[Mesa-dev] [PATCH] radeonsi: Set declared tessellation LDS size to hardware size.

Marek Olšák maraeo at gmail.com
Tue May 10 07:59:47 UTC 2016


Reviewed-by: Marek Olšák <marek.olsak at amd.com>


On Mon, May 9, 2016 at 10:41 PM, Bas Nieuwenhuizen
<bas at basnieuwenhuizen.nl> wrote:
> On Mon, May 9, 2016 at 9:55 PM, Marek Olšák <maraeo at gmail.com> wrote:
>> On Sun, May 8, 2016 at 1:00 PM, Bas Nieuwenhuizen
>> <bas at basnieuwenhuizen.nl> wrote:
>>> The calculated limit gave problems on SI as it was > 32 KiB
>>> and the hardware LDS size on SI is only 32 KiB. It isn't
>>> correct anyway when processing multiple patches in a threadgroup.
>>> As we potentially have any number of patches such that the
>>> used LDS is at most the hardware LDS size, and exact size
>>> per patch is not known at compile time, this seems like
>>> the only valid bound.
>> I think NIcolai sent out a similar patch.
> Yes, I've sent this patch as a reply.
>> Would tessellation break if we set the size to 0 or a similar small
>> number? It seems like seemingly high LDS usage can trick LLVM now or
>> in the future into thinking that it doesn't have to schedule
>> instructions for low register usage.
> The problem with smaller sizes than total LDS is that we have no
> guarantuee that it will be placed at the same position for all shader
> stages, and LLVM could introduce new LDS variables (see the promote
> alloca pass which promotes alloca to LDS memory, although it currently
> has several wrong assumptions for non-compute shaders anyway[1]).
> The LDS usage is per threadgroup though, so using the entire LDS does
> not put a limit on the number of waves that are in the threadgroup.
> Furthermore in the graphics pipeline we have to consider that even
> though the current shader has a limit on the number of active waves
> due to LDS there might be other stages that do not use LDS and could
> use the registers.
> Note that neither too small nor too large (as long as it fits in LDS)
> is likely to lead to problems at this moment.
> - Bas
> [1]:
>  - It assumes all LDS can be used. However VS as VS can't use any LDS
> at all, and PS has to leave some for the inputs.
>  - It assumes that the max threadgroup size for non-compute shaders is
> a single wave.

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