Improve arithmetic

[pepijndevos]

Currently we only generate $add cells based on the 74AC283, but Yosys has a bunch of other things that we could look into.

Summary of the discussion so far as I understand it:

• 74181 does not match $alu but could in theory be hardwired for specific functions
• 74181 is not in production it seems, so hard to get
• The 74182 should make a good $lcu
• A 7485 comparison pass was tried, but slightly worse than plain logic
• But variables take over twice as much logic, so maybe worth it
• We could generate $sub with an inverter and adder
• Yosys could maybe figure this out by itself, but experimentation shows it just generates plain logic

[pepijndevos]

I hope there is a way Yosys can be smarter about using the adder for other things. Multiplication also generates huge chunks of logic without a single adder. It'd probably be possible to implement multiplication ourselves in a techpass, but it'd be worth looking into letting Yosys do the heavy lifting.

[pepijndevos]

I'm trying to debug an issue where everything gets optimized away if I use a multiplication in one particular case. Meanwhile I found this: https://github.com/YosysHQ/yosys/blob/master/techlibs/common/techmap.v#L260 so if we make a $lcu from 74182 it would get used already. But actually it's not like I can find 74182 for sale anywhere. Hmmm, I wonder why big commercial companies stopped producing obsolete logic chips...

[pepijndevos]

Fun fact: counter <= counter + -1; synthesizes to an adder, while counter <= counter - 1; does not.

[pepijndevos]

Just a comment with random things I want to look at later so I don't forget:

• 8-input nand for comparison
• JK flip-flop (does yosys understand them? useful?)
• shift registers (what's pmux2shiftx and is shregmap something useful?)
• counters (requires changes to yosys, currently only seems to count down)

[pepijndevos]

Yosys is not happy with my J-K flip-flop. Probably not worth spending more time on. Was just a curiosity:

    // 74HC73 dual J-K flip-flop with clear
    cell(74HC73_2x1JKFFR) {
        area: 5;
        ff(IQ, IQN) {
           clocked_on: "CLK";
           next_state: "(IQ*J'*K')+(J*K')+(IQ'*J*K*)'";
           clear: "C'";
        }
        pin(CLK) { direction: input; clock: true; }
        pin(C) { direction: input; }
        pin(J) { direction: input; }
        pin(K) { direction: input; }
        pin(Q) { direction: output; function: "IQ"; }
    }

It was fine with me putting an 8-input NAND in, but never used it at all, even when writing ~&bus, this is a bit unfortunate, as it could compact some comparisons. Not sure what to do with it.

    // 74HC30 single 8-input NAND
    cell(74HC30_1x1NAND8) {
        area: 9;
        pin(A) { direction: input; }
        pin(B) { direction: input; }
        pin(C) { direction: input; }
        pin(D) { direction: input; }
        pin(E) { direction: input; }
        pin(F) { direction: input; }
        pin(G) { direction: input; }
        pin(H) { direction: input; }
        pin(Y) { direction: output; function: "((((((((A*B)*C)*D)*D)*E)*F)*G)*H)'"; }
    }