[Mesa-dev] [PATCH] radeonsi: compute the absolute value before RSQ

[Marek Olšák]

On Thu, Jan 5, 2017 at 10:06 PM, Roland Scheidegger <sroland at vmware.com> wrote:
> Am 05.01.2017 um 21:50 schrieb Samuel Pitoiset:
>>
>>
>> On 01/05/2017 09:44 PM, Marek Olšák wrote:
>>> On Thu, Jan 5, 2017 at 9:00 PM, Roland Scheidegger
>>> <sroland at vmware.com> wrote:
>>>> Am 05.01.2017 um 20:43 schrieb Samuel Pitoiset:
>>>>>
>>>>>
>>>>> On 01/05/2017 06:49 PM, Roland Scheidegger wrote:
>>>>>> Meh, I'm not really a big fan of such hacks. GPUs have support for
>>>>>> NaNs
>>>>>> since ages, and while glsl is lenient the point stands that
>>>>>> returning a
>>>>>> NaN is a more correct result, so doing extra work to get a wrong
>>>>>> result
>>>>>> doesn't look all that great to me.
>>>>>> FWIW dx10 requires NaNs as results (for both sqrt and rsq). Maybe app
>>>>>> specific quirks (if said apps can't be fixed) would be better...
>>>>>> But well, it's your driver, so whatever floats your boat.
>>>>>
>>>>> I think the main idea here is to reduce the number of bug reports when
>>>>> applications use D3D-style for RSQ/SQRT instead of what GLSL spec says.
>>>>
>>>> Well if someone ports from d3d10, he might well expect NaNs since d3d10
>>>> guarantees you get a NaN there...
>>>> (That said, with d3d10 NaNs will get converted to zeros on pixel shader
>>>> export if the RT isn't a float type, I don't know what mesa drivers to
>>>> there.)
>>>> Not saying it wouldn't make sense doing such hackery at glsl level but
>>>> imho it still should be a quirk.
>>>
>>> It looks like closed GL drivers always apply abs before rsq, so we
>>> should do so too. The GLSL compiler seems like the correct place.
>>
>> IIRC, NVIDIA blob does it as well.
> Ok, it's difficult to argue against that...
> "Everybody is actually doing it wrong so we must follow them or we'll
> see bugs..." Pretty sure this is essentially some issue coming from a
> time when gpus didn't have NaNs so everybody relied on result being
> non-NaN and now because all drivers do it still noone notices they still
> have bugs relying on that behavior.
> Anyway, I'm fine with any solution as long as the abs behavior isn't
> implied by the tgsi opcode...

No, drivers are not doing it wrong. OpenGL says that sqrt(x < 0) is
undefined, so drivers can do anything and it would be correct.

It's only unfortunate that some apps rely on a specific behavior where
no behavior is formally defined by OpenGL.

It looks like you don't like it because VMWare has GL<->D3D
translators, but I don't see how this would impact VMWare, because if
you replace Mesa with a closed driver, you'll have the same problem.

Marek