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nir: improve convert_yuv_to_rgb

Jonathan Marek requested to merge flto/mesa:nir-yuv into master

Use a different arrangement of constants to allow more ffma.

A vec4 backend will now use 3 ffma for yuv_to_rgb.

On freedreno/ir3, it is down from 10 to 7 alu (4 fma, 3 mul, 3 add to 7 fma). Other backends shouldn't be hurt.

I tested with lower_ffma on freedreno to get an idea for GPUs without fma:

  • Number of constants is the same
  • Number of alu instructions is the same (7 add and 5 mul)
  • Lower register pressure
  • Arguably worse scheduling (less optimal placement of sampling instructions)
Edited by Jonathan Marek

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