[Mesa-dev] [GSoC2016] Interested in implementing "Soft" double precision floating point support

[Roland Scheidegger]

Am 09.03.2016 um 23:51 schrieb Ian Romanick:
> On 03/09/2016 02:25 AM, tournier.elie wrote:
>> Hi everyone.
>>
>> My name is Elie TOURNIER, I am enrolled in a French Engineering school
>> (Telecom Physique Strasbourg) specialized in Medical ICT.
>> I'm interested in implementing "Soft" double precision floating point
>> support [1].
>> Taking this subject seem to be a good way to get my feet wet in the Mesa
>> code and discover how some of its components works.
>>
>> I come to you in order to become know but also to retrieve valuable
>> information for the success of this project.
>>
>> I would like to know more about the following things to understand your
>> requirements :
>> 1- "/Each double precision value would be stored in a uvec2/" The IEEE
>> double precision floating point standard representation requires a 64
>> bit: 1 for sign, 11 for exponent and the others for fraction [2].
>> -> How double precision value must be stored?
> 
> As Emil mentioned, on GLSL 1.30, a uvec2 consists of two, 32-bit
> unsigned integers.  Each double precision value would be stored in a uvec2.
> 
>> 2- Where can I find |GL_ARB_gpu_shader_fp64 |documentation|?
>> |
>>
>>
>> This is my first exposure to Mesa. Please excuse me if I am asking basic
>> questions.
> 
> For this particular project, you wouldn't need Mesa at all for quite
> some time.  All of the initial project should be done in "raw" GLSL
> 1.30, and any OpenGL implementation capable of GLSL 1.30 can be used.
> You would implement (and test!) a library of functions like 'uvec2
> addDouble(uvec2 a, uvec2 b)' that would provide all of the required
> double precision operations.
> 
> The set of required functions should be pretty small.  I think:
> 
>  - add
>  - negate
>  - absolute value
>  - multiply
>  - reciprocal
>  - convert to single precision
>  - convert from single precision
>  - pow (maybe?)
>  - exp (maybe?)
>  - log (maybe?)

I don't think you need exp/log. At least glsl dosen't require it, though
the project isn't clear about it.
(pow all hw I know of with exactly one exception (that would be intel
graphics...) implements it as log2/mul/exp2 even for f32 anyway).
I think though you need sqrt (or rsqrt). And some functions for
rounding, plus comparison operations. Maybe min/max too (albeit if you
have comparisons you can emulate them of course).

Roland


> 
> I think everything else could be implemented using those functions.
> 
> Like I mentioned in the project description, there are quite a few
> existing C implementations of these functions.  Finding one of those
> that you can understand and that has a compatible license is probably
> the best place to start.
> 
>> Please point me to the right resources so that I can better understand
>> the project. I would also be happy to fix a bug to familiarize myself 
>> with the source code. Any suggestions on bugs that are relevant to the
>> project will be of great help.
>>
>> Regards,
>> Elie
>>
>> [1]
>> http://www.x.org/wiki/SummerOfCodeIdeas/#softdoubleprecisionfloatingpointsupport
>> [2] http://steve.hollasch.net/cgindex/coding/ieeefloat.html#storage
>>
>> PS: If you have any questions, please don't hesitate to contact me.
>>
>>
>> _______________________________________________
>> mesa-dev mailing list
>> mesa-dev at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> 
> _______________________________________________
> mesa-dev mailing list
> mesa-dev at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>