Added GetCpuTime to base/ to get total CPU time consumed by process for perf tests.

webrtc/base/cpu_time.cc

+/*
+ *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/base/cpu_time.h"
+#include "webrtc/base/logging.h"
+#include "webrtc/base/timeutils.h"
+
+#if defined(WEBRTC_LINUX)
+#include <time.h>
+#elif defined(WEBRTC_MAC)
+#include <unistd.h>
+#include <sys/resource.h>
+#include <sys/times.h>
+#elif defined(WEBRTC_WIN)
+#include <windows.h>
+#endif
+
+namespace rtc {
+
+int64_t GetCpuTime() {
+#if defined(WEBRTC_LINUX)
+  struct timespec ts;
+  if (clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts) != -1) {
+    return ts.tv_sec * kNumNanosecsPerSec + ts.tv_nsec;

[nisse-webrtc, 2017/02/15 13:07:31]

I think you need a cast to int64_t on one of the factors to make sure you get a
64-bit rather than 32-bit multiply. And similarly for the rusage case below.

[ilnik, 2017/02/15 13:23:11]

kNumNanosecsPerSec and other constants are already int64_t. For simplicity
reasons I prefer no to add any additional casts.

[nisse-webrtc, 2017/02/15 14:27:35]

You're right, that's sufficient.

+  } else {
+    LOG(LS_ERROR) << "clock_gettime() failed.";

[nisse-webrtc, 2017/02/15 13:07:31]

I think you should use LOG_ERR when errno (or the corresponding thing on
windows) contains a relevant error code. I think you should use it for all three
cases, clock_gettime, getrusage, and GetProcessTimes.

[ilnik, 2017/02/15 13:23:11]

Done.

+  }
+#elif defined(WEBRTC_MAC)
+  struct rusage rusage;
+  if (getrusage(RUSAGE_SELF, &rusage) != -1) {
+    return rusage.ru_utime.tv_sec * kNumNanosecsPerSec +
+           rusage.ru_utime.tv_usec * kNumNanosecsPerMicrosec;
+  }
+#elif defined(WEBRTC_WIN)
+  //  FILETIME resolution is 100 nanosecs.
+  static const int64_t kNanosecsPerFiletime = 100;
+  FILETIME createTime;
+  FILETIME exitTime;
+  FILETIME kernelTime;
+  FILETIME userTime;
+  if (GetProcessTimes(GetCurrentProcess(), &createTime, &exitTime, &kernelTime,
+                      &userTime) != -1) {

[nisse-webrtc, 2017/02/15 13:07:31]

The docs doesn't specify which non-zero value is returned on error. So use == 0.
And perhaps change to == 0 for the the other cases too, for consistency. 

See
https://msdn.microsoft.com/en-us/library/windows/desktop/ms683223(v=vs.85).aspx

[ilnik, 2017/02/15 13:23:11]

This was actually a mistake, because it returns non-zero value ON SUCCESS.

[nisse-webrtc, 2017/02/15 14:27:35]

Ooops. Good you found it.

+    return (static_cast<uint64_t>(userTime.dwHighDateTime) << 32)
+        + userTime.dwLowDateTime) * kNanosecsPerFiletime;
+  } else {
+    LOG(LS_ERROR) << "GetProcessTimes() failed.";
+  }
+#else
+  LOG(LS_ERROR) << "No function to get CPU time";
+#endif
+  return -1;
+}
+
+}  // namespace rtc