[Mesa-dev] [PATCH] gallium: remove support for predicates from TGSI

[Marek Olšák]

On Sat, Aug 9, 2014 at 5:24 AM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> On Wed, Aug 6, 2014 at 6:29 PM, Marek Olšák <maraeo at gmail.com> wrote:
>> What IR? A flatland GLSL IR? A replacement for Mesa IR? Something else?
>
> It's a flatland IR, similar to TGSI/Direct3D style with enough GLSL
> IR-like stuff to get existing things working now and enable us to
> eventually do linking in it (so we can use the same optimizations
> before linking and after). I was hoping to get a patch series out soon
> for discussion, and I've got an experimental i965 fs backend to try it
> out. It should be able to replace Mesa IR too, but that's not the
> primary goal - it's mostly to be able to do SSA-based optimizations in
> a backend-independent way. You can find a branch here:
>
> https://github.com/cwabbott0/mesa/tree/nir
>
> but I'd like to wait on discussing it until I put out some patches.
>
>>
>> It's more of a question of which gallium hardware drivers are going to
>> support it. I don't think there will be any eager candidates.
>
> AFAIK almost every HW out there supports predication in one form or
> another - why wouldn't it make sense for a higher level part of the
> compiler to do e.g. if-conversion based on cost estimates provided by
> the driver, similar to how LLVM handles vectorization? That way, it
> can take advantage of the fact that things are predicated by being
> more aggressive with optimizations like code motion, CSE, etc. -
> that's why I originally added support for predication to NIR. Also,
> I'm sure that drivers that don't support control flow (i.e. vc4) will
> like having the frontend do if-conversion for them and pass along the
> predicated code - Eric?
>
> I'm not sure how predication in Gallium works - if it's old and crufty
> compared to how drivers do things, then that's OK but I still think
> it's useful to do some things with predication outside the driver.

Let's be realistic here. There is not enough manpower to support new
shader features like predicates in old radeon hardware drivers. The
drivers have their own IR and optimization passes, which would need to
be taught about predicates and that would a lot of (useless) work for
uncertain gain. The current radeon hardware uses LLVM and the shader
backend lives in the LLVM tree, not Mesa, so any low-level information
like predicates will be destroyed during translation to the LLVM IR.

If there is a gallium driver maintainer that would like to use
predicates generated by Mesa core, please let me know. Right now I
don't know of anybody.

Marek