[PATCH 14/25] Disable inefficient vectorization of elementwise loads/stores.

[Richard Biener]

On Mon, Sep 17, 2018 at 2:40 PM Andrew Stubbs <ams@codesourcery.com> wrote:
>
> On 17/09/18 12:43, Richard Sandiford wrote:
> > OK, sounds like the cost of vec_construct is too low then.  But looking
> > at the port, I see you have:
> >
> > /* Implement TARGET_VECTORIZE_BUILTIN_VECTORIZATION_COST.  */
> >
> > int
> > gcn_vectorization_cost (enum vect_cost_for_stmt ARG_UNUSED (type_of_cost),
> >                       tree ARG_UNUSED (vectype), int ARG_UNUSED (misalign))
> > {
> >    /* Always vectorize.  */
> >    return 1;
> > }
> >
> > which short-circuits the cost-model altogether.  Isn't that part
> > of the problem?
>
> Well, it's possible that that's a little simplistic. ;-)
>
> Although, actually the elementwise issue predates the existence of
> gcn_vectorization_cost, and the default does appear to penalize
> vec_construct somewhat.
>
> Actually, the default definition doesn't seem to do much besides
> increase vec_construct, so I'm not sure now why I needed to change it?
> Hmm, more experiments to do.
>
> Thanks for the pointer.

Btw, we do not consider to use gather/scatter for VMAT_ELEMENTWISE,
that's a missed "optimization" quite possibly because gather/scatter is so
expensive on x86.  Thus the vectorizer should consider this and use the
cheaper alternative according to the cost model (which you of course should
fill with sensible values...).

Richard.

> Andrew