On Wed, Jan 11, 2017 at 7:33 PM, Jason Ekstrand <ja...@jlekstrand.net> wrote: > On Wed, Jan 11, 2017 at 10:31 AM, Erik Faye-Lund <kusmab...@gmail.com> > wrote: >> >> On Wed, Jan 11, 2017 at 7:22 PM, Marek Olšák <mar...@gmail.com> wrote: >> > On Wed, Jan 11, 2017 at 7:09 PM, Jason Ekstrand <ja...@jlekstrand.net> >> > wrote: >> >> On Wed, Jan 11, 2017 at 9:32 AM, Samuel Pitoiset >> >> <samuel.pitoi...@gmail.com> >> >> wrote: >> >>> >> >>> >> >>> >> >>> On 01/11/2017 05:32 PM, Marek Olšák wrote: >> >>>> >> >>>> On Wed, Jan 11, 2017 at 4:33 PM, Erik Faye-Lund <kusmab...@gmail.com> >> >>>> wrote: >> >>>>> >> >>>>> On Wed, Jan 11, 2017 at 4:14 PM, Nicolai Hähnle <nhaeh...@gmail.com> >> >>>>> wrote: >> >>>>>> >> >>>>>> On 11.01.2017 13:17, Marek Olšák wrote: >> >>>>>>> >> >>>>>>> >> >>>>>>> On Tue, Jan 10, 2017 at 6:48 PM, Jason Ekstrand >> >>>>>>> <ja...@jlekstrand.net> >> >>>>>>> wrote: >> >>>>>>>> >> >>>>>>>> >> >>>>>>>> I'll be honest, I'm not a fan... Given that D3D10 has one defined >> >>>>>>>> behavior, >> >>>>>>>> D3D9 has another, and GL doesn't specify, I don't really think we >> >>>>>>>> should >> >>>>>>>> be >> >>>>>>>> making a global change to all drivers to do the D3D9 behavior >> >>>>>>>> just to >> >>>>>>>> fix >> >>>>>>>> one app. Sure, other apps probably have the same bug, but are we >> >>>>>>>> going >> >>>>>>>> to >> >>>>>>>> have apps that expect the D3D10 behavior that we've now >> >>>>>>>> explicitly >> >>>>>>>> made >> >>>>>>>> not >> >>>>>>>> work? >> >>>>>>>> >> >>>>>>>> If we're going to hack around an app bug, I would really rather >> >>>>>>>> see >> >>>>>>>> it >> >>>>>>>> behind a driconf option rather than a global change to driver >> >>>>>>>> behavior. >> >>>>>>>> Even better, it'd be cool if we could see the app get fixed. >> >>>>>>>> (Yes, I >> >>>>>>>> know >> >>>>>>>> that's not likely). >> >>>>>>> >> >>>>>>> >> >>>>>>> >> >>>>>>> I think we are not in a position to refuse this workaround, or put >> >>>>>>> more precisely, to have a different behavior from everybody else. >> >>>>>>> By >> >>>>>>> "we", I mean i965, radeonsi, svga. All closed drivers use abs. >> >>>>>>> Many >> >>>>>>> Mesa drivers also use abs internally (r300, r600, nv30, >> >>>>>>> nv50/nvc0). >> >>>>>>> This is not really a workaround for a specific application, even >> >>>>>>> though it's strongly motivated by that. It's a fix to align the >> >>>>>>> few >> >>>>>>> remaining drivers with all others. >> >>>>>>> >> >>>>>>> We talked with the publisher about this a very long time ago. >> >>>>>>> While I >> >>>>>>> don't remember the details (Nicolai?), I think they refused to fix >> >>>>>>> it >> >>>>>>> because radeonsi appeared to be the only driver not doing abs. >> >>>>>> >> >>>>>> >> >>>>>> >> >>>>>> If I remember correctly, it wasn't so much a refusal as a lack of >> >>>>>> follow-through. They even had an option in their framework to add >> >>>>>> the >> >>>>>> abs(...) when translating shaders, but somehow didn't turn it on >> >>>>>> unconditionally for some reason... >> >>>>> >> >>>>> >> >>>>> VP even says so here: >> >>>>> https://github.com/virtual-programming/specops-linux/issues/20 >> >>>>> >> >>>>> They recommend against patching mesa to do abs, though. >> >>>> >> >>>> >> >>>> We should still patch Mesa to align the behavior with closed drivers >> >>>> and gallium drivers like r600g and nouveau. In other words, it's too >> >>>> late to tell us not to patch Mesa, because r600g and nouveau have >> >>>> been >> >>>> "patched" since the beginning. >> >>>> >> >>>> We only need to decide whether we should do it in the GLSL compiler >> >>>> or >> >>>> radeonsi, i.e. whether we should exclude i965 and svga. >> >>> >> >>> >> >>> I do agree with that. >> >> >> >> >> >> I tend to disagree but I've come to the conclusion that I won't stand >> >> in the >> >> way either. If both of the other desktop vendors do it and we've >> >> already >> >> decided that no implementation we care about will have its performance >> >> impacted, it seems like a valid spec-compliant thing to do. I would >> >> prefer >> >> it to be behind a driconf option, but if it's unconditional, oh well. >> >> My >> >> disagreement is mostly philosophical. >> >> >> >> Over the last two years of working on Vulkan, I've been fighting broken >> >> tests and apps left and right. Vulkan has a huge amount of area where, >> >> if >> >> an app does something wrong, they get undefined behavior which is up to >> >> and >> >> including program termination. And basically all apps are broken in >> >> some >> >> way. Fortunately, the validation layers are finally starting to catch >> >> up to >> >> the point where I'm noticing very few bugs that the validation layers >> >> don't >> >> catch and things are getting into a better state. However, I've had >> >> more >> >> discussions than I can count with people where I have to explain to >> >> them >> >> that "No, the app is broken. It needs to be fixed. It's not my job to >> >> make >> >> it work." Once you start allowing brokenness, you can never stop >> >> allowing >> >> it and you paint yourself into a corner. Suddenly, you go to make a >> >> change, >> >> and your design decisions are not guided by the spec, they're guided by >> >> the >> >> spec *and* all of the broken apps that you have to keep working on your >> >> driver because you let something through. >> >> >> >> In the world of GLES and OpenGL conformance, we fight the same fight. >> >> When >> >> people ask me how conformance is coming, I frequently answer with, >> >> "We've >> >> got a bunch of people fixing <insert test suite name here> so that our >> >> driver passes". It's not that mesa is particularly touchy, it's that a >> >> good >> >> chunk of the rest of the industry just hacks around everything inside >> >> their >> >> driver and doesn't bother to fix the tests. Sometimes the driver that >> >> passes the conformance suite isn't even the one they ship. If we're >> >> going >> >> to have a spec and hardware vendors (or the FOSS community) are going >> >> to >> >> implement it and apps are going to write to it, then we all need to >> >> agree on >> >> what it means and play by the rules. If an app doesn't play by the >> >> rules >> >> and does something with undefined behavior, then it's a broken app. If >> >> we >> >> say, "No, it's ok, you don't have to fix it. We'll just hack around >> >> it" >> >> we're enablers for their broken behavior and the broken behavior >> >> continues. >> >> In this particular case, we're dealing with a broken app. The only >> >> real >> >> issue is that all of the drivers that point out the issues were not >> >> drivers >> >> they tested on. >> >> >> >> Another reason why I'm not a huge fan is that there is some momentum in >> >> the >> >> industry to make GLSL better defined with respect to NaN. I don't know >> >> that >> >> anything will ever come of it (because it may break apps) but if >> >> something >> >> does, we may find ourselves having to make SQRT and RSQ NaN-correct in >> >> the >> >> future and, hey look, it'll break apps. >> >> >> >> Ok, rant over. Push it if you want. You can even put my nakked-by on >> >> it if >> >> you'd like. :-) >> > >> > I agree with you completely, and I find it unfortunate too that we >> > have to add the workaround to GLSL or radeonsi to align its behavior >> > with closed drivers. >> >> Just for reference, I just tested what NVIDIA does on Windows, and >> they *don't* seem to do inversesqrt(abs(x)) on my HW/driver. > > > What about sqrt()? Do they do abs for one and not the other? Because that > would be crazy but also possible.
Not for sqrt either, it seems. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
