<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Wed, Jan 11, 2017 at 10:31 AM, Erik Faye-Lund <<a href="mailto:kusmabite@gmail.com" target="_blank">kusmabite@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 Wed, Jan 11, 2017 at 7:22 PM, Marek Olšák <<a href="mailto:maraeo@gmail.com">maraeo@gmail.com</a>> wrote: > On Wed, Jan 11, 2017 at 7:09 PM, Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> wrote: >> On Wed, Jan 11, 2017 at 9:32 AM, Samuel Pitoiset <<a href="mailto:samuel.pitoiset@gmail.com">samuel.pitoiset@gmail.com</a>> >> wrote: >>> >>> >>> >>> On 01/11/2017 05:32 PM, Marek Olšák wrote: >>>> >>>> On Wed, Jan 11, 2017 at 4:33 PM, Erik Faye-Lund <<a href="mailto:kusmabite@gmail.com">kusmabite@gmail.com</a>> >>>> wrote: >>>>> >>>>> On Wed, Jan 11, 2017 at 4:14 PM, Nicolai Hähnle <<a href="mailto:nhaehnle@gmail.com">nhaehnle@gmail.com</a>> >>>>> wrote: >>>>>> >>>>>> On 11.01.2017 13:17, Marek Olšák wrote: >>>>>>> >>>>>>> >>>>>>> On Tue, Jan 10, 2017 at 6:48 PM, Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> >>>>>>> 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: >>>>> <a href="https://github.com/virtual-programming/specops-linux/issues/20" rel="noreferrer" target="_blank">https://github.com/virtual-programming/specops-linux/issues/20</a> >>>>> >>>>> 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. </div></div>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. </blockquote></div> </div><div class="gmail_extra">What about sqrt()? Do they do abs for one and not the other? Because that would be crazy but also possible. </div></div>