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Possible improvement of MT19937 timing.

This commit is contained in:
Joris van Rantwijk 2016-10-30 16:50:00 +01:00
parent 369da67b11
commit ddafd29936
1 changed files with 44 additions and 33 deletions
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77
rtl/rng_mt19937.vhdl
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@ -107,15 +107,16 @@ architecture rng_mt19937_arch of rng_mt19937 is
-- Internal registers.
signal reg_enable: std_logic;
signal reg_reseeding: std_logic;
signal reg_reseedstate: std_logic_vector(2 downto 0);
signal reg_reseedstate: std_logic_vector(3 downto 0);
signal reg_validwait: std_logic;
signal reg_a_rdata_p: std_logic_vector(31 downto 0);
signal reg_reseed_cnt: std_logic_vector(9 downto 0);
signal reg_output_buf: std_logic_vector(31 downto 0);
signal reg_seed_a: std_logic_vector(31 downto 0);
signal reg_seed_a2: std_logic_vector(31 downto 0);
signal reg_seed_a3: std_logic_vector(31 downto 0);
signal reg_seed_b: std_logic_vector(31 downto 0);
signal reg_seed_b2: std_logic_vector(31 downto 0);
signal reg_seed_b3: std_logic_vector(31 downto 0);
signal reg_seed_c: std_logic_vector(31 downto 0);
-- Output register.
signal reg_valid: std_logic;
@ -169,9 +170,9 @@ begin
reg_a_rdata_p <= reg_a_rdata;
end if;
-- Update reseeding state (3 cycles per address step).
reg_reseedstate(2 downto 1) <= reg_reseedstate(1 downto 0);
reg_reseedstate(0) <= reg_reseedstate(2) and reg_reseeding;
-- Update reseeding state (4 cycles per address step).
reg_reseedstate(3 downto 1) <= reg_reseedstate(2 downto 0);
reg_reseedstate(0) <= reg_reseedstate(3) and reg_reseeding;
-- Update reseeding counter.
if reg_enable = '1' then
@ -185,52 +186,62 @@ begin
end if;
-- Enable state machine on next cycle
-- a) during initialization, and
-- a) every 4th cycle during initialization, and
-- b) on-demand for new output.
reg_enable <= reg_reseedstate(1) or
reg_enable <= reg_reseedstate(2) or
(not reg_reseeding and
(out_ready or not reg_valid));
-- Reseed state 1: XOR and shift previous state element.
if reg_reseedstate(0) = '1' then
if reg_reseeding = '1' then
y := reg_a_wdata;
y(1 downto 0) := y(1 downto 0) xor y(31 downto 30);
reg_seed_a <= y;
if force_const_mul then
-- Multiply by 37.
reg_seed_a2 <= std_logic_vector(
unsigned(y)
+ shift_left(unsigned(y), 2)
+ shift_left(unsigned(y), 5));
-- Multiply by (2**19 - 2**15).
reg_seed_a3 <= std_logic_vector(
shift_left(unsigned(y), 19)
- shift_left(unsigned(y), 15));
end if;
end if;
-- Reseed state 2: Multiply by constant.
if force_const_mul then
-- Finalize multiplication by 1812433253 =
-- (37 + 2**6*37 - 2**15 + 2**19 - 2**26*37)
reg_seed_b <= std_logic_vector(
unsigned(reg_seed_a2)
+ shift_left(unsigned(reg_seed_a2), 6)
+ unsigned(reg_seed_a3)
- shift_left(unsigned(reg_seed_a2), 26));
-- Multiply by 37.
reg_seed_b2 <= std_logic_vector(
unsigned(reg_seed_a)
+ shift_left(unsigned(reg_seed_a), 2)
+ shift_left(unsigned(reg_seed_a), 5));
-- Multiply by (2**19 - 2**15).
reg_seed_b3 <= std_logic_vector(
shift_left(unsigned(reg_seed_a), 19)
- shift_left(unsigned(reg_seed_a), 15));
else
-- Multiply by 1812433253.
-- Let synthesizer choose the multiplier implementation.
reg_seed_b <= std_logic_vector(mulconst(unsigned(reg_seed_a)));
-- Let synthesizer choose a multiplier implementation.
reg_seed_b <= std_logic_vector(
mulconst(unsigned(reg_seed_a)));
end if;
-- Reseed state 3: Finish multiplication by constant.
if force_const_mul then
-- Finalize multiplication by 1812433253 =
-- (37 + 2**6*37 - 2**15 + 2**19 - 2**26*37)
reg_seed_c <= std_logic_vector(
unsigned(reg_seed_b2)
+ shift_left(unsigned(reg_seed_b2), 6)
+ unsigned(reg_seed_b3)
- shift_left(unsigned(reg_seed_b2), 26));
else
reg_seed_c <= reg_seed_b;
end if;
-- TODO : try this in synthesis;
-- if not good enough, use state 4 to combine the last add step
-- with the final add of reg_reseed_cnt, then put that directly
-- into reg_a_wdata and into next seeding step.
-- Update internal RNG state.
if reg_enable = '1' then
if reg_reseeding = '1' then
-- Reseed state 3: Write next state element.
reg_a_wdata <= std_logic_vector(unsigned(reg_seed_b) +
-- Reseed state 4: Write next state element.
reg_a_wdata <= std_logic_vector(unsigned(reg_seed_c) +
unsigned(reg_reseed_cnt));
else
@ -288,7 +299,7 @@ begin
-- Start re-seeding.
if reseed = '1' then
reg_reseeding <= '1';
reg_reseedstate <= "001";
reg_reseedstate <= "0001";
reg_reseed_cnt <= std_logic_vector(to_unsigned(1, 10));
reg_enable <= '0';
reg_a_wdata <= newseed;
@ -300,7 +311,7 @@ begin
reg_a_addr <= std_logic_vector(to_unsigned(0, 10));
reg_b_addr <= std_logic_vector(to_unsigned(396, 10));
reg_reseeding <= '1';
reg_reseedstate <= "001";
reg_reseedstate <= "0001";
reg_reseed_cnt <= std_logic_vector(to_unsigned(1, 10));
reg_enable <= '0';
reg_a_wdata <= init_seed;