Generalise structs and add support for packed unions.

tests/svtypes/union_simple.sv

@@ -0,0 +1,61 @@
+module top;
+
+	typedef struct packed {
+		byte a,b,c,d;
+	} byte4_t;
+
+	typedef union packed {
+		int	x;
+		byte4_t	y;
+	} w_t;
+
+	w_t w;
+
+	assign w.x = 'h42;
+	always_comb begin
+		assert(w.y.d == 8'h42);
+	end
+
+	typedef logic[4:0] reg_addr_t;
+	typedef logic[6:0] opcode_t;
+
+	typedef struct packed {
+		bit [6:0]  func7;
+		reg_addr_t rs2;
+		reg_addr_t rs1;
+		bit [2:0]  func3;
+		reg_addr_t rd;
+		opcode_t   opcode;
+	} R_t;
+
+	typedef struct packed {
+		bit[11:0]  imm;
+		reg_addr_t rs1;
+		bit[2:0]   func3;
+		reg_addr_t rd;
+		opcode_t   opcode;
+	} I_t;
+
+	typedef struct packed {
+		bit[19:0]  imm;
+		reg_addr_t rd;
+		opcode_t   opcode;
+	} U_t;
+
+	typedef union packed {
+		R_t	r;
+		I_t	i;
+		U_t	u;
+	} instruction_t;
+
+	instruction_t ir1;
+	assign ir1 = 32'h0AA01EB7;          //	lui t4,0xAA01
+	always_comb begin
+		assert(ir1.u.opcode == 'h37);
+		assert(ir1.r.opcode == 'h37);
+		assert(ir1.u.rd == 'd29);
+		assert(ir1.r.rd == 'd29);
+		assert(ir1.u.imm == 'hAA01);
+	end
+
+endmodule