(Question 7.45 in P&H)
Why might a compiler perform the following optimization?

/* Before */
for (j=0; j<20; j++) {
  for (i=0; i<200; i++) {
    x[i][j] += 1;
  }
}

/* After */
for (i=0; i<200; i++) {
  for (j=0; j<20; j++) {
    x[i][j] += 1;
  }
}