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countzero: Reorganize to have fewer levels of logic and fewer LUTs

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By using 4:1 multiplexers rather than 2:1, this cuts the number of
levels of multiplexing from 4 to 2 and also reduces the total number
of slice LUTs required.  Because we are now handling 4 bits at each
level, including the bottom level, the logic to do the priority
encoding can be factored out into a function that is used at each
level.

This rearranges the logic so that the encoding and selection of bits
is done whether or not the input operand is zero, and the if statement
testing whether the input is zero only affects what is assigned to
result.  With this we don't get the inferred latches and we can go
back to using signals rather than variables.

Also add some comments about what is being done.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
pull/99/head
Paul Mackerras 5 years ago
parent 0a0fe03767
commit e527e3a9b7
1 changed files with 85 additions and 82 deletions
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167
countzero.vhdl
Unescape Escape View File

@ -14,97 +14,100 @@ entity zero_counter is
end entity zero_counter;

architecture behaviour of zero_counter is
begin
zerocounter0: process(all)
variable l32, r32 : std_ulogic;
variable v32 : std_ulogic_vector(31 downto 0);
variable v16 : std_ulogic_vector(15 downto 0);
variable v8 : std_ulogic_vector(7 downto 0);
variable v4 : std_ulogic_vector(3 downto 0);
variable sel : std_ulogic_vector(5 downto 0);
begin
l32 := '0';
r32 := '0';
v32 := (others => '0');
v16 := (others => '0');
v8 := (others => '0');
v4 := (others => '0');
sel := (others => '0');

l32 := or (rs(63 downto 32));
r32 := or (rs(31 downto 0));
if (l32 = '0' or is_32bit = '1') and r32 = '0' then
-- operand is zero, return 32 for 32-bit, else 64
result <= x"00000000000000" & '0' & not is_32bit & is_32bit & "00000";
else
signal y, z : std_ulogic_vector(3 downto 0);
signal v16 : std_ulogic_vector(15 downto 0);
signal v4 : std_ulogic_vector(3 downto 0);
signal sel : std_ulogic_vector(5 downto 0);

if count_right = '0' then
sel(5) := l32 and (not is_32bit);
-- Return the index of the leftmost or rightmost 1 in a set of 4 bits.
-- Assumes v is not "0000"; if it is, return (right ? "11" : "00").
function encoder(v: std_ulogic_vector(3 downto 0); right: std_ulogic) return std_ulogic_vector is
begin
if right = '0' then
if v(3) = '1' then
return "11";
elsif v(2) = '1' then
return "10";
elsif v(1) = '1' then
return "01";
else
sel(5) := (not r32) and (not is_32bit);
return "00";
end if;
if sel(5) = '1' then
v32 := rs(63 downto 32);
else
if v(0) = '1' then
return "00";
elsif v(1) = '1' then
return "01";
elsif v(2) = '1' then
return "10";
else
v32 := rs(31 downto 0);
return "11";
end if;
end if;
end;

if count_right = '0' then
sel(4) := or (v32(31 downto 16));
else
sel(4) := not (or (v32(15 downto 0)));
end if;
if sel(4) = '1' then
v16 := v32(31 downto 16);
else
v16 := v32(15 downto 0);
end if;
begin
zerocounter0: process(all)
begin
-- Test 4 groups of 16 bits each.
-- The top 2 groups are considered to be zero in 32-bit mode.
z(0) <= or (rs(15 downto 0));
z(1) <= or (rs(31 downto 16));
z(2) <= or (rs(47 downto 32));
z(3) <= or (rs(63 downto 48));
if is_32bit = '0' then
sel(5 downto 4) <= encoder(z, count_right);
else
sel(5) <= '0';
if count_right = '0' then
sel(4) <= z(1);
else
sel(4) <= not z(0);
end if;
end if;

if count_right = '0' then
sel(3) := or (v16(15 downto 8));
else
sel(3) := not (or (v16(7 downto 0)));
end if;
if sel(3) = '1' then
v8 := v16(15 downto 8);
else
v8 := v16(7 downto 0);
end if;
-- Select the leftmost/rightmost non-zero group of 16 bits
case sel(5 downto 4) is
when "00" =>
v16 <= rs(15 downto 0);
when "01" =>
v16 <= rs(31 downto 16);
when "10" =>
v16 <= rs(47 downto 32);
when others =>
v16 <= rs(63 downto 48);
end case;

if count_right = '0' then
sel(2) := or (v8(7 downto 4));
else
sel(2) := not (or (v8(3 downto 0)));
end if;
if sel(2) = '1' then
v4 := v8(7 downto 4);
else
v4 := v8(3 downto 0);
end if;
-- Test 4 groups of 4 bits
y(0) <= or (v16(3 downto 0));
y(1) <= or (v16(7 downto 4));
y(2) <= or (v16(11 downto 8));
y(3) <= or (v16(15 downto 12));
sel(3 downto 2) <= encoder(y, count_right);

if count_right = '0' then
if v4(3) = '1' then
sel(1 downto 0) := "11";
elsif v4(2) = '1' then
sel(1 downto 0) := "10";
elsif v4(1) = '1' then
sel(1 downto 0) := "01";
else
sel(1 downto 0) := "00";
end if;
result <= x"00000000000000" & "00" & (not sel(5) and not is_32bit) & not sel(4 downto 0);
else
if v4(0) = '1' then
sel(1 downto 0) := "00";
elsif v4(1) = '1' then
sel(1 downto 0) := "01";
elsif v4(2) = '1' then
sel(1 downto 0) := "10";
else
sel(1 downto 0) := "11";
end if;
result <= x"00000000000000" & "00" & sel;
end if;
-- Select the leftmost/rightmost non-zero group of 4 bits
case sel(3 downto 2) is
when "00" =>
v4 <= v16(3 downto 0);
when "01" =>
v4 <= v16(7 downto 4);
when "10" =>
v4 <= v16(11 downto 8);
when others =>
v4 <= v16(15 downto 12);
end case;

sel(1 downto 0) <= encoder(v4, count_right);

-- sel is now the index of the leftmost/rightmost 1 bit in rs
if v4 = "0000" then
-- operand is zero, return 32 for 32-bit, else 64
result <= x"00000000000000" & '0' & not is_32bit & is_32bit & "00000";
elsif count_right = '0' then
-- return (63 - sel), trimmed to 5 bits in 32-bit mode
result <= x"00000000000000" & "00" & (not sel(5) and not is_32bit) & not sel(4 downto 0);
else
result <= x"00000000000000" & "00" & sel;
end if;

end process;

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