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

base/time/time_win_unittest.cc

Index: base/time/time_win_unittest.cc
diff --git a/base/time/time_win_unittest.cc b/base/time/time_win_unittest.cc
index 46e256ead0cd6c84c571230ad8e7f0b8820b2cc3..c7ce00ef4ee7875e63e84b6b585f3fa22e1ba7bc 100644
--- a/base/time/time_win_unittest.cc
+++ b/base/time/time_win_unittest.cc
@@ -241,3 +241,30 @@ TEST(TimeTicks, DISABLED_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));
+}