Hi folks,

How can I improve / speed up this frequent function?

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

// Assumptions: x, y, z, a, b and c are all array of 10.
// Requirement: return the value of ret;
//              get all elements of array c
float fnFrequentFunction(const float* x, const float* y, const float* z,
                         const float *a, const float *b, float *c, int M)
{
    register float tmp;
    register float sum;
    register float ret = 0;
    int i;

    for (i = 0; i < M; i++)  // M == 1, 2, 4, or 8
    {
        sum = 0;

        tmp = x[0] - y[0]; sum += tmp * tmp * z[0];
        tmp = x[1] - y[1]; sum += tmp * tmp * z[1];
        tmp = x[2] - y[2]; sum += tmp * tmp * z[2];
        tmp = x[3] - y[3]; sum += tmp * tmp * z[3];
        tmp = x[4] - y[4]; sum += tmp * tmp * z[4];
        tmp = x[5] - y[5]; sum += tmp * tmp * z[5];
        tmp = x[6] - y[6]; sum += tmp * tmp * z[6];
        tmp = x[7] - y[7]; sum += tmp * tmp * z[7];
        tmp = x[8] - y[8]; sum += tmp * tmp * z[8];
        tmp = x[9] - y[9]; sum += tmp * tmp * z[9];

        ret += (c[i] = log(a[i] * b[i]) + sum);
    }

    return ret;
}

int main()
{
    float x[10] = {0.001251f, 0.563585f, 0.193304f, 0.808741f, 0.585009f, 0.479873f, 0.350291f, 0.895962f, 0.622840f, 0.746605f};
    float y[10] = {0.864406f, 0.709006f, 0.091433f, 0.995727f, 0.227180f, 0.902585f, 0.659047f, 0.865627f, 0.846767f, 0.514359f};
    float z[10] = {0.866817f, 0.581347f, 0.175542f, 0.620197f, 0.781823f, 0.778588f, 0.938688f, 0.721610f, 0.940214f, 0.811353f};
    float a[10] = {0.870205f, 0.733879f, 0.711386f, 0.588244f, 0.484176f, 0.852962f, 0.168126f, 0.684286f, 0.072573f, 0.632160f};
    float b[10] = {0.871487f, 0.998108f, 0.798608f, 0.134831f, 0.576281f, 0.410779f, 0.402936f, 0.522935f, 0.623218f, 0.193030f};
    float c[8];
    int i;

    int n = 10000000;
    long start;

    // Speed test here:
    start = clock();
    while(--n)
        fnFrequentFunction(x, y, z, a, b, c, 8);
    printf("Time used: %ld\n", clock() - start);

    printf("fnFrequentFunction == %f\n", fnFrequentFunction(x, y, z, a, b, c, 8));

    for(i = 0; i < 8; ++i)
        printf("  c[%d] == %f\n", i, c[i]);

    printf("\n");

    return 0;
}

Any suggestions are welcome :-)