Copying DOM NodeLists to genuine JS Arrays - Performance

Hi all,

I'm currently working on a script that executes over 7000 tags, in about 
7 seconds.

I analyzed it and determined that the slowest operation was this:

var length = tagsBase.length;
for(var i = 0; i < length; i++)
    tags.push(tagsBase[i]);

In other words, simply copying a DOM NodeList (fetched by 
.getElementsByTagName()) to a legit array.

It took over 4 seconds of the 7 second execution time. Another 2 seconds 
was spent performing a similar operation (with a filter applied).


The solution I applied to optimize it uses Duff's Device:
http://home.earthlink.net/~kendrasg/info/js_opt/jsOptMain.html
(bottom of that page)

I used a 64-wide version of the Modified version - here's a code snippet:

Array.appendNodeList64(array, tags) {
    // Modified Duff's Device (Tom Duff, Kendra SG, Anon)
    var iterations = tags.length;
    var iTags = iterations - 1;
    var i = iTags + array.length;

    var n = iterations % 64;
    while (n--)
    {
        array[i] = tags[iTags]; i--; iTags--;
    }
       
    n = Math.floor(iterations / 64);
    while (n--)
    {
        array[i] = tags[iTags]; i--; iTags--;
       // 64 of the above line . . .
    }
}


The Duff's Device improved performance as follows:

Firefox:
push of 7000 elements without: 4.28s
push of 7000 elements with: 2.08s

So that's pretty significant.

Of course, a simple Array.concat() of 7000 elements occurs in something 
like .2s, and I really wish I could hit a performance number closer to 
that - even 2s wasted on this trivial copy seems silly.

What performance tricks have others found to speed up this operation?

Thanks,
-Chris "SoopahMan" Moschini


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