[PATCH v4] vect/rs6000: Support vector with length cost modeling

[Richard Biener]

On Fri, Jul 31, 2020 at 2:37 PM Kewen.Lin <linkw@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.sandiford@arm.com> wrote:
> >>
> >> "Kewen.Lin" <linkw@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).

> >>> I noticed that i386 port ix86_add_stmt_cost will check stmt_info->stmt,
> >>> whether is assignment and the subcode of the expression, it provides the
> >>> chance to check the statement more fine-grain, not just as normal
> >>> scalar_stmt/vector_stmt.
> >>>
> >>> For the case here, we don't have the stmt_info, but we know the type
> >>> of computation(expression), how about to extend the hook add_stmt_cost
> >>> with one extra tree_code type argument, by default it can be some
> >>> unmeaningful code, for some needs like here, we specify the tree_code
> >>> as the code of computation, like {MIN,MAX}_EXPR, then target specific
> >>> add_stmt_cost can check this tree_code and adjust the cost accordingly.
> >>
> >> If we do that, I guess we should “promote” code_helper out of
> >> gimple-match.h and use that instead, so that we can handle
> >> internal and built-in functions too.
> >>
> >> Would like to hear Richard's opinion on the best way forward here.
> >
> > I'd say defer this to a later patch and for now simply cost one scalar
> > stmt for MIN/MAX.  I agree that if we add a tree_code we want a
> > code_helper instead.  Note that I want to eventually have a
> > full SLP tree for the final code generation where all info should be
> > there (including SLP nodes for those min/max ops) and which the
> > target could traverse.  But I'm not sure if I can make enough progress
> > on that SLP-only thing for GCC 11 even...
> >
>
> OK, I'm fine to take MIN/MAX as scalar_stmt here.  Thank both of you!
> This new SLP framework looks very promising and powerful.  :-)

As do all things that are still on paper^Win my head only ;)

Richard.

>
> BR,
> Kewen