[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 #49 from Jacob Lifshay <programmerjake at gmail.com> ---
lkcl asked me to calculate gate counts for grevlut:
just counting the grev network and luts:

latency of a single mux-lut combination:
1 mux through data input and 2 muxes through select input.

gate count of a single mux-lut combination:
4 muxes.


latency of full network:
6 muxes through data input and 12 muxes through select input plus wire delay --
will almost certainly need 2 clock cycles. this is 3x a grev/gorc combined
network. also 3x a rotator network.

gate count of full network:
4 * 6 * 64 = 1536 muxes

a grev/gorc combined network can be made by taking a grev network and writing
it in terms of the 4-input and-or-invert cells that the muxes would likely be
anyway, converting every other layer to or-and-invert, and then enabling both
and-gate inputs simultaneously instead of having them be S and ~S.

I'll make a diagram.

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