Add test using non-power-of-two vector

tests/ui/simd/repr_packed.rs

@@ -6,6 +6,9 @@
 #[repr(simd, packed)]
 struct Simd<T, const N: usize>([T; N]);
 
+#[repr(simd)]
+struct FullSimd<T, const N: usize>([T; N]);
+
 fn check_size_align<T, const N: usize>() {
     use std::mem;
     assert_eq!(mem::size_of::<Simd<T, N>>(), mem::size_of::<[T; N]>());
@@ -37,9 +40,20 @@ fn main() {
 
     unsafe {
         // powers-of-two have no padding and work as usual
-        // non-powers-of-two have padding and need to be expanded to full vectors
         let x: Simd<f64, 4> =
             simd_add(Simd::<f64, 4>([0., 1., 2., 3.]), Simd::<f64, 4>([2., 2., 2., 2.]));
         assert_eq!(std::mem::transmute::<_, [f64; 4]>(x), [2., 3., 4., 5.]);
+
+        // non-powers-of-two have padding and need to be expanded to full vectors
+        fn load<T, const N: usize>(v: Simd<T, N>) -> FullSimd<T, N> {
+            unsafe {
+                let mut tmp = core::mem::MaybeUninit::<FullSimd<T, N>>::uninit();
+                std::ptr::copy_nonoverlapping(&v as *const _, tmp.as_mut_ptr().cast(), 1);
+                tmp.assume_init()
+            }
+        }
+        let x: FullSimd<f64, 3> =
+            simd_add(load(Simd::<f64, 3>([0., 1., 2.])), load(Simd::<f64, 3>([2., 2., 2.])));
+        assert_eq!(x.0, [2., 3., 4.]);
     }
 }