Re: [PATCH v4 04/20] rust: add new `num` module with useful integer operations

["Alexandre Courbot" <acourbot@nvidia.com>]

On Wed Jun 4, 2025 at 8:02 AM JST, Benno Lossin wrote:
> On Mon Jun 2, 2025 at 3:09 PM CEST, Alexandre Courbot wrote:
>> On Thu May 29, 2025 at 4:27 PM JST, Benno Lossin wrote:
>>> On Thu May 29, 2025 at 3:18 AM CEST, Alexandre Courbot wrote:
>>>> On Thu May 29, 2025 at 5:17 AM JST, Benno Lossin wrote:
>>>>> On Wed May 21, 2025 at 8:44 AM CEST, Alexandre Courbot wrote:
>>>>>> +    /// Align `self` up to `alignment`.
>>>>>> +    ///
>>>>>> +    /// `alignment` must be a power of 2 for accurate results.
>>>>>> +    ///
>>>>>> +    /// Wraps around to `0` if the requested alignment pushes the result above the type's limits.
>>>>>> +    ///
>>>>>> +    /// # Examples
>>>>>> +    ///
>>>>>> +    /// ```
>>>>>> +    /// use kernel::num::NumExt;
>>>>>> +    ///
>>>>>> +    /// assert_eq!(0x4fffu32.align_up(0x1000), 0x5000);
>>>>>> +    /// assert_eq!(0x4000u32.align_up(0x1000), 0x4000);
>>>>>> +    /// assert_eq!(0x0u32.align_up(0x1000), 0x0);
>>>>>> +    /// assert_eq!(0xffffu16.align_up(0x100), 0x0);
>>>>>> +    /// assert_eq!(0x4fffu32.align_up(0x0), 0x0);
>>>>>> +    /// ```
>>>>>> +    fn align_up(self, alignment: Self) -> Self;
>>>>>
>>>>> Isn't this `next_multiple_of` [1] (it also allows non power of 2
>>>>> inputs).
>>>>>
>>>>> [1]: https://doc.rust-lang.org/std/primitive.u32.html#method.next_multiple_of
>>>>
>>>> It is, however the fact that `next_multiple_of` works with non powers of
>>>> two also means it needs to perform a modulo operation. That operation
>>>> might well be optimized away by the compiler, but ACAICT we have no way
>>>> of proving it will always be the case, hence the always-optimal
>>>> implementation here.
>>>
>>> When you use a power of 2 constant, then I'm very sure that it will get
>>> optimized [1]. Even with non-powers of 2, you don't get a division [2].
>>> If you find some code that is not optimized, then sure add a custom
>>> function.
>>>
>>> [1]: https://godbolt.org/z/57M9e36T3
>>> [2]: https://godbolt.org/z/9P4P8zExh
>>
>> That's impressive and would definitely work well with a constant. But
>> when the value is not known at compile-time, the division does occur
>> unfortunately: https://godbolt.org/z/WK1bPMeEx
>>
>> So I think we will still need a kernel-optimized version of these
>> alignment functions.
>
> Hmm what exactly is the use-case for a variable align amount? Could you
> store it in const generics?

Say you have an IOMMU with support for different pages sizes, the size
of a particular page can be decided at runtime.

>
> If not, there are also these two variants that are more efficient:
>
> * option: https://godbolt.org/z/ecnb19zaM
> * unsafe: https://godbolt.org/z/EqTaGov71
>
> So if the compiler can infer it from context it still optimizes it :)

I think the `Option` (and subsequent `unwrap`) is something we want to
avoid on such a common operation.

>
> But yeah to be extra sure, you need your version. By the way, what
> happens if `align` is not a power of 2 in your version?

It will just return `(self + (self - 1)) & (alignment - 1)`, which will
likely be a value you don't want.

So yes, for this particular operation we would prefer to only use powers
of 2 as inputs - if we can ensure that then it solves most of our
problems (can use `next_multiple_of`, no `Option`, etc).

Maybe we can introduce a new integer type that, similarly to `NonZero`,
guarantees that the value it stores is a power of 2? Users with const
values (90+% of uses) won't see any difference, and if working with a
runtime-generated value we will want to validate it anyway...

(I can already hear you saying "send that to upstream Rust!" ^_^;)