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

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) (See http://crbug.com/14734).  System-dependent
 // clock interface routines are defined in time_PLATFORM.cc.
 //
 // TimeDelta represents a duration of time, internally represented in
@@ skipping 212 matching lines @@
   static const int64 kNanosecondsPerSecond = kNanosecondsPerMicrosecond *
                                              kMicrosecondsPerSecond;
 
 #if !defined(OS_WIN)
   // On Mac & Linux, this value is the delta from the Windows epoch of 1601 to
   // the Posix delta of 1970. This is used for migrating between the old
   // 1970-based epochs to the new 1601-based ones. It should be removed from
   // this global header and put in the platform-specific ones when we remove the
   // migration code.
   static const int64 kWindowsEpochDeltaMicroseconds;
+#else
+  // To avoid overflow in QPC to Microseconds calculations, since we multiplying

[willchan no longer on Chromium, 2014/08/01 22:41:55]

grammar error at the end of the line. perhaps s/we multiplying/we multiply/?

[fmeawad, 2014/08/05 22:20:09]

Done.

+  // by kMicrosecondsPerSecond, then the QPC value should not exceed
+  // (2^63 - 1) / 1E6. If it exceeds that threshold, we divide then multiply.
+  static const int64 kQPCOverflowThreshold = 0x8637BD05AF7;
 #endif
 
   // Represents an exploded time that can be formatted nicely. This is kind of
   // like the Win32 SYSTEMTIME structure or the Unix "struct tm" with a few
   // additions and changes to prevent errors.
   struct BASE_EXPORT Exploded {
     int year;          // Four digit year "2007"
     int month;         // 1-based month (values 1 = January, etc.)
     int day_of_week;   // 0-based day of week (0 = Sunday, etc.)
     int day_of_month;  // 1-based day of month (1-31)
@@ skipping 495 matching lines @@
 #endif
 };
 
 inline TimeTicks TimeDelta::operator+(TimeTicks t) const {
   return TimeTicks(t.ticks_ + delta_);
 }
 
 }  // namespace base
 
 #endif  // BASE_TIME_TIME_H_