rdtsc

static inline uint64_t rdtsc(void)
{
        uint32_t a;
        uint32_t d;
        asm volatile ("rdtsc" : "=a" (a), "=d" (d));

        return (uint64_t) a | ((uint64_t) d << 32);
}

See also

• https://stackoverflow.com/questions/9887839/how-to-count-clock-cycles-with-rdtsc-in-gcc-x86/9887979