On Thu Jun 12, 2025 at 10:17 PM JST, Alexandre Courbot wrote: > On Wed Jun 4, 2025 at 4:18 PM JST, Benno Lossin wrote: >> On Wed Jun 4, 2025 at 2:05 AM CEST, Alexandre Courbot wrote: >>> 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. >> >> Makes sense. >> >>>> 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 wouldn't it be better to make users validate that they gave a >> power-of-2 alignment? >> >>> 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 like this idea. But it will mean that we have to have a custom >> function that is either standalone and const or in an extension trait :( >> But for this one we can use the name `align_up` :) >> >> Here is a cool idea for the implementation: https://godbolt.org/z/x6navM5WK > > Yeah that's close to what I had in mind.
... with one difference though: I would like to avoid the use of `unsafe` for something so basic, so the implementation is close to the C one (using masks and logical operations). I think it's a great demonstration of the compiler's abilities that we can generate an always-optimized version of `next_multiple_of`, but for our use-case it feels like jumping through hoops just to show that we can jump through these hoops. I'll reconsider if there is pushback on v5 though. :)
