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Show HN: Nova JavaScript Engine

(github.com)
269 points aapoalas | 10 comments | 17 Nov 24 23:07 UTC | HN request time: 1.561s | source | bottom

We're building a different kind of JavaScript engine, based on data-oriented design and willingness to try something quite out of left field. This is most concretely visible in our major architectural choices:

1. All data allocated on the JavaScript heap is placed into a type-specific vector. Numbers go into the numbers vector, strings into the strings vector, and so on.

2. All heap references are type-discriminated indexes: A heap number is identified by its discriminant value and the index to which it points to in the numbers vector.

3. Objects are also split up into object kind -specific vectors. Ordinary objects go into one vector, Arrays go into another, DataViews into yet another, and so on.

4. Unordinary objects' heap data does not contain ordinary object data but instead they contain an optional index to the ordinary objects vector.

5. Objects are aggressively split into parts to avoid common use-cases having to reading parts that are known to be unused.

If this sounds interesting, I've written a few blog posts on the internals of Nova over in our blog, you can jump into that here: https://trynova.dev/blog/what-is-the-nova-javascript-engine

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liontwist ◴[18 Nov 24 15:26 UTC] No.42173207[source]▶
This is a great idea! I had thought about doing this with a lisp interpreter. I had identified a few key advantages:

- homogenous allocation means no alignment gaps - linear access win in garbage collection - indices smaller than pointers - type discriminated index can save some size

I haven’t verified whether those actually work out in the details. I’ll read your blog article.

Don’t bother with these comments immediately comparing it to V8 (a multi billion dollar venture). I don’t know how many creative projects they’ve done before.

You may be be interested in looking at Fabrice Bellard’s JS engine for ideas.

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1. mbrock ◴[18 Nov 24 19:53 UTC] No.42176218[source]▶
I actually made a Lisp interpreter in Zig a couple of years ago that has each object type in a separate heap array. In fact each field of each object type has its own array: every CDR is in one contiguous array. This was mostly for fun and to experiment with data-driven techniques using Zig metaprogramming. The code turned out relatively clean and simple.

https://github.com/mbrock/wisp

GC is stop&copy which as a side effect compacts each of those arrays and improves locality. I think most lists should end up having their CDRs next to each other in memory making iteration very cache friendly. But I didn't verify any performance qualities, beyond making it efficient enough for basic use.

It also has delimited continuation control, compiles to WebAssembly, and hooks promises into the continuation system, among some other pretty cool features!

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2. mbrock ◴[18 Nov 24 20:04 UTC] No.42176308[source]▶
Oh yeah, continuation pointers also have their own array like every other field kind, which should have similar benefits as list traversal but for continuation copying... It's a really interesting design area, I think. Zig makes it easy!
replies(1): >>42176386 #
3. aapoalas ◴[18 Nov 24 20:10 UTC] No.42176372[source]▶
Well I'll be damned! That sounds very much like what I want Nova to eventually be :) We don't have fields split apart at present, mostly because Rust doesn't make that quite as easy as I would want to. Otherwise it sounds like it's very much all the same, in a good way.

I'll definitely be taking a look at wisp, thank you very much for the link! If you ever have the time, I'd love seeing a comparison of this sort of engine design against a more traditional one.

Sorry, what is "CDR" in this context though?

replies(1): >>42176472 #
4. aapoalas ◴[18 Nov 24 20:12 UTC] No.42176386[source]▶
Yeah, I'm quite envious of the MultiArrayList or whatever it was that Zig has: If only Rust had that sort of a type built-in <3
replies(1): >>42176557 #
5. mbrock ◴[18 Nov 24 20:20 UTC] No.42176472[source]▶
Quick reply to the cdr thing: car/cdr are old Lisp names for the head/tail fields of linked list cells! :)
replies(1): >>42176599 #
6. mbrock ◴[18 Nov 24 20:26 UTC] No.42176557{3}[source]▶
That's how I got interested in this kind of memory layout in the first place. I wanted a nice Lisp for WebAssembly and had recently gotten into Zig. When I started defining the word structure I remembered Andy Kelley's talk about using data-oriented design to make the Zig compiler fast, so I thought I'd try it, and the more I thought about it the more reasonable it seemed.

There are like a dozen object types with different growing multiarrays. Words are 32 bit with 1 for GC state and 27 for index and the rest are the type tag. Ints are 28 bits. Byte arrays have their own heap too, as well as general 32 bit vectors.

7. aapoalas ◴[18 Nov 24 20:29 UTC] No.42176599{3}[source]▶
Ah, of course!
8. liontwist ◴[18 Nov 24 20:43 UTC] No.42176767[source]▶
Yes. the right thing to do is to treat a list as a general case and other uses of cons as special case
replies(1): >>42177938 #
9. codr7 ◴[18 Nov 24 22:47 UTC] No.42177938[source]▶
I've flipped that idea around in a few of my own language designs, where pairs are the central feature and lists are just pairs with pair cdrs. Works fine from what I can see.
replies(1): >>42178815 #
10. liontwist ◴[19 Nov 24 00:20 UTC] No.42178815{3}[source]▶
Yes pairs is the 1980s lisp design, but it’s not good for modern caches. Both obviously work.