fix for high resolution timer on windows.

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 74 matching lines @@
 }
 
 // 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.
 const int kMinTimerIntervalHighResMs = 1;
 const int kMinTimerIntervalLowResMs = 4;
 // Track if kMinTimerIntervalHighResMs or kMinTimerIntervalLowResMs is active.
 bool g_high_res_timer_enabled = false;
 // How many times the high resolution timer has been called.
-int g_high_res_timer_count = 0;
+unsigned int g_high_res_timer_count = 0;

[jamesr, 2014/09/04 22:56:03]

uint32_t

'unsigned int' is redundant and we don't typically use just 'unsigned'

 // The lock to control access to the above two variables.
 base::LazyInstance<base::Lock>::Leaky g_high_res_lock =
     LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace
 
 // Time -----------------------------------------------------------------------
 
 // The internal representation of Time uses FILETIME, whose epoch is 1601-01-01
 // 00:00:00 UTC.  ((1970-1601)*365+89)*24*60*60*1000*1000, where 89 is the
@@ skipping 84 matching lines @@
     timeEndPeriod(kMinTimerIntervalHighResMs);
     timeBeginPeriod(kMinTimerIntervalLowResMs);
   }
 }
 
 // static
 bool Time::ActivateHighResolutionTimer(bool activating) {
   // We only do work on the transition from zero to one or one to zero so we
   // can easily undo the effect (if necessary) when EnableHighResolutionTimer is
   // called.
+  const unsigned int max = std::numeric_limits<unsigned int>::max();

[jamesr, 2014/09/04 22:56:03]

uint32_t

+
   base::AutoLock lock(g_high_res_lock.Get());
   UINT period = g_high_res_timer_enabled ? kMinTimerIntervalHighResMs
                                          : kMinTimerIntervalLowResMs;
-  int high_res_count =
+
+  // Check for overflow or underflow.
+  DCHECK((g_high_res_timer_count != 0) || activating);
+  DCHECK((g_high_res_timer_count != max) || !activating);
+
+  unsigned int high_res_count =
       activating ? ++g_high_res_timer_count : --g_high_res_timer_count;
 
   if (activating) {
     if (high_res_count == 1)
       timeBeginPeriod(period);
   } else {
     if (high_res_count == 0)
       timeEndPeriod(period);
   }
   return (period == kMinTimerIntervalHighResMs);
@@ skipping 331 matching lines @@
     return TimeTicks() + TimeDelta::FromMilliseconds(timeGetTime());
   }
 }
 
 // TimeDelta ------------------------------------------------------------------
 
 // static
 TimeDelta TimeDelta::FromQPCValue(LONGLONG qpc_value) {
   return TimeDelta(GetHighResNowSingleton()->QPCValueToMicroseconds(qpc_value));
 }