On Tue, May 21, 2019 at 12:20:50PM +0200, Richard Biener wrote: > On Tue, 21 May 2019, Kewen.Lin wrote: > > > on 2019/5/21 上午12:37, Segher Boessenkool wrote: > > > On Mon, May 20, 2019 at 08:43:59AM -0600, Jeff Law wrote: > > >>> I think we should have two hooks: one is called with the struct loop as > > >>> parameter; and the other is called for every statement in the loop, if > > >>> the hook isn't null anyway. Or perhaps we do not need that second one. > > >> I'd wait to see a compelling example from real world code where we need > > >> to scan the statements. Otherwise we're just dragging in more target > > >> specific decisions which in fact we want to minimize target stuff. > > > > > > The ivopts pass will be too optimistic about what loops will end up as a > > > doloop, and cost things accordingly. The cases where we cannot later > > > actually use a doloop are doing pretty much per iteration, so I think > > > ivopts will still make good decisions. We'll need to make the rtl part > > > not actually do a doloop then, but we probably still need that logic > > > anyway. > > > > > > Kewen, Bin, will that work satisfactorily do you think? > > > > If my understanding on this question is correct, IMHO we should try to make > > IVOPTs conservative than optimistic, since once the predict is wrong from > > too optimistic decision, the costing on the doloop use is wrong, it's very > > possible to affect the global optimal set. It looks we don't have any ways > > to recover it in RTL then? (otherwise, there should be better place to fix > > the PR). Although it's also possible to miss some good cases, it's at least > > as good as before, I'm inclined to make it conservative. > > I wonder if you could simply benchmark what happens if you make > IVOPTs _always_ create a doloop IV (if possible)?
That would help yes. > I doubt the > cases where a doloop IV is bad (calls, etc.) are too common and They are quite common I think :-( Like, more than the number of valid doloops for us. But let's see numbers :-) > that in those cases the extra simple IV hurts. That, I don't know. We do not usually need an IV that counts down, but will it be worse performance? Segher
