On Wed, Mar 08, 2017 at 11:03:53AM +0100, Richard Biener wrote:
> On Wed, 8 Mar 2017, Jakub Jelinek wrote:
>
> > On Wed, Mar 08, 2017 at 09:15:05AM +0100, Richard Biener wrote:
> > > Ok. Note that another option for the loopy case is to do
> > >
> > > for (;;)
> > > {
> > > vec >> by-one-elt;
> > > elt = BIT_FIELD_REF <vec, index-zero>;
> > > }
> >
> > Indeed, that is a possibility, but I guess I'd need to construct
> > the result similarly if resv is non-NULL. Also, not sure about big endian
> > vectors and whether BIT_FIELD_REF with zero or size - elt_size is
> > more efficient there.
> >
> > In any case, the PR was about s390 without vectors enabled, so this wouldn't
> > apply.
> >
> > > when whole-vector shifts are available (they are constructed by
> > > VEC_PERM_EXPR if vec_perm_const_ok for that mask). If you end up
> > > doing variable-index array accesses you can as well spill the
> > > vector to memory and use memory accesses on that. Not sure how
> > > to arrange that from this part of the expander.
> >
> > Shouldn't something else during the expansion force it into memory if it is
> > more efficient to expand it that way? Apparently it is forced into memory
>
> Possibly - but it might end up spilling in the loop itself and thus be
> rather inefficient?
Ok. That said, this is -fsanitize=undefined which slows down code anyway,
so making it more efficient can be done in GCC8 IMNSHO.
Jakub
