smt2: Add smtlib2_comb_expr attribute to allow user-selected smtlib2 expressions

tests/various/smtlib2_module.v

@@ -0,0 +1,33 @@
+(* smtlib2_module *)
+module smtlib2(a, b, add, sub, eq);
+	input [7:0] a, b;
+	(* smtlib2_comb_expr = "(bvadd a b)" *)
+	output [7:0] add;
+	(* smtlib2_comb_expr = "(bvadd a (bvneg b))" *)
+	output [7:0] sub;
+	(* smtlib2_comb_expr = "(= a b)" *)
+	output eq;
+endmodule
+
+(* top *)
+module uut;
+	wire [7:0] a = $anyconst, b = $anyconst, add, sub, add2, sub2;
+	wire eq;
+
+	assign add2 = a + b;
+	assign sub2 = a - b;
+
+	smtlib2 s (
+		.a(a),
+		.b(b),
+		.add(add),
+		.sub(sub),
+		.eq(eq)
+	);
+
+	always @* begin
+		assert(add == add2);
+		assert(sub == sub2);
+		assert(eq == (a == b));
+	end
+endmodule