Optimization1

Copy the contents of file MMult0.c into a file named MMult1.c and change the contents:

from

/* Create macros so that the matrices are stored in column-major order */

#define A(i,j) a[ (j)*lda + (i) ]
#define B(i,j) b[ (j)*ldb + (i) ]
#define C(i,j) c[ (j)*ldc + (i) ]

/* Routine for computing C = A * B + C */

void MY_MMult( int m, int n, int k, double *a, int lda, 
                                    double *b, int ldb,
                                    double *c, int ldc )
{
  int i, j, p;

  for ( i=0; i<m; i++ ){        /* Loop over the rows of C */
    for ( j=0; j<n; j++ ){        /* Loop over the columns of C */
      for ( p=0; p<k; p++ ){        /* Update C( i,j ) with the inner
				       product of the ith row of A and
				       the jth column of B */
	C( i,j ) = C( i,j ) +  A( i,p ) * B( p,j );
      }
    }
  }
}

to

/* Create macros so that the matrices are stored in column-major order */

#define A(i,j) a[ (j)*lda + (i) ]
#define B(i,j) b[ (j)*ldb + (i) ]
#define C(i,j) c[ (j)*ldc + (i) ]

/* Routine for computing C = A * B + C */

void AddDot( int, double *, int, double *, double * );

void MY_MMult( int m, int n, int k, double *a, int lda, 
                                    double *b, int ldb,
                                    double *c, int ldc )
{
  int i, j;

  for ( j=0; j<n; j+=1 ){        /* Loop over the columns of C */
    for ( i=0; i<m; i+=1 ){        /* Loop over the rows of C */
      /* Update the C( i,j ) with the inner product of the ith row of A
	 and the jth column of B */

      AddDot( k, &A( i,0 ), lda, &B( 0,j ), &C( i,j ) );
    }
  }
}


/* Create macro to let X( i ) equal the ith element of x */

#define X(i) x[ (i)*incx ]

void AddDot( int k, double *x, int incx,  double *y, double *gamma )
{
  /* compute gamma := x' * y + gamma with vectors x and y of length n.

     Here x starts at location x with increment (stride) incx and y starts at location y and has (implicit) stride of 1.
  */
 
  int p;

  for ( p=0; p<k; p++ ){
    *gamma += X( p ) * y[ p ];     
  }
}

Change the first lines in the makefile to

OLD  := MMult0
NEW  := MMult1

• make run

octave:3> PlotAll        % this will create the plot

This time the performance graph will look something like

We see no performance benefit. What we are doing is slowly changing the code into one where there will be a performance benefit.