Use valid /prefetch arguments for process launches on Windows. - do not submit

chrome/common/prefetch_argument_win.cc

+// Copyright (c) 2016 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/common/prefetch_argument_win.h"
+
+#include <string>
+
+#include "base/command_line.h"
+#include "base/logging.h"
+#include "base/strings/stringprintf.h"
+#include "chrome/common/chrome_switches.h"
+#include "chrome/installer/util/browser_distribution.h"
+#include "components/startup_metric_utils/common/pre_read_field_trial_utils_win.h"
+#include "content/public/common/content_switches.h"
+
+namespace chrome {
+
+void AddWindowsPrefetchArgument(base::CommandLine* command_line) {
+  DCHECK(command_line);
+
+  int pre_read_options = startup_metric_utils::GetPreReadOptions(
+      BrowserDistribution::GetDistribution()->GetRegistryPath());
+  if (pre_read_options & startup_metric_utils::NO_PREFETCH_ARGUMENT)
+    return;
+
+  const std::string process_type =
+      command_line->GetSwitchValueASCII(switches::kProcessType);
+
+  // There can be 9 prefetch profiles per executable: a profile for launches
+  // without a /prefetch argument and a profile per /prefetch:# argument, where
+  // # is [1, 8].
+  int prefetch_id;
+
+  if (process_type.empty()) {
+    // No prefetch argument when the browser process is launched in foreground.
+    if (!command_line->HasSwitch(switches::kNoStartupWindow))
+      return;
+    prefetch_id = 1;
+  } else if (process_type == switches::kRendererProcess) {
+    prefetch_id = 2;
+  } else if (process_type == switches::kGpuProcess) {
+    prefetch_id = 3;
+  } else if (process_type == switches::kPpapiPluginProcess) {
+    prefetch_id = 4;
+  } else {
+    // Default prefetch argument, shared by all other process types. It is
+    // likely that the prefetcher will never prefetch anything for these process
+    // types as it won't be able to observe consistent file reads across
+    // launches, but a least by using a custom profile they won't interfere with
+    // the default profile used by the browser process.
+    prefetch_id = 8;
+  }
+
+  DCHECK_GE(prefetch_id, 0);
+  DCHECK_LE(prefetch_id, 8);
+
+  if (prefetch_id != 0)
+    command_line->AppendArg(base::StringPrintf("/prefetch:%d", prefetch_id));
+}
+
+}  // namespace chrome