Use __rdtsc on Windows.

tools/timer/SysTimer_windows.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "SysTimer_windows.h"
 
+#include <intrin.h>
+
 static ULONGLONG win_cpu_time() {
     FILETIME createTime;
     FILETIME exitTime;
     FILETIME usrTime;
     FILETIME sysTime;
     if (0 == GetProcessTimes(GetCurrentProcess(), &createTime, &exitTime, &sysTime, &usrTime)) {
         return 0;
     }
     ULARGE_INTEGER start_cpu_sys;
     ULARGE_INTEGER start_cpu_usr;
     start_cpu_sys.LowPart  = sysTime.dwLowDateTime;
     start_cpu_sys.HighPart = sysTime.dwHighDateTime;
     start_cpu_usr.LowPart  = usrTime.dwLowDateTime;
     start_cpu_usr.HighPart = usrTime.dwHighDateTime;
     return start_cpu_sys.QuadPart + start_cpu_usr.QuadPart;
 }
 
-void SysTimer::startWall() {
-    if (0 == ::QueryPerformanceCounter(&fStartWall)) {
-        fStartWall.QuadPart = 0;
-    }
-}
 void SysTimer::startCpu() {
     fStartCpu = win_cpu_time();
 }
 
 double SysTimer::endCpu() {
     ULONGLONG end_cpu = win_cpu_time();
     return static_cast<double>(end_cpu - fStartCpu) / 10000.0L;
 }
+
+// On recent Intel chips (roughly, "has Core or Atom in its name") __rdtsc will always tick
+// at the CPU's maximum rate, even while power management clocks the CPU up and down.
+// That's great, because it makes measuring wall time super simple.
+
+void SysTimer::startWall() {
+    fStartWall = __rdtsc();
+}
+
 double SysTimer::endWall() {
-    LARGE_INTEGER end_wall;
-    if (0 == ::QueryPerformanceCounter(&end_wall)) {
-        end_wall.QuadPart = 0;
-    }
+    unsigned __int64 end = __rdtsc();
 
-    LARGE_INTEGER ticks_elapsed;
-    ticks_elapsed.QuadPart = end_wall.QuadPart - fStartWall.QuadPart;
+    // This seems to, weirdly, give the CPU frequency in kHz.  That's exactly what we want!
+    LARGE_INTEGER freq_khz;
+    QueryPerformanceFrequency(&freq_khz);
 
-    LARGE_INTEGER frequency;
-    if (0 == ::QueryPerformanceFrequency(&frequency)) {
-        return 0.0L;
-    } else {
-        return static_cast<double>(ticks_elapsed.QuadPart)
-             / static_cast<double>(frequency.QuadPart)
-             * 1000.0L;
-    }
+    return static_cast<double>(end - fStartWall) / static_cast<double>(freq_khz.QuadPart);
 }