Fix deopt caused by slicing arguments object

[STRML]

Gotta go fast

[rtsao]

Awesome, thank you for pointing this out, TIL. :smile:

By the way, I used IRHydra2 to look up the intermediate representations of your proposed change and the following (which omits the length cache):

var values = Array(arguments.length - 1);
for (var i = 1; i < arguments.length; i++) {
  values[i - 1] = arguments[i];
}

Your suggestion and the above yielded identical intermediate representations, which seems to confirm what is discussed in this post: http://mrale.ph/blog/2014/12/24/array-length-caching.html

All else being equal, I think would prefer omitting the length cache, if you wouldn't mind updating your PR.

Regardless, I'd definitely like to start benchmarking (https://github.com/rtsao/csjs/issues/12) and hopefully identify further optimizations.

[STRML]

Glad you're thinking about that. I was too. I found an old thread from about 2013 saying that preallocating the array doesn't affect performance, but in my testing and many others', it clearly does. It appears that the length access is indeed properly cached in an optimized function. V8 is a fickle yet powerful beast.

I've amended the commit.

[STRML]

By the way, thanks for pointing out IRHydra2. That is seriously, seriously awesome. Been looking for something like this to validate some of my assumptions/guesses.