#239: Bitmask conversion trait

Another approach that fixes #223, as an alternative to #238.

This adds the `ToBitMask` trait, which is implemented on a vector for each bitmask type it supports.  This includes all unsigned integers with enough bits to contain it.  The byte array variant has been separated out for now into #246 and still requires `generic_const_exprs`, but the integer variants no longer require it and can make it to nightly.

crates/core_simd/src/masks.rs

@@ -12,8 +12,10 @@
 )]
 mod mask_impl;
 
-use crate::simd::intrinsics;
-use crate::simd::{LaneCount, Simd, SimdElement, SupportedLaneCount};
+mod to_bitmask;
+pub use to_bitmask::ToBitMask;
+
+use crate::simd::{intrinsics, LaneCount, Simd, SimdElement, SupportedLaneCount};
 use core::cmp::Ordering;
 use core::{fmt, mem};
 
@@ -225,22 +227,6 @@ where
         }
     }
 
-    /// Convert this mask to a bitmask, with one bit set per lane.
-    #[cfg(feature = "generic_const_exprs")]
-    #[inline]
-    #[must_use = "method returns a new array and does not mutate the original value"]
-    pub fn to_bitmask(self) -> [u8; LaneCount::<LANES>::BITMASK_LEN] {
-        self.0.to_bitmask()
-    }
-
-    /// Convert a bitmask to a mask.
-    #[cfg(feature = "generic_const_exprs")]
-    #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
-    pub fn from_bitmask(bitmask: [u8; LaneCount::<LANES>::BITMASK_LEN]) -> Self {
-        Self(mask_impl::Mask::from_bitmask(bitmask))
-    }
-
     /// Returns true if any lane is set, or false otherwise.
     #[inline]
     #[must_use = "method returns a new bool and does not mutate the original value"]

crates/core_simd/src/masks/bitmask.rs

@@ -1,7 +1,7 @@
 #![allow(unused_imports)]
 use super::MaskElement;
 use crate::simd::intrinsics;
-use crate::simd::{LaneCount, Simd, SupportedLaneCount};
+use crate::simd::{LaneCount, Simd, SupportedLaneCount, ToBitMask};
 use core::marker::PhantomData;
 
 /// A mask where each lane is represented by a single bit.
@@ -115,20 +115,22 @@ where
         unsafe { Self(intrinsics::simd_bitmask(value), PhantomData) }
     }
 
-    #[cfg(feature = "generic_const_exprs")]
     #[inline]
-    #[must_use = "method returns a new array and does not mutate the original value"]
-    pub fn to_bitmask(self) -> [u8; LaneCount::<LANES>::BITMASK_LEN] {
-        // Safety: these are the same type and we are laundering the generic
+    pub fn to_bitmask_integer<U>(self) -> U
+    where
+        super::Mask<T, LANES>: ToBitMask<BitMask = U>,
+    {
+        // Safety: these are the same types
         unsafe { core::mem::transmute_copy(&self.0) }
     }
 
-    #[cfg(feature = "generic_const_exprs")]
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
-    pub fn from_bitmask(bitmask: [u8; LaneCount::<LANES>::BITMASK_LEN]) -> Self {
-        // Safety: these are the same type and we are laundering the generic
-        Self(unsafe { core::mem::transmute_copy(&bitmask) }, PhantomData)
+    pub fn from_bitmask_integer<U>(bitmask: U) -> Self
+    where
+        super::Mask<T, LANES>: ToBitMask<BitMask = U>,
+    {
+        // Safety: these are the same types
+        unsafe { Self(core::mem::transmute_copy(&bitmask), PhantomData) }
     }
 
     #[inline]

crates/core_simd/src/masks/full_masks.rs

@@ -2,7 +2,7 @@
 
 use super::MaskElement;
 use crate::simd::intrinsics;
-use crate::simd::{LaneCount, Simd, SupportedLaneCount};
+use crate::simd::{LaneCount, Simd, SupportedLaneCount, ToBitMask};
 
 #[repr(transparent)]
 pub struct Mask<T, const LANES: usize>(Simd<T, LANES>)
@@ -66,6 +66,23 @@ where
     }
 }
 
+// Used for bitmask bit order workaround
+pub(crate) trait ReverseBits {
+    fn reverse_bits(self) -> Self;
+}
+
+macro_rules! impl_reverse_bits {
+    { $($int:ty),* } => {
+        $(
+        impl ReverseBits for $int {
+            fn reverse_bits(self) -> Self { <$int>::reverse_bits(self) }
+        }
+        )*
+    }
+}
+
+impl_reverse_bits! { u8, u16, u32, u64 }
+
 impl<T, const LANES: usize> Mask<T, LANES>
 where
     T: MaskElement,
@@ -110,41 +127,36 @@ where
         unsafe { Mask(intrinsics::simd_cast(self.0)) }
     }
 
-    #[cfg(feature = "generic_const_exprs")]
     #[inline]
-    #[must_use = "method returns a new array and does not mutate the original value"]
-    pub fn to_bitmask(self) -> [u8; LaneCount::<LANES>::BITMASK_LEN] {
-        unsafe {
-            let mut bitmask: [u8; LaneCount::<LANES>::BITMASK_LEN] =
-                intrinsics::simd_bitmask(self.0);
-
-            // There is a bug where LLVM appears to implement this operation with the wrong
-            // bit order.
-            // TODO fix this in a better way
-            if cfg!(target_endian = "big") {
-                for x in bitmask.as_mut() {
-                    *x = x.reverse_bits();
-                }
-            }
+    pub(crate) fn to_bitmask_integer<U: ReverseBits>(self) -> U
+    where
+        super::Mask<T, LANES>: ToBitMask<BitMask = U>,
+    {
+        // Safety: U is required to be the appropriate bitmask type
+        let bitmask: U = unsafe { intrinsics::simd_bitmask(self.0) };
 
+        // LLVM assumes bit order should match endianness
+        if cfg!(target_endian = "big") {
+            bitmask.reverse_bits()
+        } else {
             bitmask
         }
     }
 
-    #[cfg(feature = "generic_const_exprs")]
     #[inline]
-    #[must_use = "method returns a new mask and does not mutate the original value"]
-    pub fn from_bitmask(mut bitmask: [u8; LaneCount::<LANES>::BITMASK_LEN]) -> Self {
-        unsafe {
-            // There is a bug where LLVM appears to implement this operation with the wrong
-            // bit order.
-            // TODO fix this in a better way
-            if cfg!(target_endian = "big") {
-                for x in bitmask.as_mut() {
-                    *x = x.reverse_bits();
-                }
-            }
+    pub(crate) fn from_bitmask_integer<U: ReverseBits>(bitmask: U) -> Self
+    where
+        super::Mask<T, LANES>: ToBitMask<BitMask = U>,
+    {
+        // LLVM assumes bit order should match endianness
+        let bitmask = if cfg!(target_endian = "big") {
+            bitmask.reverse_bits()
+        } else {
+            bitmask
+        };
 
+        // Safety: U is required to be the appropriate bitmask type
+        unsafe {
             Self::from_int_unchecked(intrinsics::simd_select_bitmask(
                 bitmask,
                 Self::splat(true).to_int(),

crates/core_simd/src/masks/to_bitmask.rs

@@ -0,0 +1,57 @@
+use super::{mask_impl, Mask, MaskElement};
+use crate::simd::{LaneCount, SupportedLaneCount};
+
+mod sealed {
+    pub trait Sealed {}
+}
+pub use sealed::Sealed;
+
+impl<T, const LANES: usize> Sealed for Mask<T, LANES>
+where
+    T: MaskElement,
+    LaneCount<LANES>: SupportedLaneCount,
+{
+}
+
+/// Converts masks to and from integer bitmasks.
+///
+/// Each bit of the bitmask corresponds to a mask lane, starting with the LSB.
+///
+/// # Safety
+/// This trait is `unsafe` and sealed, since the `BitMask` type must match the number of lanes in
+/// the mask.
+pub unsafe trait ToBitMask: Sealed {
+    /// The integer bitmask type.
+    type BitMask;
+
+    /// Converts a mask to a bitmask.
+    fn to_bitmask(self) -> Self::BitMask;
+
+    /// Converts a bitmask to a mask.
+    fn from_bitmask(bitmask: Self::BitMask) -> Self;
+}
+
+macro_rules! impl_integer_intrinsic {
+    { $(unsafe impl ToBitMask<BitMask=$int:ty> for Mask<_, $lanes:literal>)* } => {
+        $(
+        unsafe impl<T: MaskElement> ToBitMask for Mask<T, $lanes> {
+            type BitMask = $int;
+
+            fn to_bitmask(self) -> $int {
+                self.0.to_bitmask_integer()
+            }
+
+            fn from_bitmask(bitmask: $int) -> Self {
+                Self(mask_impl::Mask::from_bitmask_integer(bitmask))
+            }
+        }
+        )*
+    }
+}
+
+impl_integer_intrinsic! {
+    unsafe impl ToBitMask<BitMask=u8> for Mask<_, 8>
+    unsafe impl ToBitMask<BitMask=u16> for Mask<_, 16>
+    unsafe impl ToBitMask<BitMask=u32> for Mask<_, 32>
+    unsafe impl ToBitMask<BitMask=u64> for Mask<_, 64>
+}

crates/core_simd/tests/masks.rs

@@ -68,16 +68,16 @@ macro_rules! test_mask_api {
                 assert_eq!(core_simd::Mask::<$type, 8>::from_int(int), mask);
             }
 
-            #[cfg(feature = "generic_const_exprs")]
             #[test]
             fn roundtrip_bitmask_conversion() {
+                use core_simd::ToBitMask;
                 let values = [
                     true, false, false, true, false, false, true, false,
                     true, true, false, false, false, false, false, true,
                 ];
                 let mask = core_simd::Mask::<$type, 16>::from_array(values);
                 let bitmask = mask.to_bitmask();
-                assert_eq!(bitmask, [0b01001001, 0b10000011]);
+                assert_eq!(bitmask, 0b1000001101001001);
                 assert_eq!(core_simd::Mask::<$type, 16>::from_bitmask(bitmask), mask);
             }
         }