Christian Schoenebeck writes:
> Es geschah am Tuesday, 18. April 2006 12:04 als Andrew Haley schrieb:
> > Christian Schoenebeck writes:
> > > Es geschah am Friday, 7. April 2006 22:17 als Greg Buchholz schrieb:
> > > > Christian Schoenebeck wrote:
> > > > > b) We wondered [2] what the currently best way is to access single
> > > > > elements of one vector. Currently the only solution we saw is to use
> > > > > a union trick
> > > >
> > > > /* I don't know if this qualifies as good, but here's some things
> > > > I've used before */
> > >
> > > Oh, it officially qualified as good! We just made some benchmarks
> > > [1] to compare our old union trick vs. your straight forward type
> > > casts and your solutions were always faster, often even quite a
> > > lot. So as we thought, gcc doesn't like that union thing very much.
> >
> > It's not legal C, though. The _mm_load, _mm_set, and _mm_store
> > operations are provided to do the conversions.
>
> Well, those intrinsics would make it hardware dependent though. I
> thought the reason for introducing the vector extension was to
> avoid this. Anyway, I don't mind if this is legal C or not until it
> works^TM.
>
> Unfortunately we found a case [1] which did not work at all: a type cast from
> float vector to integer vector, like:
>
> typedef float v4sf __attribute__ ((vector_size(16),aligned(16)));
> typedef int v4i __attribute__ ((vector_size(sizeof(int)*4)));
>
> int main() {
> const v4sf v = { 1.2f ,2.2f ,3.3f, 4.4f };
> const v4i vRes = (v4i) v;
> }
>
> The resulting integer vector vRes would simply contain crap.
> Is this a bug, not implemented yet or even intentional?
I don't know, because there's not enough information here. Can you
produce a runnable test case?
Andrew.
