Some time ago I did a small experiment to automatically generate RTL from GMP (GNU multi-precision integer) programs (as in gmplib.org). These programs use API calls to perform arbitrary-precision integer arithmetic for use in cryptography, conducting/supporting mathematical proofs and for purposes of scientific computing in general.

I have developed a concept implementation that is part of HercuLeS HLS (http://www.nkavvadias.com/

In order to support for the basic GMP (multi-precision integer) API/DSL for just addition and multiplication, the following additions were needed:

- new VHDL package, mpint.vhd: MP integer and SLV (standard logic vector) operators: 500 lines of code (LOC)
- C to N-Address Code frontend extensions: 50 LOC
- Backend code generation in HercuLes: 150 LOC (in addition to what is already there)

This proof-of-concept frontend for GNU multi-precision integer programs has been implemented along with a VHDL IP library for key library functionalities such as mpz_set, mpz_add and mpz_mul.

# A simple example

The following code implements factorial with “`fgmp“`, which is under the public domain:

```
MP_INT gmp_fact(unsigned int n)
{
MP_INT prod;
int i;
mpz_init(&prod);
mpz_set_si(&prod, 1);
for (i = 2; i <= n; i++) {
mpz_mul_ui(&prod, &prod, i);
}
return (prod);
}
```

This is the unoptimized FSMD in VHDL:

```
library IEEE;
use WORK.operpack.all;
use WORK.gmp_fact_cdt_pkg.all;
use STD.textio.all;
use WORK.std_logic_textio.all;
use WORK.mpint.all;
use WORK.std_logic_1164_tinyadditions.all;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
entity gmp_fact is
port (
clk : in std_logic;
reset : in std_logic;
start : in std_logic;
n : in std_logic_vector(31 downto 0);
D_2747 : out std_logic_vector(31 downto 0);
done : out std_logic;
ready : out std_logic
);
end gmp_fact;
architecture fsmd of gmp_fact is
type state_type is (S_ENTRY, S_EXIT, S_001_001, S_001_002, S_001_003, S_001_004, S_001_005, S_002_001, S_002_002, S_002_003, S_002_004, S_002_005, S_003_001, S_003_002, S_004_001, S_004_002);
signal current_state, next_state: state_type;
signal i_21_next : std_logic_vector(31 downto 0);
signal i_21_reg : std_logic_vector(31 downto 0);
signal m_11_next : std_logic_vector(31 downto 0);
signal m_11_reg : std_logic_vector(31 downto 0);
signal i_11_next : std_logic_vector(31 downto 0);
signal i_11_reg : std_logic_vector(31 downto 0);
signal prod_next : mp_int;
signal prod_reg : mp_int;
signal i_next : std_logic_vector(31 downto 0);
signal i_reg : std_logic_vector(31 downto 0);
signal prod_11_next : mp_int;
signal prod_11_reg : mp_int;
signal prod_21_next : mp_int;
signal prod_21_reg : mp_int;
signal prod_12_next : mp_int;
signal prod_12_reg : mp_int;
signal i_2_31_next : std_logic_vector(31 downto 0);
signal i_2_31_reg : std_logic_vector(31 downto 0);
signal i_1_21_next : std_logic_vector(31 downto 0);
signal i_1_21_reg : std_logic_vector(31 downto 0);
signal D_2747_next : std_logic_vector(31 downto 0);
signal D_2747_reg : std_logic_vector(31 downto 0);
constant CNST_0 : std_logic_vector(63 downto 0) := "0000000000000000000000000000000000000000000000000000000000000000";
constant CNST_1 : std_logic_vector(63 downto 0) := "0000000000000000000000000000000000000000000000000000000000000001";
constant CNST_2 : std_logic_vector(63 downto 0) := "0000000000000000000000000000000000000000000000000000000000000010";
begin
-- current state logic
process (clk, reset)
begin
if (reset = '1') then
current_state <= S_ENTRY;
i_21_reg <= (others => '0');
m_11_reg <= (others => '0');
i_11_reg <= (others => '0');
prod_reg.data <= (others => 0);
i_reg <= (others => '0');
prod_11_reg.data <= (others => 0);
prod_21_reg.data <= (others => 0);
prod_12_reg.data <= (others => 0);
i_2_31_reg <= (others => '0');
i_1_21_reg <= (others => '0');
D_2747_reg <= (others => '0');
elsif (clk = '1' and clk'EVENT) then
current_state <= next_state;
i_21_reg <= i_21_next;
m_11_reg <= m_11_next;
i_11_reg <= i_11_next;
prod_reg <= prod_next;
i_reg <= i_next;
prod_11_reg <= prod_11_next;
prod_21_reg <= prod_21_next;
prod_12_reg <= prod_12_next;
i_2_31_reg <= i_2_31_next;
i_1_21_reg <= i_1_21_next;
D_2747_reg <= D_2747_next;
end if;
end process;
-- next state and output logic
process (current_state, start,
n,
D_2747_reg,
i_21_reg, i_21_next,
m_11_reg, m_11_next,
i_11_reg, i_11_next,
prod_reg, prod_next,
i_reg, i_next,
prod_11_reg, prod_11_next,
prod_21_reg, prod_21_next,
prod_12_reg, prod_12_next,
i_2_31_reg, i_2_31_next,
i_1_21_reg, i_1_21_next
)
begin
done <= '0';
ready <= '0';
i_21_next <= i_21_reg;
m_11_next <= m_11_reg;
i_11_next <= i_11_reg;
prod_next <= prod_reg;
i_next <= i_reg;
prod_11_next <= prod_11_reg;
prod_21_next <= prod_21_reg;
prod_12_next <= prod_12_reg;
i_2_31_next <= i_2_31_reg;
i_1_21_next <= i_1_21_reg;
D_2747_next <= D_2747_reg;
case current_state is
when S_ENTRY =>
ready <= '1';
if (start = '1') then
next_state <= S_001_001;
else
next_state <= S_ENTRY;
end if;
when S_001_001 =>
m_11_next <= CNST_1(31 downto 0);
next_state <= S_001_002;
when S_001_002 =>
prod_12_next <= mpz_set_ui(m_11_reg);
next_state <= S_001_003;
when S_001_003 =>
i_11_next <= CNST_2(31 downto 0);
prod_next <= prod_12_reg;
next_state <= S_001_004;
when S_001_004 =>
i_next <= i_11_reg(31 downto 0);
next_state <= S_001_005;
when S_001_005 =>
next_state <= S_003_001;
when S_002_001 =>
i_1_21_next(31 downto 0) <= i_reg;
next_state <= S_002_002;
when S_002_002 =>
prod_21_next <= mpz_mul_ui(prod_reg, i_1_21_reg);
next_state <= S_002_003;
when S_002_003 =>
mpz_printh(prod_21_reg);
next_state <= S_002_004;
when S_002_004 =>
i_21_next <= std_logic_vector(signed(i_reg) + signed(CNST_1(31 downto 0)));
next_state <= S_002_005;
when S_002_005 =>
prod_next <= prod_21_reg;
i_next <= i_21_reg(31 downto 0);
next_state <= S_003_001;
when S_003_001 =>
i_2_31_next(31 downto 0) <= i_reg;
next_state <= S_003_002;
when S_003_002 =>
if (i_2_31_reg <= n(31 downto 0)) then
next_state <= S_002_001;
else
next_state <= S_004_001; end if;
when S_004_001 =>
D_2747_next <= CNST_0(31 downto 0);
next_state <= S_004_002;
when S_004_002 =>
next_state <= S_EXIT;
when S_EXIT =>
done <= '1';
next_state <= S_ENTRY;
when others =>
next_state <= S_ENTRY;
end case;
end process;
D_2747 <= D_2747_reg;
end fsmd;
```