[Mesa-stable] [Mesa-dev] [PATCH 1/5] st/nine: Clamp RCP when 0*inf!=0

[Axel Davy]

On 9/9/18 9:40 PM, Ilia Mirkin wrote:
> On Sun, Sep 9, 2018 at 3:19 PM, Axel Davy <davyaxel0 at gmail.com> wrote:
>> Tests showed Intel on windows does always clamp
>> RCP, RSQ and LOG (thus preventing inf/nan generation),
>> for all shader versions (some vendor behaviours vary
>> with shader versions).
> By the way, this happens because on Intel, the ALU is put into a
> special mode where it just doesn't generate NaN's at all under any
> conditions. I don't think that other vendors operate this way.
>
Yes exactly, though the documentation, if I remember correctly,

says the flag is for shaders version <= 2.0, but tests showed the clamping

was also used for version 3.0.


The point is that games work properly on all vendors. Thus picking intel 
behaviour is safe,

even if we use a card from another vendor.


I think the inf/nan behaviour on AMD and NVidia was with compute 
applications in mind (there was no

other way to do compute back then), but games weren't interested in Infs 
and NaN.


Also our tests showed that all vendors have 0*inf = 0 and 0*NaN = 0, 
even if rcp, log and rsq are clamped.

(inf and NaN can be passed via constants or inputs for such tests).


Axel