diff --git a/llvm/lib/Support/FileOutputBuffer.cpp b/llvm/lib/Support/FileOutputBuffer.cpp
index b822312..b98152a 100644
--- a/llvm/lib/Support/FileOutputBuffer.cpp
+++ b/llvm/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);
   }