[Mesa-dev] Loop unrolling and if statement opts

[Eero Tamminen]

Hi,

On 11.07.2018 12:00, Timothy Arceri wrote:
> On 11/07/18 18:20, Eero Tamminen wrote:
>> 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?

Sorry, no. This was found a while ago when doing synthetic tests for 
things that are handled by compilers on the CPU side (GCC, LLVM...).


> If I remember correctly there are very few (if any?) shaders in shader-db
> that are not unrolled due to the limit.

Loop unrolling should probably have some limits based on shader cache 
size and how many instructions the loop content has, not on the loop 
counter, but only backend has that info...


	- Eero


> 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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