根据 http://blog.shengbin.me/posts/gcc-attribute-aligned-and-packed ，写了个测试用例，执行在 x86_64 上。理论上应该输出 80000 才对，实际执行的时候只有在 gcc 使用 -O1 及其以上优化时，才能成功。

failed_test.c

#include <stdio.h>
#include <time.h>
#include <string.h>
#include <pthread.h>

void *add_counter(void* c) {
    int i;
    for (i = 0; i < 10000; i++) {
        (*(int*)c)++;
    }
    return NULL;
}

int main() {
    const int ThreadN = 8;
    int i, ret;
    pthread_t threads[ThreadN];
    unsigned long counter __attribute__ ((aligned (64))) = 0UL;

    for (i = 0; i < ThreadN; i++) {
        if ((ret = pthread_create(&threads[i], NULL, add_counter, (void*)(&counter))) != 0) {
            fprintf(stderr, "error: %s", strerror(ret));
            return 1;
        }
    }

    for (i = 0; i < ThreadN; i++) {
        if ((ret = pthread_join(threads[i], NULL)) != 0) {
            fprintf(stderr, "error: %s", strerror(ret));
            return 1;
        }
    }

    printf("counter: %lu\n", counter);

    return 0;
}