之前的寫法在gcc 64bit會有問題~

在網路上找到下面寫法~

#include <stdio.h>
#include <stdint.h>

static inline uint64_t
rdtsc(void)
{
        uint32_t eax = 0, edx;

        __asm__ __volatile__("cpuid;"
                             "rdtsc;"
                                : "+a" (eax), "=d" (edx)
                                :
                                : "%rcx", "%rbx", "memory");

        __asm__ __volatile__("xorl %%eax, %%eax;"
                             "cpuid;"
                                :
                                :
                                : "%rax", "%rbx", "%rcx", "%rdx", "memory");

        return (((uint64_t)edx << 32) | eax);
}