[Bug middle-end/47397] Alignment of array element is not optimal in AVX mode due to use of TARGET_MEM_REFs

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47397

--- Comment #7 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-01-21 15:35:29 UTC ---
(In reply to comment #6)
> (In reply to comment #5)
> > Thus, the real reason is that we lack alignment information on
> > MEM_REFs/TARGET_MEM_REFs but only have pointer alignment information for now.
> 
> I know it won't solve this bug.  But shouldn't we at least use alignment
> of array element instead of pointer alignment?

Well, part of it is half-way re-introducing type-alignment stuff,
deviating from the intial conservative-correctness.


Thus, we run into

        align = MAX (TYPE_ALIGN (TREE_TYPE (exp)),
                     get_object_alignment (exp, BIGGEST_ALIGNMENT));

for deciding whether to use movmisalign_optab but in set_mem_attrs_minus_bitpos
we do not fall back to TYPE_ALIGNment at all (which is conservative and
by initial design).

You might argue we want consistency here.

Note that this shouldn't be an issue for AVX/SSE as unaligned moves are
as fast as aligned ones if they are really aligned (at least I hope
this is true for AVX, it definitely is for SSE on recent archs - if not
the vectorizer cost model would need adjustment).