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 492b54a93f126717ad0b2c339e75322bdeeb1532..9cd86111e78de81ae8195d797d7870d306429ae9 100644
--- a/base/time/time_win.cc
+++ b/base/time/time_win.cc
@@ -373,21 +373,24 @@ 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() {
@@ -422,16 +425,6 @@ 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;
@@ -460,7 +453,6 @@ TimeDelta HighResNowWrapper() {
 }
 
 typedef TimeDelta (*NowFunction)(void);
-NowFunction now_function = RolloverProtectedNow;
 
 bool CPUReliablySupportsHighResTime() {
   base::CPU cpu;
@@ -474,6 +466,23 @@ bool CPUReliablySupportsHighResTime() {
   return true;
 }
 
+TimeDelta InitialNowFunction();
+
+NowFunction now_function = InitialNowFunction;
+
+TimeDelta InitialNowFunction() {
+  if (!CPUReliablySupportsHighResTime()) {
+    InterlockedExchangePointer(
+        reinterpret_cast<volatile PVOID*>(&now_function),
+        reinterpret_cast<PVOID>(RolloverProtectedNow));
+    return RolloverProtectedNow();
+  }
+  InterlockedExchangePointer(
+        reinterpret_cast<volatile PVOID*>(&now_function),
+        reinterpret_cast<PVOID>(HighResNowWrapper));
+  return HighResNowWrapper();
+}
+
 }  // namespace
 
 // static
@@ -488,16 +497,6 @@ 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();
 }