Subtype Inference from Initial Values and Function Return Context - Subtype Inference from Initial Values and Function Return Context - 2026.1 English - UG901

Vivado synthesis supports VHDL-2019 subtype inference in two related areas:

1. Infer subtype from initial value:
   Example:

   entity top is
   port(in1,in2 : in unsigned(3 downto 0);
   out1 : out unsigned(3 downto 0));
   end entity top;
   architecture beh of top is
   signal s: unsigned := in1+in2;
   begin
   out1 <= s;
   end architecture;

2. Function return subtype inferred from context:
   Example:

   function get_len(b : boolean) return r of unsigned is
   begin
   --return to_unsigned(5, 5); -- this is ok
   -- when b else to_unsigned(6, 6); -- Conditional assignments forreturns not supported
   -- the following are not ok
   -- return r;
   return to_unsigned(r'length, r'length);
   end function;
   signal s1:unsigned(3 downto 0);
   begin
   s1 <= get_len(true);