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

base/time/time_win_unittest.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.
 
 #include <windows.h>
 #include <mmsystem.h>
 #include <process.h>
 
 #include "base/threading/platform_thread.h"
 #include "base/time/time.h"
@@ skipping 223 matching lines @@
     total_drift += drift_microseconds;
   }
 
   // Sanity check. We expect some time drift to occur, especially across
   // the number of iterations we do.
   EXPECT_LT(0, total_drift);
 
   printf("average time drift in microseconds: %lld\n",
          total_drift / kIterations);
 }
+
+int64 QPCValueToMicrosecondsSafely(LONGLONG qpc_value,
+                                   int64 ticks_per_second) {
+  int64 whole_seconds = qpc_value / ticks_per_second;
+  int64 leftover_ticks = qpc_value % ticks_per_second;
+  int64 microseconds = (whole_seconds * Time::kMicrosecondsPerSecond) +
+                       ((leftover_ticks * Time::kMicrosecondsPerSecond) /
+                        ticks_per_second);
+  return microseconds;
+}
+
+TEST(TimeTicks, FromQPCValue) {
+  if (!TimeTicks::IsHighResClockWorking())
+    return;
+  LARGE_INTEGER frequency;
+  QueryPerformanceFrequency(&frequency);
+  int64 ticks_per_second = frequency.QuadPart;
+  LONGLONG qpc_value = Time::kQPCOverflowThreshold;
+  TimeTicks expected_value = TimeTicks::FromInternalValue(
+    QPCValueToMicrosecondsSafely(qpc_value + 1, ticks_per_second));

[jar (doing other things), 2014/08/01 04:55:32]

nit: please add one more test, for *exactly* qpc_value.

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

Done.

+  EXPECT_EQ(expected_value,
+            TimeTicks::FromQPCValue(qpc_value + 1));
+  expected_value = TimeTicks::FromInternalValue(
+    QPCValueToMicrosecondsSafely(qpc_value - 1, ticks_per_second));
+  EXPECT_EQ(expected_value,
+            TimeTicks::FromQPCValue(qpc_value - 1));
+}