[Mesa-dev] NaN behavior in GLSL (was Re: [PATCH] glsl: always do sqrt(abs()) and inversesqrt(abs()))

[Ilia Mirkin]

On Thu, Jan 12, 2017 at 6:04 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> On 12.01.2017 23:46, Ilia Mirkin wrote:
>>
>> On Thu, Jan 12, 2017 at 4:03 PM, Matteo Bruni <matteo.mystral at gmail.com>
>> wrote:
>>>
>>> So, what would be really nice to have is a GLSL extension for some
>>> kind of switch to select the requested behavior WRT NaN. For example a
>>> three-way option with "don't generate NaN in arithmetic operations",
>>> "do generate NaN" and "don't care". It could also be a GL state if
>>> that's easier to implement with the existing hardware, since an
>>> individual application isn't supposed to require different behavior
>>> from one shader to the next.
>>>
>>> Is anyone interested in / favorable to something like this? It would
>>> solve the issue with defining NaN behavior in GLSL while making things
>>> a bit more compatible with "other API a lot of games are ported from
>>> which happens to be supported by all the desktop GPUs".
>>
>>
>> Not that I'm biased, but on the NVIDIA Tesla series (G80-GT21x), this
>> enable is handled via a global flag, not in the shader binary, so this
>> is all-or-nothing for a whole pipeline. On GF100+, I believe there is
>> also an enable via a global flag, but there are also a FMUL.FMZ (and
>> FFMA.FMZ) flag, which I *think* has the same effect. So for GF100+ hw,
>> this could be done at the instruction level.
>
>
> Well, I would also have advocated for what is effectively a
> per-program/pipeline flag anyway, even though GCN hardware can theoretically
> do it per-instruction. Tracking a per-instruction bit in the compiler
> quickly becomes fragile (e.g. there's no good way for us to model this
> information per-instruction in LLVM IR). Per-shader isn't any better than
> per-instruction due to linking, and per-shader-stage is awkward if we ever
> want to do fancier cross-stage optimizations.
>
> It's really quite simple. Introduce an extension with a name like
> MESA_shader_float_dx9. The behavior I'd suggest is:
>
> Enabling/requiring the extension in a shader causes various semantics
> changes to bring floating point behavior in line with DX9 in that shader's
> code:
>
> - 0*x = 0

Yes. But only for fp32, not for fp64.

> - sqrt/rsqrt are guaranteed to take the absolute value of their argument

Is that necessary? If the software knows about the ext, it also knows
to stick the abs() in.

> - anything else?

I'd say MESA_shader_float_zero_wins or something, if we don't stick
the sqrt stuff into it.

Here is a software model of the Tesla-era shader execution created by
Marcin (mwk):

https://github.com/envytools/envytools/blob/master/nvhw/fp.c#L168

The bit in question is "zero_wins", so just look at what that
modifies. So if you have a*b and a || b == 0 (i.e. they are +0 or -0)
then +0 is returned.

>
> It is a link error to link a program in which some shaders have the
> extension and others don't.
>
> There's funny interactions like having compute shaders with dx9 float
> semantics, but that's pretty much it :)
>
> Somebody just needs to write up a draft, but it only makes sense if we can
> at least get all the Mesa drivers and Wine behind this. If there's a simple
> global flag on NVidia hardware, then it should be easy to provide an initial
> implementation for nouveau *hint* *hint* ;)
>
> For radeonsi, more work is required (like support in LLVM).

All sounds good to me. Should be relatively straightforward for nouveau.