<div dir="ltr"> <div class="gmail_extra"> <div class="gmail_quote">On Mon, Dec 21, 2015 at 2:24 PM, Marek Olšák <<a href="mailto:maraeo@gmail.com" target="_blank">maraeo@gmail.com</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5">On Mon, Dec 21, 2015 at 11:20 PM, Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> wrote: > > On Dec 21, 2015 1:37 PM, "Marek Olšák" <<a href="mailto:maraeo@gmail.com">maraeo@gmail.com</a>> wrote: >> >> On Mon, Dec 21, 2015 at 7:48 PM, Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> >> wrote: >> > >> > On Dec 21, 2015 9:09 AM, "Connor Abbott" <<a href="mailto:cwabbott0@gmail.com">cwabbott0@gmail.com</a>> wrote: >> >> >> >> On Mon, Dec 21, 2015 at 11:45 AM, Marek Olšák <<a href="mailto:maraeo@gmail.com">maraeo@gmail.com</a>> wrote: >> >> > On Mon, Dec 21, 2015 at 4:38 PM, Connor Abbott <<a href="mailto:cwabbott0@gmail.com">cwabbott0@gmail.com</a>> >> >> > wrote: >> >> >> On Mon, Dec 21, 2015 at 6:39 AM, Marek Olšák <<a href="mailto:maraeo@gmail.com">maraeo@gmail.com</a>> >> >> >> wrote: >> >> >>> On Mon, Dec 21, 2015 at 6:48 AM, Jason Ekstrand >> >> >>> <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> >> >> >>> wrote: >> >> >>>> >> >> >>>> On Dec 20, 2015 7:43 PM, "Rob Clark" <<a href="mailto:robdclark@gmail.com">robdclark@gmail.com</a>> wrote: >> >> >>>>> >> >> >>>>> On Sun, Dec 20, 2015 at 10:29 PM, Connor Abbott >> >> >>>>> <<a href="mailto:cwabbott0@gmail.com">cwabbott0@gmail.com</a>> >> >> >>>>> wrote: >> >> >>>>> > On Sun, Dec 20, 2015 at 10:04 PM, Rob Clark >> >> >>>>> > <<a href="mailto:robdclark@gmail.com">robdclark@gmail.com</a>> >> >> >>>>> > wrote: >> >> >>>>> >> On Sun, Dec 20, 2015 at 9:12 PM, Jason Ekstrand >> >> >>>>> >> <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> >> >> >>>>> >> wrote: >> >> >>>>> >>> >> >> >>>>> >>> On Dec 19, 2015 5:55 PM, "Rob Clark" <<a href="mailto:robdclark@gmail.com">robdclark@gmail.com</a>> >> >> >>>>> >>> wrote: >> >> >>>>> >>>> >> >> >>>>> >>>> From: Rob Clark <<a href="mailto:robclark@freedesktop.org">robclark@freedesktop.org</a>> >> >> >>>>> >>>> >> >> >>>>> >>>> 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: >> >> >>>>> >> >> >> >>>>> >> >> >> >>>>> >> >> >> >>>>> >> <a href="http://lists.freedesktop.org/archives/mesa-dev/2015-October/097748.html" rel="noreferrer" target="_blank">http://lists.freedesktop.org/archives/mesa-dev/2015-October/097748.html</a> >> >> >>>> >> >> >>>> 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. </div></div>If the driver wants to modify TGSI, it does have to make a copy (tgsi_transform_shader automatically makes a copy). Regarding NIR, define and use whatever makes sense for it. </blockquote><div> </div><div>Ok, that's what I thought. I'm just trying to make sure I get it all clear. I'm not 100% sure what the right thing to do for NIR is, but knowing what's required for TGSI helps. Thanks! </div><div>--Jason </div></div> </div></div>