Hi Richard, on 2019/7/12 下午8:11, Richard Biener wrote: > On Wed, 10 Jul 2019, Kewen.Lin wrote: > >> Hi all, >> >> I'd like to gentle ping the below patch: >> https://gcc.gnu.org/ml/gcc-patches/2019-06/msg01225.html >> >> The previous version for more context/background: >> https://gcc.gnu.org/ml/gcc-patches/2019-06/msg01126.html >> >> Thanks a lot in advance! > > Again I would have hoped Bin to chime in here. > > Am I correct that doloop HW implementations are constrainted > by a decrement of one? I see no code in the patch to constrain > things this way.
If my understanding is correct, under have_count_reg_decr_p I don't think we should check the decrement one pattern, doloop can transform the loop closing to decrement by 1 since it knows the iteration total count. Since it uses special hardware register like Power count register, we don't expect it to be shared with other uses. Btw, it also doesn't require the compare should be the comp/decrement pattern, so this patch more focuses on this comp is needed or not (should be considered in selection or not). > I'm not familiar with the group code at all > but I would have expected the patch to only affect costing > of IVs of the appropriate form (decrement one and possibly > no uses besides the one in the compare/decrement). But since we select IV cand for every IV uses, we never knows this IV cand will have the only use till the whole selection done. > Since > ivcanon already adds a canonical counter IV it's not > necessary to generate an artificial candidate IV of the > wanted style (that's something I might have expected as well). This patch is only for the case guarded in have_count_reg_decr_p. It doesn't requires to have the artificial candidate IV as well as decrement-compare-jump code sequence. The code on power looks like: mtctr Rx // move Rx (which holding total_counter) // to ctr register L: loop body... bnze L // decrease ctr register and jump to L if // ctr nonzero > > The rest should be just magic from the IVOPTs side? > > There might be the need to only consider at most one counter IV > in the costing code. The current patch doesn't introduce any IV cands but focus on zeroing the cost of comp IV use since we know it will be eliminated. Still to leverage the existing candidate selection algorithm to decide the final optimal IV set. Bring back the canonical counter IV only if it's not selected by any IV uses to keep the doloop comp use rewriting correct, but it shouldn't affect anything since the use will be eliminated and is the only use, the IV and its related will be removed as well. Thanks, Kewen