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

[Erik Faye-Lund]

On Wed, Jan 11, 2017 at 9:42 PM, Erik Faye-Lund <kusmabite at gmail.com> wrote:
> On Wed, Jan 11, 2017 at 9:22 PM, Samuel Pitoiset
> <samuel.pitoiset at gmail.com> wrote:
>>
>>
>> On 01/11/2017 07:34 PM, Erik Faye-Lund wrote:
>>>
>>> On Wed, Jan 11, 2017 at 7:33 PM, Jason Ekstrand <jason at jlekstrand.net>
>>> wrote:
>>>>
>>>> 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.
>>>
>>>
>>> Not for sqrt either, it seems.
>>
>>
>> Huh? I'm sure I have seen NVIDIA doing rsq(abs()) one day. Maybe it was
>> specific to that application but I don't remember the name...
>
> Could be. But negative inputs to inversesqrt() returns NaN for me when
> writing a GLSL-shader in Render Monkey.
>
> This fragment shader produces red output:
>
> ---8<---
> void main(void)
> {
>     gl_FragColor = vec4(0.0);
>     float tmp = inversesqrt(-1.0);
>     if (tmp != tmp)
>        gl_FragColor.x = 1.0;
> }
> ---8<---
> And just to be sure, I've verified that passing the -1.0 through a
> varying doesn't change the result, neither does negative non-constant
> values.
>
> Now I've even tested on two different NVIDIA-GPUs, with two different
> driver versions, both give the same result. I even tried on Intel's
> Windows drivers, same result. I'm starting to think that this issue
> hasn't been diagnosed correctly.

Actually, the test on the Intel Windows-driver was a mistake (optimus
mixup); Intel seems to do abs on Windows.