Example
-- loop-based version
function hw_loop(v: std_logic_vector) return natural is
variable h: natural;
begin
h := 0;
for i in v'range loop
if v(i) = '1' then
h := h + 1;
end if;
end loop;
return h;
end function hw_loop;
-- recursive version
function hw_tree(v: std_logic_vector) return natural is
constant size: natural := v'length;
constant vv: std_logic_vector(size - 1 downto 0) := v;
variable h: natural;
begin
h := 0;
if size = 1 and vv(0) = '1' then
h := 1;
elsif size > 1 then
h := hw_tree(vv(size - 1 downto size / 2)) + hw_tree(vv(size / 2 - 1 downto 0));
end if;
return h;
end function hw_tree;
