yosys/passes/pmgen/peepopt_dffmux.pmg

pattern dffmux

state <IdString> muxAB

match dff
	select dff->type == $dff
	select GetSize(port(dff, \D)) > 1
endmatch

match mux
	select mux->type == $mux
	select GetSize(port(mux, \Y)) > 1
	choice <IdString> AB {\A, \B}
	//select port(mux, AB)[GetSize(port(mux, \Y))-1].wire
	index <SigSpec> port(mux, \Y) === port(dff, \D)
	define <IdString> BA (AB == \A ? \B : \A)
	index <SigSpec> port(mux, BA) === port(dff, \Q)
	set muxAB AB
endmatch

code
	SigSpec &D = mux->connections_.at(muxAB);
	SigSpec &Q = dff->connections_.at(\Q);
	int width = GetSize(D);

	SigSpec AB = port(mux, muxAB);
	if (AB[width-1] == AB[width-2]) {
		did_something = true;

		SigBit sign = D[width-1];
		bool is_signed = sign.wire;
		int i;
		for (i = width-1; i >= 2; i--) {
			if (!is_signed) {
				module->connect(Q[i], sign);
				if (D[i-1] != sign)
					break;
			}
			else {
				module->connect(Q[i], Q[i-1]);
				if (D[i-2] != sign)
					break;
			}
		}

		mux->connections_.at(\A).remove(i, width-i);
		mux->connections_.at(\B).remove(i, width-i);
		mux->connections_.at(\Y).remove(i, width-i);
		mux->fixup_parameters();
		dff->connections_.at(\D).remove(i, width-i);
		dff->connections_.at(\Q).remove(i, width-i);
		dff->fixup_parameters();

		log("dffmux pattern in %s: dff=%s, mux=%s; removed top %d bits.\n", log_id(module), log_id(dff), log_id(mux), width-i);
		accept;
	}
	else {
		int count = 0;
		for (int i = width-1; i >= 0; i--) {
			if (AB[i].wire)
				continue;
			Wire *w = Q[i].wire;
			auto it = w->attributes.find(\init);
			State init;
			if (it != w->attributes.end())
				init = it->second[Q[i].offset];
			else
				init = State::Sx;

			if (init == State::Sx || init == AB[i].data) {
				count++;
				module->connect(Q[i], AB[i]);
				mux->connections_.at(\A).remove(i);
				mux->connections_.at(\B).remove(i);
				mux->connections_.at(\Y).remove(i);
				dff->connections_.at(\D).remove(i);
				dff->connections_.at(\Q).remove(i);
			}
		}
		if (count > 0) {
			did_something = true;
			mux->fixup_parameters();
			dff->fixup_parameters();
			log("dffmux pattern in %s: dff=%s, mux=%s; removed %d constant bits.\n", log_id(module), log_id(dff), log_id(mux), count);
		}

		accept;
	}
endcode