t078.js

function t1() {
  var ary = [];
  for (var i = 0; i < 10000000; ++i) {
    ary[i] = i;
  }
  return ary;
}

function t2() {
  var ary = [];
  var i = 10000000;
  while (i--) {
    ary[i] = i;
  }
  return ary;
}

var now = Date.now();
t1();
print(Date.now() - now);

var now = Date.now();
t2();
print(Date.now() - now);

t2 is very slow. Because t2 becomes sparse and GetElementIC cannot apply it.