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

[Marek Olšák]

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.

Marek