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Add notes to avoid direct field accesses

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This commit is contained in:
Markus Everling 2023-04-22 23:02:45 +00:00
parent afad9c3f64
commit 52833ccbe8
1 changed files with 17 additions and 0 deletions
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17
crates/core_simd/src/vector.rs
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@ -76,6 +76,11 @@ use crate::simd::{
/// [`read`]: pointer::read
/// [`write`]: pointer::write
/// [as_simd]: slice::as_simd
//
// NOTE: Accessing the inner array directly in any way (e.g. by using the `.0` field syntax) or
// directly constructing an instance of the type (i.e. `let vector = Simd(array)`) should be
// avoided, as it will likely become illegal on `#[repr(simd)]` structs in the future. It also
// causes rustc to emit illegal LLVM IR in some cases.
#[repr(simd)]
pub struct Simd<T, const LANES: usize>([T; LANES])
where
@ -138,6 +143,9 @@ where
// SAFETY: Transmuting between `Simd<T, LANES>` and `[T; LANES]`
// is always valid and `Simd<T, LANES>` never has a lower alignment
// than `[T; LANES]`.
//
// NOTE: This deliberately doesn't just use `&self.0`, see the comment
// on the struct definition for details.
unsafe { &*(self as *const Self as *const [T; LANES]) }
}
@ -146,6 +154,9 @@ where
// SAFETY: Transmuting between `Simd<T, LANES>` and `[T; LANES]`
// is always valid and `Simd<T, LANES>` never has a lower alignment
// than `[T; LANES]`.
//
// NOTE: This deliberately doesn't just use `&mut self.0`, see the comment
// on the struct definition for details.
unsafe { &mut *(self as *mut Self as *mut [T; LANES]) }
}
@ -153,6 +164,9 @@ where
pub const fn from_array(array: [T; LANES]) -> Self {
// SAFETY: Transmuting between `Simd<T, LANES>` and `[T; LANES]`
// is always valid.
//
// NOTE: This deliberately doesn't just use `Self(array)`, see the comment
// on the struct definition for details.
unsafe { core::mem::transmute_copy(&array) }
}
@ -160,6 +174,9 @@ where
pub const fn to_array(self) -> [T; LANES] {
// SAFETY: Transmuting between `Simd<T, LANES>` and `[T; LANES]`
// is always valid.
//
// NOTE: This deliberately doesn't just use `self.0`, see the comment
// on the struct definition for details.
unsafe { core::mem::transmute_copy(&self) }
}