Hacker News Reader

Comment by hencq | Hacker News Reader

The article mentions that for optimizing out of bounds checks it uses constant lists or fixed length lists. Could it also optimize the check in a loop if it knows the loop ends at the end of the list? For example, in the article it mentions:

  for (int i = 0; i < v.constraints.length; i++) {
    ...
    c = t1[i]; 
    ...
  }

Does it do that optimization because it knows v.constraints is fixed length or because it knows that i can never be bigger than v.constraints.length?

View on HN

dart2js does range analyses. For example the indexing in the following example will not do any bounds checks.

    for (int i = 0; i < list.length; i++) {
      var t = list[i];
    }

Things get more complicated when there are function calls in the loop-body that could potentially change the list-length:

    for (int i = 0; i < list.length; i++) {
      foo("random call", list);  // could have side-effect.
      var t = list[i];  // needs to check bounds.
    }

However if the element is read before the call, it should still work:

    for (int i = 0; i < list.length; i++) {
      var t = list[i];  // no need for bounds-check.
      foo("random call", list);  // could have side-effect.
    }

The last example doesn't need a bounds-check, but it does need to reload the length-field of the list at every iteration (in case the length changed).

Reply on news.ycombinator.com