
+
+  /// \brief Set the flag indicating whether we should (re)build the global
+  /// module index.
+  void setBuildGlobalModuleIndex(bool Build) {
+    BuildGlobalModuleIndex = Build;
+  }
+
   /// }
   /// @name Forwarding Methods
   /// {
diff --git a/include/clang/Frontend/FrontendOptions.h b/include/clang/Frontend/FrontendOptions.h
index 00f1dc4..bb6da34 100644
--- a/include/clang/Frontend/FrontendOptions.h
+++ b/include/clang/Frontend/FrontendOptions.h
@@ -137,7 +137,9 @@
                                            /// speed up parsing in cases you do
                                            /// not need them (e.g. with code
                                            /// completion).
-
+  unsigned GenerateModuleIndex : 1;        ///< Whether to auto-generate a
+                                           ///< global module index when a new
+                                           ///< module has been built.
   CodeCompleteOptions CodeCompleteOpts;
 
   enum {
@@ -209,8 +211,9 @@
     ShowStats(false), ShowTimers(false), ShowVersion(false),
     FixWhatYouCan(false), FixOnlyWarnings(false), FixAndRecompile(false),
     FixToTemporaries(false), ARCMTMigrateEmitARCErrors(false),
-    SkipFunctionBodies(false), ARCMTAction(ARCMT_None),
-    ObjCMTAction(ObjCMT_None), ProgramAction(frontend::ParseSyntaxOnly)
+    SkipFunctionBodies(false), GenerateModuleIndex(false),
+    ARCMTAction(ARCMT_None), ObjCMTAction(ObjCMT_None),
+    ProgramAction(frontend::ParseSyntaxOnly)
   {}
 
   /// getInputKindForExtension - Return the appropriate input kind for a file
diff --git a/include/clang/Serialization/GlobalModuleIndex.h b/include/clang/Serialization/GlobalModuleIndex.h
new file mode 100644
index 0000000..cac6c3a
--- /dev/null
+++ b/include/clang/Serialization/GlobalModuleIndex.h
@@ -0,0 +1,119 @@
+//===--- GlobalModuleIndex.h - Global Module Index --------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the GlobalModuleIndex class, which manages a global index
+// containing all of the identifiers with namespace-scope bindings attached to
+// them as well as all of the selectors that name methods, across all of the
+// modules within a given subdirectory of the module cache. It is used to
+// improve the performance of queries such as "does this identifier have any
+// top-level bindings in any module?"
+//
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_CLANG_SERIALIZATION_GLOBAL_MODULE_INDEX_H
+#define LLVM_CLANG_SERIALIZATION_GLOBAL_MODULE_INDEX_H
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+#include <utility>
+
+namespace clang {
+
+class DeclarationName;
+class DirectoryEntry;
+class FileEntry;
+class FileManager;
+
+using llvm::SmallVector;
+using llvm::SmallVectorImpl;
+using llvm::StringRef;
+
+/// \brief A global index for a set of module files, providing information about
+/// the top-level identifiers and selectors within those module files.
+///
+/// The global index is an aid for name lookup into modules, offering a central
+/// place where one can look for identifiers or selectors to determine which
+/// module files contain a namespace-scope identity with that identifier or
+/// a method with that selector, respectively. This allows the client to
+/// restrict the search to only those module files known to have a binding for
+/// that identifier or selector, improving performance. Moreover, the global
+/// module index may know about module files that have not been imported, and
+/// can be queried to determine which modules the currently translation could
+/// or should load to fix a problem.
+class GlobalModuleIndex {
+  /// \brief Internal constructor. Use \c readIndex() to read an index.
+  explicit GlobalModuleIndex(FileManager &FileMgr);
+
+public:
+  /// \brief An error code returned when trying to read an index.
+  enum ErrorCode {
+    /// \brief No error occurred.
+    EC_None,
+    /// \brief The index found was out-of-date, meaning that some of the
+    /// module files are newer than the index.
+    ///
+    /// This error code is not actually fatal, because if the index is
+    /// up-to-date for any module files, it is 
+    EC_OutOfDate,
+    /// \brief No index was found.
+    EC_NotFound,
+    /// \brief Some other process is currently building the index; it is not
+    /// available yet.
+    EC_Building,
+    /// \brief There was an unspecified I/O error reading or writing the index.
+    EC_IOError
+  };
+
+  /// \brief Read a global index file for the given directory.
+  ///
+  /// \param FileMgr The file manager to use for reading files.
+  ///
+  /// \param Path The path to the specific module cache where the module files
+  /// for the intended configuration reside.
+  ///
+  /// \returns A pair containing the global module index (if it exists) and
+  /// the error code.
+  static std::pair<GlobalModuleIndex *, ErrorCode>
+  readIndex(FileManager &FileMgr, StringRef Path);
+
+  /// \brief Retrieve the set of modules that have up-to-date indexes.
+  ///
+  /// \param ModuleFiles Will be populated with the set of module files that
+  /// have been indexed.
+  void getKnownModules(SmallVectorImpl<const FileEntry *> &ModuleFiles);
+
+  /// \brief Retrieve the set of module files on which the given module file
+  /// directly depends.
+  void getModuleDependencies(const FileEntry *ModuleFile,
+                             SmallVectorImpl<const FileEntry *> &Dependencies);
+
+  /// \brief Look for all of the module files with a namespace-scope binding
+  /// for the given name, e.g., a global function, variable, or type with that
+  /// name, or declare a method with the selector.
+  ///
+  /// \param Name The name or selector to look for.
+  ///
+  /// \param DeclaringModuleFiles Will be populated with the list of module
+  /// files that declare entities with the given name.
+  ///
+  /// \returns true if any module files were found, false otherwise.
+  bool lookupName(DeclarationName Name,
+                  SmallVectorImpl<const FileEntry *> &DeclaringModuleFiles);
+
+  /// \brief Write a global index into the given
+  ///
+  /// \param FileMgr The file manager to use to load module files.
+  ///
+  /// \param Path The path to the directory containing module files, into
+  /// which the global index will be written.
+  static ErrorCode writeIndex(FileManager &FileMgr, StringRef Path);
+};
+
+}
+
+#endif
diff --git a/lib/Frontend/CompilerInstance.cpp b/lib/Frontend/CompilerInstance.cpp
index 9941eae..07c0248 100644
--- a/lib/Frontend/CompilerInstance.cpp
+++ b/lib/Frontend/CompilerInstance.cpp
@@ -48,7 +48,8 @@
 using namespace clang;
 
 CompilerInstance::CompilerInstance()
-  : Invocation(new CompilerInvocation()), ModuleManager(0) {
+  : Invocation(new CompilerInvocation()), ModuleManager(0),
+    BuildGlobalModuleIndex(false) {
 }
 
 CompilerInstance::~CompilerInstance() {
@@ -785,6 +786,7 @@
   FrontendOptions &FrontendOpts = Invocation->getFrontendOpts();
   FrontendOpts.OutputFile = ModuleFileName.str();
   FrontendOpts.DisableFree = false;
+  FrontendOpts.GenerateModuleIndex = false;
   FrontendOpts.Inputs.clear();
   InputKind IK = getSourceInputKindFromOptions(*Invocation->getLangOpts());
 
@@ -858,6 +860,12 @@
   Instance.clearOutputFiles(/*EraseFiles=*/true);
   if (!TempModuleMapFileName.empty())
     llvm::sys::Path(TempModuleMapFileName).eraseFromDisk();
+
+  // We've rebuilt a module. If we're allowed to generate or update the global
+  // module index, record that fact in the importing compiler instance.
+  if (ImportingInstance.getFrontendOpts().GenerateModuleIndex) {
+    ImportingInstance.setBuildGlobalModuleIndex(true);
+  }
 }
 
 ModuleLoadResult
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index eab9cc3..011e3e1 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -705,7 +705,8 @@
   Opts.FixAndRecompile = Args.hasArg(OPT_fixit_recompile);
   Opts.FixToTemporaries = Args.hasArg(OPT_fixit_to_temp);
   Opts.ASTDumpFilter = Args.getLastArgValue(OPT_ast_dump_filter);
-
+  Opts.GenerateModuleIndex = Args.hasArg(OPT_generate_module_index);
+  
   Opts.CodeCompleteOpts.IncludeMacros
     = Args.hasArg(OPT_code_completion_macros);
   Opts.CodeCompleteOpts.IncludeCodePatterns
diff --git a/lib/Frontend/FrontendAction.cpp b/lib/Frontend/FrontendAction.cpp
index 742caf1..4b642b3 100644
--- a/lib/Frontend/FrontendAction.cpp
+++ b/lib/Frontend/FrontendAction.cpp
@@ -23,6 +23,7 @@
 #include "clang/Parse/ParseAST.h"
 #include "clang/Serialization/ASTDeserializationListener.h"
 #include "clang/Serialization/ASTReader.h"
+#include "clang/Serialization/GlobalModuleIndex.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
@@ -380,6 +381,16 @@
   }
   else ExecuteAction();
 
+  // If we are supposed to rebuild the global module index, do so now unless
+  // an error occurred.
+  if (CI.getBuildGlobalModuleIndex() && CI.hasFileManager() &&
+      CI.hasPreprocessor() &&
+      (!CI.hasDiagnostics() || !CI.getDiagnostics().hasErrorOccurred())) {
+    GlobalModuleIndex::writeIndex(
+      CI.getFileManager(),
+      CI.getPreprocessor().getHeaderSearchInfo().getModuleCachePath());
+  }
+
   return true;
 }
 
diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index 83d894c..917b51e 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -5411,8 +5411,9 @@
         Info->second.NameLookupTableData;
       bool FoundAnything = false;
       for (ASTDeclContextNameLookupTable::data_iterator
-	     I = LookupTable->data_begin(), E = LookupTable->data_end();
-	   I != E; ++I) {
+             I = LookupTable->data_begin(), E = LookupTable->data_end();
+           I != E;
+           ++I) {
         ASTDeclContextNameLookupTrait::data_type Data = *I;
         for (; Data.first != Data.second; ++Data.first) {
           NamedDecl *ND = This->Reader.GetLocalDeclAs<NamedDecl>(M,
diff --git a/lib/Serialization/CMakeLists.txt b/lib/Serialization/CMakeLists.txt
index 1ed2474..3c68b64 100644
--- a/lib/Serialization/CMakeLists.txt
+++ b/lib/Serialization/CMakeLists.txt
@@ -11,6 +11,7 @@
   ASTWriterDecl.cpp
   ASTWriterStmt.cpp
   GeneratePCH.cpp
+  GlobalModuleIndex.cpp
   Module.cpp
   ModuleManager.cpp
   )
diff --git a/lib/Serialization/GlobalModuleIndex.cpp b/lib/Serialization/GlobalModuleIndex.cpp
new file mode 100644
index 0000000..630646f
--- /dev/null
+++ b/lib/Serialization/GlobalModuleIndex.cpp
@@ -0,0 +1,535 @@
+//===--- GlobalModuleIndex.cpp - Global Module Index ------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the GlobalModuleIndex class.
+//
+//===----------------------------------------------------------------------===//
+
+#include "ASTReaderInternals.h"
+#include "clang/Basic/FileManager.h"
+#include "clang/Basic/OnDiskHashTable.h"
+#include "clang/Serialization/ASTBitCodes.h"
+#include "clang/Serialization/GlobalModuleIndex.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/MapVector.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/Bitcode/BitstreamReader.h"
+#include "llvm/Bitcode/BitstreamWriter.h"
+#include "llvm/Support/Filesystem.h"
+#include "llvm/Support/LockFileManager.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/PathV2.h"
+using namespace clang;
+using namespace serialization;
+
+//----------------------------------------------------------------------------//
+// Shared constants
+//----------------------------------------------------------------------------//
+namespace {
+  enum {
+    /// \brief The block containing the index.
+    GLOBAL_INDEX_BLOCK_ID = llvm::bitc::FIRST_APPLICATION_BLOCKID
+  };
+
+  /// \brief Describes the record types in the index.
+  enum IndexRecordTypes {
+    /// \brief Contains version information and potentially other metadata,
+    /// used to determine if we can read this global index file.
+    METADATA,
+    /// \brief Describes a module, including its file name and dependencies.
+    MODULE,
+    /// \brief The index for identifiers.
+    IDENTIFIER_INDEX
+  };
+}
+
+/// \brief The name of the global index file.
+static const char * const IndexFileName = "modules.idx";
+
+/// \brief The global index file version.
+static const unsigned CurrentVersion = 1;
+
+//----------------------------------------------------------------------------//
+// Global module index writer.
+//----------------------------------------------------------------------------//
+
+namespace {
+  /// \brief Provides information about a specific module file.
+  struct ModuleFileInfo {
+    /// \brief The numberic ID for this module file.
+    unsigned ID;
+
+    /// \brief The set of modules on which this module depends. Each entry is
+    /// a module ID.
+    SmallVector<unsigned, 4> Dependencies;
+  };
+
+  /// \brief Builder that generates the global module index file.
+  class GlobalModuleIndexBuilder {
+    FileManager &FileMgr;
+
+    /// \brief Mapping from files to module file information.
+    typedef llvm::MapVector<const FileEntry *, ModuleFileInfo> ModuleFilesMap;
+
+    /// \brief Information about each of the known module files.
+    ModuleFilesMap ModuleFiles;
+
+    /// \brief Mapping from identifiers to the list of module file IDs that
+    /// consider this identifier to be interesting.
+    typedef llvm::StringMap<SmallVector<unsigned, 2> > InterestingIdentifierMap;
+
+    /// \brief A mapping from all interesting identifiers to the set of module
+    /// files in which those identifiers are considered interesting.
+    InterestingIdentifierMap InterestingIdentifiers;
+    
+    /// \brief Write the block-info block for the global module index file.
+    void emitBlockInfoBlock(llvm::BitstreamWriter &Stream);
+
+    /// \brief Retrieve the module file information for the given file.
+    ModuleFileInfo &getModuleFileInfo(const FileEntry *File) {
+      llvm::MapVector<const FileEntry *, ModuleFileInfo>::iterator Known
+        = ModuleFiles.find(File);
+      if (Known != ModuleFiles.end())
+        return Known->second;
+
+      unsigned NewID = ModuleFiles.size();
+      ModuleFileInfo &Info = ModuleFiles[File];
+      Info.ID = NewID;
+      return Info;
+    }
+
+  public:
+    explicit GlobalModuleIndexBuilder(FileManager &FileMgr) : FileMgr(FileMgr){}
+
+    /// \brief Load the contents of the given module file into the builder.
+    ///
+    /// \returns true if an error occurred, false otherwise.
+    bool loadModuleFile(const FileEntry *File);
+
+    /// \brief Write the index to the given bitstream.
+    void writeIndex(llvm::BitstreamWriter &Stream);
+  };
+}
+
+static void emitBlockID(unsigned ID, const char *Name,
+                        llvm::BitstreamWriter &Stream,
+                        SmallVectorImpl<uint64_t> &Record) {
+  Record.clear();
+  Record.push_back(ID);
+  Stream.EmitRecord(llvm::bitc::BLOCKINFO_CODE_SETBID, Record);
+
+  // Emit the block name if present.
+  if (Name == 0 || Name[0] == 0) return;
+  Record.clear();
+  while (*Name)
+    Record.push_back(*Name++);
+  Stream.EmitRecord(llvm::bitc::BLOCKINFO_CODE_BLOCKNAME, Record);
+}
+
+static void emitRecordID(unsigned ID, const char *Name,
+                         llvm::BitstreamWriter &Stream,
+                         SmallVectorImpl<uint64_t> &Record) {
+  Record.clear();
+  Record.push_back(ID);
+  while (*Name)
+    Record.push_back(*Name++);
+  Stream.EmitRecord(llvm::bitc::BLOCKINFO_CODE_SETRECORDNAME, Record);
+}
+
+void
+GlobalModuleIndexBuilder::emitBlockInfoBlock(llvm::BitstreamWriter &Stream) {
+  SmallVector<uint64_t, 64> Record;
+  Stream.EnterSubblock(llvm::bitc::BLOCKINFO_BLOCK_ID, 3);
+
+#define BLOCK(X) emitBlockID(X ## _ID, #X, Stream, Record)
+#define RECORD(X) emitRecordID(X, #X, Stream, Record)
+  BLOCK(GLOBAL_INDEX_BLOCK);
+  RECORD(METADATA);
+  RECORD(MODULE);
+  RECORD(IDENTIFIER_INDEX);
+#undef RECORD
+#undef BLOCK
+
+  Stream.ExitBlock();
+}
+
+namespace clang {
+  class InterestingASTIdentifierLookupTrait
+    : public serialization::reader::ASTIdentifierLookupTraitBase {
+
+  public:
+    /// \brief The identifier and whether it is "interesting".
+    typedef std::pair<StringRef, bool> data_type;
+
+    data_type ReadData(const internal_key_type& k,
+                       const unsigned char* d,
+                       unsigned DataLen) {
+      // The first bit indicates whether this identifier is interesting.
+      // That's all we care about.
+      using namespace clang::io;
+      unsigned RawID = ReadUnalignedLE32(d);
+      bool IsInteresting = RawID & 0x01;
+      return std::make_pair(k, IsInteresting);
+    }
+  };
+}
+
+bool GlobalModuleIndexBuilder::loadModuleFile(const FileEntry *File) {
+  // Open the module file.
+  OwningPtr<llvm::MemoryBuffer> Buffer;
+  Buffer.reset(FileMgr.getBufferForFile(File));
+  if (!Buffer) {
+    return true;
+  }
+
+  // Initialize the input stream
+  llvm::BitstreamReader InStreamFile;
+  llvm::BitstreamCursor InStream;
+  InStreamFile.init((const unsigned char *)Buffer->getBufferStart(),
+                  (const unsigned char *)Buffer->getBufferEnd());
+  InStream.init(InStreamFile);
+
+  // Sniff for the signature.
+  if (InStream.Read(8) != 'C' ||
+      InStream.Read(8) != 'P' ||
+      InStream.Read(8) != 'C' ||
+      InStream.Read(8) != 'H') {
+    return true;
+  }
+
+  // Record this module file and assign it a unique ID (if it doesn't have
+  // one already).
+  unsigned ID = getModuleFileInfo(File).ID;
+
+  // Search for the blocks and records we care about.
+  enum { Outer, ControlBlock, ASTBlock } State = Outer;
+  bool Done = false;
+  while (!Done) {
+    const unsigned Flags = llvm::BitstreamCursor::AF_DontPopBlockAtEnd;
+    llvm::BitstreamEntry Entry = InStream.advance(Flags);
+    switch (Entry.Kind) {
+    case llvm::BitstreamEntry::Error:
+      return true;
+
+    case llvm::BitstreamEntry::Record:
+      // In the outer state, just skip the record. We don't care.
+      if (State == Outer) {
+        InStream.skipRecord(Entry.ID);
+        continue;
+      }
+
+      // Handle potentially-interesting records below.
+      break;
+
+    case llvm::BitstreamEntry::SubBlock:
+      if (State == Outer && Entry.ID == CONTROL_BLOCK_ID) {
+        if (InStream.EnterSubBlock(CONTROL_BLOCK_ID))
+          return true;
+
+        // Found the control block.
+        State = ControlBlock;
+        continue;
+      }
+
+      if (State == Outer && Entry.ID == AST_BLOCK_ID) {
+        if (InStream.EnterSubBlock(AST_BLOCK_ID))
+          return true;
+
+        // Found the AST block.
+        State = ASTBlock;
+        continue;
+
+      }
+
+      if (InStream.SkipBlock())
+        return true;
+
+      continue;
+
+    case llvm::BitstreamEntry::EndBlock:
+      if (State == Outer) {
+        Done = true;
+      }
+      State = Outer;
+      continue;
+    }
+
+    // Read the given record.
+    SmallVector<uint64_t, 64> Record;
+    StringRef Blob;
+    unsigned Code = InStream.readRecord(Entry.ID, Record, &Blob);
+
+    // Handle module dependencies.
+    if (State == ControlBlock && Code == IMPORTS) {
+      // Load each of the imported PCH files.
+      unsigned Idx = 0, N = Record.size();
+      while (Idx < N) {
+        // Read information about the AST file.
+
+        // Skip the imported kind
+        ++Idx;
+
+        // Skip the import location
+        ++Idx;
+
+        // Retrieve the imported file name.
+        unsigned Length = Record[Idx++];
+        SmallString<128> ImportedFile(Record.begin() + Idx,
+                                      Record.begin() + Idx + Length);
+        Idx += Length;
+
+        // Find the imported module file.
+        const FileEntry *DependsOnFile = FileMgr.getFile(ImportedFile);
+        if (!DependsOnFile)
+          return true;
+
+        // Record the dependency.
+        unsigned DependsOnID = getModuleFileInfo(DependsOnFile).ID;
+        getModuleFileInfo(File).Dependencies.push_back(DependsOnID);
+      }
+
+      continue;
+    }
+
+    // Handle the identifier table
+    if (State == ASTBlock && Code == IDENTIFIER_TABLE && Record[0] > 0) {
+      typedef OnDiskChainedHashTable<InterestingASTIdentifierLookupTrait>
+        InterestingIdentifierTable;
+      llvm::OwningPtr<InterestingIdentifierTable>
+        Table(InterestingIdentifierTable::Create(
+                (const unsigned char *)Blob.data() + Record[0],
+                (const unsigned char *)Blob.data()));
+      for (InterestingIdentifierTable::data_iterator D = Table->data_begin(),
+                                                     DEnd = Table->data_end();
+           D != DEnd; ++D) {
+        std::pair<StringRef, bool> Ident = *D;
+        if (Ident.second)
+          InterestingIdentifiers[Ident.first].push_back(ID);
+      }
+    }
+
+    // FIXME: Handle the selector table.
+    
+    // We don't care about this record.
+  }
+
+  return false;
+}
+
+namespace {
+
+/// \brief Trait used to generate the identifier index as an on-disk hash
+/// table.
+class IdentifierIndexWriterTrait {
+public:
+  typedef StringRef key_type;
+  typedef StringRef key_type_ref;
+  typedef SmallVector<unsigned, 2> data_type;
+  typedef const SmallVector<unsigned, 2> &data_type_ref;
+
+  static unsigned ComputeHash(key_type_ref Key) {
+    return llvm::HashString(Key);
+  }
+
+  std::pair<unsigned,unsigned>
+  EmitKeyDataLength(raw_ostream& Out, key_type_ref Key, data_type_ref Data) {
+    unsigned KeyLen = Key.size();
+    unsigned DataLen = Data.size() * 4;
+    clang::io::Emit16(Out, KeyLen);
+    clang::io::Emit16(Out, DataLen);
+    return std::make_pair(KeyLen, DataLen);
+  }
+  
+  void EmitKey(raw_ostream& Out, key_type_ref Key, unsigned KeyLen) {
+    Out.write(Key.data(), KeyLen);
+  }
+
+  void EmitData(raw_ostream& Out, key_type_ref Key, data_type_ref Data,
+                unsigned DataLen) {
+    for (unsigned I = 0, N = Data.size(); I != N; ++I)
+      clang::io::Emit32(Out, Data[I]);
+  }
+};
+
+}
+
+void GlobalModuleIndexBuilder::writeIndex(llvm::BitstreamWriter &Stream) {
+  using namespace llvm;
+  
+  // Emit the file header.
+  Stream.Emit((unsigned)'B', 8);
+  Stream.Emit((unsigned)'C', 8);
+  Stream.Emit((unsigned)'G', 8);
+  Stream.Emit((unsigned)'I', 8);
+
+  // Write the block-info block, which describes the records in this bitcode
+  // file.
+  emitBlockInfoBlock(Stream);
+
+  Stream.EnterSubblock(GLOBAL_INDEX_BLOCK_ID, 3);
+
+  // Write the metadata.
+  SmallVector<uint64_t, 2> Record;
+  Record.push_back(CurrentVersion);
+  Stream.EmitRecord(METADATA, Record);
+
+  // Write the set of known module files.
+  for (ModuleFilesMap::iterator M = ModuleFiles.begin(),
+                                MEnd = ModuleFiles.end();
+       M != MEnd; ++M) {
+    Record.clear();
+    Record.push_back(M->second.ID);
+    Record.push_back(M->first->getSize());
+    Record.push_back(M->first->getModificationTime());
+
+    // File name
+    StringRef Name(M->first->getName());
+    Record.push_back(Name.size());
+    Record.append(Name.begin(), Name.end());
+
+    // Dependencies
+    Record.push_back(M->second.Dependencies.size());
+    Record.append(M->second.Dependencies.begin(), M->second.Dependencies.end());
+    Stream.EmitRecord(MODULE, Record);
+  }
+
+  // Write the identifier -> module file mapping.
+  {
+    OnDiskChainedHashTableGenerator<IdentifierIndexWriterTrait> Generator;
+    IdentifierIndexWriterTrait Trait;
+
+    // Populate the hash table.
+    for (InterestingIdentifierMap::iterator I = InterestingIdentifiers.begin(),
+                                            IEnd = InterestingIdentifiers.end();
+         I != IEnd; ++I) {
+      Generator.insert(I->first(), I->second, Trait);
+    }
+    
+    // Create the on-disk hash table in a buffer.
+    SmallString<4096> IdentifierTable;
+    uint32_t BucketOffset;
+    {
+      llvm::raw_svector_ostream Out(IdentifierTable);
+      // Make sure that no bucket is at offset 0
+      clang::io::Emit32(Out, 0);
+      BucketOffset = Generator.Emit(Out, Trait);
+    }
+
+    // Create a blob abbreviation
+    BitCodeAbbrev *Abbrev = new BitCodeAbbrev();
+    Abbrev->Add(BitCodeAbbrevOp(IDENTIFIER_INDEX));
+    Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32));
+    Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));
+    unsigned IDTableAbbrev = Stream.EmitAbbrev(Abbrev);
+
+    // Write the identifier table
+    Record.clear();
+    Record.push_back(IDENTIFIER_INDEX);
+    Record.push_back(BucketOffset);
+    Stream.EmitRecordWithBlob(IDTableAbbrev, Record, IdentifierTable.str());
+  }
+
+  // FIXME: Selectors.
+
+  Stream.ExitBlock();
+}
+
+GlobalModuleIndex::ErrorCode
+GlobalModuleIndex::writeIndex(FileManager &FileMgr, StringRef Path) {
+  llvm::SmallString<128> IndexPath;
+  IndexPath += Path;
+  llvm::sys::path::append(IndexPath, IndexFileName);
+
+  // Coordinate building the global index file with other processes that might
+  // try to do the same.
+  llvm::LockFileManager Locked(IndexPath);
+  switch (Locked) {
+  case llvm::LockFileManager::LFS_Error:
+    return EC_IOError;
+
+  case llvm::LockFileManager::LFS_Owned:
+    // We're responsible for building the index ourselves. Do so below.
+    break;
+
+  case llvm::LockFileManager::LFS_Shared:
+    // Someone else is responsible for building the index. We don't care
+    // when they finish, so we're done.
+    return EC_Building;
+  }
+
+  // The module index builder.
+  GlobalModuleIndexBuilder Builder(FileMgr);
+  
+  // Load each of the module files.
+  llvm::error_code EC;
+  for (llvm::sys::fs::directory_iterator D(Path, EC), DEnd;
+       D != DEnd && !EC;
+       D.increment(EC)) {
+    // If this isn't a module file, we don't care.
+    if (llvm::sys::path::extension(D->path()) != ".pcm") {
+      // ... unless it's a .pcm.lock file, which indicates that someone is
+      // in the process of rebuilding a module. They'll rebuild the index
+      // at the end of that translation unit, so we don't have to.
+      if (llvm::sys::path::extension(D->path()) == ".pcm.lock")
+        return EC_Building;
+
+      continue;
+    }
+
+    // If we can't find the module file, skip it.
+    const FileEntry *ModuleFile = FileMgr.getFile(D->path());
+    if (!ModuleFile)
+      continue;
+
+    // Load this module file.
+    if (Builder.loadModuleFile(ModuleFile))
+      return EC_IOError;
+  }
+
+  // The output buffer, into which the global index will be written.
+  SmallVector<char, 16> OutputBuffer;
+  {
+    llvm::BitstreamWriter OutputStream(OutputBuffer);
+    Builder.writeIndex(OutputStream);
+  }
+
+  // Write the global index file to a temporary file.
+  llvm::SmallString<128> IndexTmpPath;
+  int TmpFD;
+  if (llvm::sys::fs::unique_file(IndexPath + "-%%%%%%%%", TmpFD, IndexTmpPath))
+    return EC_IOError;
+
+  // Open the temporary global index file for output.
+  std::string ErrorInfo;
+  llvm::raw_fd_ostream Out(IndexTmpPath.c_str(), ErrorInfo,
+                           llvm::raw_fd_ostream::F_Binary);
+  if (Out.has_error())
+    return EC_IOError;
+
+  // Write the index.
+  Out.write(OutputBuffer.data(), OutputBuffer.size());
+  Out.close();
+  if (Out.has_error())
+    return EC_IOError;
+
+  // Remove the old index file. It isn't relevant any more.
+  bool OldIndexExisted;
+  llvm::sys::fs::remove(IndexPath.str(), OldIndexExisted);
+
+  // Rename the newly-written index file to the proper name.
+  if (llvm::sys::fs::rename(IndexTmpPath.str(), IndexPath.str())) {
+    // Rename failed; just remove the 
+    llvm::sys::fs::remove(IndexTmpPath.str(), OldIndexExisted);
+    return EC_IOError;
+  }
+
+  // We're done.
+  return EC_None;
+}
diff --git a/test/Modules/global_index.m b/test/Modules/global_index.m
new file mode 100644
index 0000000..b170aa4
--- /dev/null
+++ b/test/Modules/global_index.m
@@ -0,0 +1,8 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -Wauto-import -fmodule-cache-path %t -fdisable-module-hash -fmodules -generate-module-index -F %S/Inputs %s -verify
+// RUN: ls %t|grep modules.idx
+// REQUIRES: shell
+
+// expected-no-diagnostics
+@import DependsOnModule;
+