Constant initialisation from a user-made function synthesis takes forever but easily created in simulation - vhdl

Question

I have created a function "my_func" in a package which when inputted with x produced a matrix of integers of shape [log2(x), x]. I wish to place this slice into ROM memory for synthesis.
For the sake of synthesis I attatch a counter to the ROM and read out one register per clk cycle. I have been able to simulate this and get the expected answers. The synthesis of the code never finishes, even when the generic controling the function is very small as shown in the code.
I am confused as to why Vivado can very quickly simulate the desired design but takes forever to synthesise. I get no errors from vivado saying that the design is unsynthesisable.
Has anyone else experienced this problem, and what are the steps I can take to avoid the problem in the future?
Please see my synthesis code below and a snippet of the function code below
For the record for this question Synthesis = Elaborate Design (RTL)
library IEEE;
library WORK;
use WORK.mylib.all;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
use IEEE.math_real.all;
entity try is
generic(
x : integer := 8
);
port(
clk : in std_logic;
ouput: out integer;
);
end entity try;
architecture v1 of try is
constant sig : my_matrix_of_integers(0 to integer(log2(real(x)))-1, 0 to x-1) := my_func(x);
signal counter : unsigned(integer(log2(real(x)))-1 downto 0);
begin
process(clk)
begin
if rising_edge(clk) then
output <= sig(0, to_integer(counter));
counter <= counter + 1;
end if;
end process;
end architecture v1;
Here is a snippet of my function code
function my_func (x: integer) return mat_t is
variable y: integer := integer(log2(real(x)));
variable cluster : integer;
variable index : integer;
variable mat : my_matrix_of_integers(0 to y-1, 0 to x-1);
begin
for s in 0 to y-1 loop
index := x/(2**(s+1));
cluster := x/index;
for c in 0 to cluster - 1 loop
for i in 0 to index -1 loop
if c mod 2 = 0 then
mat(s, (c*index) + i) := 0;
else
mat(s, (c*index) + i) := i*(2**(s));
end if;
end loop;
end loop;
end loop;
return mat;
end function my_func;
and type ...
type my_matrix_of_integers is array(integer range <>, integer range <>) of integer;