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diff --git a/include/clang/Serialization/ModuleFileExtension.h b/include/clang/Serialization/ModuleFileExtension.h
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+++ b/include/clang/Serialization/ModuleFileExtension.h
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+//===-- ModuleFileExtension.h - Module File Extensions ----------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_SERIALIZATION_MODULEFILEEXTENSION_H
+#define LLVM_CLANG_SERIALIZATION_MODULEFILEEXTENSION_H
+
+#include "llvm/ADT/IntrusiveRefCntPtr.h"
+#include <memory>
+#include <string>
+
+namespace llvm {
+class BitstreamCursor;
+class BitstreamWriter;
+class hash_code;
+class raw_ostream;
+}
+
+namespace clang {
+
+class ASTReader;
+class ASTWriter;
+class Sema;
+  
+namespace serialization {
+  class ModuleFile;
+} // end namespace serialization
+
+/// Metadata for a module file extension.
+struct ModuleFileExtensionMetadata {
+  /// The name used to identify this particular extension block within
+  /// the resulting module file. It should be unique to the particular
+  /// extension, because this name will be used to match the name of
+  /// an extension block to the appropriate reader.
+  std::string BlockName;
+
+  /// The major version of the extension data.
+  unsigned MajorVersion;
+
+  /// The minor version of the extension data.
+  unsigned MinorVersion;
+
+  /// A string containing additional user information that will be
+  /// stored with the metadata.
+  std::string UserInfo;
+};
+
+class ModuleFileExtensionReader;
+class ModuleFileExtensionWriter;
+
+/// An abstract superclass that describes a custom extension to the
+/// module/precompiled header file format.
+///
+/// A module file extension can introduce additional information into
+/// compiled module files (.pcm) and precompiled headers (.pch) via a
+/// custom writer that can then be accessed via a custom reader when
+/// the module file or precompiled header is loaded.
+class ModuleFileExtension : public llvm::RefCountedBase<ModuleFileExtension> {
+public:
+  virtual ~ModuleFileExtension();
+
+  /// Retrieves the metadata for this module file extension.
+  virtual ModuleFileExtensionMetadata getExtensionMetadata() const = 0;
+
+  /// Hash information about the presence of this extension into the
+  /// module hash code.
+  ///
+  /// The module hash code is used to distinguish different variants
+  /// of a module that are incompatible. If the presence, absence, or
+  /// version of the module file extension should force the creation
+  /// of a separate set of module files, override this method to
+  /// combine that distinguishing information into the module hash
+  /// code.
+  ///
+  /// The default implementation of this function simply returns the
+  /// hash code as given, so the presence/absence of this extension
+  /// does not distinguish module files.
+  virtual llvm::hash_code hashExtension(llvm::hash_code c) const;
+
+  /// Create a new module file extension writer, which will be
+  /// responsible for writing the extension contents into a particular
+  /// module file.
+  virtual std::unique_ptr<ModuleFileExtensionWriter>
+  createExtensionWriter(ASTWriter &Writer) = 0;
+
+  /// Create a new module file extension reader, given the
+  /// metadata read from the block and the cursor into the extension
+  /// block.
+  ///
+  /// May return null to indicate that an extension block with the
+  /// given metadata cannot be read.
+  virtual std::unique_ptr<ModuleFileExtensionReader>
+  createExtensionReader(const ModuleFileExtensionMetadata &Metadata,
+                        ASTReader &Reader, serialization::ModuleFile &Mod,
+                        const llvm::BitstreamCursor &Stream) = 0;
+};
+
+/// Abstract base class that writes a module file extension block into
+/// a module file.
+class ModuleFileExtensionWriter {
+  ModuleFileExtension *Extension;
+
+protected:
+  ModuleFileExtensionWriter(ModuleFileExtension *Extension)
+    : Extension(Extension) { }
+
+public:
+  virtual ~ModuleFileExtensionWriter();
+
+  /// Retrieve the module file extension with which this writer is
+  /// associated.
+  ModuleFileExtension *getExtension() const { return Extension; }
+
+  /// Write the contents of the extension block into the given bitstream.
+  ///
+  /// Responsible for writing the contents of the extension into the
+  /// given stream. All of the contents should be written into custom
+  /// records with IDs >= FIRST_EXTENSION_RECORD_ID.
+  virtual void writeExtensionContents(Sema &SemaRef,
+                                      llvm::BitstreamWriter &Stream) = 0;
+};
+
+/// Abstract base class that reads a module file extension block from
+/// a module file.
+///
+/// Subclasses 
+class ModuleFileExtensionReader {
+  ModuleFileExtension *Extension;
+
+protected:
+  ModuleFileExtensionReader(ModuleFileExtension *Extension)
+    : Extension(Extension) { }
+
+public:
+  /// Retrieve the module file extension with which this reader is
+  /// associated.
+  ModuleFileExtension *getExtension() const { return Extension; }
+
+  virtual ~ModuleFileExtensionReader();
+};
+
+} // end namespace clang
+
+#endif // LLVM_CLANG_FRONTEND_MODULEFILEEXTENSION_H