[Mesa-dev] Loop unrolling and if statement opts

[Timothy Arceri]

On 11/07/18 18:20, Eero Tamminen wrote:
> Hi,
> 
> Have you considered partial loop unrolling support?
> 
> I.e. when loop counter is known, but too high for full unroll, doing 
> partial loop unrolling (e.g. unroll 4x times) and dividing loop counter 
> by same amount (if it didn't divide evenly, need to unroll remainder 
> outside of loop).
> 
> This is supported e.g. by Intel Windows compiler.
> 

Do you have any examples of apps this helps? If I remember correctly 
there are very few (if any?) shaders in shader-db that are not unrolled 
due to the limit.

There was a measurable impact from the unroll limits on the Talos 
benchmark for RADV. I guess it might be interesting to try partial 
unrolling with that Game, it would be good to know where else it might 
help.

> 
>      - Eero
> 
> On 11.07.2018 09:48, Timothy Arceri wrote:
>> This series started out as me trying to unrolls some useless loops I
>> spotted in some shaders from DXVK games (see patch 10), but I found
>> some other issues and improvements along the way.
>>
>> The biggest winner seem like it could be the dolphin uber shaders on
>> i965 (on radeonsi the shaders don't seem to have spilling issues).
>> The loops in the uber shaders that are unrolled are those used as
>> wrappers around switchs by GLSL IR.
>>
>> shader-db results for the series on IVB (note as the loops that are
>> unrolled only have a single iteration I enabled shader-db reporting
>> on shaders where loops are unrolled):
>>
>> total instructions in shared programs: 10018187 -> 10016468 (-0.02%)
>> instructions in affected programs: 104080 -> 102361 (-1.65%)
>> helped: 36
>> HURT: 15
>>
>> total cycles in shared programs: 220065064 -> 154529655 (-29.78%)
>> cycles in affected programs: 126063017 -> 60527608 (-51.99%)
>> helped: 51
>> HURT: 0
>>
>> total loops in shared programs: 2515 -> 2308 (-8.23%)
>> loops in affected programs: 903 -> 696 (-22.92%)
>> helped: 51
>> HURT: 0
>>
>> total spills in shared programs: 4370 -> 4124 (-5.63%)
>> spills in affected programs: 1397 -> 1151 (-17.61%)
>> helped: 9
>> HURT: 12
>>
>> total fills in shared programs: 4581 -> 4419 (-3.54%)
>> fills in affected programs: 2201 -> 2039 (-7.36%)
>> helped: 9
>> HURT: 15
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