Improve documentation

Author: calebzulawski

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -480,10 +480,15 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
             }
 
             _ if name.as_str().starts_with("simd_") => {
-                // Unpack non-power-of-2 #[repr(packed)]
+                // Unpack non-power-of-2 #[repr(packed, simd)] arguments.
+                // This gives them the expected layout of a regular #[repr(simd)] vector.
                 let mut loaded_args = Vec::new();
                 for (ty, arg) in arg_tys.iter().zip(args) {
                     loaded_args.push(
+                        // #[repr(packed, simd)] vectors are passed like arrays (as references,
+                        // with reduced alignment and no padding) rather than as immediates.
+                        // We can use a vector load to fix the layout and turn the argument
+                        // into an immediate.
                         if ty.is_simd()
                             && let OperandValue::Ref(place) = arg.val
                         {

tests/ui/simd/repr_packed.rs

@@ -36,12 +36,13 @@ fn main() {
     check_ty::<f64>();
 
     unsafe {
-        // powers-of-two have no padding and work as usual
+        // powers-of-two have no padding and have the same layout as #[repr(simd)]
         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 lesser alignment, but the intrinsic handles it
+        // non-powers-of-two should have padding (which is removed by #[repr(packed)]),
+        // but the intrinsic handles it
         let x: Simd<f64, 3> = simd_add(Simd::<f64, 3>([0., 1., 2.]), Simd::<f64, 3>([2., 2., 2.]));
         let arr: [f64; 3] = x.0;
         assert_eq!(arr, [2., 3., 4.]);