tibsar tibsar - 2 months ago 17
C Question

Optimization of C loop

I have the following function to calculate coefficients:

void CalculateCoefficients(LinearFit *DataSet, double *A, double *B)
{
/* Declare and initialize sum variables */
double S_XX = 0.0;
double S_XY = 0.0;
double S_X = 0.0;
double S_Y = 0.0;
int lcv;

/* Compute the sums */
for (lcv=0; lcv < DataSet->NextElement; lcv++)
{
S_XX += DataSet->Data_X[lcv] * DataSet->Data_X[lcv];
S_XY += DataSet->Data_X[lcv] * DataSet->Data_Y[lcv];
S_X += DataSet->Data_X[lcv];
S_Y += DataSet->Data_Y[lcv];
} /* for() */

/* Compute the parameters of the line Y = A*X + B */
(*A) = (((DataSet->NextElement * S_XY) - (S_X * S_Y)) / ((DataSet->NextElement * S_XX) - (S_X * S_X)));
(*B) = (((S_XX * S_Y) - (S_XY * S_X)) / ((DataSet->NextElement * S_XX) - (S_X * S_X)));
} /* CalculateCoefficients() */


I am looking to optimize the loop. I tried loop unrolling but it didn't do much. What else can I do?

Answer

You could try:

double dsdx, dsdy;
....
dsdx = DataSet->Data_X[lcv];
dsdy = DataSet->Data_y[lcv];
S_XX += dsdx * dsdx;
S_XY += dsdx * dsdy;
S_X  += dsdx;
S_Y  += dsdy;
...

This way you only get the values out of your struct once in each iteration of the loop.