On Mon, 5 Feb 2007, Paolo Bonzini wrote:
>
> > As we also only vectorize innermost loops I believe doing a
> > complete unrolling pass early will help in general (I pushed
> > for this some time ago).
> >
> > Thoughts?
>
> It might also hurt, though, since we don't have a basic block vectorizer.
> IIUC the vectorizer is able to turn
>
> for (i = 0; i < 4; i++)
> v[i] = 0.0;
>
> into
>
> *(vector double *)v = (vector double){0.0, 0.0, 0.0, 0.0};
That's true. But we can not do constant propagation out of this
(and the vectorizer leaves us with a lot of cruft which is only
removed much later).
The above case would also ask for an early vectorization pass if the
loop was wrapped into another.
Finding a good heuristic for which loops to completely unroll early
is not easy, though for odd small numbers of iterations it is
probably always profitable.
Richard.
--
Richard Guenther <[EMAIL PROTECTED]>
Novell / SUSE Labs
