Initialize the now_funciton to the HighResNowWrapper in case High Res is supported

base/time/time_win.cc

Index: base/time/time_win.cc
diff --git a/base/time/time_win.cc b/base/time/time_win.cc
index 5fa899d1f97f69be69d1df65ba854565d7240b53..27bcd1876da0d414ad14764c96b88d98561da722 100644
--- a/base/time/time_win.cc
+++ b/base/time/time_win.cc
@@ -359,21 +359,24 @@ bool IsBuggyAthlon(const base::CPU& cpu) {
 class HighResNowSingleton {
  public:
   HighResNowSingleton()
-    : ticks_per_second_(0),
-      skew_(0) {
-    InitializeClock();
+      : ticks_per_second_(0),
+        skew_(0) {
 
     base::CPU cpu;
     if (IsBuggyAthlon(cpu))
-      DisableHighResClock();
-  }
+      return;
 
-  bool IsUsingHighResClock() {
-    return ticks_per_second_ != 0.0;
+    // Synchronize the QPC clock with GetSystemTimeAsFileTime.
+    LARGE_INTEGER ticks_per_sec = {0};
+    if (!QueryPerformanceFrequency(&ticks_per_sec))
+      return; // QPC is not available.
+    ticks_per_second_ = ticks_per_sec.QuadPart;
+
+    skew_ = UnreliableNow() - ReliableNow();
   }
 
-  void DisableHighResClock() {
-    ticks_per_second_ = 0.0;
+  bool IsUsingHighResClock() {
+    return ticks_per_second_ != 0;
   }
 
   TimeDelta Now() {
@@ -408,16 +411,6 @@ class HighResNowSingleton {
   }
 
  private:
-  // Synchronize the QPC clock with GetSystemTimeAsFileTime.
-  void InitializeClock() {
-    LARGE_INTEGER ticks_per_sec = {0};
-    if (!QueryPerformanceFrequency(&ticks_per_sec))
-      return;  // Broken, we don't guarantee this function works.
-    ticks_per_second_ = ticks_per_sec.QuadPart;
-
-    skew_ = UnreliableNow() - ReliableNow();
-  }
-
   // Get the number of microseconds since boot in an unreliable fashion.
   int64 UnreliableNow() {
     LARGE_INTEGER now;
@@ -446,7 +439,6 @@ TimeDelta HighResNowWrapper() {
 }
 
 typedef TimeDelta (*NowFunction)(void);
-NowFunction now_function = RolloverProtectedNow;
 
 bool CPUReliablySupportsHighResTime() {
   base::CPU cpu;
@@ -460,6 +452,14 @@ bool CPUReliablySupportsHighResTime() {
   return true;
 }
 
+NowFunction GetNowFunction() {
+  if (!CPUReliablySupportsHighResTime())
+    return RolloverProtectedNow;
+  return HighResNowWrapper;
+}
+
+NowFunction now_function = GetNowFunction();

[willchan no longer on Chromium, 2014/08/25 23:07:25]

FYI: this results in a static initializer. This may slow down Windows Chrome
startup. You may want to use a function local static instead since it won't
create a static initializer. Of course, since we don't lock function local
static initialization, that will possibly lead to races, depending on how this
code gets run.

[M-A Ruel, 2014/08/26 02:03:19]

Furthermore, the static initializer is unneeded:

TimeDelta InitialNowFunction() {
  if (!CPUReliablySupportsHighResTime()) {
    InterlockedExchange(&now_function, RolloverProtectedNow);
    return RolloverProtectedNow();
  }
  InterlockedExchange(&now_function, HighResNowWrapper);
  return HighResNowWrapper();
}

NowFunction now_function = InitialNowFunction;

+
 }  // namespace
 
 // static
@@ -474,16 +474,6 @@ TimeTicks::TickFunctionType TimeTicks::SetMockTickFunction(
 }
 
 // static
-bool TimeTicks::SetNowIsHighResNowIfSupported() {
-  if (!CPUReliablySupportsHighResTime()) {
-    return false;
-  }
-
-  now_function = HighResNowWrapper;
-  return true;
-}
-
-// static
 TimeTicks TimeTicks::Now() {
   return TimeTicks() + now_function();
 }