testalign.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/time.h>
#include <stdint.h>

#ifdef __MACH__
#define CLOCK_REALTIME 0 
#include <sys/time.h>
//clock_gettime is not implemented on OSX
int clock_gettime(int clk_id, struct timespec* t) {
    struct timeval now;
    int rv = gettimeofday(&now, NULL);
    if (rv) return rv;
    t->tv_sec  = now.tv_sec;
    t->tv_nsec = now.tv_usec * 1000;
    return 0;
}
#endif

#define LEN 512
typedef struct __attribute__ ((__packed__)) {
		int32_t N;
		char _padding[60];
} padded;

typedef struct __attribute__ ((__packed__)) {
        padded X[LEN];
        padded Y[LEN];
} EVILXY;

typedef struct __attribute__ ((__packed__)) {
        int32_t X;
        int32_t Y;
} XY;

int main(void) {
        EVILXY *evil;
        XY *good;
        int i, loop;
        struct timespec begin;
        struct timespec end;

        posix_memalign((void**)&evil, 4096, sizeof(EVILXY));
        posix_memalign((void**)&good, 4096, LEN*sizeof(XY));

		clock_gettime(CLOCK_REALTIME, &begin);
        for (i=0;i<LEN;i++) {
                evil->X[i].N^=evil->Y[i].N;
                evil->Y[i].N^=evil->X[i].N;
                evil->X[i].N^=evil->Y[i].N;
        }
		clock_gettime(CLOCK_REALTIME, &end);
        printf("nanoseconds of evil prog:%ld\n", end.tv_nsec - begin.tv_nsec);

		clock_gettime(CLOCK_REALTIME, &begin);
        for (i=0;i<LEN;i++) {
                good[i].X^=good[i].Y;
                good[i].Y^=good[i].X;
                good[i].X^=good[i].Y;
        }

		clock_gettime(CLOCK_REALTIME, &end);
        printf("nanoseconds of good prog:%ld\n", end.tv_nsec - begin.tv_nsec);
}