Implement ThreadTicks::Now on Windows.

Use QueryThreadCycleTime() to get the number of CPU clock cycles used
by the current thread. Convert it to microseconds using a measured
TSC frequency.

The value returned by QueryThreadCycleTime() is based on the rdtsc
instruction. For several years, Intel has been shipping CPUs with a
constant-rate counter, which means that the QueryThreadCycleTime()
results are directly proportional to wall-clock time on most systems
(see crbug.com/280743#c15). ThreadTicks::IsSupported() will return
false if the CPU doesn't have a constant rate TSC.

BUG=280743
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1390743002

Cr-Commit-Position: refs/heads/master@{#354213}

1 files changed, 28 insertions, 1 deletions

diff --git a/base/time/time.h b/base/time/time.h
index e0435d0..8da08ff 100644
--- a/base/time/time.h
+++ b/base/time/time.h
@@ -79,6 +79,8 @@
 // For FILETIME in FromFileTime, until it moves to a new converter class.
 // See TODO(iyengar) below.
 #include <windows.h>
+
+#include "base/gtest_prod_util.h"
 #endif
 
 #include <limits>
@@ -733,16 +735,28 @@ class BASE_EXPORT ThreadTicks : public time_internal::TimeBase<ThreadTicks> {
 #if (defined(_POSIX_THREAD_CPUTIME) && (_POSIX_THREAD_CPUTIME >= 0)) || \
     (defined(OS_MACOSX) && !defined(OS_IOS)) || defined(OS_ANDROID)
     return true;
+#elif defined(OS_WIN)
+    return IsSupportedWin();
 #else
     return false;
 #endif
   }
 
+  // Waits until the initialization is completed. Needs to be guarded with a
+  // call to IsSupported().
+  static void WaitUntilInitialized() {
+#if defined(OS_WIN)
+    WaitUntilInitializedWin();
+#endif
+  }
+
   // Returns thread-specific CPU-time on systems that support this feature.
   // Needs to be guarded with a call to IsSupported(). Use this timer
   // to (approximately) measure how much time the calling thread spent doing
   // actual work vs. being de-scheduled. May return bogus results if the thread
-  // migrates to another CPU between two calls.
+  // migrates to another CPU between two calls. Returns an empty ThreadTicks
+  // object until the initialization is completed. If a clock reading is
+  // absolutely needed, call WaitUntilInitialized() before this method.
   static ThreadTicks Now();
 
  private:
@@ -752,6 +766,19 @@ class BASE_EXPORT ThreadTicks : public time_internal::TimeBase<ThreadTicks> {
   // and testing.
   explicit ThreadTicks(int64 us) : TimeBase(us) {
   }
+
+#if defined(OS_WIN)
+  FRIEND_TEST_ALL_PREFIXES(TimeTicks, TSCTicksPerSecond);
+
+  // Returns the frequency of the TSC in ticks per second, or 0 if it hasn't
+  // been measured yet. Needs to be guarded with a call to IsSupported().
+  // This method is declared here rather than in the anonymous namespace to
+  // allow testing.
+  static double TSCTicksPerSecond();
+
+  static bool IsSupportedWin();
+  static void WaitUntilInitializedWin();
+#endif
 };
 
 // For logging use only.