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

[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.

>
> Marek
>