I have to admit, I was skeptical but my complier did pick SIMD for the second and not the first and it did make a huge difference. % cat t.c #include <sys/

I have to admit, I was skeptical but my complier did pick SIMD for the second and not the first and it did make a huge difference. % cat t.c #include <sys/time.h> #include <sys/resource.h> #include <stdio.h> double get_time() { struct timeval t; struct timezone tzp; gettimeofday(&t, &tzp); return t.tv_sec + t.tv_usec*1e-6; } int blah[SIZE][SIZE]; int whatever() { static int i=0; return ++i; } int main() { double t0 = get_time(); for(int i=0; i<SIZE; i++){ for(int j=0; j<SIZE; j++){ blah[j][i] = whatever(); // Column oriented is very slow. } } double t1 = get_time(); for(int j=0; j<SIZE; j++){ for(int i=0; i<SIZE; i++){ // Very fast: We move row-wise now. With luck, this is SIMD-vectorized by your compiler. blah[j][i] = whatever(); } } double t2 = get_time(); printf("SIZE=%5d dt1=%4.3e dt2=%4.3e\n",SIZE,t1-t0,t2-t1); return 0; } % for (( i=1; i<100000 ; i*=10 )) do gcc -DSIZE=$i -O t.c; ./a.out; done SIZE= 1 dt1=4.179e-04 dt2=0.000e+00 SIZE= 10 dt1=5.190e-04 dt2=0.000e+00 SIZE= 100 dt1=5.062e-04 dt2=9.537e-07 SIZE= 1000 dt1=4.014e-03 dt2=1.490e-04 SIZE=10000 dt1=1.347e+00 dt2=4.349e-02 2.8G Core i7 1600MHz DDR3