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Comment by hurril | Hacker News Reader

I've been seriously at it with Rust for about six months now and really loving it. What do you mean by "use an arena for the AST?" What is an arena in this context?

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I believe it means flattening the AST, here is a nice blog post about this technique https://www.cs.cornell.edu/~asampson/blog/flattening.html

Nice! Thank you! I'd forgotten about that technique - think I read about it in a ancient compiler book (by french authors no less!)

EDIT: this was a really interesting read!

https://docs.rs/bumpalo/latest/bumpalo/

If you're curious about arena allocators, look at the rust compiler itself. It uses one for all of its allocations as regular heap allocation was not performant enough.

i assume they're talking about an Arena Allocator

https://blog.logrocket.com/guide-using-arenas-rust/

I figured as much but was unsure. So the gain is that you still take heap allocations, but you do it in a "novelty" heap allocator that you will probably dipose of entirely at some point?

Allocation happens roughly like this:

If current top of heap + allocation size > buffer size, allocate an extra buffer for the arena. Save the current top in a temp variable. Add the allocation size to the top Return the temp variable.

It's fast, and the amortized memory overhead per allocation is near zero because you don't need to track allocation sizes or free lists, since you only free the whole arena at once (one or more buffers).

It's ideal for anything where the lifetime of allocations is known to be roughly the same. E.g. a data structure you'll free all at once.

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