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

[Marek Olšák]

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.

Marek