[Mesa-dev] [PATCH] radeonsi: pass alpha_ref value to PS in the user sgpr

[Vadim Girlin]

On 10/10/2013 08:10 PM, Christian König wrote:
> Am 10.10.2013 18:02, schrieb Vadim Girlin:
>> On 10/10/2013 02:11 PM, Michel Dänzer wrote:
>>> On Don, 2013-10-10 at 12:49 +0400, Vadim Girlin wrote:
>>>> Currently it's hardcoded in the shader, so every change requires
>>>> compilation of the shader variant, killing the performance
>>>> in Serious Sam 3 and probably other apps.
>>>>
>>>> This patch passes alpha_ref in the user sgpr and removes it from
>>>> the shader key.
>>>>
>>>> Signed-off-by: Vadim Girlin <vadimgirlin at gmail.com>
>>>
>>> Reviewed-by: Michel Dänzer <michel.daenzer at amd.com>
>>>
>>> I presume this causes no regressions with piglit quick.tests.
>>
>> Yes, there are no regressions with piglit. Thanks for reviewing.
>>
>> By the way, I'm also not sure if this is the right way of doing it,
>> especially if we'll need to pass more parameters for any new features.
>>
>> Possibly some other ways could be more preferable, e.g. to put it with
>> any other data that we may need in the future into internal const
>> buffer (like we do in r600g for clip planes etc), or maybe there are
>> other ways on SI that I'm not aware of yet?
>
> That strongly depends on how often we use a parameter. The docs speak of
> a penalty associated with loading each SGPR so we should try to use as
> less as possible, but loading something from constant space is also
> costly without proper support for the constant IB.

By the way, AFAICS some SGPR inputs are often not used at all in the 
shaders, I guess we might want to use per-shader mapping of parameters 
to SGPRs so that we'll only load actually used values for each shader. 
Compiler will need to pack required input SGPRs to lowest indices and 
provide the parameter->SGPR map to the driver. OTOH this would slightly 
increase the amount of driver's work, so I'm not really sure yet if it's 
worth it, looks like we already have a pretty significant overhead.

Vadim