Forum libero

        ;; ecx <- source[i]
        movl    $1374389535, %edx
        movl    %ecx, %eax
        imull   %edx
        sarl    $3, %edx
        movl    %ecx, %eax
        sarl    $31, %eax
        subl    %eax, %edx
        movslq  %ebx,%rax
        ;; edx -> dest[i]

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

#define M 10000000
#define REP 50

int source[M];
int dest[M];
int divtab[65536];

int main(void) {
  int i, m;
  time_t start;

  for (i = 0; i < M; ++i) source[i] = ((double)rand() / INT_MAX) * 65535;
  for (i = 0; i < 65536; ++i) divtab[i] = i / 25;

  start = time(NULL);

  printf("Inizio divisioni: %ld\n", start);

  for (m = 0; m < REP; ++m) 
    for (i = 0; i < M; ++i) dest[i] = source[i] / 25;

  printf("Divisioni: %ld\n", time(NULL) - start);

  start = time(NULL);

  printf("Inizio lookups: %ld\n", start);

  for (m = 0; m < REP; ++m) 
    for (i = 0; i < M; ++i) dest[i] = divtab[source[i]];
    
  printf("Lookups: %ld\n", time(NULL) - start);

  return 0;
}

#include <algorithm>

using namespace std;

int filtro(int img_in[][COL], int img_out[][COL], int rig)
{
    for(int i=0;i<rig;i++)
    {     
      int lim_inf_r = max(0, i-2), lim_sup_r = min(rig-1, i+2);
      for(int j=0;j<COL;j++) {
    int lim_inf_c = max(0, j-2), lim_sup_c = min(COL-1, j+2);
    long subtotale=0; //per sommare dei numeri grandi, ci vuole una variabile grande
    int n_elementi;
    
    //Ora che si sono calcolati gli estremi, si calcola la media della sottomatrice
    for (int sub_r=lim_inf_r; sub_r<=lim_sup_r; sub_r++)
      for (int sub_c=lim_inf_c; sub_c<=lim_sup_c; sub_c++)
        subtotale+=img_in[sub_r][sub_c];
    
    n_elementi=(lim_sup_r-lim_inf_r+1)*(lim_sup_c-lim_inf_c+1);
    img_out[i][j]=subtotale/n_elementi;
      }        
    }
    return 0;
}