write_aiger: Detect and error out on combinational loops

Without this it will overflow the stack when loops are present.

backends/aiger/aiger.cc

@@ -54,6 +54,8 @@ struct AigerWriter
 
 	vector<pair<int, int>> aig_gates;
 	vector<int> aig_latchin, aig_latchinit, aig_outputs;
+	vector<SigBit> bit2aig_stack;
+	size_t next_loop_check = 1024;
 	int aig_m = 0, aig_i = 0, aig_l = 0, aig_o = 0, aig_a = 0;
 	int aig_b = 0, aig_c = 0, aig_j = 0, aig_f = 0;
 
@@ -81,6 +83,23 @@ struct AigerWriter
 			return it->second;
 		}
 
+		if (bit2aig_stack.size() == next_loop_check) {
+			for (size_t i = 0; i < next_loop_check; ++i)
+			{
+				SigBit report_bit = bit2aig_stack[i];
+				if (report_bit != bit)
+					continue;
+				for (size_t j = i; j < next_loop_check; ++j) {
+					report_bit = bit2aig_stack[j];
+					if (report_bit.is_wire() && report_bit.wire->name.isPublic())
+						break;
+				}
+				log_error("Found combinational logic loop while processing signal %s.\n", log_signal(report_bit));
+			}
+			next_loop_check *= 2;
+		}
+		bit2aig_stack.push_back(bit);
+
 		// NB: Cannot use iterator returned from aig_map.insert()
 		//     since this function is called recursively
 
@@ -101,6 +120,8 @@ struct AigerWriter
 			a = initstate_ff;
 		}
 
+		bit2aig_stack.pop_back();
+
 		if (bit == State::Sx || bit == State::Sz)
 			log_error("Design contains 'x' or 'z' bits. Use 'setundef' to replace those constants.\n");