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Show HN: Faster.js – a micro-optimizing JavaScript compiler (github.com/vzhou842)
89 points by vzhou842 on April 20, 2018 | hide | past | favorite | 32 comments

This is cool, but array methods are so rarely the bottleneck; Even in the highly-specialized demo, the difference is near-negligible.

It's a fun project and might have application in specialized situations (and maybe for Node, where time spent in synchronous iteration blocks all other requests?). But I feel like this does more harm than good in most cases (larger bundle, extra build step, less readable output code can make debugging harder, etc)

I agree, while this is a cool idea it doesn't seem to be useful beyond being a proof of concept of an optimization technique. For example, my browser rendered the faster.js version 50ms slower at this point: https://i.imgur.com/G5b10Ns.jpg. It does appear to perform better in the long run (https://i.imgur.com/u3ApYYl.jpg) but it's an improvement of less than 1%, and when your renders are taking 30+ms there are probably other things you should be looking at :P

Also, unless I'm misunderstanding the README, using the plugin introduces the huge caveat of breaking code that uses "restricted names" (certain names of Array functions) - the example given is a class that defines a map() function, which apparently would cause some kind of failure. In larger codebases where you don't control all code used in production this seems like a big problem.

Agreed for the most part - this realistically does not have a good generalized use case. IMO the only real harm is the slightly larger bundle, though - I think the extra build time is negligible and you could just disable this optimization when developing.

My main motivation for building this was to make a better way to use an optimization library like fast.js (https://github.com/codemix/fast.js), which inspired faster.js. A huge issue with the fast.js library is that you have to rewrite your entire codebase in order to use it, whereas enabling or disabling faster.js is a one line change.

To the extent this tool will make it appear that there is less usage of functional JS patterns browser vendors deciding how to optimize their engines, I'm not a fan. Compiling away a useful feature or pattern just because it's not yet optimized creates a risk of a self-fulfilling prophecy. Obviously this is not a zero-sum game and optimizing for one pattern doesn't necessarily mean that something else must suffer, but prioritization of what to optimize is sometimes based on instrumentation measuring deployed usage and not just artificial benchmarks.

The performance gap between a `for` loop and and a `.forEach` (also map) is much narrower than it used to be, and that is very encouraging.

It’s seems like it does harm in JS environments that have JITs. But it would be interesting in strictly interpreted environments. Some devices run JS but disallow JIT due to security concerns.

Optimizing JavaScript is hard.

v8 for instance, has the interpreter (ignition), and optimizing compiler (turbofan), that have a lot of undocumented behavior that people just to try to probe via microbenchmarks.

- https://en.wikipedia.org/wiki/Inline_caching

- https://static.googleusercontent.com/media/research.google.c...

If you want to understand what is going on the only way is to read v8's source code or running a profiler.

For example, did you know that touching the prototype of a RegExp object makes it slow? You had no way of knowing that.

Prepack, https://prepack.io, is also in the same realm of an optimizing Javascript compiler.

Prepack is somewhat different though in that it precalculates what can be. That's much less problematic than what the linked library does.

Of course prepack also isn't even close to production ready yet either.

Prepack is rather nice. It allows you to execute part of your code at compile time, like a macro.

Closure compiler in ADVANCED_OPTIMIZATIONS mode does this too.

    function add5(x) {
      return x + 5;

compiles down to


Prepack is doing much more than GCC can because it effectively evaluates the whole code and then serialises its heap. This is both its biggest strength and weakness.

Unlike GCC, it can unroll any kind of metaprogramming, but it needs to have a model of the environment (e.g. it won’t execute code relying on DOM) and it can produce larger code (in terms of absolute size).

I would avoid using the name GCC to refer to the closure compiler, since there's already an extremely well-known compiler named GCC.

Admittedly it doesn't take JS as input, but I was somewhat confused when I read your comment nonetheless.

oh, cool. thanks for the info.

Incidentally, http://www.scala-js.org compiler has constant folding too.

Also if you need faster numeric calculations https://github.com/non/spire (has scala.js support)

On the demo page in Chrome, Faster.js is actually slower.

For me faster.js was faster for 2 seconds then it became slower and slower in Firefox

faster.js was about 9ms faster on my mobile device.

For me faster.js seems to be slower in Safari about 2-3ms.

it works out to 20-25% for me, but I'm on 10.13.5 beta, not sure what changes have been made in safari.

I'd like to be able to specify restrictions on type usage in Javascript. For instance, in some cases I know that my array will always be 6 items larges - no smaller, no larger, never sparse. But I can't specify this, yet it allows for certain optimizations (e.g. loop unrolling).

Lately I was memory profiling an application in Javascript using Chrome. The fact that it used ES6 classes really helped, as some anonymous classes where hard to find memory leaks with, but the typed classes was a simple text query.

I can imagine ES6 classes with decorators may enable specialized optimizations too.

Probably, you can optimize .forEach even more. You can get 'arr.length' and 'let i = 0' outside of the for-loop.

It could be faster according to this test: https://jsperf.com/extended-array-loops-performance

Why is this called a "compiler", though?

Their first example

for (let _i = 0; _i < arr.length; _i++) { results.push(_f(arr[_i], _i, arr)); }

This is recalculating length on each iteration

The mystery of JIT optimization. The optimizer will turn that into a dereferenced register value. I remember reading that v8 used to add an extra instruction if you assigned the variable yourself.

Are you using this in a real application?

I've been trying it out in prod on a small site I run: http://geoarena.online

The demo page basically shows this is pointless practically - and sometimes _slower_.

These kind of optimizations were very useful in hot paths even just a few years ago. However browser engines have optimized functional patterns (what this mostly rewrites) to be within absolute microseconds of imperative versions.

In some cases it can even make useful assumptions in an FP method (immutability, etc) that it can't in the standard loops - which is why sometimes this is slower.

Long story short: concentrate on shipping less JavaScript and good practical solutions/algorithms and not stuff like this.

I thought I was in new and not the front page. You're telling me that as a demonstration of perf gains, in a graphics context I win back .8 ms in exchange for another dependency and unnecessary cognitive load?

This is a good example of premature optimization.

I think this is an example of _optimization_. An optimization that is easily applied as a transform for Babel, but that's just it.

Whether it is premature or not, it would depend on if you apply it carelessly without knowing whether you need it, or if you apply it when you know you need. Right?

The optimization is transforming all code that matches the pattern independent from whether the code is hot or not.

The transformation comes with cognitive cost for the developer as it can turn correct code into incorrect code (sparse arrays).

By enabling this one trades complexity for potential performance gains.

This is how I understand premature.

Is it an optimization when it makes things slower?

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