Hi! On Tue, Sep 21, 2021 at 11:24:08AM +0800, Kewen.Lin wrote: > on 2021/9/18 上午6:01, Segher Boessenkool wrote: > > On Thu, Sep 16, 2021 at 09:14:15AM +0800, Kewen.Lin wrote: > >> 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 scale, this patch > >> doesn't use nunits * stmt_cost any more, but it still keeps > >> nunits since there are actually nunits scalar loads there. So > >> it uses one cost adjusted from stmt_cost, since the current > >> stmt_cost sort of considers nunits, we can stablize the cost > >> for big nunits and retain the cost for small nunits. After > >> some tries, this patch gets the adjusted cost as: > >> > >> stmt_cost / (log2(nunits) * log2(nunits)) > > > > So for V16QI it gives *16/(4*4) so *1 > > V8HI it gives *8/(3*3) so *8/9 > > V4SI it gives *4/(2*2) so *1 > > V2DI it gives *2/(1*1) so *2 > > and for V1TI it gives *1/(0*0) which is UB (no, does not crash for us, > > just gives wildly wrong answers; the div returns 0 on recent systems). > > I don't expected we will have V1TI for strided/elementwise load, > if it's one unit vector, it's the whole vector itself. > Besides, the below assertion should exclude it already.
Yes. But ignoring the UB for unexpectedly large vector components, the 1 / 1.111 / 1 / 2 scoring does not make much sense. The formulas "look" smooth and even sort of reasonable, but as soon as you look at what it *means*, and realise the domain if the function is discrete (only four or five possible inputs), and then see how the function behaves on that... Hrm :-) > > This of course is assuming nunits will always be a power of 2, but I'm > > sure that we have many other places in the compiler assuming that > > already, so that is fine. And if one day this stops being true we will > > get a nice ICE, pretty much the best we could hope for. > > Yeah, exact_log2 returns -1 for non power of 2 input, for example: Exactly. > >> + unsigned int adjusted_cost = stmt_cost / nunits_sq; > > > > But this can divide by 0. Or are we somehow guaranteed that nunits > > will never be 1? Yes the log2 check above, sure, but that ICEs if this > > is violated; is there anything that actually guarantees it is true? > > As I mentioned above, I don't expect we can have nunits 1 strided/ew load, > and the ICE should check this and ensure dividing by zero never happens. :) Can you assert that *directly* then please? > > A magic crazy formula like this is no good. If you want to make the > > cost of everything but V2D* be the same, and that of V2D* be twice that, > > that is a weird heuristic, but we can live with that perhaps. But that > > beats completely unexplained (and unexplainable) magic! > > > > Sorry. > > That's all right, thanks for the comments! let's improve it. :) I like that spirit :-) > How about just assigning 2 for V2DI and 1 for the others for the > penalized_cost_per_load with some detailed commentary, it should have > the same effect with this "magic crazy formula", but I guess it can > be more clear. That is fine yes! (Well, V2DF the same I guess? Or you'll need very detailed commentary :-) ) It is fine to say "this is just a heuristic without much supporting theory" in places. That is what most of our --param= are as well, for example. If counting two-element vectors as twice as expensive as all other vectors helps performance, then so be it: if there is no better way to cost things (or we do not know one), then what else are we to do? Segher
