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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 known to the various modules within a given
+// subdirectory of the module cache. It is used to improve the performance of
+// queries such as "do any modules know about this identifier?"
+//
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_CLANG_SERIALIZATION_GLOBAL_MODULE_INDEX_H
+#define LLVM_CLANG_SERIALIZATION_GLOBAL_MODULE_INDEX_H
+
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/OwningPtr.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringRef.h"
+#include <utility>
+
+namespace llvm {
+class BitstreamCursor;
+class MemoryBuffer;
+}
+
+namespace clang {
+
+class DirectoryEntry;
+class FileEntry;
+class FileManager;
+
+namespace serialization {
+  class ModuleFile;
+}
+
+using llvm::SmallVector;
+using llvm::SmallVectorImpl;
+using llvm::StringRef;
+using serialization::ModuleFile;
+
+/// \brief A global index for a set of module files, providing information about
+/// the identifiers 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 determine which
+/// module files contain any information about that identifier. This
+/// allows the client to restrict the search to only those module files known
+/// to have a information about that identifier, 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 current
+/// translation could or should load to fix a problem.
+class GlobalModuleIndex {
+  /// \brief Buffer containing the index file, which is lazily accessed so long
+  /// as the global module index is live.
+  llvm::OwningPtr<llvm::MemoryBuffer> Buffer;
+
+  /// \brief The hash table.
+  ///
+  /// This pointer actually points to a IdentifierIndexTable object,
+  /// but that type is only accessible within the implementation of
+  /// GlobalModuleIndex.
+  void *IdentifierIndex;
+
+  /// \brief Information about a given module file.
+  struct ModuleInfo {
+    ModuleInfo() : File(), Size(), ModTime() { }
+
+    /// \brief The module file, once it has been resolved.
+    ModuleFile *File;
+
+    /// \brief The module file name.
+    std::string FileName;
+
+    /// \brief Size of the module file at the time the global index was built.
+    off_t Size;
+
+    /// \brief Modification time of the module file at the time the global
+    /// index was built.
+    time_t ModTime;
+
+    /// \brief The module IDs on which this module directly depends.
+    /// FIXME: We don't really need a vector here.
+    llvm::SmallVector<unsigned, 4> Dependencies;
+  };
+
+  /// \brief A mapping from module IDs to information about each module.
+  ///
+  /// This vector may have gaps, if module files have been removed or have
+  /// been updated since the index was built. A gap is indicated by an empty
+  /// file name.
+  llvm::SmallVector<ModuleInfo, 16> Modules;
+
+  /// \brief Lazily-populated mapping from module files to their
+  /// corresponding index into the \c Modules vector.
+  llvm::DenseMap<ModuleFile *, unsigned> ModulesByFile;
+
+  /// \brief The set of modules that have not yet been resolved.
+  ///
+  /// The string is just the name of the module itself, which maps to the
+  /// module ID.
+  llvm::StringMap<unsigned> UnresolvedModules;
+
+  /// \brief The number of identifier lookups we performed.
+  unsigned NumIdentifierLookups;
+
+  /// \brief The number of identifier lookup hits, where we recognize the
+  /// identifier.
+  unsigned NumIdentifierLookupHits;
+  
+  /// \brief Internal constructor. Use \c readIndex() to read an index.
+  explicit GlobalModuleIndex(llvm::MemoryBuffer *Buffer,
+                             llvm::BitstreamCursor Cursor);
+
+  GlobalModuleIndex(const GlobalModuleIndex &) LLVM_DELETED_FUNCTION;
+  GlobalModuleIndex &operator=(const GlobalModuleIndex &) LLVM_DELETED_FUNCTION;
+
+public:
+  ~GlobalModuleIndex();
+
+  /// \brief An error code returned when trying to read an index.
+  enum ErrorCode {
+    /// \brief No error occurred.
+    EC_None,
+    /// \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 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(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<ModuleFile *> &ModuleFiles);
+
+  /// \brief Retrieve the set of module files on which the given module file
+  /// directly depends.
+  void getModuleDependencies(ModuleFile *File,
+                             SmallVectorImpl<ModuleFile *> &Dependencies);
+
+  /// \brief A set of module files in which we found a result.
+  typedef llvm::SmallPtrSet<ModuleFile *, 4> HitSet;
+  
+  /// \brief Look for all of the module files with information about the given
+  /// identifier, e.g., a global function, variable, or type with that name.
+  ///
+  /// \param Name The identifier to look for.
+  ///
+  /// \param Hits Will be populated with the set of module files that have
+  /// information about this name.
+  ///
+  /// \returns true if the identifier is known to the index, false otherwise.
+  bool lookupIdentifier(StringRef Name, HitSet &Hits);
+
+  /// \brief Note that the given module file has been loaded.
+  ///
+  /// \returns false if the global module index has information about this
+  /// module file, and true otherwise.
+  bool loadedModuleFile(ModuleFile *File);
+
+  /// \brief Print statistics to standard error.
+  void printStats();
+
+  /// \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