Add a PartialPreReadImage function to file_util (on Windows).

chrome/app/image_pre_reader_win.h

Index: chrome/app/image_pre_reader_win.h
diff --git a/chrome/app/image_pre_reader_win.h b/chrome/app/image_pre_reader_win.h
new file mode 100644
index 0000000000000000000000000000000000000000..5a16a54f7ab029359d64c2edd8f09aabfa9ae9e2
--- /dev/null
+++ b/chrome/app/image_pre_reader_win.h
@@ -0,0 +1,74 @@
+// Copyright (c) 2006-2011 The Chromium Authors. All rights reserved.

[chrisha, 2012/02/03 16:31:08]

New file: Copyright (c) 2012.

[Roger McFarlane (Chromium), 2012/02/03 17:45:42]

Done.

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This file defines utility functions pre-read a PE Image in order to avoid
+// hard page faults when the image is subsequently loaded into memory for
+// execution.
+
+#ifndef CHROME_APP_IMAGE_PRE_READER_WIN_H_
+#define CHROME_APP_IMAGE_PRE_READER_WIN_H_
+#pragma once
+
+#include "base/basictypes.h"
+
+// This class defines static helper functions to pre-read a PE Image in order
+// to avoid hard page faults when the image is subsequently loaded into memory
+// for execution.
+class ImagePreReader {
+ public:
+  // Reads the file passed in as a PE Image and touches pages to avoid
+  // subsequent hard page faults during LoadLibrary. The size to be pre read

[Sigurður Ásgeirsson, 2012/02/03 16:36:39]

nit: pre read->pre-read

[Roger McFarlane (Chromium), 2012/02/03 17:45:42]

Done.

+  // is passed in. If it is 0 then the whole file is paged in. The step size
+  // which indicates the number of bytes to skip after every page touched is
+  // also passed in.
+  //
+  // This function checks the Windows version to determine which pre-reading
+  // mechanism to use.
+  static bool PreReadImage(const wchar_t* file_path,
+                           size_t size_to_read,
+                           size_t step_size);
+
+  // Loads the file passed in as PE Image and touches a percentage of the
+  // pages in each of the image's sections to avoid subsequent hard page
+  // faults during LoadLibrary.
+  //
+  // This function checks the Windows version to determine which pre-reading
+  // mechanism to use.
+  //
+  // The percentage of the file to be read is an integral value between 0 and

[Sigurður Ásgeirsson, 2012/02/03 16:36:39]

great comment!

[Roger McFarlane (Chromium), 2012/02/03 17:45:42]

Thanks!

+  // 100, inclusive. If it is 0 then this is a NOP, if it is 100 (or greater)
+  // then the whole file is paged in sequentially via PreReadImage. Otherwise,
+  // for each section, in order, the given percentage of the blocks in that
+  // section are paged in, starting at the beginning of each section. For
+  // example: if percentage is 30 and there is a .text section and a .data
+  // section, then the first 30% of .text will be paged and the first 30% of
+  // .data will be paged in.
+  //
+  // The max_chunk_size indicates the number of bytes to read off the disk in
+  // each step (for Vista and greater, where where this is the way the pages

[chrisha, 2012/02/03 16:31:08]

where where

[Roger McFarlane (Chromium), 2012/02/03 17:45:42]

Done.

+  // are warmed).
+  //
+  // This function is intended to be used in the context of a PE image with
+  // an optimized layout, such that the blocks in each section are arranged
+  // with the data and code most needed for startup moved to the front.
+  // See also: http://code.google.com/p/chromium/issues/detail?id=98508
+  static bool PartialPreReadImage(const wchar_t* file_path,
+                                  uint8 percentage,
+                                  size_t max_chunk_size);
+
+  // Helper function used by PartialPreReadImage on Windows versions (Vista+)
+  // where reading through the file on disk serves to warm up the page cache.
+  // Exported for unit-testing purposes.
+  static bool PartialPreReadImageOnDisk(const wchar_t* file_path,
+                                        uint8 percentage,
+                                        size_t max_chunk_size);
+
+  // Helper function used by PartialPreReadImage on Windows versions (XP) where
+  // cheaply loading the image then stepping through its address space serves
+  // to warm up the page cache. Exported for unit-testing purposes.
+  static bool PartialPreReadImageInMemory(const wchar_t* file_path,
+                                          uint8 percentage);
+};  // namespace internal
+
+#endif  // CHROME_APP_IMAGE_PRE_READER_WIN_H_

[chrisha, 2012/02/03 16:31:08]

Great comments, btw!

[Roger McFarlane (Chromium), 2012/02/03 17:45:42]

Thanks!