Matt Turner <matts...@gmail.com> writes: > The docs say that flipping this bit on for shaders that don't do SIMD > branching (i.e., non-uniform control flow) will save us some power. > > An easy first step is turning this on when we don't see control flow. > > In the future with more infrastructure in place, we can determine if > all branching conditions are uniformly constant and turn on SPF. > > Hopefully this saves some power and extends battery life, but I'm not > sure how to accurately quantify this, short of printing i915_energy_uJ > before and after some workload. Even then I don't have any expectation > for how much energy the GPU would use for, say a piglit run. Is > 200 ~ 300 Joules reasonable (over 220 seconds)?
I tried this once myself, and found no power difference (n=17). I think when SPF actually helps you is when you have flow control and the SPF flag set: then the HW gets to do one compare of pcip instead of 8 or 16. It might still be worth merging as progress toward SPF with control flow.
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