RFC: No (opsem) Magic Boxes

[chorman0773]

Summary

Currently, the operational semantics of the type alloc::boxed::Box<T> is in dispute, but the compiler adds llvm noalias to it. To support it, the current operational semantics models have the type use a special form of the Unique (Stacked Borrows) or Active (Tree Borrows) tag, which has aliasing implications, validity implications, and also presents some unique complications in the model and in improvements to the type (e.g. Custom Allocators). We propose that, for the purposes of the runtime semantics of Rust, Box is treated as no more special than a user-defined smart pointer you can write today¹. In particular, it is given similar behaviour on a typed copy to a raw pointer.

Rendered

Footnotes

1. We maintain some trivial validity invariants (such as alignment and address space limits) that a user cannot define, but these invariants only depend upon the value of the Box itself, rather than on memory. ↩

[clarfonthey]

It feels odd that one of the clear options is left out: why not expose a Unique<T> that has the same semantics as Box<T>, so any smart pointer type can use it?

Like, I definitely agree that Box shouldn't have special semantics that you can't reproduce elsewhere. But among the options, it feels pretty limiting to come to the conclusion that we should eliminate those semantics, rather than just making them reproducible elsewhere.

I agree that whatever happens shouldn't be specific to the Global allocator, though.

[scottmcm]

We maintain some trivial validity invariants (such as alignment and address space limits) that a user cannot define

I, at least, fully expect us to eventually have some way of writing alignment-obeying raw pointers in Rust in some way. If nothing else, slice::Iter is less good than it could be because of the lack of them, and optimizing that type is super-important.

(Transmuting between NonNull<T> and unsafe<'a> &'a T is one thing that might work, for example, though it's also possible that the opsem for that will end up saying that it doesn't and that something else is required instead.)

EDIT: added a word to try to communicate that I wasn't expecting this RFC to include such a type.

[chorman0773]

I, at least, fully expect us to have some way of writing alignment-obeying raw pointers in Rust in some way. If nothing else, slice::Iter is less good than it could be because of the lack of them, and optimizing that type is super-important.

That's my hope for the future as well, but to avoid the RFC becoming too cluttered, I am refraining from defining such a type in this RFC.

[juntyr]

Is there a list of optimisations that depend on noalias being emitted for Box’es?

[clarfonthey]

The RFC seems pretty clear that noalias hasn't really provided many benefits compared to being an extra burden to uphold for implementers, but maybe it is worth seeing if there are any sources that can provide a bit more detail on that.

[kennytm]

sorry but is the term "typed copy" explained somewhere?

(the explanations I could find are from pretty unofficial places like a reddit¹ and urlo² post)

Footnotes

1. "they are like memcpy, but the copy occurs with a type, giving the compiler some extra power." ↩

2. "a typed copy consists of essentially decoding the AM-bytes into the abstract value then encoding that abstract value back to AM-bytes at the new location." ↩

[chorman0773]

The urlo definition is probably good.
It's defined in the opsem, but I don't know if we have a very good written record of that other than spread arround zulip threads and github issues.

[scottmcm]

I'm a fan of this. I think that people moving from Vec<T> to Box<[T]> having to deal with drastically-different soundness rules is a giant footgun, and getting rid of the special [ST]B behaviour here sounds good to me.

[nikomatsakis]

My general take:

The two "endpoints" here are

• Efficient end-user abstractions: this allows most safe code to run faster. This would have strong alias requirements and would not expose raw/unsafe details. This permits non-obvious optimizations (e.g., small string optimization or 0-length capacity).
• Building blocks for unsafe code: this exposes raw/unsafe details.

From what I can tell, we current orient Box as the former but Vec and String as the latter. That seems backwards, since if anything those are far more useful as abstractions than Box is.

If I could go back in time, I think I would favor end-user abstractions and offer different types (e.g., RawVec or RawBuffer or something) that exposed their innards, but I think that ship has sailed, and we might as well embrace the current situation (which is nice in some ways too).

[nikomatsakis]

The RFC would benefit from some attempt to quantify the impact on performance, though our lack of standardized runtime benchmarks makes that hard.

[traviscross]

@rust-lang/opsem: We were curious in our discussions, does this RFC represent an existing T-opsem consensus?

[chorman0773]

We were curious in our discussions, does this RFC represent an existing T-opsem consensus?

It does not represent any FCP done by T-opsem, which is why I've included them here. The claims I make, including those about the impact on the operation semantics, are included in the request for comment and consensus.

The RFC would benefit from some attempt to quantify the impact on performance, though our lack of standardized runtime benchmarks makes that hard.

I recall some perf PR's (using the default rustc-perf suite) being done to determine the impact, which showed negligible impact. I can probably pull them up at some point in the RFC's lifecycle.

• [perf experiment] Don't emit noalias for box when compiling rustc itself rust#99527 is such PR, and showed only a secondary regression of 1.9% in Cycle Count, but an improvement in Max RSS of 4.0% on average.

[RalfJung]

The Reference has been adjusted a while ago to state validity invariants positively, i..e by listing what must be true, instead of listing what must not be false. IMO that's more understandable, and the RFC should be updated to also do that.

[RalfJung]

I agree that whatever happens shouldn't be specific to the Global allocator, though.

There are patterns of using a custom per-Box allocator that are incompatible with the aliasing requirements, at least under our current aliasing models. See rust-lang/miri#3341 for an example. So if we always make Box be unique, we have to declare those allocators to be UB.

Is there a list of optimisations that depend on noalias being emitted for Box’es?

It's "every LLVM optimization that looks at alias information". The question is how much that matters in practice, which is hard to evaluate.

We were curious in our discussions, does this RFC represent an existing T-opsem consensus?

As Connor said, not in any formal sense. Several opsem members have expressed repeatedly that they want to see noalias on Box go, but I don't know whether we have team consensus on this.

My own position is that I love how this simplifies the model and Miri, I am just slightly concerned about this being an irreversible loss of optimization potential that we might regret later. Absence of evidence of optimization benefit is not evidence of absence. Our benchmarks likely just don't have a lot of functions that take Box<T> by value. However, that in itself is an indication that the optimization benefit is likely limited.

Is there a way we can query the ecosystem for functions taking Box<T> by value?

[RalfJung]

This is actually the status quo, since rust-lang/rust#122018

[clarfonthey]

Just to follow up on some of the discussion, it wasn't immediately clear to me that types similar to Box, like Vec and Arc, genuinely don't have these semantics, even though it is implied by the fact that Box is unique in this regard. I think this is worth emphasising more in the text of the RFC, since in a very real sense, we've been going without these semantics for most^{[citation needed]} parts of Rust totally fine, which further emphasises less will be lost by removing it for Box.

I would still love if there's more data showing the lack of returns on noalias optimisations, since it feels wrong that something with a lot of history and usage isn't helping that much, but the fact that Vec and other types don't have this optimisation at least helps us understand that it's not going to cause any performance issues if removed.

[chorman0773]

I am just slightly concerned about this being an irreversible loss of optimization potential that we might regret later

FTR, speaking right now as one of the main developers of lccc, my opinion is that the best way to mitigate any loss of future optimization potential is to just be far more granular with &mut. I don't think spamming extra noalias on parameters is necessary if we just emit more metadata on derefs (llvm's scoped noalias and dereferenceable, and lccc's unique and dereferenceable type attributes). If there are problems with doing that, I don't think this is a place where we should necessarily bend the whole language to this one attribute on one backend, especially given the complexity of maintaining the feature as-is, where we keep running into fundamental issues with the special treatment of Box. If there are legitimate optimizations lost as a result of the function level noalias, that can't be made up with more granular scoping on dereferences, that may be a different consideration, but my view is that the language after this change has the semantics necessary to justify at the very least a majority of the optimizations that may ultimately be lost, and its up to rustc and llvm (and other compilers) to make use of those semantics if they wish to.

Just to follow up on some of the discussion, it wasn't immediately clear to me that types similar to Box, like Vec and Arc, genuinely don't have these semantics, even though it is implied by the fact that Box is unique in this regard. I think this is worth emphasising more in the text of the RFC, since in a very real sense, we've been going without these semantics for most parts of Rust totally fine, which further emphasises less will be lost by removing it for Box.

I mentioned Vec<T> in particular, and also mentioned std::unique_ptr<T> from C++, which is the most closely equivalent type, and lacks the same semantic implications (and also optimizations).