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

[Erik Faye-Lund]

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.