[Libre-soc-dev] [RFC] REMAP, SHAPE, complex (full) FFT

[Luke Kenneth Casson Leighton]

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crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

On Mon, Jul 19, 2021 at 12:29 PM Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:

> the next trick to investigate, therefore, is to *pre-load* the data with
> a *pre-reversed* top half, such that the MULs can be done as:
>
> 0 MUL 7 --> +/- 0 7
> 1 MUL 6 -> +/- 1 6
> 2 MUL 5 -> +/- 2 5
> 3 MUL 4 -> +/- 3 4

done, with this patch:
https://git.libre-soc.org/?p=openpower-isa.git;a=commitdiff;h=3c7f345703fc55bfc3590329828752be5c49204c

i say "done", i mean "it's demonstrated the direction that's being
taken by doing a manual swap.

so the order is done:

0 MUL 7 --> +/- 0 7
1 MUL 6 -> +/- 1 6
2 MUL 5 -> +/- 2 5
3 MUL 4 -> +/- 3 4

but then the following "swaps" are carried out, afterwards:

   7 <-> 4
   6 <-> 5

and that turns the data into this:

0 MUL 7 --> +/- 0 4
1 MUL 6 -> +/- 1 5
2 MUL 5 -> +/- 2 6
3 MUL 4 -> +/- 3 7

what i would like to do is, have the data *already* pre-swapped
7 6 5 4 rather than 4 5 6 7.

then down the iteration levels, same thing.

l.