On Dec 21, 2015 1:37 PM, "Marek Olšák" <mar...@gmail.com> wrote: > > On Mon, Dec 21, 2015 at 7:48 PM, Jason Ekstrand <ja...@jlekstrand.net> wrote: > > > > On Dec 21, 2015 9:09 AM, "Connor Abbott" <cwabbo...@gmail.com> wrote: > >> > >> On Mon, Dec 21, 2015 at 11:45 AM, Marek Olšák <mar...@gmail.com> wrote: > >> > On Mon, Dec 21, 2015 at 4:38 PM, Connor Abbott <cwabbo...@gmail.com> > >> > wrote: > >> >> On Mon, Dec 21, 2015 at 6:39 AM, Marek Olšák <mar...@gmail.com> wrote: > >> >>> On Mon, Dec 21, 2015 at 6:48 AM, Jason Ekstrand < ja...@jlekstrand.net> > >> >>> wrote: > >> >>>> > >> >>>> On Dec 20, 2015 7:43 PM, "Rob Clark" <robdcl...@gmail.com> wrote: > >> >>>>> > >> >>>>> On Sun, Dec 20, 2015 at 10:29 PM, Connor Abbott > >> >>>>> <cwabbo...@gmail.com> > >> >>>>> wrote: > >> >>>>> > On Sun, Dec 20, 2015 at 10:04 PM, Rob Clark < robdcl...@gmail.com> > >> >>>>> > wrote: > >> >>>>> >> On Sun, Dec 20, 2015 at 9:12 PM, Jason Ekstrand > >> >>>>> >> <ja...@jlekstrand.net> > >> >>>>> >> wrote: > >> >>>>> >>> > >> >>>>> >>> On Dec 19, 2015 5:55 PM, "Rob Clark" <robdcl...@gmail.com> > >> >>>>> >>> wrote: > >> >>>>> >>>> > >> >>>>> >>>> From: Rob Clark <robcl...@freedesktop.org> > >> >>>>> >>>> > >> >>>>> >>>> Jason, > >> >>>>> >>>> > >> >>>>> >>>> How much do you hate this idea? Seems like an easy alternative > >> >>>>> >>>> to > >> >>>>> >>>> using ralloc ctx's to clean up nir variants/clones, which would > >> >>>>> >>>> let > >> >>>>> >>>> us drop the parent memctx for nir_shader_create()/clone(), > >> >>>>> >>>> making > >> >>>>> >>>> it easier to introduce reference counting. > >> >>>>> >>> > >> >>>>> >>> I think "hate" is a but strong. I don't like it but it works. > >> >>>>> >>> If we > >> >>>>> >>> really > >> >>>>> >>> want nir_shader refcounted, we'll have to do something. > >> >>>>> >> > >> >>>>> >> I suppose the alternate idea of moving the nir_shader_clone() out > >> >>>>> >> of > >> >>>>> >> brw_compile_xyz(), and always passing in the clone would be a > >> >>>>> >> cleaner > >> >>>>> >> way. It looks like each of the brw_compile_xyz() has exactly one > >> >>>>> >> call-site, so doing the nir_shader_clone() inside doesn't really > >> >>>>> >> buy > >> >>>>> >> anything. > >> >>>> > >> >>>> Your forgetting that there may be *cough* other users of this API... > >> >>>> We can > >> >>>> change those too but I would like the needs of the compiler users to > >> >>>> drive > >> >>>> the API, not the cloning. I still have some details to work out > >> >>>> there. In > >> >>>> any case, it doesn't really matter; we can figure something out. > >> >>>> > >> >>>>> >>> About refcounting... The more I think about it the more I'm not > >> >>>>> >>> convinced > >> >>>>> >>> it's useful. As it stands, we have no use for it an I'm not > >> >>>>> >>> convinced > >> >>>>> >>> you > >> >>>>> >>> do either. We'll see if I can convince you. :-) > >> >>>>> >>> > >> >>>>> >>> In the history of i965 using NIR, we've had about three > >> >>>>> >>> different ways > >> >>>>> >>> of > >> >>>>> >>> doing things: > >> >>>>> >>> > >> >>>>> >>> 1) GLSL is the gold copy and we run glsl_to_nir for every > >> >>>>> >>> shader/variant > >> >>>>> >>> compile. This is what we did when we first stated using NIR > >> >>>>> >>> because > >> >>>>> >>> it was > >> >>>>> >>> easy and didn't involve reworking any plumbing. > >> >>>>> >>> > >> >>>>> >>> 2) Lowered NIR is the gold copy; variants are done entirely in > >> >>>>> >>> the > >> >>>>> >>> back-end > >> >>>>> >>> IR. This is what we did up until about a month ago. Because > >> >>>>> >>> variants > >> >>>>> >>> are > >> >>>>> >>> done in the back-end, we can run gksl_to_nir and do all of our > >> >>>>> >>> optimizing > >> >>>>> >>> and lowering at link time. Going from NIR to the final shader > >> >>>>> >>> binary > >> >>>>> >>> is > >> >>>>> >>> then a read-only operation as far as NIR is concerned. > >> >>>>> >>> > >> >>>>> >>> 3) Optimized but not lowered NIR is the gold copy; variants are > >> >>>>> >>> sometimes > >> >>>>> >>> done in NIR. This is the scheme we use now. We call > >> >>>>> >>> glsl_to_nir and > >> >>>>> >>> do > >> >>>>> >>> some of the optimization and lowering at link time but leave it > >> >>>>> >>> in SSA > >> >>>>> >>> form. > >> >>>>> >>> When we go to compile the final shader, we make a copy, apply > >> >>>>> >>> variants, do > >> >>>>> >>> the final lowering and then go into the back-end IR. > >> >>>>> >>> > >> >>>>> >>> In each of these cases, we know exactly where we need to make a > >> >>>>> >>> copy > >> >>>>> >>> without > >> >>>>> >>> the help of reference counting. In the first, we get a fresh > >> >>>>> >>> copy > >> >>>>> >>> each time > >> >>>>> >>> so we are free to destroy the copy. In the second, we never > >> >>>>> >>> have to > >> >>>>> >>> modify > >> >>>>> >>> the NIR so no copy. In the third scheme, we always have to make > >> >>>>> >>> a > >> >>>>> >>> copy > >> >>>>> >>> because, even if variants are a no-op, we still have to go out > >> >>>>> >>> of SSA > >> >>>>> >>> form > >> >>>>> >>> and do final lowering. You could say that we could avoid making > >> >>>>> >>> that > >> >>>>> >>> copy. > >> >>>>> >>> However, the work to determine when we don't need variants and > >> >>>>> >>> can do > >> >>>>> >>> all > >> >>>>> >>> our lowering up-front is far more than the work saved by > >> >>>>> >>> reference > >> >>>>> >>> counting. > >> >>>>> >>> > >> >>>>> >>> How about gallium? Here's how I imagine it would work (please > >> >>>>> >>> correct > >> >>>>> >>> me of > >> >>>>> >>> I'm wrong): > >> >>>>> >>> > >> >>>>> >>> 1) In the TGSI case, tgsi_to_nir gets called for each compile so > >> >>>>> >>> you > >> >>>>> >>> get a > >> >>>>> >>> fresh mutable shader each time. In this case, you are free to > >> >>>>> >>> do > >> >>>>> >>> whatever > >> >>>>> >>> you want with the shader without making a copy. > >> >>>>> >>> > >> >>>>> >>> 2) In the GLSL case, you run glsl_to_nir and do some basic > >> >>>>> >>> optimizations at > >> >>>>> >>> link time and hold onto the NIR shader. (Hold a reference of > >> >>>>> >>> you'd > >> >>>>> >>> like.) > >> >>>>> >>> When you go to compile it in the back-end, it needs to do it's > >> >>>>> >>> own > >> >>>>> >>> lowering > >> >>>>> >>> so it takes a reference and ends up making a copy. > >> >>>>> >>> > >> >>>>> >>> If this description is anywhere close to correct, then I don't > >> >>>>> >>> think > >> >>>>> >>> you > >> >>>>> >>> really need it either. Determining whether or not you need to > >> >>>>> >>> copy is > >> >>>>> >>> simply "if (comes_from_tgsi)”. Maybe there's something subtle > >> >>>>> >>> about > >> >>>>> >>> the > >> >>>>> >>> gallium layer that I don't know that makes refcounting the best > >> >>>>> >>> solution. > >> >>>>> >>> Please enlighten me of there is. > >> >>>>> >> > >> >>>>> >> This issue is that we *potentially* have both the state tracker > >> >>>>> >> and > >> >>>>> >> the driver both doing some of there own variant management. > >> >>>>> >> (Which > >> >>>>> >> tbh, isn't awesome, it would have been nice if someone realized > >> >>>>> >> earlier on that nearly every driver is going to have to do some > >> >>>>> >> sort > >> >>>>> >> of variant mgmt and figured out a way just to push it all down to > >> >>>>> >> the > >> >>>>> >> driver.. but I can't see a good way to get there from here.) > >> >>>>> >> > >> >>>>> >> With TGSI as the IR, driver just unconditionally does > >> >>>>> >> tgsi_dup_tokens().. because of the whole thing where st does > >> >>>>> >> variants > >> >>>>> >> in some cases, things are defined that driver doesn't own the > >> >>>>> >> copy of > >> >>>>> >> the TGSI IR passed in after the fxn call to driver returns. > >> >>>>> >> > >> >>>>> >> With NIR I was hoping to fix this, esp. since nir_shader_clone() > >> >>>>> >> is > >> >>>>> >> more heavyweight than tgsi_dup_tokens() (memcpy()). > >> >>>>> >> > >> >>>>> >> Refcnt'ing is a nice solution so that we can pass the driver a > >> >>>>> >> reference that it owns. In cases where state tracker isn't doing > >> >>>>> >> variant mgmt, we pass it the one-and-only ref (avoiding clone). > >> >>>>> >> > >> >>>>> >> I'd suggested that in cases where st does variant mgmt, that st > >> >>>>> >> should > >> >>>>> >> do the clone/dup. But that was shot down: > >> >>>>> >> > >> >>>>> >> > >> >>>>> >> http://lists.freedesktop.org/archives/mesa-dev/2015-October/097748.html > >> >>>> > >> >>>> It sounds like Marek's argument there is more about lifetime > >> >>>> management than > >> >>>> anything. He wants gallium modules to be able to create IR, call > >> >>>> into the > >> >>>> driver, and then throw it away. In particular, he doesn't want them > >> >>>> to have > >> >>>> to think about cloning. In a lot of ways it sounds a lot like what > >> >>>> i965 > >> >>>> wants too. I really like having brw_compile_foo take a const > >> >>>> nir_shader. > >> >>>> The difference is that i965 basically always wants to clone whereas a > >> >>>> gallium driver may not have to if gallium doesn't care what happens > >> >>>> to the > >> >>>> shader when it's done. How common is this case? How important is it > >> >>>> to > >> >>>> optimize for? I don't know. > >> >>>> > >> >>>> One other thing that bothers me a bit: From Marek's comment, it > >> >>>> sounds like > >> >>>> the components want to just pass in IR and be agnostic about whether > >> >>>> the > >> >>>> driver wants its own copy or wants to change it or whatever. This > >> >>>> seems > >> >>>> like an argument for always cloning to me. From the perspective of a > >> >>>> gallium module, "I want to hang in to this, I'll keep a reference" > >> >>>> seems > >> >>>> exactly the same as "I want to hang onto this, I'll give the driver a > >> >>>> copy". > >> >>>> How are they actually different given that the driver basically has > >> >>>> to > >> >>>> modify what you give it in order to do lowering? > >> >>>> > >> >>>>> > Ugh... I didn't read this at the time, but I don't like Marek's > >> >>>>> > response. My understanding of the situation, based on this thread, > >> >>>>> > is > >> >>>>> > that there are some cases where the st knows that there's only > >> >>>>> > going > >> >>>>> > to be one variant and can throw away the (NIR or TGSI) shader > >> >>>>> > after it > >> >>>>> > hands it to the driver, while at other times it has to hold onto > >> >>>>> > all > >> >>>>> > the variants and only give the driver a read-only copy (or > >> >>>>> > duplicate > >> >>>> > >> >>>> As per above, my interpretation of Marek's comment is that he doesn't > >> >>>> want > >> >>>> the st to have to think about cloning ever. He wants it to assume > >> >>>> that > >> >>>> compilation never modifies the IR so the driver should always clone. > >> >>>> You > >> >>>> have to keep in mind that Marek is most likely thinking about caching > >> >>>> the > >> >>>> TGSI rather than doing in-place lowering in it. > >> >>>> > >> >>>> If I'm understanding Marek correctly, then it sounds like shader > >> >>>> compilation > >> >>>> should never touch the IR that's passed in. If this is the case, it > >> >>>> sounds > >> >>>> like always cloning is the way to go. At least its not *that* > >> >>>> expensive. > >> >>> > >> >>> Note that st/mesa needs to keep the FS IR because of glDrawPixels and > >> >>> glBitmap, and the VS IR because of edge flags, glRasterPos evaluation, > >> >>> selection and feedback modes. The last three are done with Draw/LLVM > >> >>> and only support TGSI. > >> >>> > >> >>> Therefore, st/mesa always hangs onto the IR and drivers can't modify > >> >>> it. It also needs VS in TGSI to be able to do everything correctly. > >> >>> > >> >>> What other Gallium modules want or not want is not that important, but > >> >>> changing the current semantics will require fixing a lot of places. > >> >>> (state trackers - mesa, nine, xa; modules - blitter, draw, hud, > >> >>> postprocess, tests, vl) > >> >>> > >> >>> You really better think about whether changing all those and the risk > >> >>> of breaking them is worth it. > >> >>> > >> >>> Marek > >> >> > >> >> Well, we're talking about passing NIR here, not TGSI, so none of those > >> >> places will need to be updated. NIR is a much more heavyweight IR, and > >> >> copying is much more expensive, so it makes more sense there for the > >> >> st to duplicate it and let the driver own the IR it passes in, to > >> >> reduce copying when there's no variant management necessary. > >> > > >> > My main concern was about TGSI, not so much about NIR. > >> > >> Yes, exactly -- we don't plan on changing the semantics of passing in > >> TGSI, so this only matters when the user passes in a NIR shader. > > > > 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. > > > > 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. > > > > 2) gallium passes read-only IR to the back-end and it always makes a copy. > > Not always. The copy is optional. Drivers are encouraged not to make a > copy if they don't need it. Or they can keep a copy in a different IR, > which is the same scenario for the original IR.
Let's suppose that the driver always has to modify the IR before it can be used. Does it have to make a copy then? I know that may not be the case for TGSI but it is currently the case for NIR. --Jason
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