[PATCH v5 05/23] rust: num: add the `fls` operation

[Benno Lossin]

On Thu Jun 12, 2025 at 4:01 PM CEST, Alexandre Courbot wrote:
> Add an equivalent to the `fls` (Find Last Set bit) C function to Rust
> unsigned types.

Have you tried to upstream this?

> It is to be first used by the nova-core driver.
>
> Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
> ---
>  rust/kernel/num.rs | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
>
> diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs
> index ee0f67ad1a89e69f5f8d2077eba5541b472e7d8a..934afe17719f789c569dbd54534adc2e26fe59f2 100644
> --- a/rust/kernel/num.rs
> +++ b/rust/kernel/num.rs
> @@ -171,3 +171,34 @@ fn borrow(&self) -> &T {
>          &self.0
>      }
>  }
> +
> +macro_rules! impl_fls {
> +    ($($t:ty),+) => {
> +        $(
> +            ::kernel::macros::paste! {
> +            /// Find Last Set Bit: return the 1-based index of the last (i.e. most significant) set
> +            /// bit in `v`.
> +            ///
> +            /// Equivalent to the C `fls` function.
> +            ///
> +            /// # Examples
> +            ///
> +            /// ```
> +            /// use kernel::num::fls_u32;
> +            ///
> +            /// assert_eq!(fls_u32(0x0), 0);
> +            /// assert_eq!(fls_u32(0x1), 1);
> +            /// assert_eq!(fls_u32(0x10), 5);
> +            /// assert_eq!(fls_u32(0xffff), 16);
> +            /// assert_eq!(fls_u32(0x8000_0000), 32);
> +            /// ```
> +            #[inline(always)]
> +            pub const fn [<fls_ $t>](v: $t) -> u32 {

Can we name this `find_last_set_bit_ $t`? When the upstream function
lands, we should also rename this one.

---
Cheers,
Benno

> +                $t::BITS - v.leading_zeros()
> +            }
> +            }
> +        )+
> +    };
> +}
> +
> +impl_fls!(usize, u8, u16, u32, u64, u128);