linux-kernel - Re: [PATCH 09/20] rust: types: implement `Unique<T>`

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Message-ID: <8bfd9934-6227-4584-a11c-3f3c881b7fac@proton.me>
Date: Sat, 06 Jul 2024 13:37:02 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Danilo Krummrich <dakr@...hat.com>, ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...sung.com, aliceryhl@...gle.com, daniel.almeida@...labora.com, faith.ekstrand@...labora.com, boris.brezillon@...labora.com, lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com, acurrid@...dia.com, cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com, ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 09/20] rust: types: implement `Unique<T>`

On 06.07.24 14:40, Miguel Ojeda wrote:
> On Sat, Jul 6, 2024 at 12:59 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>>
>> The `Unique` from std is special, in the sense that the Rust compiler
>> will emit the `noalias` LLVM attribute.
>>
>> This gives std's `Box` type a possible performance advantage (IIRC Gary
>> had a compiler explorer example that showed different instruction
>> count).
> 
> The example in question: https://godbolt.org/z/n93vePqMj -- there is
> one less memory load.
> 
> One can also easily craft examples where the compiler e.g. removes an
> entire loop: https://godbolt.org/z/c8ncbdKMe
> 
> But, of course, it depends on whether we will actually encounter these
> situations in real code, as you say. It could also be that today we
> don't find any relevant benefit, but there may exist situations later
> (perhaps because we have more code, or perhaps because codegen
> backends change).

It would be a good idea to ask the Rust folks if they could give us a
unique type that does what we need.

> From a quick look, there are still quite a few open issues about the
> exact properties of `Box` and `Unique`, including whether `Box` has a
> derefencability requirement
> (https://github.com/rust-lang/unsafe-code-guidelines/issues/145).

Yes, this is why I wasn't sure if we could do the workaround I
mentioned. My guess is that it doesn't work or that at least it isn't
supported (i.e. could change at any point).

> What properties would we want, today, from our `Box` types, if we
> could pick any? Should we have several kinds of `Box`es if there is no
> unique answer? Is it worth diverging from the standard one(s) in
> either direction (i.e. more invariants or less)?

I would expect that `Box<T>` behaves like `&mut T` except that it owns
the memory.

For starters we probably want this "normal" Box. If we get a unique
pointer type, then we can introduce more, but I don't think we need that
at the moment.

---
Cheers,
Benno