1 files changed, 10 insertions, 4 deletions

diff --git a/contrib/llvm/lib/Support/FileOutputBuffer.cpp b/contrib/llvm/lib/Support/FileOutputBuffer.cpp
index b176a8b45ab3..307ff09afedc 100644
--- a/contrib/llvm/lib/Support/FileOutputBuffer.cpp
+++ b/contrib/llvm/lib/Support/FileOutputBuffer.cpp
@@ -11,11 +11,10 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/Support/Errc.h"
-#include "llvm/ADT/STLExtras.h"
-#include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/FileOutputBuffer.h"
-#include "llvm/Support/raw_ostream.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/Errc.h"
 #include <system_error>
 
 #if !defined(_MSC_VER) && !defined(__MINGW32__)
@@ -77,9 +76,16 @@ FileOutputBuffer::create(StringRef FilePath, size_t Size,
   if (EC)
     return EC;
 
+#ifndef LLVM_ON_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.
   EC = sys::fs::resize_file(FD, Size);
   if (EC)
     return EC;
+#endif
 
   auto MappedFile = llvm::make_unique<mapped_file_region>(
       FD, mapped_file_region::readwrite, Size, 0, EC);