Hi Richard, Richard Guenther <[EMAIL PROTECTED]> wrote on 05/02/2007 17:27:03:
... > > ... > > and we are later not able to do constant propagation to the > second loop which we can do if we first unroll such small loops. > > As we also only vectorize innermost loops by the way, we are working on vectorization of outer-loops > I believe doing a > complete unrolling pass early will help in general (I pushed > for this some time ago). > > Thoughts? > My initial thought was that it's probably not the right thing to do because: 1. the problems with constant propagation and aliasing really calls for improving/extending these optimization (and probably also have the vectorizer convey more of the information it has), rather than work around the problem. 2. the vectorizer should be able to decide, based on a cost model, whether it is profitable to vectorize a loop, and if not - as in the case above - leave it unvectorized. 3. In the meantime, you can use --param min-vect-loop-bound=2 to disallow vectorization of this loop. In fact, maybe we should make that the deafult (instead of the current default - 0). However..., I have seen cases in which complete unrolling before vectorization enabled constant propagation, which in turn enabled significant simplification of the code, thereby, in fact making a previously unvectorizable loop (at least on some targets, due to the presence of divisions, unsupported in the vector unit), into a loop (in which the divisions were replaced with constants), that can be vectorized. Also, given that we are working on "SLP" kind of technology (straight line code vectorization), which would make vectorization less sensitive to unrolling, I think maybe it's not such a bad idea after all... One option is to increase the default value of --param min-vect-loop-bound for now, and when SLP is incorporated, go ahead and schedule early complete unrolling. However, since SLP implementation may take some time (hopefully within the time frame of 4.3 though) - we could just go ahead and schedule early complete unrolling right now. (I can't believe I'm in favor of this idea, but that loop I was talking about before - improved by a factor over 20x when early complete unrolling + subsequent vectorization were applied...) dorit > Thanks, > Richard. > > -- > Richard Guenther <[EMAIL PROTECTED]> > Novell / SUSE Labs
