[Bug tree-optimization/95845] Failure to optimize vector load made in separate operations to single load

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95845

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's now vectorized at -O3 or with -ftree-slp-vectorize.  In particular
vect_slp_check_for_constructors now matches

      else if (code == BIT_INSERT_EXPR
               && VECTOR_TYPE_P (TREE_TYPE (rhs))
               && TYPE_VECTOR_SUBPARTS (TREE_TYPE (rhs)).is_constant ()
               && TYPE_VECTOR_SUBPARTS (TREE_TYPE (rhs)).to_constant () > 1
               && integer_zerop (gimple_assign_rhs3 (assign))
               && useless_type_conversion_p
                    (TREE_TYPE (TREE_TYPE (rhs)),
                     TREE_TYPE (gimple_assign_rhs2 (assign)))
               && bb_vinfo->lookup_def (gimple_assign_rhs2 (assign)))
        {
          /* We start to match on insert to lane zero but since the
             inserts need not be ordered we'd have to search both
             the def and the use chains.  */

this matching could be factored out and used by forwprop to build a
vector CTOR.  So I don't think it's fully fixed yet and there's an
opportunity to improve things earlier.

Partial defs of otherwise uninitialized vectors might also be an interesting
target.  When not keying on lane zero to start the match the possibility
is to start matching on the insert that does not have a single immediate
use of the result in another BIT_INSERT_EXPR.