time: Add a static TimeTicks method that returns the underlying clock

base/time/time.h

 // 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.
 
 // Time represents an absolute point in coordinated universal time (UTC),
 // internally represented as microseconds (s/1,000,000) since the Windows epoch
 // (1601-01-01 00:00:00 UTC). System-dependent clock interface routines are
 // defined in time_PLATFORM.cc. Note that values for Time may skew and jump
 // around as the operating system makes adjustments to synchronize (e.g., with
 // NTP servers). Thus, client code that uses the Time class must account for
@@ skipping 624 matching lines @@
 }
 
 // For logging use only.
 BASE_EXPORT std::ostream& operator<<(std::ostream& os, Time time);
 
 // TimeTicks ------------------------------------------------------------------
 
 // Represents monotonically non-decreasing clock time.
 class BASE_EXPORT TimeTicks : public time_internal::TimeBase<TimeTicks> {
  public:
+  // The underlying clock used to generate new TimeTicks.
+  enum class Clock {
+    UNKNOWN,

[danakj, 2016/03/30 19:51:58]

Not sure that you really need the UNKNOWN here. It just makes you write it in
switches and NOTREACHED() which is unneeded complexity. We never actually return
that. I think you could let the global win one just default init to 0.

LGTM otherwise.

[charliea (OOO until 10-5), 2016/03/30 20:07:44]

Done.

+    LINUX_CLOCK_MONOTONIC,
+    IOS_CF_ABSOLUTE_TIME_MINUS_KERN_BOOTTIME,
+    MAC_MACH_ABSOLUTE_TIME,
+    WIN_QPC,
+    WIN_ROLLOVER_PROTECTED_TIME_GET_TIME
+  };
+
   TimeTicks() : TimeBase(0) {
   }
 
   // Platform-dependent tick count representing "right now." When
   // IsHighResolution() returns false, the resolution of the clock could be
   // as coarse as ~15.6ms. Otherwise, the resolution should be no worse than one
   // microsecond.
   static TimeTicks Now();
 
   // Returns true if the high resolution clock is working on this system and
@@ skipping 18 matching lines @@
   // the same value for the duration of the application, but will be different
   // in future application runs.
   static TimeTicks UnixEpoch();
 
   // Returns |this| snapped to the next tick, given a |tick_phase| and
   // repeating |tick_interval| in both directions. |this| may be before,
   // after, or equal to the |tick_phase|.
   TimeTicks SnappedToNextTick(TimeTicks tick_phase,
                               TimeDelta tick_interval) const;
 
+  // Returns an enum indicating the underlying clock being used to generate
+  // TimeTicks timestamps. This function should only be used for debugging and
+  // logging purposes.
+  static Clock GetClock();
+
 #if defined(OS_WIN)
  protected:
   typedef DWORD (*TickFunctionType)(void);
   static TickFunctionType SetMockTickFunction(TickFunctionType ticker);
 #endif
 
  private:
   friend class time_internal::TimeBase<TimeTicks>;
 
   // Please use Now() to create a new object. This is for internal use
@@ skipping 62 matching lines @@
   static void WaitUntilInitializedWin();
 #endif
 };
 
 // For logging use only.
 BASE_EXPORT std::ostream& operator<<(std::ostream& os, ThreadTicks time_ticks);
 
 }  // namespace base
 
 #endif  // BASE_TIME_TIME_H_