On Mon, Dec 28, 2015 at 1:20 PM, Jason Ekstrand <ja...@jlekstrand.net> wrote: > > > 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.
It is a mix.. I do texcoord saturate, clip-plane, and 2-sided color lowering in NIR. But flat-shading, binning-pass, and half vs full precision color output in ir3. I do as much lowering in NIR as I can, in an effort to do as much as possible at compile time, vs draw time. I do the first round of lowering/opt w/ null shader key, which is enough for the common cases. Pretty much independent, I suppose, of whether I came out of SSA or not first. Although binning-pass variant and the instruction scheduling I do are easier in SSA. Somewhat unrelated, but I may end up converting array access to registers, but leave everything else in SSA, so I can benefit from converting multi-dimensional offsets into a single offset.. this is still one open issue w/ gallium glsl_to_nir.. right now I have a hacked up version of nir_lower_io that converts global/local load/store_var's into load/store_var2 which take an offset as a src (like load_input/store_output) instead of deref chain.. not sure yet whether this will be the permanent solution, but at least it fixes a huge heap of variable-indexing piglits and lets me continue w/ implementing lowering passes for everything else that was previously done in glsl->tgsi or tgsi->tgsi passes. BR, -R >> >> >>> >> >>> 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 > > _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
