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 <acour...@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);
