Index: llvm/trunk/include/llvm/Support/FileOutputBuffer.h
===================================================================
--- llvm/trunk/include/llvm/Support/FileOutputBuffer.h
+++ llvm/trunk/include/llvm/Support/FileOutputBuffer.h
@@ -33,11 +33,6 @@
   enum {
     /// set the 'x' bit on the resulting file
     F_executable = 1,
-
-    /// the contents of the new file are initialized from the file that exists
-    /// at the location (if present).  This allows in-place modification of an
-    /// existing file.
-    F_modify = 2
   };
 
   /// Factory method to create an OutputBuffer object which manages a read/write
Index: llvm/trunk/lib/Support/FileOutputBuffer.cpp
===================================================================
--- llvm/trunk/lib/Support/FileOutputBuffer.cpp
+++ llvm/trunk/lib/Support/FileOutputBuffer.cpp
@@ -116,30 +116,24 @@
 }
 
 static Expected<std::unique_ptr<OnDiskBuffer>>
-createOnDiskBuffer(StringRef Path, size_t Size, bool InitExisting,
-                   unsigned Mode) {
+createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
   Expected<fs::TempFile> FileOrErr =
       fs::TempFile::create(Path + ".tmp%%%%%%%", Mode);
   if (!FileOrErr)
     return FileOrErr.takeError();
   fs::TempFile File = std::move(*FileOrErr);
 
-  if (InitExisting) {
-    if (auto EC = sys::fs::copy_file(Path, File.FD))
-      return errorCodeToError(EC);
-  } else {
 #ifndef _WIN32
-    // On Windows, CreateFileMapping (the mmap function on Windows)
-    // automatically extends the underlying file. We don't need to
-    // extend the file beforehand. _chsize (ftruncate on Windows) is
-    // pretty slow just like it writes specified amount of bytes,
-    // so we should avoid calling that function.
-    if (auto EC = fs::resize_file(File.FD, Size)) {
-      consumeError(File.discard());
-      return errorCodeToError(EC);
-    }
-#endif
+  // On Windows, CreateFileMapping (the mmap function on Windows)
+  // automatically extends the underlying file. We don't need to
+  // extend the file beforehand. _chsize (ftruncate on Windows) is
+  // pretty slow just like it writes specified amount of bytes,
+  // so we should avoid calling that function.
+  if (auto EC = fs::resize_file(File.FD, Size)) {
+    consumeError(File.discard());
+    return errorCodeToError(EC);
   }
+#endif
 
   // Mmap it.
   std::error_code EC;
@@ -163,15 +157,6 @@
   fs::file_status Stat;
   fs::status(Path, Stat);
 
-  if ((Flags & F_modify) && Size == size_t(-1)) {
-    if (Stat.type() == fs::file_type::regular_file)
-      Size = Stat.getSize();
-    else if (Stat.type() == fs::file_type::file_not_found)
-      return errorCodeToError(errc::no_such_file_or_directory);
-    else
-      return errorCodeToError(errc::invalid_argument);
-  }
-
   // Usually, we want to create OnDiskBuffer to create a temporary file in
   // the same directory as the destination file and atomically replaces it
   // by rename(2).
@@ -186,7 +171,7 @@
   case fs::file_type::regular_file:
   case fs::file_type::file_not_found:
   case fs::file_type::status_error:
-    return createOnDiskBuffer(Path, Size, !!(Flags & F_modify), Mode);
+    return createOnDiskBuffer(Path, Size, Mode);
   default:
     return createInMemoryBuffer(Path, Size, Mode);
   }
Index: llvm/trunk/unittests/Support/FileOutputBufferTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/FileOutputBufferTest.cpp
+++ llvm/trunk/unittests/Support/FileOutputBufferTest.cpp
@@ -122,53 +122,4 @@
   // Clean up.
   ASSERT_NO_ERROR(fs::remove(TestDirectory.str()));
 }
-
-TEST(FileOutputBuffer, TestModify) {
-  // Create unique temporary directory for these tests
-  SmallString<128> TestDirectory;
-  {
-    ASSERT_NO_ERROR(
-      fs::createUniqueDirectory("FileOutputBuffer-modify", TestDirectory));
-  }
-
-  SmallString<128> File1(TestDirectory);
-  File1.append("/file");
-  // First write some data.
-  {
-    Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
-      FileOutputBuffer::create(File1, 10);
-    ASSERT_NO_ERROR(errorToErrorCode(BufferOrErr.takeError()));
-    std::unique_ptr<FileOutputBuffer> &Buffer = *BufferOrErr;
-    memcpy(Buffer->getBufferStart(), "AAAAAAAAAA", 10);
-    ASSERT_NO_ERROR(errorToErrorCode(Buffer->commit()));
-  }
-
-  // Then re-open the file for modify and change only some bytes.
-  {
-    Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
-        FileOutputBuffer::create(File1, size_t(-1), FileOutputBuffer::F_modify);
-    ASSERT_NO_ERROR(errorToErrorCode(BufferOrErr.takeError()));
-    std::unique_ptr<FileOutputBuffer> &Buffer = *BufferOrErr;
-    ASSERT_EQ(10U, Buffer->getBufferSize());
-    uint8_t *Data = Buffer->getBufferStart();
-    Data[0] = 'X';
-    Data[9] = 'X';
-    ASSERT_NO_ERROR(errorToErrorCode(Buffer->commit()));
-  }
-
-  // Finally, re-open the file for read and verify that it has the modified
-  // contents.
-  {
-    ErrorOr<std::unique_ptr<MemoryBuffer>> BufferOrErr = MemoryBuffer::getFile(File1);
-    ASSERT_NO_ERROR(BufferOrErr.getError());
-    std::unique_ptr<MemoryBuffer> Buffer = std::move(*BufferOrErr);
-    ASSERT_EQ(10U, Buffer->getBufferSize());
-    EXPECT_EQ(StringRef("XAAAAAAAAX"), Buffer->getBuffer());
-  }
-
-  // Clean up.
-  ASSERT_NO_ERROR(fs::remove(File1));
-  ASSERT_NO_ERROR(fs::remove(TestDirectory));
-}
-
 } // anonymous namespace