[Mesa-dev] RFC uniform packing for gallium V2

[Nicolai Hähnle]

On 25.06.2017 09:18, Nicolai Hähnle wrote:
> On 25.06.2017 03:31, Timothy Arceri wrote:
>> There are still a handful of piglit tests failing and I'm yet to test
>> that there are no regressions in the non-packed path, but I'd really
>> like some feedback on the approach as Dave has flagged it as a possible
>> controversial tgsi change.
>>
>> In order to avoid complicated swizzling and array element adjustments
>> when dealing with arrays, this series simply adjusts the constant buffer
>> index to point to the right location. There are some small changes to
>> deal with indirect indexing but these also remain very simple and easy
>> to follow.
>>
>> Dave has raised concerns that others might not like this as it doesn't
>> strictly follow the tgsi approach that everything is a vec4. I would
>> argue however that this is by far the simplest approch.
>> Doing this with swizzles and array adjustments is going to require
>> something like lower_packed_varyings.cpp which would be unnecessarily
>> complicated IMO, I started off down that track and soon changed
>> direction.
> 
> Yeah, I don't like the approach either. All register files are by vec4 
> in TGSI, and changing that feels pretty wrong.

Actually, just thinking about the fact that there are more sources of 
TGSI than just GLSL means that this is a definite no-go. Think of nine 
and some of the blitter shaders, for example. Just use LOAD, please.

Cheers,
Nicolai

> 
> I would suggest lowering loads from CONST[0] to LOAD instructions, in 
> the same way that is used for SSBOs. This has the additional advantage 
> that we could then use the same code paths to support std430 packing for 
> UBOs (via a GL extension, I suppose).
> 
> 
>> The main goal of this series is to reduce the cpu overhead cause by
>> _mesa_propagate_uniforms_to_driver_storage(). The function is slow 
>> since we
>> need to deal with strides etc because we are copying packed data to an
>> unpacked destination. It's also copying data that we have only just 
>> copied
>> to anouther duplicate uniform storage that gets created by the linker.
> 
> The duplicate copy is necessary unless we start using the same constant 
> buffer for all shaders in a program, which actually might not be such a 
> bad idea.
> 
> Cheers,
> Nicolai
> 
> 
>> This series fixes both of these issues and also reduces the size of the
>> drivers const buffer as a side effect.
>>
>> Patches 2-3 just rework the way we use the param list.
>>
>> The remaining add the packing support enabled by the
>> PackedDriverUniformStorage const.
>>
>> You can get the series in my test4 branch [1].
>>
>> [1] https://github.com/tarceri/Mesa.git
>>
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>>
> 
> 


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