Store PreRead options obtained from the registry in a global variable.

components/startup_metric_utils/common/pre_read_field_trial_utils_win.h

 // Copyright 2015 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.
 
 #ifndef COMPONENTS_STARTUP_METRIC_UTILS_COMMON_PRE_READ_FIELD_TRIAL_UTILS_WIN_H_
 #define COMPONENTS_STARTUP_METRIC_UTILS_COMMON_PRE_READ_FIELD_TRIAL_UTILS_WIN_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
 #include "base/strings/string16.h"
 
 // Utility functions to support the PreRead field trial. The PreRead field trial
 // changes the way DLLs are pre-read during startup.
 
 namespace startup_metric_utils {
 
 // Callback to register a synthetic field trial.
 using RegisterPreReadSyntheticFieldTrialCallback =
     const base::Callback<bool(const std::string&, const std::string&)>;
 
-// Get DLL pre-reading options. |product_registry_path| is the registry path
-// under which the registry key for this field trial resides. The |no_pre_read|
-// option is set if DLLs should not be pre-read. The |high_priority| option is
-// set if pre-reading should be done with a high thread priority. The
-// |only_if_cold| option is set if only cold DLLs should be pre-read. The
-// |prefetch_virtual_memory| option is set if the
-// ::PrefetchVirtualMemory function should be used to pre-read DLLs, if
-// available.
-void GetPreReadOptions(const base::string16& product_registry_path,
-                       bool* no_pre_read,
-                       bool* high_priority,
-                       bool* only_if_cold,
-                       bool* prefetch_virtual_memory);
+// The options controlled by the PreRead field trial.
+enum PreReadOptions {
+  // No explicit DLL pre-reading.
+  PRE_READ_OPTION_NO_PRE_READ = 1 << 0,

[grt (UTC plus 2), 2016/01/21 04:24:00]

use a series of "const uint32_t kFooTheBar = n;" values rather than an enum for
bits in a bitfield. that said, i think using a struct, as mentioned earlier, may
be cleaner.

[fdoray, 2016/01/21 16:04:16]

Done (now using a struct).

+
+  // Pre-read DLLs with a high thread priority.
+  PRE_READ_OPTION_HIGH_PRIORITY = 1 << 1,
+
+  // Pre-read DLLs only when they are cold.
+  PRE_READ_OPTION_ONLY_IF_COLD = 1 << 2,
+
+  // Pre-read DLLs using the ::PrefetchVirtualMemory function, if available.
+  PRE_READ_OPTION_PREFETCH_VIRTUAL_MEMORY = 1 << 3,
+
+  // Set when the pre-read options have not been initialized yet.
+  PRE_READ_OPTION_UNINITIALIZED = 1 << 4,
+};
+
+// Initializes DLL pre-reading options from the registry.
+// |product_registry_path| is the registry path under which the registry key for
+// this field trial resides.
+void InitializePreReadOptions(const base::string16& product_registry_path);
+
+// Returns the bitfield of the DLL pre-reading options to use for the current
+// process. InitializePreReadOptions() must have been called before this.
+int GetPreReadOptions();

[grt (UTC plus 2), 2016/01/21 04:24:00]

use uint32_t rather than int for the bitfield. bit twiddling on unsigned ints is
more obvious than signed. that said, maybe the struct-based bitfield is cleaner.

[fdoray, 2016/01/21 16:04:16]

Done (now using a struct).

 
 // Updates DLL pre-reading options in the registry with the latest info for the
 // next startup. |product_registry_path| is the registry path under which the
 // registry key for this field trial resides.
 void UpdatePreReadOptions(const base::string16& product_registry_path);
 
 // Registers a synthetic field trial with the PreRead group currently stored in
 // the registry. This must be done before the first metric log
 // (metrics::MetricsLog) is created. Otherwise, UMA metrics generated during
 // startup won't be correctly annotated. |product_registry_path| is the registry
 // path under which the key for this field trial resides.
 void RegisterPreReadSyntheticFieldTrial(
     const base::string16& product_registry_path,
     const RegisterPreReadSyntheticFieldTrialCallback&
         register_synthetic_field_trial);
 
 }  // namespace startup_metric_utils
 
 #endif  // COMPONENTS_STARTUP_METRIC_UTILS_COMMON_PRE_READ_FIELD_TRIAL_UTILS_WIN_H_