[Mesa-dev] [PATCH 0/5] Gallium: Passing drirc options to create_screen() and fixing Rocket League

[Nicolai Hähnle]

On 22.06.2017 10:14, Michel Dänzer wrote:
> On 22/06/17 04:34 PM, Nicolai Hähnle wrote:
>> On 22.06.2017 03:38, Rob Clark wrote:
>>> On Wed, Jun 21, 2017 at 8:15 PM, Marek Olšák <maraeo at gmail.com> wrote:
>>>> On Wed, Jun 21, 2017 at 10:37 PM, Rob Clark <robdclark at gmail.com> wrote:
>>>>> On Tue, Jun 20, 2017 at 6:54 PM, Marek Olšák <maraeo at gmail.com> wrote:
>>>>>> Hi,
>>>>>>
>>>>>> This series updates pipe loaders so that flags such as drirc options
>>>>>> can be passed to create_screen(). I have compile-tested everything
>>>>>> except clover.
>>>>>>
>>>>>> The first pipe_screen flag is a drirc option to fix incorrect grass
>>>>>> rendering in Rocket League for radeonsi. Rocket League expects DirectX
>>>>>> behavior for partial derivative computations after discard/kill, but
>>>>>> radeonsi implements the more efficient but stricter OpenGL behavior
>>>>>> and that will remain our default behavior. The new screen flag forces
>>>>>> radeonsi to use the DX behavior for that game.
>>>>>>
>>>>>
>>>>> do we really want this to be a *global* option for the screen?
>>>>
>>>> Yes. Shaders are pipe_screen (global) objects in radeonsi, so a
>>>> compiler option also has to be global. We can't look at the context
>>>> during the TGSI->LLVM translation.
>>>
>>> well, I didn't really mean per-screen vs per-context, as much as
>>> per-screen vs per-shader (or maybe more per-screen vs
>>> per-instruction?)
>>
>> I honestly don't think it's worth the trouble. Applications that are
>> properly coded against GLSL can benefit from the relaxed semantics, and
>> applications that get it wrong in one shader are rather likely to get it
>> wrong everywhere.
>>
>> Since GLSL simply says derivatives are undefined after non-uniform
>> discard, and this option makes it defined instead, setting this flag can
>> never break the behavior of a correctly written shader.
> 
> BTW, how expensive is the radeonsi workaround when it isn't needed?
> 
> I'm starting to wonder if we shouldn't just make it always safe and call
> it a day, saving the trouble of identifying broken apps and plumbing the
> info through the API layers...

As-is, the workaround can be *very* expensive in the worst case. A large 
number of pixels could be disabled by a discard early in the shader, and 
we're now moving the discard down, which means a lot of unnecessary 
texture fetches may be happening.

Also, I think I spoke too soon about this flag not having negative 
effects: if a shader has an image/buffer write after a discard, that 
write is now no longer disabled.

A more efficient workaround can be done at the LLVM level by doing the 
discard early, but then re-enabling WQM "relative to" the new set of 
active pixels. It's a bit involved, especially when the discard itself 
happens in a branch, and still a little more expensive, but it's an option.

Cheers,
Nicolai
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