The strange speed of a program!

[bear]

hi all,
I have a program whose speed is so strange to me. It is maily used
to
calculate a output image so from four images s0,s1,s2,s3 where
so=(s0-s2)^2+ (s1-s3)^2. I compile it with gcc (no optimization). the
codec between /***********/ is the initialization code. What supprise
me a lot
is the code with initialization( io==1) is much faster than without
initialization( io!=1). The initialization code should takes some time
and it should slow down the program. But the result confuse me. Why?

Code is listed below
=============== =============== =============== =============== ======
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
clock_t start,end;
double cpu_time_used;
short *s0,*s1,*s2,*s3 ;
int *so;
unsigned int i,j;
unsigned int times;
unsigned int length;
int io;
if( argc<4 )
{
fprintf( stderr,"USAGE: %s times width (1 initialize, otherwize
noinitialize)\n ",argv[0] );
exit(1);
}
else
{
times = atoi( argv[1] );
length = atoi( argv[2] );
length = length*length;
io = atoi( argv[3] );
}
s0 = (short *)malloc( length*sizeof(s hort) );
s1 = (short *)malloc( length*sizeof(s hort) );
s2 = (short *)malloc( length*sizeof(s hort) );
s3 = (short *)malloc( length*sizeof(s hort) );
s3 = (short *)malloc( length*sizeof(s hort) );
so = (int *)malloc( length*sizeof(i nt) );
start = clock();
for( i=0; i<times; ++i)
{
/*************** *************** *************** *****/
if( io==1 )
{
for( j=0; j<length; ++j )
{
s0[j] = i+j;
s1[j] = length-1-j;
s2[j] = 2*j;
s3[j] = 3*j;
}
}
/*************** *************** *************** *****/
for( j=0; j<length; ++j )
{
int tmp1,tmp2;
tmp1 = s0[j]-s2[j];
tmp1 = tmp1*tmp1;
tmp2 = s1[j]-s3[j];
tmp2 = tmp2*tmp2;
so[j] = tmp1+tmp2;
}
}
end = clock();
cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
printf("CPU time: %f sec\n",cpu_time _used);
free( s0 );
free( s1 );
free( s2 );
free( s3 );
free( so );
return 0;
}

[Lawrence Kirby]

On Sat, 11 Jun 2005 18:41:56 +0100, Lawrence Kirby wrote:

Please ignore this last posting which is incomplete and incorrect. It was
an old working copy that was posted by accident.

Lawrence