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

base/time/time_win.cc

Index: base/time/time_win.cc
diff --git a/base/time/time_win.cc b/base/time/time_win.cc
index dc968ad63980edbbe0280519a654447c121a74ae..e0307c560212edfe39983b57f8b543813799b4c4 100644
--- a/base/time/time_win.cc
+++ b/base/time/time_win.cc
@@ -86,6 +86,9 @@ void InitializeClock() {
   initial_time = CurrentWallclockMicroseconds();
 }
 
+const char* kTimeGetTimeClockId = "WIN_ROLLOVER_PROTECTED_TIME_GET_TIME_MICROS";
+const char* kQPCClockId = "WIN_QPC_MICROS";
+
 // The two values that ActivateHighResolutionTimer uses to set the systemwide
 // timer interrupt frequency on Windows. It controls how precise timers are
 // but also has a big impact on battery life.
@@ -403,6 +406,8 @@ TimeDelta InitialNowFunction();
 // See "threading notes" in InitializeNowFunctionPointer() for details on how
 // concurrent reads/writes to these globals has been made safe.
 NowFunction g_now_function = &InitialNowFunction;
+const char* g_now_clock_id = "";
+
 int64_t g_qpc_ticks_per_second = 0;
 
 // As of January 2015, use of <atomic> is forbidden in Chromium code. This is
@@ -459,12 +464,15 @@ void InitializeNowFunctionPointer() {
   // Otherwise, Now uses the high-resolution QPC clock. As of 21 August 2015,
   // ~72% of users fall within this category.
   NowFunction now_function;
+  const char* now_clock_id;
   base::CPU cpu;
   if (ticks_per_sec.QuadPart <= 0 ||
       !cpu.has_non_stop_time_stamp_counter() || IsBuggyAthlon(cpu)) {
     now_function = &RolloverProtectedNow;
+    now_clock_id = kTimeGetTimeClockId;
   } else {
     now_function = &QPCNow;
+    now_clock_id = kQPCClockId;
   }
 
   // Threading note 1: In an unlikely race condition, it's possible for two or
@@ -479,6 +487,7 @@ void InitializeNowFunctionPointer() {
   g_qpc_ticks_per_second = ticks_per_sec.QuadPart;
   ATOMIC_THREAD_FENCE(memory_order_release);
   g_now_function = now_function;
+  g_now_clock_id = now_clock_id;

[danakj, 2016/03/26 00:40:33]

Why is this after the fence?

[charliea (OOO until 10-5), 2016/03/28 15:40:51]

I'm not sure, but if the above comment is any indicator, it looks like it has to
do with making sure g_qpc_ticks_per_second is changed before the function
pointers. Naively, I'd expect that things should be okay as long as we change
g_now_function and g_now_clock_id in the same block.

[danakj, 2016/03/28 17:53:50]

Yeah, I stared at this for a while, and AFAICT the reason is because the now
function can use g_qpc_ticks_p_s, so if the pointer is updated we want the
g_qpc.. to be updated as well.

However, the function does not use the clock id. The fence is magical, so
putting things below makes them now depend on magic. So I'd prefer this was
added above then. Maybe just above g_qpc_t_p_s?

[charliea (OOO until 10-5), 2016/03/28 21:07:36]

Done.

 }
 
 TimeDelta InitialNowFunction() {
@@ -512,6 +521,13 @@ bool TimeTicks::IsHighResolution() {
 }
 
 // static
+std::string TimeTicks::ClockId() {
+  if (g_now_function == &InitialNowFunction)
+    InitializeNowFunctionPointer();
+  return g_now_clock_id;
+}
+
+// static
 ThreadTicks ThreadTicks::Now() {
   DCHECK(IsSupported());