Do not inherit TaskPriority in TaskTraits.

base/task_scheduler/task_traits.h

 // Copyright 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.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 #define BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 
 #include <stdint.h>
 
 #include <iosfwd>
@@ skipping 124 matching lines @@
 
   // Applies |shutdown_behavior| to tasks with these traits.
   TaskTraits& WithShutdownBehavior(TaskShutdownBehavior shutdown_behavior);
 
   // Returns true if tasks with these traits may block.
   bool may_block() const { return may_block_; }
 
   // Returns true if tasks with these traits may use base/ sync primitives.
   bool with_base_sync_primitives() const { return with_base_sync_primitives_; }
 
+  // Returns true if the priority was set explicitly.
+  bool priority_set_explicitly() const { return priority_set_explicitly_; }
+
   // Returns the priority of tasks with these traits.
   TaskPriority priority() const { return priority_; }
 
   // Returns the shutdown behavior of tasks with these traits.
   TaskShutdownBehavior shutdown_behavior() const { return shutdown_behavior_; }
 
  private:
-  bool may_block_;
-  bool with_base_sync_primitives_;
+  bool may_block_ : 1;
+  bool with_base_sync_primitives_ : 1;
+  bool priority_set_explicitly_ : 1;

[gab, 2017/04/25 17:16:41]

Let's ask Etienne to make sure these bit optimizations are worth it (and not
causing undesired churn).

[etienneb, 2017/04/25 17:45:59]

Using 
bool x : 1;
bool y : 1;

This will use less memory space for every allocated objects.
But every access will use more code as the compiler need to emit code to extract
the bit.

So, this is a tradeoff between code size and memory size.

[fdoray, 2017/04/25 17:57:48]

Since there will be very few accesses (ignoring constexpr constructors) and a
few instances in memory at all times, I'd say it's worth it.

[etienneb, 2017/04/25 18:05:47]

If this is not "largely" used, I'll stay with the simple version.
"bool" and not "bool : 1"

[fdoray, 2017/04/25 18:40:25]

Done.

   TaskPriority priority_;
   TaskShutdownBehavior shutdown_behavior_;
 };
 
 // Returns string literals for the enums defined in this file. These methods
 // should only be used for tracing and debugging.
 BASE_EXPORT const char* TaskPriorityToString(TaskPriority task_priority);
 BASE_EXPORT const char* TaskShutdownBehaviorToString(
     TaskShutdownBehavior task_priority);
 
 // Stream operators so that the enums defined in this file can be used in
 // DCHECK and EXPECT statements.
 BASE_EXPORT std::ostream& operator<<(std::ostream& os,
                                      const TaskPriority& shutdown_behavior);
 BASE_EXPORT std::ostream& operator<<(
     std::ostream& os,
     const TaskShutdownBehavior& shutdown_behavior);
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRAITS_H_