Makes TraceTicks use lowres times on Win when highres are buggy or slow

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 418 matching lines @@
 
 void InitializeNowFunctionPointers() {
   LARGE_INTEGER ticks_per_sec = {};
   if (!QueryPerformanceFrequency(&ticks_per_sec))
     ticks_per_sec.QuadPart = 0;
 
   // If Windows cannot provide a QPC implementation, both TimeTicks::Now() and
   // TraceTicks::Now() must use the low-resolution clock.
   //
   // If the QPC implementation is expensive and/or unreliable, TimeTicks::Now()
-  // will use the low-resolution clock, but TraceTicks::Now() will use the QPC
-  // (in the hope that it is still useful for tracing purposes). A CPU lacking a
-  // non-stop time counter will cause Windows to provide an alternate QPC
-  // implementation that works, but is expensive to use. Certain Athlon CPUs are
-  // known to make the QPC implementation unreliable.
+  // and TraceTicks::Now() will still use the low-resolution clock. A CPU
+  // lacking a non-stop time counter will cause Windows to provide an alternate
+  // QPC implementation that works, but is expensive to use. Certain Athlon CPUs
+  // are known to make the QPC implementation unreliable.
   //
   // Otherwise, both Now functions can use the high-resolution QPC clock. As of
-  // 4 January 2015, ~68% of users fall within this category.
+  // 21 August 2015, ~72% of users fall within this category.
   NowFunction now_function;
   NowFunction system_trace_now_function;
   base::CPU cpu;
-  if (ticks_per_sec.QuadPart <= 0) {
+  if (ticks_per_sec.QuadPart <= 0 ||
+      !cpu.has_non_stop_time_stamp_counter() || IsBuggyAthlon(cpu)) {
     now_function = system_trace_now_function = &RolloverProtectedNow;

[fmeawad, 2015/08/21 21:24:40]

Can you explain why you still need 2 variable (now_function and
system_trace_now_function)

-  } else if (!cpu.has_non_stop_time_stamp_counter() || IsBuggyAthlon(cpu)) {
-    now_function = &RolloverProtectedNow;
-    system_trace_now_function = &QPCNow;
   } else {
     now_function = system_trace_now_function = &QPCNow;
   }
 
   // Threading note 1: In an unlikely race condition, it's possible for two or
   // more threads to enter InitializeNowFunctionPointers() in parallel. This is
   // not a problem since all threads should end up writing out the same values
   // to the global variables.
   //
   // Threading note 2: A release fence is placed here to ensure, from the
@@ skipping 56 matching lines @@
 TimeTicks TimeTicks::FromQPCValue(LONGLONG qpc_value) {
   return TimeTicks() + QPCValueToTimeDelta(qpc_value);
 }
 
 // TimeDelta ------------------------------------------------------------------
 
 // static
 TimeDelta TimeDelta::FromQPCValue(LONGLONG qpc_value) {
   return QPCValueToTimeDelta(qpc_value);
 }