On Mon, Dec 28, 2015 at 9:37 AM, Connor Abbott <cwabbo...@gmail.com> wrote:
> On Mon, Dec 28, 2015 at 10:13 AM, Rob Clark <robdcl...@gmail.com> wrote: > > On Tue, Dec 22, 2015 at 10:11 PM, Connor Abbott <cwabbo...@gmail.com> > wrote: > >> On Tue, Dec 22, 2015 at 9:55 PM, Rob Clark <robdcl...@gmail.com> wrote: > >>> On Tue, Dec 22, 2015 at 9:47 PM, Connor Abbott <cwabbo...@gmail.com> > wrote: > >>>> On Tue, Dec 22, 2015 at 9:02 PM, Rob Clark <robdcl...@gmail.com> > wrote: > >>>>> On Mon, Dec 21, 2015 at 1:48 PM, Jason Ekstrand < > ja...@jlekstrand.net> wrote: > >>>>>> > >>>>>> I think two different concepts of ownership are getting conflated > here: > >>>>>> Right/responsibility to delete and right to modify. > >>>>>> > >>>>>> The way I understand it, gallium, as it stands, gives neither to > the driver. > >>>>>> A back-end using NIR requires the right to modify but who deletes > it doesn't > >>>>>> ultimately matter. I think it's dangerous to pass one of these > rights to > >>>>>> the driver and not the other but we need to think about both. > >>>>> > >>>>> yeah, uneasy about driver modifying the IR if the state tracker is > >>>>> still going to potentially spin off variants of the IR.. that sounds > >>>>> like madness. > >>>>> > >>>>> The refcnt'ing I proposed does deal w/ right to modify vs delete via > >>>>> nir_shader(_is)_mutable() which returns something that is guaranteed > >>>>> to be safe to modify (ie. has only a single reference) > >>>>> > >>>>>> What I'm trying to say is that we have two options here: > >>>>>> > >>>>>> 1) gallium passes IR to the back-end that it is free to modify and > is > >>>>>> required to delete when it's done. > >>>>> > >>>>> with refcnt'ing, s/delete/unref/ > >>>>> > >>>>> The idea is, the st transfers ownership of the reference it passes to > >>>>> the driver. If the st wants to hang on to a reference itself, it > must > >>>>> increment the refcnt before passing to the driver (backend). > >>>>> > >>>>> Without refcnt'ing, I suppose we could (since we don't have to follow > >>>>> TGSI semantics), just decree that the driver always takes ownership > of > >>>>> the copy passed in, and if the st wants to hang on to a copy too, > then > >>>>> it must clone. I suppose this would work well enough for > >>>>> freedreno/vc4, which both end up generating variants later. It does > >>>>> force an extra clone for drivers that immediately translate into > their > >>>>> own backend IR and don't need to keep the NIR around, for example. > >>>>> Maybe that is not worth caring about (since at this point it is > >>>>> hypothetical). > >>>> > >>>> While always cloning does have this disadvantage, I don't think it's > >>>> really relevant here. Even if the driver throws away the NIR > >>>> immediately after consuming it, almost invariably it's going to want > >>>> to modify it. The generic NIR passed in by the state tracker (other > >>>> IR -> NIR + some optimizations) is almost never going to be the same > >>>> as the NIR after going through driver-specific lowering passes, which > >>>> means that drivers are never going to want a read-only version of the > >>>> IR. In light of that, I think making the driver own the IR passed in > >>>> seems like the most sensible thing. > >>> > >>> well, unless the driver is already doing it's own lowering in it's own > >>> native IR.. > >> > >> Well, if you're not doing any lowering in NIR, then you aren't really > >> taking any advantage of it. I can't see a plausible scenario where all > >> the lowering is done in the driver's own IR -- and as soon as you do > >> anything in NIR, you need the driver-owns-IR semantics. > > > > When it comes to shader variants, I have a mix, with some things > > lowered in nir and others just handled in backend.. > > > > The re-work / cleanup that I have had on a branch for a while now > > (since it is currently blocked on refcnt'ing) does a first round of > > variant-key independent NIR lowering/opt passes. And then at draw > > time, if the variant key has anything that is lowered in nir, I do a > > second round. > Just to be clear, your key-dependent lowering happens after all of your other lowering? If this is the case, then I guarantee you that you're unique in this since i965 and vc4 need to at least run out-of-SSA afterwards. To be honest, I completely forgot that a driver could use fully ssa NIR. > >>> > >>> Maybe it is too much of a hypothetical.. I still think refcnt'ing > >>> gives some nice flexibility to deal with various scenarios, and having > >>> to call nir_shader_unref() isn't so much of a burden. > >> > >> Still, I can't see how this flexibility is at all useful, and it seems > >> like overkill since the driver will always want a mutable version of > >> the IR anyways. > > > > Well, due to the structure I mentioned above, at draw time when I need > > to generate a variant with nothing lowered in NIR, I simply incr the > > refcnt on the IR which has already gone through first round of NIR > > passes, and pass that in to my back end compiler. At the end, once > > the shader binary is generated, I can unconditionally unref the > > nir_shader without having to care. > > > > Without refcnt'ing I'd either have to generate a pointless clone or > > keep track that the nir_shader should not actually be free'd. Not > > impossible, just a bit more ugly. > > Assuming you do all your variant management in your driver's IR, then > you don't need to do anything. If you do some variant management in > NIR, then in the function where you do the NIR-based lowering you can > check if you need to do any lowering based on the shader key, clone > first, and give the NIR->ir3 function the cloned IR and then free it. > It might be a "bit more ugly," but it's really not that much different > from the refcounting, and when the extra shader gets created/freed is > made explicit. > > > > > (The gallium glsl_to_nir stuff is also currently using refcnt'ing, > > although at least for freedreno/ir3 it isn't strictly needed.. I could > > just unconditionally clone in the state tracker. That said, I'm still > > of the opinion that refcnt'ing could be useful to some other driver > > someday) > > "It could be useful to some driver someday" isn't a good argument for > adding stuff today. We've already had enough examples of things in NIR > that we added because we thought it was useful, but turned out not to > be. > > > > > BR, > > -R > > > >>> > >>> BR, > >>> -R > >>> > >>>>> > >>>>> (I guess nouveau is the one driver, that if it ever consumed NIR, > >>>>> would translate immediately into it's own backend IR?) > >>>>> > >>>>>> 2) gallium passes read-only IR to the back-end and it always makes > a copy. > >>>>> > >>>>> This is how it is w/ TGSI, but I think with NIR we are free to make a > >>>>> clean break. And we *definitely* want to avoid > >>>>> the-driver-always-copies semantics.. > >>>>> > >>>>>> It sounds like, from what Marek is saying, that gallium is > currently doing > >>>>>> (2) and changing it to (1) would be painful. I think reference > counting is > >>>>>> more like an awkward option 1.5 than option 3. Reference counting > would > >>>>>> mean that gallium passes a reference to the driver which it is > expected to > >>>>>> unref but may keep a second reference if it wants to keep the > driver from > >>>>>> modifying it. Then the driver may or may not make a copy based on > the > >>>>>> number of references. Why don't we just make it explicit and add a > >>>>>> read-only bit and call it a day. > >>>>>> > >>>>>> One of the reasons I don't like passing a reference is that it > effectively > >>>>>> puts allocation and freeing in different components of the driver. > >>>>> > >>>>> With refcnt'ing you should talk in terms of ref/unref rather than > >>>>> allocate/free.. imho. Although maybe that is what you meant. (In > >>>>> which case, yes, that was my idea, that passing in to driver > transfers > >>>>> ownership of the passed reference.) > >>>>> > >>>>>> This > >>>>>> means that if and driver doesn't care at all about the shader that > gets > >>>>>> passed in, it still has to under it to avoid a memory leak. You > can't have > >>>>>> the driver take the reference because then, either it comes in with > a > >>>>>> recount of 0 and should have been deleted, or the "can I modify > this" check > >>>>>> becomes "recount <= 2" which makes no sense. > >>>>> > >>>>> hmm, no, if ownership of the reference is transferred to the driver, > >>>>> then it becomes "refcount == 1" (and refcount == 0 should be an > >>>>> assert, because something has gone horribly wrong) > >>>>> > >>>>> BR, > >>>>> -R >
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