Count bits in VHDL, with loop and unrolled loop produces different results

A

a s

Dear all,

I have come up with 2 solutions in VHDL, how to count number of bits
in input data.
The thing I don't understand is why the 2 solutions produce different
results, at least with Xilinx ISE and its XST.
There is quite a substantial difference in required number of slices/
LUTs.

1. solution with unrolled loop: 41 slices, 73 LUTs
2. solution with loop: 54 slices, 100 LUTs

The entity of both architectures is the same:

entity one_count is
Port ( din : in STD_LOGIC_vector(31 downto 0);
dout : out STD_LOGIC_vector(5 downto 0)
);
end one_count;


The architecture with an unrolled loop is the following:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

entity one_count is
Port ( din : in STD_LOGIC_vector(31 downto 0);
dout : out STD_LOGIC_vector(5 downto 0)
);
end one_count;

architecture one_count_unrolled_arch of one_count is

signal cnt : integer range 0 to 32;

begin

cnt <= to_integer(unsigned(din( 0 downto 0))) +
to_integer(unsigned(din( 1 downto 1))) +
to_integer(unsigned(din( 2 downto 2))) +
to_integer(unsigned(din( 3 downto 3))) +
to_integer(unsigned(din( 4 downto 4))) +
to_integer(unsigned(din( 5 downto 5))) +
to_integer(unsigned(din( 6 downto 6))) +
to_integer(unsigned(din( 7 downto 7))) +
to_integer(unsigned(din( 8 downto 8))) +
to_integer(unsigned(din( 9 downto 9))) +
to_integer(unsigned(din(10 downto 10))) +
to_integer(unsigned(din(11 downto 11))) +
to_integer(unsigned(din(12 downto 12))) +
to_integer(unsigned(din(13 downto 13))) +
to_integer(unsigned(din(14 downto 14))) +
to_integer(unsigned(din(15 downto 15))) +
to_integer(unsigned(din(16 downto 16))) +
to_integer(unsigned(din(17 downto 17))) +
to_integer(unsigned(din(18 downto 18))) +
to_integer(unsigned(din(19 downto 19))) +
to_integer(unsigned(din(20 downto 20))) +
to_integer(unsigned(din(21 downto 21))) +
to_integer(unsigned(din(22 downto 22))) +
to_integer(unsigned(din(23 downto 23))) +
to_integer(unsigned(din(24 downto 24))) +
to_integer(unsigned(din(25 downto 25))) +
to_integer(unsigned(din(26 downto 26))) +
to_integer(unsigned(din(27 downto 27))) +
to_integer(unsigned(din(28 downto 28))) +
to_integer(unsigned(din(29 downto 29))) +
to_integer(unsigned(din(30 downto 30))) +
to_integer(unsigned(din(31 downto 31)));

dout <= std_logic_vector(to_unsigned(cnt,6));

end one_count_unrolled_arch ;


And the architecture with a loop is the following:

architecture one_count_loop_arch of one_count_loop is

signal cnt : integer range 0 to 32;

begin

process(din) is
variable tmp : integer range 0 to 32;
begin
tmp := to_integer(unsigned(din(0 downto 0)));
for i in 1 to 31 loop
tmp := tmp + to_integer(unsigned(din(i downto i)));
end loop;
cnt <= tmp;
end process;

dout <= std_logic_vector(to_unsigned(cnt,6));

end one_count_loop_arch ;


I would be really grateful if somebody could point out what I did
wrong with the 2. solution with loop.
It certainly must be my mistake, but I can not find it...

Additionally, I know that this "brute-force" one counting might not be
the optimal approach,
but this is just my first attempt to get the job done. If somebody has
a better solution, I would
appreciate it if you could share it.

Regards,
Peter
 
T

Tricky

Dear all,

I have come up with 2 solutions in VHDL, how to count number of bits
in input data.
The thing I don't understand is why the 2 solutions produce different
results, at least with Xilinx ISE and its XST.
There is quite a substantial difference in required number of slices/
LUTs.

1. solution with unrolled loop:        41 slices,  73 LUTs
2. solution with loop:                    54 slices, 100 LUTs

The entity of both architectures is the same:

entity one_count is
  Port ( din : in  STD_LOGIC_vector(31 downto 0);
         dout : out  STD_LOGIC_vector(5 downto 0)
        );
end one_count;

The architecture with an unrolled loop is the following:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

entity one_count is
  Port ( din : in  STD_LOGIC_vector(31 downto 0);
         dout : out  STD_LOGIC_vector(5 downto 0)
        );
end one_count;

architecture one_count_unrolled_arch of one_count is

  signal  cnt : integer range 0 to 32;

begin

   cnt <= to_integer(unsigned(din( 0 downto  0))) +
          to_integer(unsigned(din( 1 downto  1))) +
          to_integer(unsigned(din( 2 downto  2))) +
          to_integer(unsigned(din( 3 downto  3))) +
          to_integer(unsigned(din( 4 downto  4))) +
          to_integer(unsigned(din( 5 downto  5))) +
          to_integer(unsigned(din( 6 downto  6))) +
          to_integer(unsigned(din( 7 downto  7))) +
          to_integer(unsigned(din( 8 downto  8))) +
          to_integer(unsigned(din( 9 downto  9))) +
          to_integer(unsigned(din(10 downto 10))) +
          to_integer(unsigned(din(11 downto 11))) +
          to_integer(unsigned(din(12 downto 12))) +
          to_integer(unsigned(din(13 downto 13))) +
          to_integer(unsigned(din(14 downto 14))) +
          to_integer(unsigned(din(15 downto 15))) +
          to_integer(unsigned(din(16 downto 16))) +
          to_integer(unsigned(din(17 downto 17))) +
          to_integer(unsigned(din(18 downto 18))) +
          to_integer(unsigned(din(19 downto 19))) +
          to_integer(unsigned(din(20 downto 20))) +
          to_integer(unsigned(din(21 downto 21))) +
          to_integer(unsigned(din(22 downto 22))) +
          to_integer(unsigned(din(23 downto 23))) +
          to_integer(unsigned(din(24 downto 24))) +
          to_integer(unsigned(din(25 downto 25))) +
          to_integer(unsigned(din(26 downto 26))) +
          to_integer(unsigned(din(27 downto 27))) +
          to_integer(unsigned(din(28 downto 28))) +
          to_integer(unsigned(din(29 downto 29))) +
          to_integer(unsigned(din(30 downto 30))) +
          to_integer(unsigned(din(31 downto 31)));

   dout <= std_logic_vector(to_unsigned(cnt,6));

end one_count_unrolled_arch ;

And the architecture with a loop is the following:

architecture one_count_loop_arch of one_count_loop is

signal  cnt : integer range 0 to 32;

begin

  process(din) is
    variable  tmp : integer range 0 to 32;
    begin
      tmp := to_integer(unsigned(din(0 downto 0)));
      for i in 1 to 31 loop
          tmp := tmp + to_integer(unsigned(din(i downto i)));
      end loop;
      cnt <= tmp;
  end process;

  dout <= std_logic_vector(to_unsigned(cnt,6));

end one_count_loop_arch ;

I would be really grateful if somebody could point out what I did
wrong with the 2. solution with loop.
It certainly must be my mistake, but I can not find it...

Additionally, I know that this "brute-force" one counting might not be
the optimal approach,
but this is just my first attempt to get the job done. If somebody has
a better solution, I would
appreciate it if you could share it.

Regards,
Peter
Click to expand...

see what you get with this function instead (a function I have used
before):

function count_ones(slv : std_logic_vector) return natural is
varaible n_ones : natural := 0;
begin
for i in slv'range loop
if slv(i) = '1' then
n_ones := n_ones + 1;
end if;
end loop;

return n_ones;
end function count_ones;

.....

inside architecture, no process needed:

dout <= std_logic_vector( to_unsigned(count_ones(din), dout'length) );


The beauty with this is the function will work with a std_logic_vector
of any length.
 
A

Andy

A good synthesis tool should be able to optimize either version to the
same implementation. But there are semantic differences that Xilinx
may be getting hung up on.

In the unrolled version, you have a long expression, and there is
freedom within vhdl to evaluate it in different orders or groups of
operations. In the loop version, since you are continually updating
tmp, you are describing an explicitly sequential order in which the
calculation takes place. Like I said, a good synthesis tool should be
able to handle either one equally well, but you get what you pay for
in synthesis tools.

If you are looking for a general solution to the problem for any size
vector, try a recursive function implentation of a binary tree and see
what happens.

Just for kicks, you might also put the whole calculation in the loop
(0 to 31), with temp set to 0 before the loop. Shouldn't make any
difference, but then again, we're already seeing differences where
there should be none.

On the other hand, if what you have works (fits and meets timing), use
the most maintainable, understandable version. It will save you time
(=money) in the long run. It is often interesting to find out what is
"the optimal" way to code something such that it results in the
smallest/fastest circuit. But in the big picture, it most often does
not matter, especially when you write some cryptic code to squeeze the
last pS/LUT out of it, and you had plenty of slack and space to spare
anyway. Nevertheless, knowing how to squeeze every pS/LUT comes in
handy every once in a while.

Andy
 
A

a s

Dear Andy, Tricky,

thank you both for your valuable input. Please find my comments below.

A good synthesis tool should be able to optimize either version to the
same implementation. But there are semantic differences that Xilinx
may be getting hung up on.
Click to expand...

Aha, that's a good thing, it means that I did not make some obvious
mistake. ;-)
If you are looking for a general solution to the problem for any size
vector, try a recursive function implentation of a binary tree and see
what happens.
Click to expand...

OK, I didn't quite get that but will consider it again.
Just for kicks, you might also put the whole calculation in the loop
(0 to 31), with temp set to 0 before the loop. Shouldn't make any
difference, but then again, we're already seeing differences where
there should be none.
Click to expand...

Sorry I didn't tell you before. I have already tried that and in this
case XST produces the same result.
On the other hand, if what you have works (fits and meets timing), use
the most maintainable, understandable version. It will save you time
(=money) in the long run. It is often interesting to find out what is
"the optimal" way to code something such that it results in the
smallest/fastest circuit. But in the big picture, it most often does
not matter, especially when you write some cryptic code to squeeze the
last pS/LUT out of it, and you had plenty of slack and space to spare
anyway. Nevertheless, knowing how to squeeze every pS/LUT comes in
handy every once in a while.
Click to expand...

Andy, I completely agree with what you have written above.
One should strive for maintainable and understandable version.
Although, on my particular case, I have to find a good solution
in terms of LUT resources, because I need 8 instances of
one counters with 64-bit input data. And the device is getting full...


Tricky, your approach does indeed look very neat. I like it.
Although it is far less efficient than mine. For the same input/output
ports, your version with function requires 171 Slices in 313 LUTs.
(The minimum that I get with unrolled version is 41 Slices and 73
LUTs).
 
J

johnp

Here's a slightly different approach to your problem.... It tries to
take advantage
of the fact that the LUTs are pretty good as lookup tables. It's in
Verilog, but
you should easily be able to convert it to VHDL.

`define V1

module tst (
input [31:0] data,
output [5:0] cnt
);


`ifdef V1

/*
Device utilization summary:
---------------------------

Selected Device : 3s50pq208-5

Number of Slices: 35 out of 768
4%
Number of 4 input LUTs: 62 out of 1536
4%
Number of IOs: 38
Number of bonded IOBs: 0 out of 124
0%
*/


function [1:0] cnt3;
input [2:0] d_in;
begin
case (d_in)
3'h0: cnt3 = 2'h0;
3'h1: cnt3 = 2'h1;
3'h2: cnt3 = 2'h1;
3'h3: cnt3 = 2'h2;
3'h4: cnt3 = 2'h1;
3'h5: cnt3 = 2'h2;
3'h6: cnt3 = 2'h2;
3'h7: cnt3 = 2'h3;
endcase
end
endfunction

assign cnt = cnt3(data[2:0])
+ cnt3(data[5:3])
+ cnt3(data[8:6])
+ cnt3(data[11:9])
+ cnt3(data[14:12])
+ cnt3(data[17:15])
+ cnt3(data[20:18])
+ cnt3(data[23:21])
+ cnt3(data[26:24])
+ cnt3(data[29:27])
+ cnt3({1'b0, data[31:30]})
;

`endif


`ifdef V2
/*
Selected Device : 3s50pq208-5

Number of Slices: 44 out of 768
5%
Number of 4 input LUTs: 79 out of 1536
5%
Number of IOs: 38
Number of bonded IOBs: 0 out of 124
0%
*/

function [2:0] cnt4;
input [3:0] d_in;
begin
case (d_in)
4'h0: cnt4 = 3'h0;
4'h1: cnt4 = 3'h1;
4'h2: cnt4 = 3'h1;
4'h3: cnt4 = 3'h2;
4'h4: cnt4 = 3'h1;
4'h5: cnt4 = 3'h2;
4'h6: cnt4 = 3'h2;
4'h7: cnt4 = 3'h3;

4'h8: cnt4 = 3'h1;
4'h9: cnt4 = 3'h2;
4'ha: cnt4 = 3'h2;
4'hb: cnt4 = 3'h3;
4'hc: cnt4 = 3'h2;
4'hd: cnt4 = 3'h3;
4'he: cnt4 = 3'h3;
4'hf: cnt4 = 3'h4;

endcase
end
endfunction

assign cnt = cnt4(data[3:0])
+ cnt4(data[7:4])
+ cnt4(data[11:8])
+ cnt4(data[15:12])
+ cnt4(data[19:16])
+ cnt4(data[23:20])
+ cnt4(data[27:24])
+ cnt4(data[31:28])
;


`endif
endmodule


John Providenza
 
A

Andy

If you are looking for a general solution to the problem for any size
OK, I didn't quite get that but will consider it again.
Click to expand...

The synthesis tool may not be able to figure out that it need not
carry all the bits of the sum through every caculation in the loop. A
binary tree implementation can manage the sum width at every stage.

For a recursive binary tree implementation, define a function that
divides the vector into two ~equal parts (i.e. n/2 and n/2 + n mod 2),
calls itself on each one, and returns the sum of the two results. Stop
recursion when the size of the incoming vector is 1 (just return 1 if
the bit is set, and 0 if not). This is synthesizeable as long as the
recursion is statically bound (which it is, by the size of the
original vector).

It should work out pretty close to what johnp's approach does, except
work for any size input vector.

Andy
 
G

glen herrmannsfeldt

The synthesis tool may not be able to figure out that it need not
carry all the bits of the sum through every caculation in the loop. A
binary tree implementation can manage the sum width at every stage.
Click to expand...

Is that the same as a Carry Save Adder tree?

Maybe not. The CSA has three inputs and two outputs, so it
isn't exactly binary.

-- glen
 
B

Brian Davis

Peter said:
If somebody has a better solution, I would appreciate it if you could share it.
Click to expand...
When I looked at this some years back, XST worked well enough
at creating an adder cascade using the old "mask and add" software
trick that I never bothered writing something more optimal:

gen_bcnt32: if ( ALU_WIDTH = 32 ) generate
begin

process(din)
-- multiple variables not needed, but make intermediate steps
visible in simulation
variable temp : unsigned (ALU_MSB downto 0);
variable temp1 : unsigned (ALU_MSB downto 0);
variable temp2 : unsigned (ALU_MSB downto 0);
variable temp3 : unsigned (ALU_MSB downto 0);
variable temp4 : unsigned (ALU_MSB downto 0);
variable temp5 : unsigned (ALU_MSB downto 0);

begin
temp := unsigned(din);

temp1 := (temp AND X"5555_5555") + ( ( temp srl 1) AND
X"5555_5555"); -- 0..2 out x16

temp2 := (temp1 AND X"3333_3333") + ( ( temp1 srl 2) AND
X"3333_3333"); -- 0..4 out x8

temp3 := (temp2 AND X"0707_0707") + ( ( temp2 srl 4) AND
X"0707_0707"); -- 0..8 out x4

temp4 := (temp3 AND X"001f_001f") + ( ( temp3 srl 8) AND
X"001f_001f"); -- 0..16 out x2

temp5 := (temp4 AND X"0000_003f") + ( ( temp4 srl 16) AND
X"0000_003f"); -- 0..32 out

cnt <= std_logic_vector(temp5(5 downto 0));

end process;

end generate gen_bcnt32;


Brian
 
G

glen herrmannsfeldt

When I looked at this some years back, XST worked well enough
at creating an adder cascade using the old "mask and add" software
trick that I never bothered writing something more optimal:
(snip)

temp1 := (temp AND X"5555_5555") + ( ( temp srl 1) AND
X"5555_5555"); -- 0..2 out x16

temp2 := (temp1 AND X"3333_3333") + ( ( temp1 srl 2) AND
X"3333_3333"); -- 0..4 out x8
Click to expand...

OK, that would be a binary tree. I believe the CSA adder tree
is slightly more efficient, though it might depend on the number
of inputs.

The binary tree cascade works especially well on word oriented
machines, and can be easily written in many high-level languages
(with the assumption of knowing the word size).

The first stage of a CSA tree starts with N inputs, and generates
N/3 two bit outputs that are the sums and carries from N/3 full adders.
(If N isn't a multiple of three, then one bit may bypass the stage,
and two bits go into a half adder.)

The next stage takes the N/3 ones and N/3 twos, and generates
N/9 ones, 2N/9 twos, and N/9 fours. You can continue until
there is only one bit of each, or sometimes there are other
optimizations near the end. Last time I did one, I only needed
to know zero, one, two, three, or more than three, which simplifies
it slightly.

It also pipelines well, but then so does the binary tree.

-- glen
 
A

a s

Andy nailed it again when he said you get what you pay for
regarding synthesis tool. Initially I was synthesising with
ISE12.4 and the results were, hm, inconsistent. After
switching the synthesis tool to SynplifyPro v2010.03 the
results were as expected and, of course, even better than that.

Please see the table below. Tricky's version is denoted "funct",
where there are major differences between the 2 tools:

---------- 32-bit input data --------
unrolled: XST 74 LUTs, 41 slices
unrolled: SynplifyPro 57 LUTs, 34 slices

loop: XST 100 LUTs, 54 slices
loop: SynplifyPro 57 LUTs, 34 slices

funct: XST 317 LUTs, 161 slices
funct: SynplifyPro 58 LUTs, 34 slices

---------- 64-bit input data --------
unrolled: XST 174 LUTs, 96 slices
unrolled: SynplifyPro 129 LUTs, 80 slices

loop: XST 227 LUTs, 121 slices
loop: SynplifyPro 129 LUTs, 80 slices

funct: XST 813 LUTs, 436 slices
funct: SynplifyPro 130 LUTs, 82 slices



Peter
 
A

a s

Johnp, Brian, thank you too for your input! Much appreciated.

I have ran your code through 2 synthesisers and have updated the table
of required resources.

-------------- 32-bit input data --------------
unrolled: XST 74 LUTs, 41 slices
unrolled: SynplifyPro 57 LUTs, 34 slices

loop: XST 100 LUTs, 54 slices
loop: SynplifyPro 57 LUTs, 34 slices

funct: XST 317 LUTs, 161 slices
funct: SynplifyPro 58 LUTs, 34 slices

JohnpV1: XST 62 LUTs, 35 slices
JohnpV1: SynplifyPro 57 LUTs, 33 slices

JohnpV2: XST 78 LUTs, 43 slices
JohnpV2: SynplifyPro 54 LUTs, 32 slices

Brian: XST 57 LUTs, 39 slices
Brian: SynplifyPro 57 LUTs, 41 slices


The latest 3 pairs of results are interesting because even
XST produces good results, especially in Brian's version
where XST is surprisingly even slightly better. But anyway,
it's not that XST is so clever, it is a clever coding of the design.

Regards,
Peter
 
A

Andy

Thanks for publishing your results!

It is interesting how little variance there is in the SynplifyPro
results from widely different RTL implementations. This allows you,
within limits, to code something so that it makes sense functionally
to you and others, and the synthesis tool will still get pretty darn
close to "the optimal" implementation so that it will work even in
demanding environments (speed and/or space constrained). This also
allows reuse of the same code across more environments.

Andy
 
G

glen herrmannsfeldt

In comp.arch.fpga Andy said:
It is interesting how little variance there is in the SynplifyPro
results from widely different RTL implementations. This allows you,
within limits, to code something so that it makes sense functionally
to you and others, and the synthesis tool will still get pretty darn
close to "the optimal" implementation so that it will work even in
demanding environments (speed and/or space constrained). This also
allows reuse of the same code across more environments.
Click to expand...

As far as I know, yes, the tools are pretty good at optimizing
combinatorial logic. This problem can be pipelined, though, and
as far as I know the tools don't have a way to optimize that.

You would at least have to specify the number of pipeline stages.
It would be nice to have a tool that would optimize the partition
between stages. Even more, given the clock frequency, it would be
nice to have a tool that would find the optimal number of pipeline
stages.

-- glen
 
A

Andy

As far as I know, yes, the tools are pretty good at optimizing
combinatorial logic.  This problem can be pipelined, though, and
as far as I know the tools don't have a way to optimize that.

You would at least have to specify the number of pipeline stages.
It would be nice to have a tool that would optimize the partition
between stages.  Even more, given the clock frequency, it would be
nice to have a tool that would find the optimal number of pipeline
stages.  

-- glen
Click to expand...

Depending on which tools to which you are referring, yes they can be
very good. But there is a large difference between some tools even in
their ability to optimize purely combinatorial circuits, as shown in
Peter's results.

The Synplicity and Mentor tools have the capability to optimize logic
across register boundaries (re-balancing). Some do it over more than
one boundary (moving logic more than one clock cycle forward/back).

You still have to model the pipeline stages, but that can be a simple
register array tacked onto the beginning or end of the logic, and you
just let the tool redistribute the logic amongst them.

Latency often affects other portions of the design, so unless the
entire design is "floated" WRT to latency (clock cycles or pipeline
stages), it makes little sense for a local optimizing routine to pick
the number of stages.

The behavioral C synthesis tools (Catapult-C and others) take a
completely untimed C algorithm in the form of a function, and allow
you to trade resources, clock speed, latency, etc. with the help of
different views including a Gant chart of various resources and their
utilization. They also can synthesize different types of hardware
interfaces, including registers, streaming data, memories (single and
dual port), etc. around the function.

Andy
 
G

glen herrmannsfeldt

(after I wrote)
Depending on which tools to which you are referring, yes they can be
very good. But there is a large difference between some tools even in
their ability to optimize purely combinatorial circuits, as shown in
Peter's results.
Click to expand...
The Synplicity and Mentor tools have the capability to optimize logic
across register boundaries (re-balancing). Some do it over more than
one boundary (moving logic more than one clock cycle forward/back).
Click to expand...
You still have to model the pipeline stages, but that can be a simple
register array tacked onto the beginning or end of the logic, and you
just let the tool redistribute the logic amongst them.
Click to expand...

That does sound pretty nice of them. Lately I mostly use Xilinx
ISE which, as far as I know, doesn't do that.
Latency often affects other portions of the design, so unless the
entire design is "floated" WRT to latency (clock cycles or pipeline
stages), it makes little sense for a local optimizing routine to pick
the number of stages.
Click to expand...

I have done systolic array designs that do, as you say, float.
The constraint is on the clock period. Though in addition there
is the question of the number of unit cells that can fit into
one FPGA. Throughput is clock frequency times (cells/FPGA).
The behavioral C synthesis tools (Catapult-C and others) take a
completely untimed C algorithm in the form of a function, and allow
you to trade resources, clock speed, latency, etc. with the help of
different views including a Gant chart of various resources and their
utilization. They also can synthesize different types of hardware
interfaces, including registers, streaming data, memories (single and
dual port), etc. around the function.
Click to expand...

Are there tools that will convert a sequential C code
dynamic programming algorithm to a systolic array?
That would be a pretty amazing optimization.

-- glen
 
J

JustJohn

Andy, I completely agree with what you have written above.
One should strive for maintainable and understandable version.
Although, on my particular case, I have to find a good solution
in terms of LUT resources, because I need 8 instances of
one counters with 64-bit input data. And the device is getting full...
Click to expand...

Peter,
It looks like you've solved your problem simply by moving to a
better synthesis tool, so this may not be of interest anymore.
However, in addition to space, finding a more compact implementation
often leads to a speed increase as well, AND power savings.
Additionally, I like this counting bits problem, it turns up often
enough that it deserves some attention. As to maintainability, nothing
promotes that more than good comments. Once the function is written it
can be stuffed away in a package, never dealt with again (If anyone
copies the code, please include credit).

See my other post for the most compact/fastest way to implement the 32-
bit sum using 4-LUTs and taking advantage of carry logic fabric.
Gabor's post of the 35 LUT number when using 6-LUTs got me looking at
that case. Here are the results (Spartan 3 for the 4-LUTs, Spartan 6
for the 6-LUTs, XST for both, I'd be curious if any other synthesis
tool does better). Synthesizers continually improve, but nothing beats
a good look at the problem, as the 6-LUT case illustrates with a
better than 2:1 savings:

vec32_sum2: 4-input LUTs: 53 Slices: 31
vec32_sum3: 6-input LUTs: 15 Slices: 4

Finally, this is a neat problem because it's nice to make the little
things count.

Best regards all,
John L. Smith

Code for 6-LUT based 32 bit sum:
function vec32_sum3( in_vec : in UNSIGNED ) return UNSIGNED is
type Leaf_type is array ( 0 to 63 ) of UNSIGNED( 2 downto
0 );
-- Each ROM entry is sum of address bits:
constant Leaf_ROM : Leaf_type := (
"000", "001", "001", "010", "001", "010", "010", "011",
"001", "010", "010", "011", "010", "011", "011", "100",
"001", "010", "010", "011", "010", "011", "011", "100",
"010", "011", "011", "100", "011", "100", "100", "101",
"001", "010", "010", "011", "010", "011", "011", "100",
"010", "011", "011", "100", "011", "100", "100", "101",
"010", "011", "011", "100", "011", "100", "100", "101",
"011", "100", "100", "101", "100", "101", "101", "110" );
type S3_Type is array ( 0 to 4 ) of UNSIGNED( 2 downto 0 );
variable S3 : S3_Type;
variable result : UNSIGNED( 5 downto 0 );
variable Leaf_Bits : natural range 0 to 63;
variable S4_1 : UNSIGNED( 3 downto 0 );
variable S4_2 : UNSIGNED( 3 downto 0 );
variable S5_1 : UNSIGNED( 4 downto 0 );
begin
-- Form five 3-bit sums using three 6-LUTs each:
for i in 0 to 4 loop
Leaf_Bits := TO_INTEGER( UNSIGNED( in_vec( 6 * i + 5 downto 6 *
i ) ) );
S3( i ) := Leaf_ROM( Leaf_Bits );
end loop;
-- Add two 3-bit sums + leftover leaf bits as a carry in to get 4
bit sums:
S4_1 := ( "0" & S3( 0 ) ) + ( "0" & S3( 1 ) )
+ ( "000" & in_vec( 30 ) );
S4_2 := ( "0" & S3( 2 ) ) + ( "0" & S3( 3 ) )
+ ( "000" & in_vec( 31 ) );
-- Add 4 bit sums to get 5 bit sum:
S5_1 := ( "0" & S4_1 ) + ( "0" & S4_2 );
-- Add leftover 3 bit sum to get 5 bit result:
result := ( "0" & S5_1 )
+ ( "000" & S3( 4 ) );
return result;
end vec32_sum3;
 
J

JustJohn

Peter,
  It looks like you've solved your problem simply by moving to a
better synthesis tool, so this may not be of interest anymore.
However, in addition to space, finding a more compact implementation
often leads to a speed increase as well, AND power savings.
Additionally, I like this counting bits problem, it turns up often
enough that it deserves some attention. As to maintainability, nothing
promotes that more than good comments. Once the function is written it
can be stuffed away in a package, never dealt with again (If anyone
copies the code, please include credit).

See my other post for the most compact/fastest way to implement the 32-
bit sum using 4-LUTs and taking advantage of carry logic fabric.
Gabor's post of the 35 LUT number when using 6-LUTs got me looking at
that case. Here are the results (Spartan 3 for the 4-LUTs, Spartan 6
for the 6-LUTs, XST for both, I'd be curious if any other synthesis
tool does better). Synthesizers continually improve, but nothing beats
a good look at the problem, as the 6-LUT case illustrates with a
better than 2:1 savings:

  vec32_sum2:  4-input LUTs: 53  Slices: 31
  vec32_sum3:  6-input LUTs: 15  Slices: 4

Finally, this is a neat problem because it's nice to make the little
things count.

Best regards all,
John L. Smith
Click to expand...
Chagrinned OOPS!, synthesizer was throwing the leaf ROMs into BRAMs
for the 6-LUT case. Actual numbers for the 6-LUT case without BRAMs
are (and this is not a synthesis benchmark, it is an illustraion of
efficient circuit structure expressed via coding style to reduce LUT
usage, applicable across all synthesis tools, thanks for the warning
Philippe, and BTW I AGREE, EULAs forbidding benchmarks are evil):

vec32_sum3: 6-input LUTs: 30 Slices: 4

Still tops 35, although some may consider the code too complex for
marginal improvement.

Regards,
John
 

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