We have noticed a clock shift when QPC was deployed. It shows as a regression on the perf bots

https://chromeperf.appspot.com/report?masters=ChromiumPerf&bots=chromium-rel-win8-dual&tests=startup.warm.dirty.blank_page%2Fwindow_display_time&rev=286928

It is not a real regression, the initial_time and initial_ticks are not properly initialized when switching to HighResolution (i.e. QPC).
This CL initializes the now_function to the HighResNowWrapper instead of setting it to RolloverProtectedNow then to the HighResNowWrapper.

By doing that, we avoid getting an incorrect initial_time and initial_ticks using the RolloverProtectedNow and avoid having to reinitialize.

BUG=158234

Review URL: https://codereview.chromium.org/446203002

Cr-Commit-Position: refs/heads/master@{#291974}

3 files changed, 20 insertions, 39 deletions

diff --git a/base/test/test_suite.cc b/base/test/test_suite.cc
index 0084d1e..c7a921a 100644
--- a/base/test/test_suite.cc
+++ b/base/test/test_suite.cc
@@ -135,7 +135,6 @@ void TestSuite::InitializeFromCommandLine(int argc, wchar_t** argv) {
 void TestSuite::PreInitialize(bool create_at_exit_manager) {
 #if defined(OS_WIN)
   testing::GTEST_FLAG(catch_exceptions) = false;
-  base::TimeTicks::SetNowIsHighResNowIfSupported();
 #endif
   base::EnableTerminationOnHeapCorruption();
 #if defined(OS_LINUX) && defined(USE_AURA)
diff --git a/base/time/time.h b/base/time/time.h
index 79346f1..4d72a19 100644
--- a/base/time/time.h
+++ b/base/time/time.h
@@ -639,14 +639,6 @@ class BASE_EXPORT TimeTicks {
   // This is only for testing.
   static bool IsHighResClockWorking();
 
-  // Enable high resolution time for TimeTicks::Now(). This function will
-  // test for the availability of a working implementation of
-  // QueryPerformanceCounter(). If one is not available, this function does
-  // nothing and the resolution of Now() remains 1ms. Otherwise, all future
-  // calls to TimeTicks::Now() will have the higher resolution provided by QPC.
-  // Returns true if high resolution time was successfully enabled.
-  static bool SetNowIsHighResNowIfSupported();
-
   // Returns a time value that is NOT rollover protected.
   static TimeTicks UnprotectedNow();
 #endif
diff --git a/base/time/time_win.cc b/base/time/time_win.cc
index 5fa899d..27bcd18 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();
+
 }  // 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();
 }