diff --git a/llvm/lib/Support/FileOutputBuffer.cpp b/llvm/lib/Support/FileOutputBuffer.cpp
index de55538..1214b5a 100644
--- a/llvm/lib/Support/FileOutputBuffer.cpp
+++ b/llvm/lib/Support/FileOutputBuffer.cpp
@@ -110,24 +110,30 @@
 }
 
 static Expected<std::unique_ptr<OnDiskBuffer>>
-createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
+createOnDiskBuffer(StringRef Path, size_t Size, bool InitExisting,
+                   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);
-  }
+    // 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;
@@ -151,6 +157,15 @@
   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).
@@ -165,7 +180,7 @@
   case fs::file_type::regular_file:
   case fs::file_type::file_not_found:
   case fs::file_type::status_error:
-    return createOnDiskBuffer(Path, Size, Mode);
+    return createOnDiskBuffer(Path, Size, !!(Flags & F_modify), Mode);
   default:
     return createInMemoryBuffer(Path, Size, Mode);
   }