If I want to calculate the Boolean difference by assigning a signal to 1 and 0 respectively, then XOR the outputs, how should I do it?

[yujning]

Yosys doesn't seem to have a direct command for calculating Boolean difference. Could you give me some suggestions on how to do this? Thank you.

[KrystalDelusion]

I don't understand the question, can you give an example?

[nakengelhardt]

Your question seems to be assuming knowledge of some larger context that we don't have. Can you give an example input verilog (or RTLIL) design, and what the transformed netlist output would look like after the pass you are looking for?

[yujning]

For example,
F = L1·S1 + X + S2

The Boolean difference with respect to S1 is:
(L1 + X + S2) ⊕ (X + S2) = L1·(~X)·(~S2)

The Boolean difference with respect to S2 is:
(1) ⊕ (L1·S1 + X) = (~(L1·S1))·(~X)

If L is to make both S1 and S2 unobservable, the following condition must be satisfied:
L1·(~X)·(S2) + ((L1·S1))·(~X) = 0

The solution for L1 that satisfies this equation is unsolvable. When L1 = 0, only S1 becomes irrelevant (unobservable).
How to solve for L?

[georgerennie]

This is probably something it doesn't make a ton of sense to do at the Yosys level. Yosys doesn't really have infrastructure for computing projections of functions or in general much for manipulating symbolic functions - you may be able to get part of the way there using Yosys' SAT infrastructure but its probably easiest to do this in abc.

[nakengelhardt]

I will go further and say Yosys does not represent functions at all, and only a subset of Yosys designs is even possible to express as functions (and it's a whole own task to transform it into that when we try to export to a functional representation). You seem to be working in an entirely different problem domain...