Transform ImagePreReader into PreReadFile.

chrome/app/file_pre_reader_win.cc

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/app/file_pre_reader_win.h"
+
+#include <windows.h>
+
+#include "base/files/file.h"
+#include "base/files/memory_mapped_file.h"
+#include "base/logging.h"
+#include "base/threading/thread_restrictions.h"
+#include "base/win/windows_version.h"
+
+namespace {
+
+// A helper function to touch all pages in the range
+// [base_addr, base_addr + length).
+void TouchPagesInRange(const void* base_addr, size_t length) {
+  DCHECK(base_addr);
+  DCHECK_GT(length, static_cast<size_t>(0));
+
+  // Get the system info so we know the page size. Also, make sure we use a
+  // non-zero value for the page size; GetSystemInfo() is hookable/patchable,
+  // and you never know what shenanigans someone could get up to.
+  SYSTEM_INFO system_info = {};
+  ::GetSystemInfo(&system_info);
+  if (system_info.dwPageSize == 0)
+    system_info.dwPageSize = 4096;
+
+  // We don't want to read outside the byte range (which could trigger an
+  // access violation), so let's figure out the exact locations of the first
+  // and final bytes we want to read.
+  volatile uint8_t const* touch_ptr =
+      reinterpret_cast<uint8_t const*>(base_addr);
+  volatile uint8_t const* final_touch_ptr = touch_ptr + length - 1;
+
+  // Read the memory in the range [touch_ptr, final_touch_ptr] with a stride
+  // of the system page size, to ensure that it's been paged in.
+  uint8_t dummy;
+  for (; touch_ptr < final_touch_ptr; touch_ptr += system_info.dwPageSize)
+    dummy = *touch_ptr;
+  dummy = *final_touch_ptr;
+}
+
+}  // namespace
+
+bool PreReadFile(const base::FilePath& file_path, int step_size) {
+  DCHECK_GT(step_size, 0);
+  base::ThreadRestrictions::AssertIOAllowed();
+
+  if (base::win::GetVersion() > base::win::VERSION_XP) {
+    // Vista+ branch. On these OSes, the forced reads through the DLL actually
+    // slows warm starts. The solution is to sequentially read file contents.
+    base::File file(file_path, base::File::FLAG_OPEN | base::File::FLAG_READ |
+                                   base::File::FLAG_SEQUENTIAL_SCAN);
+    if (!file.IsValid())
+      return false;
+
+    LPVOID buffer = ::VirtualAlloc(nullptr, static_cast<DWORD>(step_size),
+                                   MEM_COMMIT, PAGE_READWRITE);
+    if (!buffer)
+      return false;
+
+    while (file.ReadAtCurrentPos(reinterpret_cast<char*>(buffer), step_size) >

[grt (UTC plus 2), 2015/11/03 21:32:50]

does this end up looking better if buffer is a char* (with the cast on line 60)?

[fdoray, 2015/11/03 22:09:16]

Yes, I think it looks better with the cast on line 60.

+           0) {}
+
+    ::VirtualFree(buffer, 0, MEM_RELEASE);
+  } else {
+    // WinXP branch. Here, reading the DLL from disk doesn't do what we want so
+    // instead we pull the pages into memory and touch pages at a stride. We use
+    // the system's page size as the stride, ignoring the passed in step_size,
+    // to make sure each page in the range is touched.
+    base::MemoryMappedFile file_memory_map;
+    if (!file_memory_map.Initialize(file_path))
+      return false;
+    TouchPagesInRange(file_memory_map.data(), file_memory_map.length());
+  }
+
+  return true;
+}