On Fri, Apr 17, 2020 at 10:51 PM Segher Boessenkool <seg...@kernel.crashing.org> wrote: > > On Fri, Apr 17, 2020 at 08:53:08AM +0200, Richard Biener wrote: > > On Thu, Apr 16, 2020 at 7:46 PM Segher Boessenkool > > <seg...@kernel.crashing.org> wrote: > > > > > > On Thu, Apr 16, 2020 at 10:33:45AM +0200, Richard Biener wrote: > > > > On Wed, Apr 15, 2020 at 11:23 PM Segher Boessenkool > > > > > On a general note, we shouldn't depend on some pass that may or may > > > > > not > > > > > clean up the mess we make, when we could just avoid making a mess in > > > > > the > > > > > first place. > > > > > > > > True - but the issue at hand is not trivial given you have to care for > > > > partial defs, uses outside of the loop (or across the backedge), etc. > > > > So there's plenty of things to go "wrong" here. > > > > > > Certainly. But *all* RTL passes before RA should leave things in "web > > > form" (compare to SSA form). The code in web.c is probably just fine; > > > but we shouldn't have one web pass, *all* passes should leave things in > > > a useful form! > > > > Yeah well, but RTL is not in SSA form > > "Webs" are not the *same* as SSA, in a few crucial ways; but they serve > similar purposes: they both make code transformations easier to do. > Both do this by having more different (independent) names. > > > and there's no RTL IL verification > > in place to track degradation. > > Adding this isn't hard in principle. But currently it is violated all > over the place, including in backend code. > > > And we even work in the opposite way > > when expanding to RTL from SSA, coalescing as much as we can ... > > Which often is bad. A lot of what expand does is premature optimisation > that a) is much too complicated, and b) results in worse code in the end. > Expand should not try to do *anything* "interesting", it should "merely" > translate to RTL. > > (And in a way that can be debugged sanely at all, if possible ;-) ) > > > > > > The web pass belongs immediately after expand; but ideally, even > > > > > expand > > > > > would not reuse pseudos anyway. > > > > > > > > But for example when lower-subreg decomposes things in a way turning > > > > partial defs into full defs new opportunities to split the web arise. > > > > > > But that is only for the new registers it creates, or what am I missing? > > > > No idea, just made up the example that maintaing "SSA" RTL is > > not automagic. > > Oh sure, but in most cases, it is. > > My point is that making web form only once (and messing it up after that) > isn't great. > > > > > > Maybe it would be better as some utility routines, not a pass? > > > > > > > > Sure, but then when do we apply it? > > > > > > All of the time! Ideally every pass would leave things in a good shape. > > > Usually it is cheapest as well as easiest to do things manually, but for > > > some harder cases, such helper routines can be used. > > > > > > > Ideally scheduling would to > > > > register renaming itself and thus not rely on the used pseudos > > > > (I'm not sure if it tracks false dependences - I guess it must if it > > > > isn't able to rename regs). That would be a much better place > > > > for improvements? > > > > > > sched2 runs after RA, so it has nothing to do with webs? And sched1 > > > doesn't do much relevant here (it doesn't move insns much). > > > > > > I don't see how this is directly related to register renaming either? > > > > If scheduling ignores "false" dependences (anti dependence with > > full defs) then when it schedules across such defs needs to perform > > renaming. Maybe I'm using bogus terms here. > > Your terminology is fine afaic, but I don't see what you mean, sorry. > Maybe I don't know sched well enough... I thought it would just refuse > to move something if that would result in broken code?
Yes, I think that's what it does. And I was suggesting it should learn to schedule the use after the def in (use (reg:SI 101)) (set (reg:SI 101) (...)) by renaming the pseudo, removing the false/anti dependence. After RA this means the ability to do simple local register allocation of course. Before RA the trivial way to implement this is to not have those false dependences, aka have webs / SSA ... ;) Richard. > > Segher
