Why vectorizing the loop does not have performance improvement

Question

I am investigating the effect of vectorization on the performance of the program. In this regard, I have written following code:

#include 
#in         


        

Answer 1

Just in case a[] b[] and c[] are fighting for the L2 cache ::

#include  /* for memcpy */

 ...

 gettimeofday(&stTime, NULL);

    for(k = 0; k < LEN; k += 4) {
        double a4[4], b4[4], c4[4];
        memcpy(a4,a+k, sizeof a4);
        memcpy(b4,b+k, sizeof b4);
        c4[0] = a4[0] * b4[0];
        c4[1] = a4[1] * b4[1];
        c4[2] = a4[2] * b4[2];
        c4[3] = a4[3] * b4[3];
        memcpy(c+k,c4, sizeof c4);
        }

    gettimeofday(&endTime, NULL);

Reduces the running time from 98429.000000 to 67213.000000; unrolling the loop 8-fold reduces it to 57157.000000 here.