[Libre-soc-isa] [Bug 1074] create twin-butterfly research page into DCT/FFT instructions

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1074

--- Comment #7 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
although it looks horrible can i suggest this instead?

    prod1 <- MUL(RB, sum)   # RB = c
    prod2 <- MUL(RB, diff)  # TODO: Pick high half?
    res1 <- ROTL64(prod1, XLEN-SH)
    res2 <- ROTL64(prod2, XLEN-SH)

==>

    prod1 <- MUL(RB, sum)   # RB = c
    prod2 <- MUL(RB, diff)  # TODO: Pick high half?

    res1 <- prod1[XLEN/2-SH:XLEN-1-SH]
    res2 <- prod2[XLEN/2-SH:XLEN-1-SH]

because the ROTL64 actually requires masking out of the top
LSBs.

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