VHDL declaration expression cannot contain relational operator?

Question

I try to code a counter with a range from a to b that could be flipped top to bottom when the two generic parameters a and b fulfill a>b. Whether flipping occurs could be determined at compile time by comparing the values and setting a constant to hold the result (used later in the logic). However, the comparison fails. I have distilled the following minimal failing code example:

library IEEE;
use IEEE.std_logic_1164.all;

entity test is
end entity test;

architecture b_test of test is
begin
    process
            constant t : bit := 3 < 7;
    begin
            wait;
    end process;
end architecture b_test;

ghdl analyze complains about the line with the constant declaration:

no function declarations for operator "<"

The same error is reported if instead a constant, a variable is declared like

variable t : bit := 1 < 3;

How can I precompute a bit typed variable based on comparison of integer values a and b (actually a and b would be generic paramters of a subtype integer range)?

Answer 1

The constant should be defined with type boolean:

constant t : boolean := 3 < 7;

Relational operators return that type, see e.g.:

• https://edg.uchicago.edu/~tang/VHDLref.pdf#page=106
• http://www.pldworld.com/_hdl/2/_ref/vhdl_lrm/1076_7.HTM#7.2.2