Re: [PATCH v8 02/10] rust: num: add `shr` and `shl` methods to `Bounded`

[Yury Norov]

On Tue, Mar 10, 2026 at 12:13:59AM +0900, Alexandre Courbot wrote:
> Shifting a `Bounded` left or right changes the number of bits required
> to represent the value. Add methods that perform the shift and return a
> `Bounded` with the appropriately adjusted bit width.
> 
> These methods are particularly useful for bitfield extraction.
> 
> Suggested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
> Reviewed-by: Daniel Almeida <daniel.almeida@xxxxxxxxxxxxx>
> Tested-by: Dirk Behme <dirk.behme@xxxxxxxxxxxx>
> Acked-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>

Acked-by: Yury Norov <ynorov@xxxxxxxxxx>

> ---
>  rust/kernel/num/bounded.rs | 42 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs
> index fa81acbdc8c2..2f5f13ecd3d6 100644
> --- a/rust/kernel/num/bounded.rs
> +++ b/rust/kernel/num/bounded.rs
> @@ -473,6 +473,48 @@ pub fn cast<U>(self) -> Bounded<U, N>
>          // `N` bits, and with the same signedness.
>          unsafe { Bounded::__new(value) }
>      }
> +
> +    /// Right-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, RES>`, where `RES >=
> +    /// N - SHIFT`.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::num::Bounded;
> +    ///
> +    /// let v = Bounded::<u32, 16>::new::<0xff00>();
> +    /// let v_shifted: Bounded::<u32, 8> = v.shr::<8, _>();
> +    ///
> +    /// assert_eq!(v_shifted.get(), 0xff);
> +    /// ```
> +    pub fn shr<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
> +        const { assert!(RES + SHIFT >= N) }
> +
> +        // SAFETY: We shift the value right by `SHIFT`, reducing the number of bits needed to
> +        // represent the shifted value by as much, and just asserted that `RES >= N - SHIFT`.
> +        unsafe { Bounded::__new(self.0 >> SHIFT) }
> +    }
> +
> +    /// Left-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, RES>`, where `RES >=
> +    /// N + SHIFT`.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::num::Bounded;
> +    ///
> +    /// let v = Bounded::<u32, 8>::new::<0xff>();
> +    /// let v_shifted: Bounded::<u32, 16> = v.shl::<8, _>();
> +    ///
> +    /// assert_eq!(v_shifted.get(), 0xff00);
> +    /// ```
> +    pub fn shl<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
> +        const { assert!(RES >= N + SHIFT) }
> +
> +        // SAFETY: We shift the value left by `SHIFT`, augmenting the number of bits needed to
> +        // represent the shifted value by as much, and just asserted that `RES >= N + SHIFT`.
> +        unsafe { Bounded::__new(self.0 << SHIFT) }
> +    }
>  }
>  
>  impl<T, const N: u32> Deref for Bounded<T, N>
> 
> -- 
> 2.53.0