c1.c

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

double A[1024][1024];

int
main(){
	int i,j,k;
	k = 0;
	clock_t start,end;
	start = clock();

	while (k < 1000){
		for (i=0; i<1000; i++){
			for (j=0; j<1000; j++){
				A[i][j] +=1;
			}
		}
		k++;
	}
	end = clock();
	printf("%.2f sec.\n",(double)(end-start)/CLOCKS_PER_SEC);
}

c2.c

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

double A[1024][1024];

int
main(){
	int i,j,k;
	k = 0;
	clock_t start,end;
	start = clock();

	while (k < 1000){
		for (i=0; i<1000; i++){
			for (j=0; j<1000; j++){
				A[j][i] +=1;
			}
		}
		k++;
	}
	end = clock();
	printf("%.2f sec.\n",(double)(end-start)/CLOCKS_PER_SEC);
}

gistfile1.txt

$ gcc -o c1 c1.c 
$ gcc -o c1 c1.c 
$ ./c1
4.11 sec.
$ ./c2
26.96 sec.