This is a fantastic idea. AST works well in an array based allocation block since it has no need for freeing individual nodes. It’s an add-only allocation.

"Instead of allocating Expr objects willy-nilly on the heap, we’ll pack them into a single, contiguous array." Zig compiler pipeline (AST, Zir, Air, Sema) does exactly this on all layers. Not only contiguous, but instead of array-of-structs it is struct-of-arrays, so walking the tree is even more cache friendly. For AST see: https://github.com/ziglang/zig/blob/master/lib/std/zig/Ast.z...

Amazing article, very good advice to keep your data structures flat.

Adding to that, it also makes editing the AST vastly more efficient.

I have discovered that principle on my own when I worked on an editor that directly operated on the AST instead of text. I found manipulating the tree-style AST so painful, constantly traversing the tree and all. Once I made it flat, my life was a hell lot easier. You can just directly index any part of AST in linear time.

Rust-analyzer uses a similar technique for parsing https://github.com/rust-lang/rust-analyzer/blob/master/crate... which then gets fed into https://github.com/rust-analyzer/rowan (lossless syntax tree)

Cool! Carbon is doing exactly this. I had asked leads if there was a paper on this approach, but they didn't have anything for me. I'll send them this post!

> Instead of allocating Expr objects willy-nilly on the heap, we’ll pack them into a single, contiguous array.

This happens naturally if you bump-allocate them in a garbage-collected run-time, particularly under a copying collector. Free lists also tend to co-locate because they are produced during sweep phases of GC which run through heaps in order of address.

Don't make me bring out the L word for the billionth time.

> A flat array of Exprs can make it fun and easy to implement hash consing

OK, it's not a case of L-ignorance, just willful neglect.

As the article mentions, this makes it quite similar to a bytecode vm. I think the traditional wisdom is that an AST walker is easy to write, but for speed you'd want a bytecode interpreter. It'd be interesting to see how close the performance gets with this flattened AST.

In practice I think there are more differences. E.g. AST interpreters tend to pass environments around while bytecode interpreters often store these on a stack (though I guess there's nothing stopping you from doing this with an AST either). I wonder if there's some goldilocks zone for ease of implementation with decent performance.

Related:

Flattening ASTs and other compiler data structures (2023) - https://news.ycombinator.com/item?id=42181603 - Nov 2024 (2 comments)

Flattening ASTs and other compiler data structures - https://news.ycombinator.com/item?id=36559346 - July 2023 (81 comments)

I thought a reddit comment on this article had an interesting point:

https://www.reddit.com/r/rust/comments/1d3b356/my_new_favori...

[–]Timzhy0 3 points 7 months ago

Btw I think one can go a step further than the author, there is no need to keep two explicit ExprRef baked in a binary node (lhs, rhs). You can exploit locality, basically the AST, seen it the LISP way, is just an arbitrarily nestable list, where elements are atoms or other lists. Hence all you need to know is where each list ends (and if it's an atom you can assume it spans one node) and actually one bit to know if it is the last entry in the list is quite ergonomic as well (because then you can distinguish whether moving next slot in the AST means there is a sibling). Basically it's easier to keep it sync while constructing and takes up less memory per node. I pay 40 bits per node, stored interleaved for best cache locality (some unaligned accesses but I think it's still worthwhile), 8 bits for the tag, 32 for the data, if data is bigger, 32 is an index into some auxiliary segment (basically a ptr).