static uint64_t get_timer_bits(void)
{
    uint64_t res = OPENSSL_rdtsc();

    if (res != 0)
        return res;
#if defined(_WIN32)
    {
        LARGE_INTEGER t;
        FILETIME ft;

        if (QueryPerformanceCounter(&t) != 0)
            return t.QuadPart;
        GetSystemTimeAsFileTime(&ft);
        return TWO32TO64(ft.dwHighDateTime, ft.dwLowDateTime);
    }
#elif defined(__sun) || defined(__hpux)
    return gethrtime();
#elif defined(_AIX)
    {
        timebasestruct_t t;

        read_wall_time(&t, TIMEBASE_SZ);
        return TWO32TO64(t.tb_high, t.tb_low);
    }
#else

# if defined(OSSL_POSIX_TIMER_OKAY)
    {
        struct timespec ts;
        clockid_t cid;

#  ifdef CLOCK_BOOTTIME
        cid = CLOCK_BOOTTIME;
#  elif defined(_POSIX_MONOTONIC_CLOCK)
        cid = CLOCK_MONOTONIC;
#  else
        cid = CLOCK_REALTIME;
#  endif

        if (clock_gettime(cid, &ts) == 0)
            return TWO32TO64(ts.tv_sec, ts.tv_nsec);
    }
# endif
# if defined(__unix__) \
     || (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L)
    {
        struct timeval tv;

        if (gettimeofday(&tv, NULL) == 0)
            return TWO32TO64(tv.tv_sec, tv.tv_usec);
    }
# endif
    {
        time_t t = time(NULL);
        if (t == (time_t)-1)
            return 0;
        return t;
    }
#endif
}