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rust: error: optimize error type to use nonzero
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Optimize `Result<(), Error>` size by changing `Error` type to
`NonZero*` for niche optimization.

This reduces the space used by the `Result` type, as the `NonZero*`
type enables the compiler to apply more efficient memory layout.
For example, the `Result<(), Error>` changes size from 8 to 4 bytes.

Link: #1120
Signed-off-by: Filipe Xavier <felipe_life@live.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Link: https://lore.kernel.org/r/BL0PR02MB4914B9B088865CF237731207E9732@BL0PR02MB4914.namprd02.prod.outlook.com
[ Removed unneeded block around `match`, added backticks in panic
  message and added intra-doc link. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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felipeagger authored and ojeda committed Oct 7, 2024
1 parent 6f56651 commit c9b5ce6
Showing 1 changed file with 28 additions and 9 deletions.
37 changes: 28 additions & 9 deletions rust/kernel/error.rs
Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@ use crate::{alloc::AllocError, str::CStr};
use alloc::alloc::LayoutError;

use core::fmt;
use core::num::NonZeroI32;
use core::num::TryFromIntError;
use core::str::Utf8Error;

Expand All @@ -20,7 +21,11 @@ pub mod code {
$(
#[doc = $doc]
)*
pub const $err: super::Error = super::Error(-(crate::bindings::$err as i32));
pub const $err: super::Error =
match super::Error::try_from_errno(-(crate::bindings::$err as i32)) {
Some(err) => err,
None => panic!("Invalid errno in `declare_err!`"),
};
};
}

Expand Down Expand Up @@ -88,7 +93,7 @@ pub mod code {
///
/// The value is a valid `errno` (i.e. `>= -MAX_ERRNO && < 0`).
#[derive(Clone, Copy, PartialEq, Eq)]
pub struct Error(core::ffi::c_int);
pub struct Error(NonZeroI32);

impl Error {
/// Creates an [`Error`] from a kernel error code.
Expand All @@ -107,45 +112,59 @@ impl Error {

// INVARIANT: The check above ensures the type invariant
// will hold.
Error(errno)
// SAFETY: `errno` is checked above to be in a valid range.
unsafe { Error::from_errno_unchecked(errno) }
}

/// Creates an [`Error`] from a kernel error code.
///
/// Returns [`None`] if `errno` is out-of-range.
const fn try_from_errno(errno: core::ffi::c_int) -> Option<Error> {
if errno < -(bindings::MAX_ERRNO as i32) || errno >= 0 {
return None;
}

// SAFETY: `errno` is checked above to be in a valid range.
Some(unsafe { Error::from_errno_unchecked(errno) })
}

/// Creates an [`Error`] from a kernel error code.
///
/// # Safety
///
/// `errno` must be within error code range (i.e. `>= -MAX_ERRNO && < 0`).
unsafe fn from_errno_unchecked(errno: core::ffi::c_int) -> Error {
const unsafe fn from_errno_unchecked(errno: core::ffi::c_int) -> Error {
// INVARIANT: The contract ensures the type invariant
// will hold.
Error(errno)
// SAFETY: The caller guarantees `errno` is non-zero.
Error(unsafe { NonZeroI32::new_unchecked(errno) })
}

/// Returns the kernel error code.
pub fn to_errno(self) -> core::ffi::c_int {
self.0
self.0.get()
}

#[cfg(CONFIG_BLOCK)]
pub(crate) fn to_blk_status(self) -> bindings::blk_status_t {
// SAFETY: `self.0` is a valid error due to its invariant.
unsafe { bindings::errno_to_blk_status(self.0) }
unsafe { bindings::errno_to_blk_status(self.0.get()) }
}

/// Returns the error encoded as a pointer.
pub fn to_ptr<T>(self) -> *mut T {
#[cfg_attr(target_pointer_width = "32", allow(clippy::useless_conversion))]
// SAFETY: `self.0` is a valid error due to its invariant.
unsafe {
bindings::ERR_PTR(self.0.into()) as *mut _
bindings::ERR_PTR(self.0.get().into()) as *mut _
}
}

/// Returns a string representing the error, if one exists.
#[cfg(not(testlib))]
pub fn name(&self) -> Option<&'static CStr> {
// SAFETY: Just an FFI call, there are no extra safety requirements.
let ptr = unsafe { bindings::errname(-self.0) };
let ptr = unsafe { bindings::errname(-self.0.get()) };
if ptr.is_null() {
None
} else {
Expand Down

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