VHDL shift operators?

Question

I'm still trying to get used to some of the quirks of VHDL and I'm having a bit of an issue. First off, I understand that shift operators like rol, ror, ssl, srl, etc. are not synthesizeable. The purpose of this lab is to use a golden model to check against a synthesizeable version of the same thing in a testbench.

Now, the purpose of this program is to convert thermometer code into a 3-bit binary number. So, in other words, thermometer code "00000001" = "001", "00000011" = "010", "00000111" = "011", etc. I'm basically trying to count the number of 1's in the string from right to left. There will be no case where a '0' is placed between the string of 1's, so the vector "00011101" is invalid and will never occur.

I've devised a non-synthesizeable (and so far, non-compile-able) algorithm that I can't figure out how to get working. Basically, the idea is to read the thermometer code, shift it right and increment a counter until the thermometer code equals zero, and then assign the counter value to the 3-bit std_logic_vector. Below is the code I've done so-far.

library ieee;
use ieee.std_logic_1164.all; 
use ieee.numeric_std.all;

entity therm2bin_g is
    port(therm : inout std_logic_vector(6 downto 0); -- thermometer code
         bin : out std_logic_vector(2 downto 0); -- binary code 
         i : integer range 0 to 7);
end therm2bin_g;    

architecture behavioral_g of therm2bin_g is
begin

golden : process(therm)
begin

    while(therm /= "00000000") loop
        therm <= therm srl 1;
        i = i + 1;      
    end loop;

    bin <= std_logic'(to_unsigned(i,3));

end process golden;
behavioral_g;

Answer 1

here's a version that is synthesisable. the while loop is replaced by a for loop. srl is implemented explicitly:

entity therm2bin_g is
port(therm : inout std_logic_vector(6 downto 0); -- thermometer code
     bin : out std_logic_vector(2 downto 0); -- binary code 
     i : out integer range 0 to 7);
end therm2bin_g;    

architecture behavioral_g of therm2bin_g is
begin

golden : process(therm)
    variable i_internal: integer range 0 to 7;
begin
    i_internal:=0;
    for idx in 0 to therm'length loop
        if therm/="0000000" then
            therm<='0' & therm(therm'left downto 1);
            i_internal := i_internal + 1;     
        end if;
    end loop;

    bin<=std_logic_vector(to_unsigned(i_internal,bin'length));
    i<=i_internal;

end process golden;
end behavioral_g;

Answer 2

"... operators like rol, ror, ssl, srl, etc. are not synthesizeable..." Who says that on who's authority? Have you checked? On which synthesis tool? Was it a recent version, or a version from the early 1990s?

Note that the argument that some tools might not support it is just silly. The fact that some kitchens might not have an oven does not stop people from writing recipes for cake.