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Document alternatives to Mask<T, LANES> that guarantee layout. Fixes #332 #333

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Document alternatives to Mask<T, LANES> that guarantee layout. Fixes #332 #333

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Document alternatives to Mask<T, LANES> that guarantee layout.
reinerp Mar 8, 2023
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s/layout/size. Mention BitMaskArray.
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4 changes: 4 additions & 0 deletions crates/core_simd/src/masks.rs
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Expand Up @@ -88,6 +88,10 @@ impl_element! { isize }
/// The layout of this type is unspecified, and may change between platforms
/// and/or Rust versions, and code should not assume that it is equivalent to
/// `[T; LANES]`.
///
/// For a type with layout guaranteed equivalent to `[T; LANES]`, use
/// `SIMD<T, LANES>`. For a type with layout guaranteed to use 1 bit per
/// lane (padded up to full bytes), use `LANES::BitMask`.
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So, the exact number of bytes is an implementation detail (we've used u64 before and we might choose it again... basically we want to retain the freedom to use an integer of a reasonable size, OR to use a byte array, depending on what strikes the best ergonomics/codegen balance). Right now, this seems like it might suggest it uses exactly the next byte in size. I am not sure what would be most clear to express our desired reservation while providing you the minimum of guarantees you want (or if you even consider that acceptable).

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The type is actually ToBitMask::BitMask: https://doc.rust-lang.org/std/simd/trait.ToBitMask.html#associatedtype.BitMask. It's not padded up to the next byte, but the next integer.

#[repr(transparent)]
pub struct Mask<T, const LANES: usize>(mask_impl::Mask<T, LANES>)
where
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