yosys/passes/pmgen/peepopt_dffmux.pmg

pattern dffmux

state <IdString> cemuxAB rstmuxBA
state <SigSpec> sigD

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

match rstmux
	select rstmux->type == $mux
	select GetSize(port(rstmux, \Y)) > 1
	index <SigSpec> port(rstmux, \Y) === port(dff, \D)
	choice <IdString> BA {\B, \A}
	select port(rstmux, BA).is_fully_const()
	set rstmuxBA BA
	optional
endmatch

code sigD
	if (rstmux)
		sigD = port(rstmux, rstmuxBA == \B ? \A : \B);
	else
		sigD = port(dff, \D);
endcode

match cemux
	select cemux->type == $mux
	select GetSize(port(cemux, \Y)) > 1
	index <SigSpec> port(cemux, \Y) === sigD
	choice <IdString> AB {\A, \B}
	index <SigSpec> port(cemux, AB) === port(dff, \Q)
	set cemuxAB AB
endmatch

code
	SigSpec D = port(cemux, cemuxAB == \A ? \B : \A);
	SigSpec Q = port(dff, \Q);
	Const rst;
	if (rstmux)
		rst = port(rstmux, rstmuxBA).as_const();
	int width = GetSize(D);

	SigSpec &ceA = cemux->connections_.at(\A);
	SigSpec &ceB = cemux->connections_.at(\B);
	SigSpec &ceY = cemux->connections_.at(\Y);
	SigSpec &dffD = dff->connections_.at(\D);
	SigSpec &dffQ = dff->connections_.at(\Q);

	if (D[width-1] == D[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 || (rst.size() && rst[i-1] != rst[width-1]))
					break;
			}
			else {
				module->connect(Q[i], Q[i-1]);
				if (D[i-2] != sign || (rst.size() && rst[i-1] != rst[width-1]))
					break;
			}
		}

		ceA.remove(i, width-i);
		ceB.remove(i, width-i);
		ceY.remove(i, width-i);
		cemux->fixup_parameters();
		dffD.remove(i, width-i);
		dffQ.remove(i, width-i);
		dff->fixup_parameters();

		log("dffcemux pattern in %s: dff=%s, cemux=%s; removed top %d bits.\n", log_id(module), log_id(dff), log_id(cemux), width-i);
		accept;
	}
	else {
		int count = 0;
		for (int i = width-1; i >= 0; i--) {
			if (D[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 == D[i].data) {
				count++;
				module->connect(Q[i], D[i]);
				ceA.remove(i);
				ceB.remove(i);
				ceY.remove(i);
				dffD.remove(i);
				dffQ.remove(i);
			}
		}
		if (count > 0) {
			did_something = true;
			cemux->fixup_parameters();
			dff->fixup_parameters();
			log("dffcemux pattern in %s: dff=%s, cemux=%s; removed %d constant bits.\n", log_id(module), log_id(dff), log_id(cemux), count);
		}

		accept;
	}
endcode