Bug 155146 – Array destructuring is very slow

• Status: RESOLVED
• Resolution: DUPLICATE
• Component: JavaScriptCore

[Joseph Pecoraro]

Created attachment 273235 [details]
[TEST] Array Destructuring Benchmark

* SUMMARY
Array destructuring is very slow.

There is a 20x performance difference between using destructuring with Arrays versus not. Object destructuring does not seem to have this cost.

    [Log] Array - Normal - 2ms
    [Log] Array - Destructuring - 48ms
    [Log] Object - Normal - 2ms
    [Log] Object - Destructuring - 1ms

* TEST (full test attached)

    bench("Array - Normal", function(arr) {
        var sum = 0;
        for (var i = 0; i < arr.length; ++i) {
            var list = arr[i];
            var a = list[0], b = list[1], c = list[2], d = list[3], e = list[4];
            sum += (a + b + c + d + e);
        }
        return sum;
    }, listOfLists);

    bench("Array - Destructuring", function(arr) {
        var sum = 0;
        for (var i = 0; i < arr.length; ++i) {
            var [a, b, c, d, e] = arr[i];
            sum += (a + b + c + d + e);
        }
        return sum;
    }, listOfLists);

[Joseph Pecoraro]

In Safari Technology Preview array destructuring has improved greatly! However it is still considerably slower than normal array access: (~7x down from ~20x).

    [Log] Array - Normal - 2ms
    [Log] Array - Destructuring - 15ms
    [Log] Object - Normal - 1ms
    [Log] Object - Destructuring - 1ms

[Saam Barati]

(In reply to comment #1)
> In Safari Technology Preview array destructuring has improved greatly!
> However it is still considerably slower than normal array access: (~7x down
> from ~20x).
> 
>     [Log] Array - Normal - 2ms
>     [Log] Array - Destructuring - 15ms
>     [Log] Object - Normal - 1ms
>     [Log] Object - Destructuring - 1ms

I think we really need more comprehensive support for making the iterator protocol fast. Whatever we do to make for-of faster, should also make this faster.

[Saam Barati]

*** This bug has been marked as a duplicate of bug 169030 ***