Non Overridable (“Private or output”) Generics

Is your capability/feature request related to a problem?

Motivation

Many designs require a public generic A and a derived value B, where:

•             B computed A

•             B used in the entity’s port clause, and perhaps architecture

•             B to not be overridden by instantiation

•             B defined once, not repeated in multiple places

Avoiding:

•             duplicated expressions in port declarations

•             accidental misuse (overriding B inconsistently with A)

•             cluttered entity interfaces

•             inability to enforce relationships between generics

Describe the solution you'd like

entity foo is

  generic (

    X         : natural := 4;

    Y         : natural := 2;

  W : private natural := X * Y -- or W : out natural, which could share a constant back up to the higher level. There could be better syntax, but this should give context.

  );

  port (

    data_in  : in  std_logic_vector(W-1 downto 0)

    data_out : out std_logic_vector(W-1 downto 0)

  );

end entity;

Describe alternatives you've considered

packages and calculations in packages

Additional context

There may be other constants that an instantiation could calculate (not limited to bus widths), akin to a construction of an object.

Other bus width functions could be

Provide the largest number the logic vector is required to represent, have a standard lib way to calc the required width.

Have a standard INTEGER_VECTOR (or natural) that can be defined on a port as

my_integers : in (0 TO SIZE - 1)(RANGE <lower> TO <upper>);