[PATCH 2/3] rust: num: add the `last_set_bit` operation

[Alice Ryhl]

On Fri, Jun 20, 2025 at 10:14:52PM +0900, Alexandre Courbot wrote:
> Add an equivalent to the `fls` (Find Last Set bit) C function to Rust
> unsigned types.
> 
> It is to be first used by the nova-core driver.
> 
> Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>

Reviewed-by: Alice Ryhl <aliceryhl at google.com>

>  rust/kernel/num.rs | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs
> index 6ecff037893dd25420a6369ea0cd6adbe3460b29..95766201a7cf208989f37ecbc6d54d264385a754 100644
> --- a/rust/kernel/num.rs
> +++ b/rust/kernel/num.rs
> @@ -161,3 +161,41 @@ pub const fn align_up(self, value: $t) -> $t {
>  }
>  
>  power_of_two_impl!(usize, u8, u16, u32, u64, u128);
> +
> +macro_rules! impl_last_set_bit {
> +    ($($t:ty),+) => {
> +        $(
> +            ::kernel::macros::paste! {

I think this would read slightly nicer with the paste! invocation on the
outer scope.

$(::kernel::macros::paste! {
    ...
})+

> +            /// 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
> +            ///
> +            /// ```
> +            #[doc = concat!("use kernel::num::last_set_bit_", stringify!($t), ";")]
> +            ///
> +            #[doc = concat!("assert_eq!(last_set_bit_", stringify!($t), "(0x0), 0);")]
> +            #[doc = concat!("assert_eq!(last_set_bit_", stringify!($t), "(0x1), 1);")]
> +            #[doc = concat!("assert_eq!(last_set_bit_", stringify!($t), "(0x10), 5);")]
> +            #[doc = concat!("assert_eq!(last_set_bit_", stringify!($t), "(0x1f), 5);")]
> +            #[doc = concat!(
> +                "assert_eq!(last_set_bit_",
> +                stringify!($t),
> +                "(",
> +                stringify!($t),
> +                "::MAX), ",
> +                stringify!($t), "::BITS);"
> +            )]
> +            /// ```
> +            #[inline(always)]
> +            pub const fn [<last_set_bit_ $t>](v: $t) -> u32 {
> +                $t::BITS - v.leading_zeros()
> +            }
> +            }
> +        )+
> +    };
> +}
> +
> +impl_last_set_bit!(usize, u8, u16, u32, u64, u128);
> 
> -- 
> 2.49.0
>