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

[Miguel Ojeda]

On Sat, Jul 6, 2024 at 12:59 PM Benno Lossin <benno.lossin@xxxxxxxxx> 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).

>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).

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)?

Cheers,
Miguel