[PATCH v9 02/10] rust: num: add `shr` and `shl` methods to `Bounded`

Alexandre Courbot
2026-03-14T01:06:38Z

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@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Yury Norov <ynorov@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
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(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