Hi, Gentle ping this:
https://gcc.gnu.org/pipermail/gcc-patches/2021-September/580358.html BR, Kewen on 2021/9/28 下午4:16, Kewen.Lin via Gcc-patches wrote: > Hi, > > This patch follows the discussions here[1][2], where Segher > pointed out the existing way to guard the extra penalized > cost for strided/elementwise loads with a magic bound does > not scale. > > The way with nunits * stmt_cost can get one much > exaggerated penalized cost, such as: for V16QI on P8, it's > 16 * 20 = 320, that's why we need one bound. To make it > better and more readable, the penalized cost is simplified > as: > > unsigned adjusted_cost = (nunits == 2) ? 2 : 1; > unsigned extra_cost = nunits * adjusted_cost; > > For V2DI/V2DF, it uses 2 penalized cost for each scalar load > while for the other modes, it uses 1. It's mainly concluded > from the performance evaluations. One thing might be > related is that: More units vector gets constructed, more > instructions are used. It has more chances to schedule them > better (even run in parallelly when enough available units > at that time), so it seems reasonable not to penalize more > for them. > > The SPEC2017 evaluations on Power8/Power9/Power10 at option > sets O2-vect and Ofast-unroll show this change is neutral. > > Bootstrapped and regress-tested on powerpc64le-linux-gnu Power9. > > Is it ok for trunk? > > [1] https://gcc.gnu.org/pipermail/gcc-patches/2021-September/579121.html > [2] https://gcc.gnu.org/pipermail/gcc-patches/2021-September/580099.html > v1: https://gcc.gnu.org/pipermail/gcc-patches/2021-September/579529.html > > BR, > Kewen > ----- > gcc/ChangeLog: > > * config/rs6000/rs6000.c (rs6000_update_target_cost_per_stmt): Adjust > the way to compute extra penalized cost. Remove useless parameter. > (rs6000_add_stmt_cost): Adjust the call to function > rs6000_update_target_cost_per_stmt. > > > --- > gcc/config/rs6000/rs6000.c | 31 ++++++++++++++++++------------- > 1 file changed, 18 insertions(+), 13 deletions(-) > > diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c > index dd42b0964f1..8200e1152c2 100644 > --- a/gcc/config/rs6000/rs6000.c > +++ b/gcc/config/rs6000/rs6000.c > @@ -5422,7 +5422,6 @@ rs6000_update_target_cost_per_stmt (rs6000_cost_data > *data, > enum vect_cost_for_stmt kind, > struct _stmt_vec_info *stmt_info, > enum vect_cost_model_location where, > - int stmt_cost, > unsigned int orig_count) > { > > @@ -5462,17 +5461,23 @@ rs6000_update_target_cost_per_stmt (rs6000_cost_data > *data, > { > tree vectype = STMT_VINFO_VECTYPE (stmt_info); > unsigned int nunits = vect_nunits_for_cost (vectype); > - unsigned int extra_cost = nunits * stmt_cost; > - /* As function rs6000_builtin_vectorization_cost shows, we have > - priced much on V16QI/V8HI vector construction as their units, > - if we penalize them with nunits * stmt_cost, it can result in > - an unreliable body cost, eg: for V16QI on Power8, stmt_cost > - is 20 and nunits is 16, the extra cost is 320 which looks > - much exaggerated. So let's use one maximum bound for the > - extra penalized cost for vector construction here. */ > - const unsigned int MAX_PENALIZED_COST_FOR_CTOR = 12; > - if (extra_cost > MAX_PENALIZED_COST_FOR_CTOR) > - extra_cost = MAX_PENALIZED_COST_FOR_CTOR; > + /* Don't expect strided/elementwise loads for just 1 nunit. */ > + gcc_assert (nunits > 1); > + /* i386 port adopts nunits * stmt_cost as the penalized cost > + for this kind of penalization, we used to follow it but > + found it could result in an unreliable body cost especially > + for V16QI/V8HI modes. To make it better, we choose this > + new heuristic: for each scalar load, we use 2 as penalized > + cost for the case with 2 nunits and use 1 for the other > + cases. It's without much supporting theory, mainly > + concluded from the broad performance evaluations on Power8, > + Power9 and Power10. One possibly related point is that: > + vector construction for more units would use more insns, > + it has more chances to schedule them better (even run in > + parallelly when enough available units at that time), so > + it seems reasonable not to penalize that much for them. */ > + unsigned int adjusted_cost = (nunits == 2) ? 2 : 1; > + unsigned int extra_cost = nunits * adjusted_cost; > data->extra_ctor_cost += extra_cost; > } > } > @@ -5510,7 +5515,7 @@ rs6000_add_stmt_cost (class vec_info *vinfo, void > *data, int count, > cost_data->cost[where] += retval; > > rs6000_update_target_cost_per_stmt (cost_data, kind, stmt_info, where, > - stmt_cost, orig_count); > + orig_count); > } > > return retval; > -- > 2.27.0 >
