--- a/include/llvm/Support/MemoryBuffer.h	Sun Dec 23 19:23:36 2018 +0900
+++ b/include/llvm/Support/MemoryBuffer.h	Wed Aug 14 19:46:37 2019 +0900
@@ -1,9 +1,8 @@
 //===--- MemoryBuffer.h - Memory Buffer Interface ---------------*- C++ -*-===//
 //
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
@@ -20,6 +19,7 @@
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/CBindingWrapping.h"
 #include "llvm/Support/ErrorOr.h"
+#include "llvm/Support/FileSystem.h"
 #include <cstddef>
 #include <cstdint>
 #include <memory>
@@ -42,14 +42,14 @@
   const char *BufferStart; // Start of the buffer.
   const char *BufferEnd;   // End of the buffer.
 
-
 protected:
   MemoryBuffer() = default;
 
   void init(const char *BufStart, const char *BufEnd,
             bool RequiresNullTerminator);
 
-  static constexpr bool Writable = false;
+  static constexpr sys::fs::mapped_file_region::mapmode Mapmode =
+      sys::fs::mapped_file_region::readonly;
 
 public:
   MemoryBuffer(const MemoryBuffer &) = delete;
@@ -90,7 +90,7 @@
   /// MemoryBuffer. The slice is specified by an \p Offset and \p MapSize.
   /// Since this is in the middle of a file, the buffer is not null terminated.
   static ErrorOr<std::unique_ptr<MemoryBuffer>>
-  getOpenFileSlice(int FD, const Twine &Filename, uint64_t MapSize,
+  getOpenFileSlice(sys::fs::file_t FD, const Twine &Filename, uint64_t MapSize,
                    int64_t Offset, bool IsVolatile = false);
 
   /// Given an already-open file descriptor, read the file and return a
@@ -100,7 +100,7 @@
   /// can change outside the user's control, e.g. when libclang tries to parse
   /// while the user is editing/updating the file or if the file is on an NFS.
   static ErrorOr<std::unique_ptr<MemoryBuffer>>
-  getOpenFile(int FD, const Twine &Filename, uint64_t FileSize,
+  getOpenFile(sys::fs::file_t FD, const Twine &Filename, uint64_t FileSize,
               bool RequiresNullTerminator = true, bool IsVolatile = false);
 
   /// Open the specified memory range as a MemoryBuffer. Note that InputData
@@ -148,15 +148,16 @@
   MemoryBufferRef getMemBufferRef() const;
 };
 
-/// This class is an extension of MemoryBuffer, which allows writing to the
-/// underlying contents.  It only supports creation methods that are guaranteed
-/// to produce a writable buffer.  For example, mapping a file read-only is not
-/// supported.
+/// This class is an extension of MemoryBuffer, which allows copy-on-write
+/// access to the underlying contents.  It only supports creation methods that
+/// are guaranteed to produce a writable buffer.  For example, mapping a file
+/// read-only is not supported.
 class WritableMemoryBuffer : public MemoryBuffer {
 protected:
   WritableMemoryBuffer() = default;
 
-  static constexpr bool Writable = true;
+  static constexpr sys::fs::mapped_file_region::mapmode Mapmode =
+      sys::fs::mapped_file_region::priv;
 
 public:
   using MemoryBuffer::getBuffer;
@@ -209,13 +210,61 @@
   using MemoryBuffer::getSTDIN;
 };
 
+/// This class is an extension of MemoryBuffer, which allows write access to
+/// the underlying contents and committing those changes to the original source.
+/// It only supports creation methods that are guaranteed to produce a writable
+/// buffer.  For example, mapping a file read-only is not supported.
+class WriteThroughMemoryBuffer : public MemoryBuffer {
+protected:
+  WriteThroughMemoryBuffer() = default;
+
+  static constexpr sys::fs::mapped_file_region::mapmode Mapmode =
+      sys::fs::mapped_file_region::readwrite;
+
+public:
+  using MemoryBuffer::getBuffer;
+  using MemoryBuffer::getBufferEnd;
+  using MemoryBuffer::getBufferStart;
+
+  // const_cast is well-defined here, because the underlying buffer is
+  // guaranteed to have been initialized with a mutable buffer.
+  char *getBufferStart() {
+    return const_cast<char *>(MemoryBuffer::getBufferStart());
+  }
+  char *getBufferEnd() {
+    return const_cast<char *>(MemoryBuffer::getBufferEnd());
+  }
+  MutableArrayRef<char> getBuffer() {
+    return {getBufferStart(), getBufferEnd()};
+  }
+
+  static ErrorOr<std::unique_ptr<WriteThroughMemoryBuffer>>
+  getFile(const Twine &Filename, int64_t FileSize = -1);
+
+  /// Map a subrange of the specified file as a ReadWriteMemoryBuffer.
+  static ErrorOr<std::unique_ptr<WriteThroughMemoryBuffer>>
+  getFileSlice(const Twine &Filename, uint64_t MapSize, uint64_t Offset);
+
+private:
+  // Hide these base class factory function so one can't write
+  //   WritableMemoryBuffer::getXXX()
+  // and be surprised that he got a read-only Buffer.
+  using MemoryBuffer::getFileAsStream;
+  using MemoryBuffer::getFileOrSTDIN;
+  using MemoryBuffer::getMemBuffer;
+  using MemoryBuffer::getMemBufferCopy;
+  using MemoryBuffer::getOpenFile;
+  using MemoryBuffer::getOpenFileSlice;
+  using MemoryBuffer::getSTDIN;
+};
+
 class MemoryBufferRef {
   StringRef Buffer;
   StringRef Identifier;
 
 public:
   MemoryBufferRef() = default;
-  MemoryBufferRef(MemoryBuffer& Buffer)
+  MemoryBufferRef(const MemoryBuffer& Buffer)
       : Buffer(Buffer.getBuffer()), Identifier(Buffer.getBufferIdentifier()) {}
   MemoryBufferRef(StringRef Buffer, StringRef Identifier)
       : Buffer(Buffer), Identifier(Identifier) {}