[Libre-soc-bugs] [Bug 653] investigate FFT, DCT, etc for REMAP in SVP64

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=653

--- Comment #14 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Luke Kenneth Casson Leighton from comment #13)
> TODO, create partial recursive partial iterative FFT

leaving this one alone for now, the simpler RADIX8 or RADIX16
is good enough.

moving on to DCT, the bink diagram and nayuki code
shows that DCT can be broken down into phases:

1) a butterfly with inversion on the top half
   indices, 0 1 2 3 7 6 5 4 where for the lower
   half the multiply constants can be set to zero

2) a bitreversed iterative sum on the top half
   of the output, and bitreversed copy on the lower
   half.

it is quite complex, but can be done in two halves.
it *might* be possible to do the 2nd part in-place,
if WaR hazards can be tolerated for the iterative
sum.

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