[Bug target/65832] Inefficient vector construction

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65832

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
On a related note, store_constructor handles VECTOR_TYPE constructors through
the vec_init optab but that doesn't work for vector elements:

            /* Don't use vec_init<mode> if some elements have VECTOR_TYPE.  */
            if (icode != CODE_FOR_nothing)
              {
                tree value;

                FOR_EACH_CONSTRUCTOR_VALUE (CONSTRUCTOR_ELTS (exp), idx, value)
                  if (TREE_CODE (TREE_TYPE (value)) == VECTOR_TYPE)
                    {
                      icode = CODE_FOR_nothing;

instead it could, for say a V16QI constructor of four V4QI elements pun
the elements to SI mode, create a V4SI mode vector via vec_init and then
pun it to V16QI.  All punning via subregs.

The vectorizer currently creates such vector constructors from grouped strided
load/store support.