jlong os::javaTimeNanos() {

  {- -------------------------------------------
  (1) clock_gettime() が使用できるなら, Linux::clock_gettime() で取得した値を返す.
      そうでなければ, gettimeofday() で取得した値を返す.
      ---------------------------------------- -}

      if (Linux::supports_monotonic_clock()) {
        struct timespec tp;
        int status = Linux::clock_gettime(CLOCK_MONOTONIC, &tp);
        assert(status == 0, "gettime error");
        jlong result = jlong(tp.tv_sec) * (1000 * 1000 * 1000) + jlong(tp.tv_nsec);
        return result;
      } else {
        timeval time;
        int status = gettimeofday(&time, NULL);
        assert(status != -1, "linux error");
        jlong usecs = jlong(time.tv_sec) * (1000 * 1000) + jlong(time.tv_usec);
        return 1000 * usecs;
      }
    }