[Libre-soc-bugs] [Bug 755] add grev instruction (OP_GREV)

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=755

--- Comment #40 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #39)
> https://libre-soc.org/openpower/sv/grevlut2x2.jpg
> 
> a separate left-right LUT2 table gives a lot more options
> both for creating constants (RA=0) as well as in more general
> circumstsances.

in order to actually have a grev, that lut2 version would need a different
lookup table for each row of the butterfly network...that's a prohibitive
number of immediates (2*4*6 = 24 bits) for a lut2butterfly instruction. also,
we'd still want grev as a separate instruction.

The other issue is the same as in the shift/rotate circuit: a lut2 has more
delay than a mux, making grev slower than 1 clock cycle (assuming the
shift/rotate barely fits in 1 cycle)

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