Thanks. Another question. Is there any plan to vectorize
the loops like the following ones?
for (i=127; i>=0; i--) {
x[i] = y[i] + z[i];
}
I found that GCC trunk still cannot handle negative step
for store. Even it can, it won't be efficient by introducing
redundant permutations on load and store.
Cheers,
Bingfeng
> -----Original Message-----
> From: Ira Rosen [mailto:i...@il.ibm.com]
> Sent: 10 February 2011 17:22
> To: Bingfeng Mei
> Cc: gcc@gcc.gnu.org
> Subject: Re: Vector permutation only deals with # of vector elements
> same as mask?
>
>
> Hi,
>
> "Bingfeng Mei" <b...@broadcom.com> wrote on 10/02/2011 05:35:45 PM:
> >
> > Hi,
> > I noticed that vector permutation gets more use in GCC
> > 4.6, which is great. It is used to handle negative step
> > by reversing vector elements now.
> >
> > However, after reading the related code, I understood
> > that it only works when the # of vector elements is
> > the same as that of mask vector in the following code.
> >
> > perm_mask_for_reverse (tree-vect-stmts.c)
> > ...
> > mask_type = get_vectype_for_scalar_type (mask_element_type);
> > nunits = TYPE_VECTOR_SUBPARTS (vectype);
> > if (!mask_type
> > || TYPE_VECTOR_SUBPARTS (vectype) != TYPE_VECTOR_SUBPARTS
> (mask_type))
> > return NULL;
> > ...
> >
> > For PowerPC altivec, the mask_type is V16QI. It means that
> > compiler can only permute V16QI type. But given the capability of
> > altivec vperm instruction, it can permute any 128-bit type
> > (V8HI, V4SI, etc). We just need convert in/out V16QI from
> > given types and a bit more extra work in producing mask.
> >
> > Do I understand correctly or miss something here?
>
> Yes, you are right. The support of reverse access is somewhat limited.
> Please see vect_transform_slp_perm_load() in tree-vect-slp.c for
> example of
> all type permutation support.
>
> But, anyway, reverse accesses are not supported for altivec's load
> realignment scheme.
>
> Ira
>
> >
> > Thanks,
> > Bingfeng Mei
> >
> >
> >
> >
>
