[Mesa-dev] [PATCH 03/10] radeonsi: disable NaNs for LS and HS

[Marek Olšák]

On Sun, Oct 11, 2015 at 4:29 AM, Roland Scheidegger <sroland at vmware.com> wrote:
> FWIW I'm still baffled by this shader bit.
> NaNs are absolutely required to be generated and handled as NaNs in
> shaders (albeit conversion to ints will make them 0) by DX10 (there's
> plenty of tests which actually check for this). And generally, you
> really want to generate NaNs for newer glsl versions too I think, albeit
> this may not be strictly required (of course, currently you can't
> distinguish this in tgsi, but particularly gs/ls/hs will always be newer
> glsl versions).
> So I'm REALLY wondering why there's a shader bit named that way...

I don't know either. The bit basically converts NaNs to 0 before an
instruction result is written into a register.

>From GL_ARB_shader_precision:

"Operations and built-in functions that operate on a NaN are not
required to return a NaN as the result." I think that NaNs in
conditional expressions are still handled as NaNs, e.g. if you get
NaNs from a constant buffer.

Besides that, GLSL optimizations passes and the GLSL->TGSI conversion
are incorrect with regard to NaNs, so even if the hardware returned
NaNs according to IEEE, the NaN behavior would different from IEEE.

Marek