Make QPCValueToMicroseconds faster in case the value is less than 44 bits

base/time/time_win.cc

 // 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.
 
 
 // Windows Timer Primer
 //
 // A good article:  http://www.ddj.com/windows/184416651
 // A good mozilla bug:  http://bugzilla.mozilla.org/show_bug.cgi?id=363258
 //
@@ skipping 375 matching lines @@
 
   int64 GetQPCDriftMicroseconds() {
     if (!IsUsingHighResClock())
       return 0;
     return abs((UnreliableNow() - ReliableNow()) - skew_);
   }
 
   int64 QPCValueToMicroseconds(LONGLONG qpc_value) {
     if (!ticks_per_second_)
       return 0;
-
-    // Intentionally calculate microseconds in a round about manner to avoid
-    // overflow and precision issues. Think twice before simplifying!
+    // If the QPC Value is below the overflow threshold, we proceed with
+    // simple multiply and divide.
+    if (qpc_value < Time::kQPCOverflowThreshold)
+      return qpc_value * Time::kMicrosecondsPerSecond / ticks_per_second_;
+    // Otherwise, calculate microseconds in a round about manner to avoid
+    // overflow and precision issues.
     int64 whole_seconds = qpc_value / ticks_per_second_;
-    int64 leftover_ticks = qpc_value % ticks_per_second_;
+    int64 leftover_ticks = qpc_value - (whole_seconds * ticks_per_second_);
     int64 microseconds = (whole_seconds * Time::kMicrosecondsPerSecond) +
                          ((leftover_ticks * Time::kMicrosecondsPerSecond) /
                           ticks_per_second_);
     return microseconds;
   }
 
  private:
   // Synchronize the QPC clock with GetSystemTimeAsFileTime.
   void InitializeClock() {
     LARGE_INTEGER ticks_per_sec = {0};
@@ skipping 29 matching lines @@
 
 TimeDelta HighResNowWrapper() {
   return GetHighResNowSingleton()->Now();
 }
 
 typedef TimeDelta (*NowFunction)(void);
 NowFunction now_function = RolloverProtectedNow;
 
 bool CPUReliablySupportsHighResTime() {
   base::CPU cpu;
-  if (!cpu.has_non_stop_time_stamp_counter())
+  if (!cpu.has_non_stop_time_stamp_counter() ||
+      !GetHighResNowSingleton()->IsUsingHighResClock())
     return false;
 
   if (IsBuggyAthlon(cpu))
     return false;
 
   return true;
 }
 
 }  // namespace
 
@@ skipping 66 matching lines @@
     return TimeTicks() + TimeDelta::FromMilliseconds(timeGetTime());
   }
 }
 
 // TimeDelta ------------------------------------------------------------------
 
 // static
 TimeDelta TimeDelta::FromQPCValue(LONGLONG qpc_value) {
   return TimeDelta(GetHighResNowSingleton()->QPCValueToMicroseconds(qpc_value));
 }