Better handling of splat func arguments

[summivox]

According to Petka, calling slice$ on arguments kills v8 optimization. He suggested to do a naive indexed for loop on arguments instead.

Currently:

f = (a, ...b, c) ->

compiles to:

f = function(a){
  var i$, b, c;
  b = 1 < (i$ = arguments.length - 1) ? slice$.call(arguments, 1, i$) : (i$ = 1, []), c = arguments[i$];
};

[summivox]

I did a patch (summivox/LiveScript@e6591cf) but it currently relies on #859 being merged. It can be done without #859 but I'm too lazy 😉

This patch itself does not change any semantics -- the only change is inlining slicing of arguments. Therefore I did not add/change any tests.

It works by inlining slice$ as a list comprehension. Above example compiles to:

f = function(a){
  var i$, b, res$, j$, c;
  res$ = [];
  for (j$ = 1 < (i$ = arguments.length - 1) ? 1 : (i$ = 1); j$ < i$; ++j$) {
    res$.push(arguments[j$]);
  }
  b = res$; c = arguments[i$];
};

According to http://jsperf.com/arguments-to-array/43

• prealloc-assign is the fastest on Chrome
• ES6 varargs (native splat) is the fastest on Firefox, followed by empty-push then prealloc-assign
• slice$.call(arguments) is universally slow

However, as long as we get rid of deoptimization, the cost of a single slice should be negligible. Prealloc-assign is much harder to implement than list comprehension/empty-push.

[isiahmeadows]

Ping @gkz (Babel and TypeScript do this by default, and CoffeeScript has been annoying people with the slow progress)

[summivox]

@isiahmeadows : currently @vendethiel is in charge (and yes he did comment on that issue too).

BTW: #859 has been merged, so I can actually make a pull request now.

[isiahmeadows]

@summivox Okay. I didn't know this.

(I don't keep up with this as much as I used to. LS 2.0 does seem pretty interesting, though.)