https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116973
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note the load in question isn't lowered because of
/* When the load permutation accesses a contiguous unpermuted,
power-of-two aligned and sized chunk leave the load alone.
We can likely (re-)load it more efficiently rather than
extracting it from the larger load.
??? Long-term some of the lowering should move to where
the vector types involved are fixed. */
if (ld_lanes_lanes == 0
&& contiguous
&& (SLP_TREE_LANES (load) > 1 || loads.size () == 1)
&& pow2p_hwi (SLP_TREE_LANES (load))
&& SLP_TREE_LOAD_PERMUTATION (load)[0] % SLP_TREE_LANES (load) == 0
&& group_lanes % SLP_TREE_LANES (load) == 0)
{
final_perm.release ();
continue;
}
which I added as part of r15-3442-g7164d982663738 that enables lowering
of single loads which specifically exempted some cases to avoid regressions.
gcc.dg/vect/slp-gap-1.c is the testcase that benefits from the above.
That case asks for lowering being more aware of gaps I guess, while we
now track those with NULL scalar stmt in the lanes the lowering code
doesn't track the "do-not-care" state of such lanes but it probably should.
