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diff --git a/include/clang/Serialization/GlobalModuleIndex.h b/include/clang/Serialization/GlobalModuleIndex.h
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+//===--- 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