Use intrinsics for Mask::{to,from}_array

This significantly simplifies codegen and should improve mask perf.
Co-authored-by: NJacob Lifshay <programmerjake@gmail.com>

crates/core_simd/src/masks.rs

@@ -12,9 +12,10 @@
 )]
 mod mask_impl;
 
+use crate::simd::intrinsics;
 use crate::simd::{LaneCount, Simd, SimdElement, SupportedLaneCount};
 use core::cmp::Ordering;
-use core::fmt;
+use core::{fmt, mem};
 
 mod sealed {
     use super::*;
@@ -105,22 +106,39 @@ pub fn splat(value: bool) -> Self {
         Self(mask_impl::Mask::splat(value))
     }
 
-    /// Converts an array to a SIMD vector.
+    /// Converts an array of bools to a SIMD mask.
     pub fn from_array(array: [bool; LANES]) -> Self {
-        let mut vector = Self::splat(false);
-        for (i, v) in array.iter().enumerate() {
-            vector.set(i, *v);
+        // SAFETY: Rust's bool has a layout of 1 byte (u8) with a value of
+        //     true:    0b_0000_0001
+        //     false:   0b_0000_0000
+        // Thus, an array of bools is also a valid array of bytes: [u8; N]
+        // This would be hypothetically valid as an "in-place" transmute,
+        // but these are "dependently-sized" types, so copy elision it is!
+        unsafe {
+            let bytes: [u8; LANES] = mem::transmute_copy(&array);
+            let bools: Simd<i8, LANES> =
+                intrinsics::simd_ne(Simd::from_array(bytes), Simd::splat(0u8));
+            Mask::from_int_unchecked(intrinsics::simd_cast(bools))
         }
-        vector
     }
 
-    /// Converts a SIMD vector to an array.
+    /// Converts a SIMD mask to an array of bools.
     pub fn to_array(self) -> [bool; LANES] {
-        let mut array = [false; LANES];
-        for (i, v) in array.iter_mut().enumerate() {
-            *v = self.test(i);
+        // This follows mostly the same logic as from_array.
+        // SAFETY: Rust's bool has a layout of 1 byte (u8) with a value of
+        //     true:    0b_0000_0001
+        //     false:   0b_0000_0000
+        // Thus, an array of bools is also a valid array of bytes: [u8; N]
+        // Since our masks are equal to integers where all bits are set,
+        // we can simply convert them to i8s, and then bitand them by the
+        // bitpattern for Rust's "true" bool.
+        // This would be hypothetically valid as an "in-place" transmute,
+        // but these are "dependently-sized" types, so copy elision it is!
+        unsafe {
+            let mut bytes: Simd<i8, LANES> = intrinsics::simd_cast(self.to_int());
+            bytes &= Simd::splat(1i8);
+            mem::transmute_copy(&bytes)
         }
-        array
     }
 
     /// Converts a vector of integers to a mask, where 0 represents `false` and -1