linux-kernel - Re: [PATCH v2 2/4] rust: add `Alignment` type

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Message-Id: <DBTQN3RW1H1S.1G8KB3ZD8A58F@kernel.org>
Date: Mon, 04 Aug 2025 17:11:30 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Miguel Ojeda" <miguel.ojeda.sandonis@...il.com>, "Alexandre Courbot"
 <acourbot@...dia.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Danilo Krummrich" <dakr@...nel.org>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <nouveau@...ts.freedesktop.org>
Subject: Re: [PATCH v2 2/4] rust: add `Alignment` type

On Mon Aug 4, 2025 at 4:17 PM CEST, Miguel Ojeda wrote:
> On Mon, Aug 4, 2025 at 1:45 PM Alexandre Courbot <acourbot@...dia.com> wrote:
>> +        if !self.0.is_power_of_two() {
>> +            // SAFETY: per the invariants, `self.0` is always a power of two so this block will
>> +            // never be reached.
>> +            unsafe { core::hint::unreachable_unchecked() }
>> +        }
>
> I guess this one is here to help optimize users after they inline the
> cal? Is there a particular case you noticed? i.e. it may be worth
> mentioning it.

I suggested this in the previous version [1]. For example, it optimizes
division to only be a left shift.

[1]: https://lore.kernel.org/all/DBL1ZGZCSJF3.29HNS9BSN89C6@kernel.org

---
Cheers,
Benno