[Mesa-dev] [GSoC] Update about floating point library

[Connor Abbott]

On Sat, Jul 23, 2016 at 3:32 PM, tournier.elie <tournier.elie at gmail.com> wrote:
> Hello,
>
> I hope you are well and that you enjoy these beautiful summer days.
>
> You can found a new post on my blog : https://hopetech.github.io/ .
> During these few days, I implemented eq_fp64, le_fp64, lt_fp64, mul_fp64,
> add_fp64 and fp64-to-fp32-conversion. The code can be found on
> https://github.com/Hopetech/libSoftFloat .
>
> Like I say in my post, I will try to make a Docker file for Mesa and Piglit
> developers.
> The goal is to have an "easy to install" working environment.
> If you have any ideas/recommandations, please don't hesitate to contact me.
>
> Have a nice day.
> Elie Tournier
>
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>

Hi,

Here are a few things to consider:

1. If you want your implementation to be pure GLSL 1.30, then you need
to add "#version 130" at the top of all your shaders. Otherwise,
you'll nominally get GLSL 1.10 (which doesn't support bit-twiddling at
all), but certain vendors (*cough* nvidia *cough*) will take it to
mean "just turn all of the extensions on." If you don't add the
version tag, then you might accidentally do something that won't fly
on other implementations. For example, you're using pointers all over
the place in function parameters when that's not valid GLSL 1.30 (or
any other unextended version of GLSL) since GLSL doesn't have the
concept of pointers. Instead, you should be using out/inout parameters
(e.g. "out uint zExpPtr").

2. I wouldn't worry too much about underflow, etc. since GLSL doesn't
require it. The goal in the end is to implement fp64 in a way that
satisfies the GLSL spec, so you don't need to worry about implementing
all of IEEE if GLSL says it isn't required.

2. For sqrt, rsqrt, mod, and so on, I'd check out Intel's
implementation of them in our compiler:
https://cgit.freedesktop.org/mesa/mesa/tree/src/compiler/nir/nir_lower_double_ops.c
Intel GPU's are missing a lot of the fp64 operations required by GLSL
(they basically only have the ones you already implemented, plus fma),
so we had to emulate the others ourselves. Once you get fma working,
you can just follow the code there to implement the rest, since your
goal is very similar to ours. This should be much more straightforward
than basing your code on another softfloat library, since, like you,
we assume the existence of all the fp32 operations and don't try to be
exact when GLSL doesn't require us to be.

Connor