Alignment operations are very common in the kernel. Since they are
always performed using a power of two value, enforcing this invariant
through a dedicated type leads to less bugs and can lead to improved
generated code.
Introduce the `Alignment` type, inspired by the nightly Rust feature of
the same name. It provides the same interface as its upstream namesake,
while extending it with `align_up` and `align_down` operations that are
usable on any integer type.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
rust/kernel/lib.rs | 1 +
rust/kernel/ptr.rs | 213 +++++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 214 insertions(+)
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 2955f65da1278dd4cba1e4272ff178b8211a892c..0e66b55cde66ee1b274862cd78ad465a572dc5d9 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -100,6 +100,7 @@
pub mod platform;
pub mod prelude;
pub mod print;
+pub mod ptr;
pub mod rbtree;
pub mod revocable;
pub mod security;
diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
new file mode 100644
index 0000000000000000000000000000000000000000..6d941db58944619ea5b05676af06981a3ceaaca8
--- /dev/null
+++ b/rust/kernel/ptr.rs
@@ -0,0 +1,213 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Types and functions to work with pointers and addresses.
+
+use core::fmt::Debug;
+use core::num::NonZero;
+use core::ops::{BitAnd, Not};
+
+use crate::build_assert;
+use crate::num::CheckedAdd;
+
+/// Type representing an alignment, which is always a power of two.
+///
+/// It be used to validate that a given value is a valid alignment, and to perform masking and
+/// align down/up operations. The alignment operations are done using the [`align_up!`] and
+/// [`align_down!`] macros.
+///
+/// Heavily inspired by the [`Alignment`] nightly feature from the Rust standard library, and
+/// hopefully to be eventually replaced by it.
+///
+/// [`Alignment`]: https://github.com/rust-lang/rust/issues/102070
+///
+/// # Invariants
+///
+/// An alignment is always a power of two.
+#[repr(transparent)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Alignment(NonZero<usize>);
+
+impl Alignment {
+ /// Validates that `align` is a power of two at build-time, and returns an [`Alignment`] of the
+ /// same value.
+ ///
+ /// A build error is triggered if `align` cannot be asserted to be a power of two.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// let v = Alignment::new(16);
+ /// assert_eq!(v.as_usize(), 16);
+ /// ```
+ #[inline(always)]
+ pub const fn new(align: usize) -> Self {
+ build_assert!(align.is_power_of_two());
+
+ // INVARIANT: `align` is a power of two.
+ // SAFETY: `align` is a power of two, and thus non-zero.
+ Self(unsafe { NonZero::new_unchecked(align) })
+ }
+
+ /// Validates that `align` is a power of two at runtime, and returns an
+ /// [`Alignment`] of the same value.
+ ///
+ /// [`None`] is returned if `align` was not a power of two.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new_checked(16), Some(Alignment::new(16)));
+ /// assert_eq!(Alignment::new_checked(15), None);
+ /// assert_eq!(Alignment::new_checked(1), Some(Alignment::new(1)));
+ /// assert_eq!(Alignment::new_checked(0), None);
+ /// ```
+ #[inline(always)]
+ pub const fn new_checked(align: usize) -> Option<Self> {
+ if align.is_power_of_two() {
+ // INVARIANT: `align` is a power of two.
+ // SAFETY: `align` is a power of two, and thus non-zero.
+ Some(Self(unsafe { NonZero::new_unchecked(align) }))
+ } else {
+ None
+ }
+ }
+
+ /// Returns the alignment of `T`.
+ #[inline(always)]
+ pub const fn of<T>() -> Self {
+ // INVARIANT: `align_of` always returns a power of 2.
+ Self(unsafe { NonZero::new_unchecked(align_of::<T>()) })
+ }
+
+ /// Returns the base-2 logarithm of the alignment.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::of::<u8>().log2(), 0);
+ /// assert_eq!(Alignment::new(16).log2(), 4);
+ /// ```
+ #[inline(always)]
+ pub const fn log2(self) -> u32 {
+ self.0.ilog2()
+ }
+
+ /// Returns this alignment as a [`NonZero`].
+ ///
+ /// It is guaranteed to be a power of two.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(16).as_nonzero().get(), 16);
+ /// ```
+ #[inline(always)]
+ pub const fn as_nonzero(self) -> NonZero<usize> {
+ if !self.0.is_power_of_two() {
+ // SAFETY: per the invariants, `self.0` is always a power of two so this block will
+ // never be reached.
+ unsafe { core::hint::unreachable_unchecked() }
+ }
+ self.0
+ }
+
+ /// Returns this alignment as a `usize`.
+ ///
+ /// It is guaranteed to be a power of two.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(16).as_usize(), 16);
+ /// ```
+ #[inline(always)]
+ pub const fn as_usize(self) -> usize {
+ self.as_nonzero().get()
+ }
+
+ /// Returns the mask corresponding to `self.as_usize() - 1`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(0x10).mask(), 0xf);
+ /// ```
+ #[inline(always)]
+ pub const fn mask(self) -> usize {
+ // INVARIANT: `self.as_usize()` is guaranteed to be a power of two (i.e. non-zero), thus
+ // `1` can safely be substracted from it.
+ self.as_usize() - 1
+ }
+
+ /// Aligns `value` down to this alignment.
+ ///
+ /// If the alignment contained in `self` is too large for `T`, then `0` is returned, which
+ /// is correct as it is also the result that would have been returned if it did.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(0x10).align_down(0x2f), 0x20);
+ /// assert_eq!(Alignment::new(0x10).align_down(0x30), 0x30);
+ /// assert_eq!(Alignment::new(0x1000).align_down(0xf0u8), 0x0);
+ /// ```
+ #[inline(always)]
+ pub fn align_down<T>(self, value: T) -> T
+ where
+ T: TryFrom<usize> + BitAnd<Output = T> + Not<Output = T> + Default,
+ {
+ T::try_from(self.mask())
+ .map(|mask| value & !mask)
+ .unwrap_or(T::default())
+ }
+
+ /// Aligns `value` up to this alignment, returning `None` if aligning pushes the result above
+ /// the limits of `value`'s type.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(0x10).align_up(0x4f), Some(0x50));
+ /// assert_eq!(Alignment::new(0x10).align_up(0x40), Some(0x40));
+ /// assert_eq!(Alignment::new(0x10).align_up(0x0), Some(0x0));
+ /// assert_eq!(Alignment::new(0x10).align_up(u8::MAX), None);
+ /// assert_eq!(Alignment::new(0x100).align_up(0x10u8), None);
+ /// assert_eq!(Alignment::new(0x100).align_up(0x0u8), Some(0x0));
+ /// ```
+ #[inline(always)]
+ pub fn align_up<T>(self, value: T) -> Option<T>
+ where
+ T: TryFrom<usize>
+ + BitAnd<Output = T>
+ + Not<Output = T>
+ + Default
+ + PartialEq
+ + Copy
+ + CheckedAdd,
+ {
+ let aligned_down = self.align_down(value);
+ if value == aligned_down {
+ Some(aligned_down)
+ } else {
+ T::try_from(self.as_usize())
+ .ok()
+ .and_then(|align| aligned_down.checked_add(align))
+ }
+ }
+}
--
2.50.1On Mon, Aug 4, 2025 at 1:45 PM Alexandre Courbot <acourbot@nvidia.com> wrote:
>
> +/// align down/up operations. The alignment operations are done using the [`align_up!`] and
> +/// [`align_down!`] macros.
These intra-doc links don't work (they are not macros in this version at least).
> + /// Returns the alignment of `T`.
> + #[inline(always)]
> + pub const fn of<T>() -> Self {
> + // INVARIANT: `align_of` always returns a power of 2.
> + Self(unsafe { NonZero::new_unchecked(align_of::<T>()) })
Missing safety comment (`CLIPPY=1` spots it).
Also, cannot we use `new()` here? i.e. the value will be known at compile-time.
> + if !self.0.is_power_of_two() {
> + // SAFETY: per the invariants, `self.0` is always a power of two so this block will
> + // never be reached.
> + unsafe { core::hint::unreachable_unchecked() }
> + }
I guess this one is here to help optimize users after they inline the
cal? Is there a particular case you noticed? i.e. it may be worth
mentioning it.
> + pub const fn mask(self) -> usize {
> + // INVARIANT: `self.as_usize()` is guaranteed to be a power of two (i.e. non-zero), thus
> + // `1` can safely be substracted from it.
> + self.as_usize() - 1
> + }
I am not sure why there is `// INVARIANT` here, since we are not
creating a new `Self`.
I guess by "safely" you are trying to say there is no overflow risk --
I would be explicit and avoid "safe", since it is safe to overflow.
Typo: subtracted
Cheers,
Miguel
On Mon Aug 4, 2025 at 4:17 PM CEST, Miguel Ojeda wrote:
> On Mon, Aug 4, 2025 at 1:45 PM Alexandre Courbot <acourbot@nvidia.com> wrote:
>> + if !self.0.is_power_of_two() {
>> + // SAFETY: per the invariants, `self.0` is always a power of two so this block will
>> + // never be reached.
>> + unsafe { core::hint::unreachable_unchecked() }
>> + }
>
> I guess this one is here to help optimize users after they inline the
> cal? Is there a particular case you noticed? i.e. it may be worth
> mentioning it.
I suggested this in the previous version [1]. For example, it optimizes
division to only be a left shift.
[1]: https://lore.kernel.org/all/DBL1ZGZCSJF3.29HNS9BSN89C6@kernel.org
---
Cheers,
Benno
On Mon Aug 4, 2025 at 11:17 PM JST, Miguel Ojeda wrote:
> On Mon, Aug 4, 2025 at 1:45 PM Alexandre Courbot <acourbot@nvidia.com> wrote:
>>
>> +/// align down/up operations. The alignment operations are done using the [`align_up!`] and
>> +/// [`align_down!`] macros.
>
> These intra-doc links don't work (they are not macros in this version at least).
Oops, these are remnants of some previous attempt at making this work,
which I could swear I removed. That and the sentence's grammar as a
whole is incorrect. Let me rework this.
>
>> + /// Returns the alignment of `T`.
>> + #[inline(always)]
>> + pub const fn of<T>() -> Self {
>> + // INVARIANT: `align_of` always returns a power of 2.
>> + Self(unsafe { NonZero::new_unchecked(align_of::<T>()) })
>
> Missing safety comment (`CLIPPY=1` spots it).
>
> Also, cannot we use `new()` here? i.e. the value will be known at compile-time.
We can indeed! Brilliant.
>
>> + if !self.0.is_power_of_two() {
>> + // SAFETY: per the invariants, `self.0` is always a power of two so this block will
>> + // never be reached.
>> + unsafe { core::hint::unreachable_unchecked() }
>> + }
>
> I guess this one is here to help optimize users after they inline the
> cal? Is there a particular case you noticed? i.e. it may be worth
> mentioning it.
This was a suggestion from Benno [1], to give more hints to the
compiler. Let me add a comment to justify its presence.
[1] https://lore.kernel.org/rust-for-linux/DBL1ZGZCSJF3.29HNS9BSN89C6@kernel.org/
>
>> + pub const fn mask(self) -> usize {
>> + // INVARIANT: `self.as_usize()` is guaranteed to be a power of two (i.e. non-zero), thus
>> + // `1` can safely be substracted from it.
>> + self.as_usize() - 1
>> + }
>
> I am not sure why there is `// INVARIANT` here, since we are not
> creating a new `Self`.
>
> I guess by "safely" you are trying to say there is no overflow risk --
> I would be explicit and avoid "safe", since it is safe to overflow.
I just wanted to justify that we cannot substract from 0. Maybe an
`unchecked_sub` would be better here? The `unsafe` block would also
justify the safety comment.
... mmm actually that would be `checked_sub().unwrap_unchecked()`, since
`unchecked_sub` appeared in Rust 1.79.
On Tue Aug 5, 2025 at 3:13 PM CEST, Alexandre Courbot wrote:
> On Mon Aug 4, 2025 at 11:17 PM JST, Miguel Ojeda wrote:
>> On Mon, Aug 4, 2025 at 1:45 PM Alexandre Courbot <acourbot@nvidia.com> wrote:
>>> + pub const fn mask(self) -> usize {
>>> + // INVARIANT: `self.as_usize()` is guaranteed to be a power of two (i.e. non-zero), thus
>>> + // `1` can safely be substracted from it.
>>> + self.as_usize() - 1
>>> + }
>>
>> I am not sure why there is `// INVARIANT` here, since we are not
>> creating a new `Self`.
>
>>
>> I guess by "safely" you are trying to say there is no overflow risk --
>> I would be explicit and avoid "safe", since it is safe to overflow.
>
> I just wanted to justify that we cannot substract from 0. Maybe an
> `unchecked_sub` would be better here? The `unsafe` block would also
> justify the safety comment.
>
> ... mmm actually that would be `checked_sub().unwrap_unchecked()`, since
> `unchecked_sub` appeared in Rust 1.79.
No need to do that, the compiler already knows that there won't be
underflow and optimizes it accordingly (since self.as_usize() converts a
`NonZero<usize>`). [1] (it also works when removing the
`is_power_of_two` check, but if we only stored a `usize`, I bet the
compiler would also optimize this given that check)
I'd just add a normal comment that mentions no underflow can occur. This
shouldn't need unsafe.
[1]: https://godbolt.org/z/M5x1W49nn
---
Cheers,
Benno
Hi Alex,
> On 4 Aug 2025, at 08:45, Alexandre Courbot <acourbot@nvidia.com> wrote:
>
> Alignment operations are very common in the kernel. Since they are
> always performed using a power of two value, enforcing this invariant
> through a dedicated type leads to less bugs and can lead to improved
> generated code.
>
> Introduce the `Alignment` type, inspired by the nightly Rust feature of
> the same name. It provides the same interface as its upstream namesake,
> while extending it with `align_up` and `align_down` operations that are
> usable on any integer type.
>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
> rust/kernel/lib.rs | 1 +
> rust/kernel/ptr.rs | 213 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 214 insertions(+)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 2955f65da1278dd4cba1e4272ff178b8211a892c..0e66b55cde66ee1b274862cd78ad465a572dc5d9 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -100,6 +100,7 @@
> pub mod platform;
> pub mod prelude;
> pub mod print;
> +pub mod ptr;
> pub mod rbtree;
> pub mod revocable;
> pub mod security;
> diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..6d941db58944619ea5b05676af06981a3ceaaca8
> --- /dev/null
> +++ b/rust/kernel/ptr.rs
> @@ -0,0 +1,213 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Types and functions to work with pointers and addresses.
> +
> +use core::fmt::Debug;
> +use core::num::NonZero;
> +use core::ops::{BitAnd, Not};
> +
> +use crate::build_assert;
> +use crate::num::CheckedAdd;
> +
> +/// Type representing an alignment, which is always a power of two.
> +///
> +/// It be used to validate that a given value is a valid alignment, and to perform masking and
> +/// align down/up operations. The alignment operations are done using the [`align_up!`] and
Nit: I’d go with “align up or align down operations” instead of using a slash.
> +/// [`align_down!`] macros.
> +///
> +/// Heavily inspired by the [`Alignment`] nightly feature from the Rust standard library, and
> +/// hopefully to be eventually replaced by it.
It’s a bit hard to parse this.
Also, I wonder if we should standardize some syntax for TODOs so we can parse
them using a script? This way we can actually keep track and perhaps pipe them
to our GitHub page as “good first issues” or just regular issues.
I guess a simple "// TODO: “ here will do, for example.
> +///
> +/// [`Alignment`]: https://github.com/rust-lang/rust/issues/102070
> +///
> +/// # Invariants
> +///
> +/// An alignment is always a power of two.
> +#[repr(transparent)]
> +#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
> +pub struct Alignment(NonZero<usize>);
> +
> +impl Alignment {
> + /// Validates that `align` is a power of two at build-time, and returns an [`Alignment`] of the
> + /// same value.
> + ///
> + /// A build error is triggered if `align` cannot be asserted to be a power of two.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::ptr::Alignment;
> + ///
> + /// let v = Alignment::new(16);
> + /// assert_eq!(v.as_usize(), 16);
> + /// ```
> + #[inline(always)]
> + pub const fn new(align: usize) -> Self {
> + build_assert!(align.is_power_of_two());
> +
> + // INVARIANT: `align` is a power of two.
> + // SAFETY: `align` is a power of two, and thus non-zero.
> + Self(unsafe { NonZero::new_unchecked(align) })
> + }
> +
> + /// Validates that `align` is a power of two at runtime, and returns an
> + /// [`Alignment`] of the same value.
> + ///
> + /// [`None`] is returned if `align` was not a power of two.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::ptr::Alignment;
> + ///
> + /// assert_eq!(Alignment::new_checked(16), Some(Alignment::new(16)));
> + /// assert_eq!(Alignment::new_checked(15), None);
> + /// assert_eq!(Alignment::new_checked(1), Some(Alignment::new(1)));
> + /// assert_eq!(Alignment::new_checked(0), None);
> + /// ```
> + #[inline(always)]
> + pub const fn new_checked(align: usize) -> Option<Self> {
> + if align.is_power_of_two() {
> + // INVARIANT: `align` is a power of two.
> + // SAFETY: `align` is a power of two, and thus non-zero.
> + Some(Self(unsafe { NonZero::new_unchecked(align) }))
> + } else {
> + None
> + }
> + }
> +
> + /// Returns the alignment of `T`.
> + #[inline(always)]
> + pub const fn of<T>() -> Self {
> + // INVARIANT: `align_of` always returns a power of 2.
> + Self(unsafe { NonZero::new_unchecked(align_of::<T>()) })
> + }
> +
> + /// Returns the base-2 logarithm of the alignment.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::ptr::Alignment;
> + ///
> + /// assert_eq!(Alignment::of::<u8>().log2(), 0);
> + /// assert_eq!(Alignment::new(16).log2(), 4);
> + /// ```
> + #[inline(always)]
> + pub const fn log2(self) -> u32 {
> + self.0.ilog2()
> + }
> +
> + /// Returns this alignment as a [`NonZero`].
> + ///
> + /// It is guaranteed to be a power of two.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::ptr::Alignment;
> + ///
> + /// assert_eq!(Alignment::new(16).as_nonzero().get(), 16);
> + /// ```
> + #[inline(always)]
> + pub const fn as_nonzero(self) -> NonZero<usize> {
> + if !self.0.is_power_of_two() {
> + // SAFETY: per the invariants, `self.0` is always a power of two so this block will
> + // never be reached.
> + unsafe { core::hint::unreachable_unchecked() }
> + }
> + self.0
> + }
> +
> + /// Returns this alignment as a `usize`.
> + ///
> + /// It is guaranteed to be a power of two.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::ptr::Alignment;
> + ///
> + /// assert_eq!(Alignment::new(16).as_usize(), 16);
> + /// ```
> + #[inline(always)]
> + pub const fn as_usize(self) -> usize {
> + self.as_nonzero().get()
> + }
> +
> + /// Returns the mask corresponding to `self.as_usize() - 1`.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::ptr::Alignment;
> + ///
> + /// assert_eq!(Alignment::new(0x10).mask(), 0xf);
> + /// ```
> + #[inline(always)]
> + pub const fn mask(self) -> usize {
> + // INVARIANT: `self.as_usize()` is guaranteed to be a power of two (i.e. non-zero), thus
> + // `1` can safely be substracted from it.
> + self.as_usize() - 1
> + }
> +
> + /// Aligns `value` down to this alignment.
> + ///
> + /// If the alignment contained in `self` is too large for `T`, then `0` is returned, which
> + /// is correct as it is also the result that would have been returned if it did.
I half get this, but still: If it did what?
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::ptr::Alignment;
> + ///
> + /// assert_eq!(Alignment::new(0x10).align_down(0x2f), 0x20);
> + /// assert_eq!(Alignment::new(0x10).align_down(0x30), 0x30);
> + /// assert_eq!(Alignment::new(0x1000).align_down(0xf0u8), 0x0);
> + /// ```
> + #[inline(always)]
> + pub fn align_down<T>(self, value: T) -> T
> + where
> + T: TryFrom<usize> + BitAnd<Output = T> + Not<Output = T> + Default,
> + {
> + T::try_from(self.mask())
> + .map(|mask| value & !mask)
> + .unwrap_or(T::default())
> + }
> +
> + /// Aligns `value` up to this alignment, returning `None` if aligning pushes the result above
> + /// the limits of `value`'s type.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::ptr::Alignment;
> + ///
> + /// assert_eq!(Alignment::new(0x10).align_up(0x4f), Some(0x50));
> + /// assert_eq!(Alignment::new(0x10).align_up(0x40), Some(0x40));
> + /// assert_eq!(Alignment::new(0x10).align_up(0x0), Some(0x0));
> + /// assert_eq!(Alignment::new(0x10).align_up(u8::MAX), None);
> + /// assert_eq!(Alignment::new(0x100).align_up(0x10u8), None);
> + /// assert_eq!(Alignment::new(0x100).align_up(0x0u8), Some(0x0));
> + /// ```
> + #[inline(always)]
> + pub fn align_up<T>(self, value: T) -> Option<T>
> + where
> + T: TryFrom<usize>
> + + BitAnd<Output = T>
> + + Not<Output = T>
> + + Default
> + + PartialEq
> + + Copy
> + + CheckedAdd,
> + {
> + let aligned_down = self.align_down(value);
> + if value == aligned_down {
> + Some(aligned_down)
> + } else {
> + T::try_from(self.as_usize())
> + .ok()
> + .and_then(|align| aligned_down.checked_add(align))
> + }
> + }
> +}
>
> --
> 2.50.1
>
>
Everything else looks fine, IMHO.
— Daniel
On Tue Aug 5, 2025 at 12:47 AM JST, Daniel Almeida wrote:
<snip>
>> +/// [`align_down!`] macros.
>> +///
>> +/// Heavily inspired by the [`Alignment`] nightly feature from the Rust standard library, and
>> +/// hopefully to be eventually replaced by it.
>
> It’s a bit hard to parse this.
>
> Also, I wonder if we should standardize some syntax for TODOs so we can parse
> them using a script? This way we can actually keep track and perhaps pipe them
> to our GitHub page as “good first issues” or just regular issues.
>
> I guess a simple "// TODO: “ here will do, for example.
FWIW, in Nova we tag each TODO items with a 4-letter identifier (i.e.
`TODO[ABCD]:` that is defined in our `todo.rst` file. This makes
grepping all the sites relevant to a given item easy.
>
>> +///
>> +/// [`Alignment`]: https://github.com/rust-lang/rust/issues/102070
>> +///
>> +/// # Invariants
>> +///
>> +/// An alignment is always a power of two.
>> +#[repr(transparent)]
>> +#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
>> +pub struct Alignment(NonZero<usize>);
>> +
>> +impl Alignment {
>> + /// Validates that `align` is a power of two at build-time, and returns an [`Alignment`] of the
>> + /// same value.
>> + ///
>> + /// A build error is triggered if `align` cannot be asserted to be a power of two.
>> + ///
>> + /// # Examples
>> + ///
>> + /// ```
>> + /// use kernel::ptr::Alignment;
>> + ///
>> + /// let v = Alignment::new(16);
>> + /// assert_eq!(v.as_usize(), 16);
>> + /// ```
>> + #[inline(always)]
>> + pub const fn new(align: usize) -> Self {
>> + build_assert!(align.is_power_of_two());
>> +
>> + // INVARIANT: `align` is a power of two.
>> + // SAFETY: `align` is a power of two, and thus non-zero.
>> + Self(unsafe { NonZero::new_unchecked(align) })
>> + }
>> +
>> + /// Validates that `align` is a power of two at runtime, and returns an
>> + /// [`Alignment`] of the same value.
>> + ///
>> + /// [`None`] is returned if `align` was not a power of two.
>> + ///
>> + /// # Examples
>> + ///
>> + /// ```
>> + /// use kernel::ptr::Alignment;
>> + ///
>> + /// assert_eq!(Alignment::new_checked(16), Some(Alignment::new(16)));
>> + /// assert_eq!(Alignment::new_checked(15), None);
>> + /// assert_eq!(Alignment::new_checked(1), Some(Alignment::new(1)));
>> + /// assert_eq!(Alignment::new_checked(0), None);
>> + /// ```
>> + #[inline(always)]
>> + pub const fn new_checked(align: usize) -> Option<Self> {
>> + if align.is_power_of_two() {
>> + // INVARIANT: `align` is a power of two.
>> + // SAFETY: `align` is a power of two, and thus non-zero.
>> + Some(Self(unsafe { NonZero::new_unchecked(align) }))
>> + } else {
>> + None
>> + }
>> + }
>> +
>> + /// Returns the alignment of `T`.
>> + #[inline(always)]
>> + pub const fn of<T>() -> Self {
>> + // INVARIANT: `align_of` always returns a power of 2.
>> + Self(unsafe { NonZero::new_unchecked(align_of::<T>()) })
>> + }
>
>> +
>> + /// Returns the base-2 logarithm of the alignment.
>> + ///
>> + /// # Examples
>> + ///
>> + /// ```
>> + /// use kernel::ptr::Alignment;
>> + ///
>> + /// assert_eq!(Alignment::of::<u8>().log2(), 0);
>> + /// assert_eq!(Alignment::new(16).log2(), 4);
>> + /// ```
>> + #[inline(always)]
>> + pub const fn log2(self) -> u32 {
>> + self.0.ilog2()
>> + }
>> +
>> + /// Returns this alignment as a [`NonZero`].
>> + ///
>> + /// It is guaranteed to be a power of two.
>> + ///
>> + /// # Examples
>> + ///
>> + /// ```
>> + /// use kernel::ptr::Alignment;
>> + ///
>> + /// assert_eq!(Alignment::new(16).as_nonzero().get(), 16);
>> + /// ```
>> + #[inline(always)]
>> + pub const fn as_nonzero(self) -> NonZero<usize> {
>> + if !self.0.is_power_of_two() {
>> + // SAFETY: per the invariants, `self.0` is always a power of two so this block will
>> + // never be reached.
>> + unsafe { core::hint::unreachable_unchecked() }
>> + }
>> + self.0
>> + }
>> +
>> + /// Returns this alignment as a `usize`.
>> + ///
>> + /// It is guaranteed to be a power of two.
>> + ///
>> + /// # Examples
>> + ///
>> + /// ```
>> + /// use kernel::ptr::Alignment;
>> + ///
>> + /// assert_eq!(Alignment::new(16).as_usize(), 16);
>> + /// ```
>> + #[inline(always)]
>> + pub const fn as_usize(self) -> usize {
>> + self.as_nonzero().get()
>> + }
>> +
>> + /// Returns the mask corresponding to `self.as_usize() - 1`.
>> + ///
>> + /// # Examples
>> + ///
>> + /// ```
>> + /// use kernel::ptr::Alignment;
>> + ///
>> + /// assert_eq!(Alignment::new(0x10).mask(), 0xf);
>> + /// ```
>> + #[inline(always)]
>> + pub const fn mask(self) -> usize {
>> + // INVARIANT: `self.as_usize()` is guaranteed to be a power of two (i.e. non-zero), thus
>> + // `1` can safely be substracted from it.
>> + self.as_usize() - 1
>> + }
>> +
>> + /// Aligns `value` down to this alignment.
>> + ///
>> + /// If the alignment contained in `self` is too large for `T`, then `0` is returned, which
>> + /// is correct as it is also the result that would have been returned if it did.
>
> I half get this, but still: If it did what?
I also stumbled while re-reading this sentence. :) Fixed.
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