[Libre-soc-isa] [Bug 213] SimpleV Standard writeup needed

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=213

--- Comment #101 from Jacob Lifshay <programmerjake at gmail.com> ---
One additional note for swizzling: it's very common to want to put constant 0
or 1 in elements, so, if there's space, I think we should try to encode that in
the swizzles.

I would expect there to be circuitry in the instruction decoder to calculate
which input elements are actually used by the swizzle and skip reading
registers for the unused input elements, the circuit shouldn't be more than a
few dozen gates. That way, we don't have to use up additional bits on something
we could trivially calculate.

Taking both those into account: 6 options for 4 elements gives 6^4 = 1296
combinations -- 11 bits. I'm sure we could find a relatively simple encoding
for that.

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