[Mesa-dev] [PATCH 4/5] gallium: Add PIPE_SHADER_CAP_DOUBLES

[Dave Airlie]

On 18 June 2014 23:50, Roland Scheidegger <sroland at vmware.com> wrote:
> Am 18.06.2014 01:54, schrieb Dave Airlie:
>> On 18 June 2014 05:08, Roland Scheidegger <sroland at vmware.com> wrote:
>>> This looks ok to me though since tgsi currently doesn't have any double
>>> opcodes (well the docs have them...) it doesn't really apply to most
>>> drivers (at least I assume you don't want to add support for it for tgsi).
>>
>> I've mostly forward ported the old gallium double code, and have
>> written most of ARB_gpu_shader_fp64 on top,
>>
>> Though the question I did want to ask Tom is if he is just going to
>> expose hw that has doubles, or does
>> he plan on emulating doubles.
>>
>> For a lot of GLSL4.0 GPUs from AMD fglrx emulates doubles using
>> massive magic shaders, I'm unsure
>> if we should have a lowering pass above/below the TGSI line for these
>> types of situations and what that
>> would mean for this CAP.
>
> Oh that's interesting. I always thought drivers didn't emulate that, and
> if apps want doubles but the device doesn't provide them it needs to do
> that itself. For which chips does fglrx do that?

Quite a lot of the evergreen family, only CAYMAN and CYPRESS seem
to have native FP64 support in the hw according to the version of the
AMD shader compiler I'm using, all other VLIW4/5 chips seem to emulate
fp64. I assume so they could advertise GL 4.0/SM5. They also expose
the fp64 extension on rv670, rv790, rv770 and rv740 gpus.

> If you'd want to emulate this, the other question is if you can do it at
> the tgsi level, or if this was exploiting some hw specific bits (well of
> course you could still do it at tgsi level, but if the hw has some bits
> to make this easier, then this isn't efficient). In any case I guess
> this could be decided later.

Yeah I'm not sure where would be the best place to lower it, doing it at
the GLSL level might be more generic, but I'm not really sure what algorithm
fglrx uses to do it and so if it takes advantage of other hw features to help.

Dave.