[Mesa-dev] [PATCH] glsl: always do sqrt(abs()) and inversesqrt(abs())

Wed Jan 11 18:33:06 UTC 2017

On Wed, Jan 11, 2017 at 10:31 AM, Erik Faye-Lund <kusmabite at gmail.com>
wrote:

> On Wed, Jan 11, 2017 at 7:22 PM, Marek Olšák <maraeo at gmail.com> wrote:
> > On Wed, Jan 11, 2017 at 7:09 PM, Jason Ekstrand <jason at jlekstrand.net>
> wrote:
> >> On Wed, Jan 11, 2017 at 9:32 AM, Samuel Pitoiset <
> samuel.pitoiset at 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 <kusmabite at gmail.com>
> >>>> wrote:
> >>>>>
> >>>>> On Wed, Jan 11, 2017 at 4:14 PM, Nicolai Hähnle <nhaehnle at gmail.com>
> >>>>> wrote:
> >>>>>>
> >>>>>> On 11.01.2017 13:17, Marek Olšák wrote:
> >>>>>>>
> >>>>>>>
> >>>>>>> On Tue, Jan 10, 2017 at 6:48 PM, Jason Ekstrand <
> jason at 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.
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