Index: llvm/include/llvm/Support/FileSystem.h
===================================================================
--- llvm/include/llvm/Support/FileSystem.h
+++ llvm/include/llvm/Support/FileSystem.h
@@ -411,6 +411,21 @@
 ///          platform-specific error_code.
 std::error_code resize_file(int FD, uint64_t Size);
 
+/// Resize \p FD to \p Size before mapping \a mapped_file_region::readwrite. On
+/// non-Windows, this calls \a resize_file(). On Windows, this is a no-op,
+/// since the subsequent mapping (via \c CreateFileMapping) automatically
+/// extends the file.
+inline std::error_code resize_file_before_mapping_readwrite(int FD,
+                                                            uint64_t Size) {
+#ifdef _WIN32
+  (void)FD;
+  (void)Size;
+  return std::error_code();
+#else
+  return resize_file(FD, Size);
+#endif
+}
+
 /// Compute an MD5 hash of a file's contents.
 ///
 /// @param FD Input file descriptor.
Index: llvm/unittests/Support/Path.cpp
===================================================================
--- llvm/unittests/Support/Path.cpp
+++ llvm/unittests/Support/Path.cpp
@@ -1284,6 +1284,31 @@
   ASSERT_NO_ERROR(fs::remove(TempPath));
 }
 
+TEST_F(FileSystemTest, ResizeBeforeMapping) {
+  // Create a temp file.
+  int FD;
+  SmallString<64> TempPath;
+  ASSERT_NO_ERROR(fs::createTemporaryFile("prefix", "temp", FD, TempPath));
+  ASSERT_NO_ERROR(fs::resize_file_before_mapping_readwrite(FD, 123));
+
+  // Map in temp file. On Windows, fs::resize_file_before_mapping_readwrite is
+  // a no-op and the mapping itself will resize the file.
+  std::error_code EC;
+  {
+    fs::mapped_file_region mfr(fs::convertFDToNativeFile(FD),
+                               fs::mapped_file_region::readwrite, 4096, 0, EC);
+    ASSERT_NO_ERROR(EC);
+    // Unmap temp file
+  }
+
+  // Check the size.
+  fs::file_status Status;
+  ASSERT_NO_ERROR(fs::status(FD, Status));
+  ASSERT_EQ(Status.getSize(), 123U);
+  ::close(FD);
+  ASSERT_NO_ERROR(fs::remove(TempPath));
+}
+
 TEST_F(FileSystemTest, MD5) {
   int FD;
   SmallString<64> TempPath;
@@ -1305,7 +1330,8 @@
   ASSERT_NO_ERROR(
       fs::createTemporaryFile("prefix", "temp", FileDescriptor, TempPath));
   unsigned Size = 4096;
-  ASSERT_NO_ERROR(fs::resize_file(FileDescriptor, Size));
+  ASSERT_NO_ERROR(
+      fs::resize_file_before_mapping_readwrite(FileDescriptor, Size));
 
   // Map in temp file and add some content
   std::error_code EC;