On Mon, Jul 17, 2017 at 11:56:50AM +0200, Richard Biener wrote: > On Thu, Jul 13, 2017 at 2:18 AM, Kugan Vivekanandarajah > <kugan.vivekanandara...@linaro.org> wrote: > > I am looking into reversing loop to increased efficiency. There is > > already a PR22041 for this and an old patch > > https://gcc.gnu.org/ml/gcc-patches/2006-01/msg01851.html by Zdenek > > which never made it to mainline. > > > > For constant loop count, ivcanon pass is adding reverse iv but this > > not selected by ivopt. > > ivopt will never do loop reversal, if it would select this IV it would need to > keep the original one as well or compute the old i based on the new. > > loop reversal needs dependence analysis and it's not clear if for the copy > case you quote would be profitable (HW prefetchers like that?). For your > case it's also invalid as a and c may overlap.
If the goal is to make the induction variable end at 0, you can transform [0..N) into [-N..0). I have some old patches to do that; it probably is better to start from scratch if you want it though. Two issues that had to be solved for this: 1) ivopts didn't cost the cheaper loop ending condition correctly; 2) where you had say a[i] in the loop it will be (a+N)[i] afterwards, and the calculation of a+N wasn't properly hoisted out of the loop. Segher
