Rust Optimization: `Vec::into_iter().collect()`
from arendjr@programming.dev to rust@programming.dev on 10 Apr 2024 09:00
https://programming.dev/post/12598716
from arendjr@programming.dev to rust@programming.dev on 10 Apr 2024 09:00
https://programming.dev/post/12598716
I just had a random thought: a common pattern in Rust is to things such as:
let vec_a: Vec<String> = /* ... */; let vec_b: Vec<String> = vec_a.into_iter().filter(some_filter).collect();
Usually, we need to be aware of the fact that Iterator::collect() allocates for the container we are collecting into. But in the snippet above, we’ve consumed a container of the same type. And since Rust has full ownership of the vector, in theory the memory allocated by vec_a could be reused to store the collected results of vec_b, meaning everything could be done in-place and no additional allocation is necessary.
It’s a highly specific optimization though, so I wonder if such a thing has been implemented in the Rust compiler. Anybody who has an idea about this?
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I don’t know and don’t think so, but what you are doing is better done with retain anyways.
I mean, the actual operation is just an example, of course. Feel free to make it a
.map()operation instead. The strings couldn’t be reused then, but the vector’s allocation still could… in theory.map()can still be used withVec::iter_mut(),filter_map()can be replaced withVec::retain_mut().Yeah, that’s helpful if I would be currently optimizing a hot loop now. But I was really just using it as an example. Also,
retain_mut()doesn’t compose as well.I’d much rather write:
let vec_a: Vec<String> = /* ... */; let vec_b: Vec<String> = vec_a .into_iter() .filter(some_filter) .map(some_map_fn) .collect();Over:
let mut vec_a: Vec<String> = /* ... */; vec_a.retain_mut(|x| if some_filter(x) { *x = some_map_fn(*x); // Yikes, cannot move out of reference. true } else { false });And it would be nice if that would be optimized the same. After all, the point of Rust’s iterators is to provide zero-cost abstractions. In my opinion, functions like
retain_mut()represent a leakiness to that abstraction, because the alternative turns out to not be zero cost.blog.polybdenum.com/…/identifying-the-collect-vec… might be relevant to your question.
along with the related github.com/rust-lang/rust/issues/120091
Thanks! That’s very much what I was looking for!
Is it really fair to say retain doesn’t compose as well just because it requires reference-based update instead of move-based? I also think using move semantics for in-place updates makes it harder to optimise things like a single field being updated on a large struct.
It also seems harsh to say iterators aren’t a zero-cost abstraction if they miss an optimisation that falls outside what the API promises. It’s natural to expect
collectto allocate, no?But I’m only writing this because I wonder if I haven’t understood your point fully.
(Side note: I think you could implement the API you want on top of
retain_mutby usingstd::mem::replacewith a default value, but you’d be hoping that the compiler optimises away all thereplacecalls when it inlines and sees the code can’t panic. Idk if that would actually work.)The composability doesn’t have much to do with whether it’s a reference or a move, it’s because it bypasses usage of the
Iteratormethods. Iterators chains can consist offilter,mapand other operations, which allows various functions and/or closures to be composed together. Whereas withretain_mut()there isn’t really a chain and functions you may otherwise use in an iterator chain become harder to (re)use.You’re right, I wouldn’t say iterators aren’t a zero-cost abstraction. But most abstractions are also leaky – it’s just the extent in which the leakiness is exposed that makes them more or less effective. As such, saying to just use
retain_mutinstead of the iterator approach highlights the shortcoming of the abstraction. But if the same results could be achieved while still using the same iterator, that would make that abstraction more useful and consistent. And that’s great, because that means we need to worry less when using iterators :)To be fair, these alternatives are also limited to the case where the item type stays the same.
I think the better solution would be to use
Vec::retain().This blog post goes into some specifics of Rust reusing Vec allocations and some of the consequences. I think it’s really worth a read to better understand Vecs. From what I understand, it is possible that Rust will reuse the allocation of
vec_ain your case, but it ultimately is quite complicated.That was a super interesting and informative read! Exactly what I was hoping to find when I posted this, thanks!
I thought this was the point of
vec.drain(…).collect().The standard library does have some specialisation internally for certain iterators and collection combinations. Not sure if it will optimise that one specifically, but
Vec::into_iter().collect::<Vec>()is optimised (it may look silly, but it comes up with functions returningimpl Iterator