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 > > > >
