[Mesa-dev] [PATCH 0/6] RadeonSI: Let's just stop spilling SGPRs

[Nicolai Hähnle]

On 13.09.2016 19:13, Marek Olšák wrote:
> This is quite easy because we just have to get rid of all of
> the preloading at the beginning of shaders.
>
> I also removed preloading of PS inputs with literal indexing, which
> has almost the same effect as sinking interp instructions.
>
> I'm slightly concerned that LICM won't move interps because they are
> not considered speculatively-executable (=movable) by LLVM, but
> the shader-db stats show that it doesn't matter.
>
> LLVM is smart enough to do CSE where needed for both descriptor loads
> and interps. In fact, it's the CSE which is responsible for some of
> the remaining SGPR spills. (It makes sense if you think about it)
>
> The compile time increased by 6% because CSE has a lot more work,
> but it's certainly worth it.
>
>
> shader-db stats:
>
> [PATCH 4/6] radeonsi: get rid of img/buf/sampler descriptor
>     https://people.freedesktop.org/~mareko/no_preload1.html
> [PATCH 5/6] radeonsi: get rid of constant buffer preloading
>     https://people.freedesktop.org/~mareko/no_preload2.html
> [PATCH 6/6] radeonsi: reload PS inputs with direct indexing at each
>     https://people.freedesktop.org/~mareko/no_preload3_ps.html
>
> Total diff:
>     https://people.freedesktop.org/~mareko/no_preload_total.html

Those numbers are impressive.

We do have to be slightly careful, I noticed that LLVM didn't lift some 
constant loads out of loops with the earlier preload removal, in shaders 
where SGPR pressure wasn't an issue at all.

I think the right way to deal with this is to improve heuristics in 
LLVM, so I'm fine with changing Mesa in this way.

Some comments sent in reply to the respective patches. Patches 2, 3, and 
5 are

Reviewed-by: Nicolai Hähnle <nicolai.haehnle at amd.com>

> Please review.
>
> Marek
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