static inline uint64_t rdtsc()
{
  uint64_t tsc;

  __asm__ __volatile__(
    "lfence;"
    "rdtsc;"
    "shl $32, %%rdx;"
    "or %%rdx,%%rax"
    : "=a" (tsc)
    :
    : "%rcx", "%rdx", "memory"
  );

  return tsc;
}