Bug 827287 - make it possible to use TimeStamp without performance concerns (TimeStamp::NowLoRes), r=ehsan, sr=roc

xpcom/ds/TimeStamp.h

@@ -211,8 +211,17 @@ public:
    * Return a timestamp reflecting the current elapsed system time. This
    * is monotonically increasing (i.e., does not decrease) over the
    * lifetime of this process' XPCOM session.
+   *
+   * Now() is trying to ensure the best possible precision on each platform,
+   * at least one millisecond.
+   *
+   * NowLoRes() has been introduced to workaround performance problems of
+   * QueryPerformanceCounter on the Windows platform.  NowLoRes() is giving
+   * lower precision, usually 15.6 ms, but with very good performance benefit.
+   * Use it for measurements of longer times, like >200ms timeouts.
    */
-  static TimeStamp Now();
+  static TimeStamp Now() { return Now(true); }
+  static TimeStamp NowLoRes() { return Now(false); }
   /**
    * Compute the difference between two timestamps. Both must be non-null.
    */
@@ -298,6 +307,8 @@ private:
 
   TimeStamp(TimeStampValue aValue) : mValue(aValue) {}
 
+  static TimeStamp Now(bool aHighResolution);
+
   /**
    * When built with PRIntervalTime, a value of 0 means this instance
    * is "null". Otherwise, the low 32 bits represent a PRIntervalTime,

xpcom/ds/TimeStamp_darwin.cpp

@@ -156,7 +156,7 @@ TimeStamp::Shutdown()
 }
 
 TimeStamp
-TimeStamp::Now()
+TimeStamp::Now(bool aHighResolution)
 {
   return TimeStamp(ClockTime());
 }

xpcom/ds/TimeStamp_posix.cpp

@@ -168,7 +168,7 @@ TimeStamp::Shutdown()
 }
 
 TimeStamp
-TimeStamp::Now()
+TimeStamp::Now(bool aHighResolution)
 {
   return TimeStamp(ClockTimeNs());
 }

xpcom/ds/TimeStamp_windows.cpp

@@ -537,10 +537,10 @@ TimeStamp::Shutdown()
 }
 
 TimeStamp
-TimeStamp::Now()
+TimeStamp::Now(bool aHighResolution)
 {
   // sUseQPC is volatile
-  bool useQPC = sUseQPC;
+  bool useQPC = (aHighResolution && sUseQPC);
 
   // Both values are in [mt] units.
   ULONGLONG QPC = useQPC ? PerformanceCounter() : uint64_t(0);