on 2020/7/31 下午9:01, Richard Biener wrote: > On Fri, Jul 31, 2020 at 2:37 PM Kewen.Lin <li...@linux.ibm.com> wrote: >> >> Hi Richards, >> >> on 2020/7/31 下午7:20, Richard Biener wrote: >>> On Fri, Jul 31, 2020 at 1:03 PM Richard Sandiford >>> <richard.sandif...@arm.com> wrote: >>>> >>>> "Kewen.Lin" <li...@linux.ibm.com> writes: >>>>>>> + bool niters_known_p = LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo); >>>>>>> + bool need_iterate_p >>>>>>> + = (!LOOP_VINFO_EPILOGUE_P (loop_vinfo) >>>>>>> + && !vect_known_niters_smaller_than_vf (loop_vinfo)); >>>>>>> + >>>>>>> + /* Init min/max, shift and minus cost relative to single >>>>>>> + scalar_stmt. For now we only use length-based partial vectors on >>>>>>> + Power, target specific cost tweaking may be needed for other >>>>>>> + ports in future. */ >>>>>>> + unsigned int min_max_cost = 2; >>>>>>> + unsigned int shift_cost = 1, minus_cost = 1; >>>>>> >>>>>> Please instead add a scalar_min_max to vect_cost_for_stmt, and use >>>>>> scalar_stmt for shift and minus. There shouldn't be any Power things >>>>>> hard-coded into target-independent code. >>>>>> >>>>> >>>>> Agree! It's not good to leave them there. I thought to wait and see >>>>> if other targets which support vector with length can reuse this, or >>>>> move it to target specific codes then if not sharable. But anyway >>>>> it looks not good, let's fix it. >>> >>> In other generic places like this we simply use three generic scalar_stmt >>> costs. At least I don't see how min_max should be different from it >>> when shift can be the same as minus. Note this is also how we treat >> >> Yeah, normally they (min/max/minus/shift) are taken as scalar_stmt, excepting >> for fine-grain tuning like i386 port, they will use the same cost. On >> Power9, >> to implement min/max it takes double cycles of the normal scalar operations >> like add/shift, I was trying to model it more fine-grained since we probably >> generate a few min/max here, if the loop body cost is small, I was worried >> the decision isn't good enough. But yeah, in other generic places, the small >> loop could also suffer this similar off, they are the same essentially. >> >>> vectorization of MAX_EXPR - scalar cost is one scalar_stmt and >>> vector cost is one vector_stmt. As you say below the add_stmt_cost >>> hook can adjust based on the actual GIMPLE stmt -- if one is >>> available (which indeed it isn't here). >>> >>> I'm somewhat lacking context here as well - we actually GIMPLE >>> code-generate the min/max/shift/minus and only the eventual >>> AND is defered to the target, right? >>> >> >> Yes, min/max/shift/minus are all GIMPLE code, targets like Power >> will have its target specific cost for shift which moves length >> to high bits 0:7. >> >> One typical case is as below: >> >> <bb 3> [local count: 105119324]: >> _26 = n_11(D) * 4; >> _37 = MAX_EXPR <_26, 16>; >> _38 = _37 + 18446744073709551600; >> _40 = MIN_EXPR <_26, 16>; >> >> <bb 4> [local count: 630715945]: >> # ivtmp_35 = PHI <0(3), ivtmp_36(4)> >> # loop_len_30 = PHI <_40(3), _44(4)> >> _19 = &MEM[base: a_12(D), index: ivtmp_35, offset: 0B]; >> vect_24 = .LEN_LOAD (_19, 4B, loop_len_30); >> vect__3.7_23 = VIEW_CONVERT_EXPR<vector(4) unsigned int>(vect_24); >> _1 = &MEM[base: b_13(D), index: ivtmp_35, offset: 0B]; >> vect_17 = .LEN_LOAD (_1, 4B, loop_len_30); >> vect__5.10_9 = VIEW_CONVERT_EXPR<vector(4) unsigned int>(vect_17); >> vect__7.11_8 = vect__5.10_9 + vect__3.7_23; >> vect_28 = VIEW_CONVERT_EXPR<vector(16) unsigned char>(vect__7.11_8); >> _2 = &MEM[base: c_14(D), index: ivtmp_35, offset: 0B]; >> .LEN_STORE (_2, 4B, loop_len_30, vect_28); >> _42 = MIN_EXPR <ivtmp_35, _38>; >> _43 = _38 - _42; >> _44 = MIN_EXPR <_43, 16>; >> ivtmp_36 = ivtmp_35 + 16; >> if (ivtmp_35 < _38) >> goto <bb 4>; [83.33%] >> else >> goto <bb 5>; [16.67%] >> >> >>>>> I had some concerns on vect_cost_for_stmt way, since it seems to allow >>>>> more computations similar to min/max to be added like this, in long >>>>> term it probably leads to the situtation like: scalar_min_max, >>>>> scalar_div_expr, scalar_mul_expr ... an enum (cost types) bloat, it >>>>> seems not good to maintain. >>>> >>>> I guess doing that doesn't seem so bad to me :-) I think it's been >>>> a recurring problem that the current classification isn't fine-grained >>>> enough for some cases. >>> >>> But we eventually want to get rid of this classification enum in favor >>> of the add_stmt_cost hook ... >>> >> >> Nice, sounds like each target has to handle it fine-grain. :) >> IIUC, the current modeling doesn't consider the instruction dependency and >> execution resource etc. like scheduling, even if all costs are fine-grained >> enough, the decision could be sub-optimal. > > That's what the finish_cost hook is for - the target can collect > all stmts during add_stmt and then apply adjustments at the end > (IIRC power does already for shift operation resource constraints). >
oh, forgot that we have that hook, though I just imagined some generic codes would do this. Good to know we have a way to make it. Nice! BR, Kewen
