Making mp3 decoder VHDL code synthesizable - mp3

Question

I'm building an MP3 decoder using VHDL and I want to make it synthesizable knowing that for loops of variable size and selecting a variable size from a variable position in std_logic_vector is also not synthesizable
so how do I get a loop to be synthesizable and how to make selecting a variable size from a std_logic_vector synthesizable
for j in 0 to switch_point_1-1 loop
bits:= data(ptr+current+scale_lengths(j)-1 downto ptr+current);
scalefac(j)(0) := to_integer(UNSIGNED(bits(scale_lengths(j)-1 downto 0)));
scalefac(j)(1) := to_integer(UNSIGNED(bits(scale_lengths(j)-1 downto 0)));
scalefac(j)(2) := to_integer(UNSIGNED(bits(scale_lengths(j)-1 downto 0)));
current := current+ scale_lengths(j);
end loop;
And another one
for j in switch_point_s-1 to cb_max loop
for k in 0 to 2 loop
-- window
bits:=data(ptr+current+scale_lengths(j)-1 downto ptr+current);
scalefac(j)(k) := to_integer(UNSIGNED(bits(scale_lengths(j)-1 downto 0)));
current := current+ scale_lengths(j);
end loop;
end loop;
This is one of the loops I have and there is a select in itbits:=data(ptr+current+scale_lengths(j)-1 downto ptr+current); is also not synthesizable how can I make them synthesizable ?
Full code:
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_unsigned.all;
use ieee.numeric_std.all;
use std.textio.all;
use work.txt_util.all;
--use txt_util.all;
entity extract_main is
port(
data:in std_logic_vector(4095 downto 0);
en: in std_logic;
gr,clk:in std_logic;
enabled:in std_logic;
scsfi:in std_logic_vector(20 downto 0);
ptr: in integer;
windows_switching: in std_logic;
block_type: in std_logic_vector(1 downto 0);
mixed_block_flag: in std_logic;
part2_3_length: in std_logic_vector(11 downto 0);
slen1,slen2: in integer;
saved_scalefac_in: in integer;
saved_scalefac_out: out integer;
ptr_out: out integer;
enablenext:out std_logic
);
end extract_main;
architecture a of extract_main is
type intarr is array(20 downto 0) of integer;
type scalef is array(20 downto 0) of intarr;
begin
process(clk)
variable scalefac : scalef;
variable scale_lengths : intarr;
variable mainInfoBegin,mainInfoSize : integer;
variable blockType : integer;
variable part2_length :integer;
variable switch_point_1: integer;
variable switch_point_s: integer;
variable cb_max : integer;
variable current: integer:=0; -- iterator on data
variable it: integer;
variable bits: std_logic_vector(3 downto 0):="0000";
variable ptrout: integer;
begin
if enabled ='1' then
--size of main data
if falling_edge(clk) then
if en = '1' then
mainInfoSize := to_integer(UNSIGNED(part2_3_length));
if windows_switching = '0' then
blockType := 0;
else
blockType := to_integer(UNSIGNED(block_type));
end if;
if blockType = 2 and mixed_block_flag = '1' then
part2_length := 17 * slen1 + 18 * slen2;
scale_lengths(16) :=slen2;
scale_lengths(15) :=slen2;
scale_lengths(14) :=slen2;
scale_lengths(13) :=slen2;
scale_lengths(12) :=slen2;
scale_lengths(11) :=slen2;
scale_lengths(10) :=slen1;
scale_lengths(9) :=slen1;
scale_lengths(8) :=slen1;
scale_lengths(7) :=slen1;
scale_lengths(6) :=slen1;
scale_lengths(5) :=slen1;
scale_lengths(4) :=slen1;
scale_lengths(3) :=slen1;
scale_lengths(2) :=slen1;
scale_lengths(1) :=slen1;
scale_lengths(0) :=slen1;
cb_max := 16;
switch_point_1 := 8;
switch_point_s := 9;
elsif blockType = 2 and mixed_block_flag = '0' then
part2_length := 18 * slen1 + 18 * slen2;
scale_lengths(11) :=slen2;
scale_lengths(10) :=slen2;
scale_lengths(9) :=slen2;
scale_lengths(8) :=slen2;
scale_lengths(7) :=slen2;
scale_lengths(6) :=slen2;
scale_lengths(5) :=slen1;
scale_lengths(4) :=slen1;
scale_lengths(3) :=slen1;
scale_lengths(2) :=slen1;
scale_lengths(1) :=slen1;
scale_lengths(0) :=slen1;
switch_point_1 := 0;
switch_point_s := 1;
cb_max := 11;
else
part2_length := 11 * slen1 + 10 * slen2;
--scalefactor_bands(20 downto 11) <= slen2;
--scalefactor_bands(10 downto 0)<= slen1;
scale_lengths(20) :=slen2;
scale_lengths(19) :=slen2;
scale_lengths(18) :=slen2;
scale_lengths(17) :=slen2;
scale_lengths(16) :=slen2;
scale_lengths(15) :=slen2;
scale_lengths(14) :=slen2;
scale_lengths(13) :=slen2;
scale_lengths(12) :=slen2;
scale_lengths(11) :=slen2;
scale_lengths(10) :=slen1;
scale_lengths(9) :=slen1;
scale_lengths(8) :=slen1;
scale_lengths(7) :=slen1;
scale_lengths(6) :=slen1;
scale_lengths(5) :=slen1;
scale_lengths(4) :=slen1;
scale_lengths(3) :=slen1;
scale_lengths(2) :=slen1;
scale_lengths(1) :=slen1;
scale_lengths(0) :=slen1;
cb_max := 20;
end if;
if blockType = 2 and windows_switching = '1' then
for j in 0 to switch_point_1-1 loop
-- it:=0;
--for e in ptr+current to ptr+current+scale_lengths(j)-1 loop
-- bits(it) := ram(e)(0);
--it := it+1;
--end loop;
bits:= data(ptr+current+scale_lengths(j)-1 downto ptr+current);
scalefac(j)(0) := to_integer(UNSIGNED(bits(scale_lengths(j)-1 downto 0)));
scalefac(j)(1) := to_integer(UNSIGNED(bits(scale_lengths(j)-1 downto 0)));
scalefac(j)(2) := to_integer(UNSIGNED(bits(scale_lengths(j)-1 downto 0)));
current := current+ scale_lengths(j);
end loop;
for j in switch_point_s-1 to cb_max loop
for k in 0 to 2 loop
-- window
-- it:=0;
-- for e in ptr+current to ptr+current+scale_lengths(j)-1 loop
-- bits(it) := ram(e)(0);
-- it := it+1;
--end loop;
bits:=data(ptr+current+scale_lengths(j)-1 downto ptr+current);
scalefac(j)(k) := to_integer(UNSIGNED(bits(scale_lengths(j)-1 downto 0)));
current := current+ scale_lengths(j);
end loop;
end loop;
else
for j in 0 to 20 loop
if gr ='0' or scsfi(j) ='0' then
-- it:=0;
-- for e in ptr+current to ptr+current+scale_lengths(j)-1 loop
-- bits(it) := ram(e)(0);
-- it := it+1;
--end loop;
bits:=data(ptr+current+scale_lengths(j)-1 downto ptr+current);
scalefac(j)(0):= to_integer(UNSIGNED(bits(scale_lengths(j)-1 downto 0)));
current := current+ scale_lengths(j);
else
scalefac(j)(0):= saved_scalefac_in;
end if;
end loop;
if gr ='1' then
if scsfi(0) /= '0' or scsfi(6) /='0' or scsfi(11)/='0' or scsfi(16)/='0' then
saved_scalefac_out<=scalefac(0)(0);
end if;
end if;
end if;
ptrout:=ptr+current;
ptr_out<=ptrout;
enablenext<='1';
end if;
end if;
else enablenext<='0';
end if;
end process;
end;

Answer 1

With the necessary parts of the code, we can start to help.
It is now clear that there ARE upper bounds on the ranges of the loops, though the full code goes out of its way to hide them. However, the loop ranges must be bound by the fact that the loop variable is used to address a fixed size array.
First : is there a good reason to address your integer arrays in reverse? I am tentatively going to assume not, and simplify things a bit. Second, I am going to assume the length of intarr and scalef are dependent; again this may not be true but if it is, things get simpler.
And now I write the key declarations as:
subtype index is natural range 0 to 20;
type intarr is array(index) of integer;
type scalef is array(index) of intarr;
then
process(clk)
variable scalefac : scalef;
variable scale_lengths : intarr;
variable switch_point_1: index;
variable switch_point_s: index;
at which point the bounds on these variables is obvious...
if blockType = 2 and mixed_block_flag = '1' then
scale_lengths(11 to 16) := (others => slen2);
scale_lengths(0 to 10) := (others => slen1);
or scale_lengths(0 to 10) := (0 to 10 => slen1);
or scale_lengths := (0 to 10 => slen1, 11 to 16 => slen2, others => default_value);
depending what you want to do with the uninitialised entries.
Not because it's synthesisable but because it saves a lot of code...
And we can now work on those loops:
Make their bounds "locally static" i.e. constant and determinable from this unit alone.
And since we have a convenient and correct definition of the bounds,
for j in 0 to switch_point_1-1 loop
...
end loop;
now becomes
for j in index'range loop
if j <= switch_point_1-1 then
...
end if;
end loop;
which is potentially synthesisable, depending on the loop body actions.