Hacker News Reader

Comment by gsnedders | Hacker News Reader

No, the primary optimization asm.js brings (at least in OdinMonkey) is AOT compilation.

Fully typed code should produce the same code from IonMonkey without OdinMonkey: the problem is you have to get up to 10k calls (at least last I looked!) of a function before you get there; but once you are there it is just as good. It's the build-up, through the interpreter and the less efficient code-generators (in terms of code generated, that is; they're more efficient in terms of their own runtime), which really hurts.

View on HN

No, the primary optimization asm.js brings (at least in OdinMonkey) is AOT compilation.

But it can do that precisely because of the static typing, which loops back to exactly what I said. Looser, more dynamic language always have a cost associated with that flexibility.

Arbitrary JS can be as statically typed as asm.js is (because, well, asm.js is just a subset). There's no reason why one couldn't compile AOT something like:

  function add_and_double(a, b) {
    a = a | 0;
    b = b | 0;
    var sum = (a + b) | 0;
    return (sum + sum) | 0;
  }

Yes, it'd be trivial to fall off the AOT-compilation path (esp. without a defined subset that got AOT compilation and hence likely would differ between implementations), so static verification that you're on the AOT-compilation path is valuable (which asm.js provides), but it doesn't fundamentally alter the semantics of the language.

You seem to be saying that the AOT path requires constrained typing and a strict subset of the language semantics, which agrees with the parent's point that static type semantics matter in terms of performance.

They matter, yes. But they aren't in any way specific to asm.js — it was something that was already true in generic JS.

But it is extremely specific to asm.js -- the optimizations possible can only happen because static typing is mandated. It is absolutely a subset of JS, but JS can't make those optimizations because the guarantees aren't there.

Reply on news.ycombinator.com