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divider: Do absolute-value ops in divider instead of decode

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This moves the negation of negative operands for signed divide and
modulus operations out of the decode2 stage and into the divider.
If either of the operands for a signed divide or modulus operation
is negative, the divider now takes an extra cycle to negate the
operands that are negative.

The interface to the divider now has an 'is_signed' signal rather
than a 'neg_result' signal, and the dividend and divisor can be
negative, so divider_tb had to be updated for the new interface.

The reason for doing this is that one of the worst timing violations
on the Arty A7-100 at 100MHz involved the carry chain in the adders
that did the negation of the dividend and divisor in the decode stage.
Moving the negations to a separate cycle fixes that and also seems to
reduce the total number of slice LUTs used.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
jtag-port
Paul Mackerras 5 years ago
parent e6536d4b8b
commit 25b9450475
4 changed files with 94 additions and 162 deletions
Show Stats Download Patch File Download Diff File

4
common.vhdl
Unescape Escape View File

@ -81,13 +81,13 @@ package common is
write_reg: std_ulogic_vector(4 downto 0);
dividend: std_ulogic_vector(63 downto 0);
divisor: std_ulogic_vector(63 downto 0);
neg_result: std_ulogic;
is_signed: std_ulogic;
is_32bit: std_ulogic;
is_extended: std_ulogic;
is_modulus: std_ulogic;
rc: std_ulogic;
end record;
constant Decode2ToDividerInit: Decode2ToDividerType := (valid => '0', neg_result => '0', is_32bit => '0', is_extended => '0', is_modulus => '0', rc => '0', others => (others => '0'));
constant Decode2ToDividerInit: Decode2ToDividerType := (valid => '0', is_signed => '0', is_32bit => '0', is_extended => '0', is_modulus => '0', rc => '0', others => (others => '0'));

type Decode2ToRegisterFileType is record
read1_enable : std_ulogic;

51
decode2.vhdl
Unescape Escape View File

@ -221,9 +221,6 @@ begin
variable v_int : reg_internal_type;
variable mul_a : std_ulogic_vector(63 downto 0);
variable mul_b : std_ulogic_vector(63 downto 0);
variable dividend : std_ulogic_vector(63 downto 0);
variable divisor : std_ulogic_vector(63 downto 0);
variable absdend : std_ulogic_vector(31 downto 0);
variable decoded_reg_a : decode_input_reg_t;
variable decoded_reg_b : decode_input_reg_t;
variable decoded_reg_c : decode_input_reg_t;
@ -308,60 +305,36 @@ begin
-- s = 1 for signed, 0 for unsigned (for div*)
-- t = 1 for 32-bit, 0 for 64-bit
-- r = RC bit (record condition code)
-- For signed division/modulus, we take absolute values and
-- tell the divider what the sign of the result should be,
-- which is the dividend sign for modulus, and the XOR of
-- the dividend and divisor signs for division.
dividend := decoded_reg_a.data;
divisor := decoded_reg_b.data;
v.d.write_reg := decode_output_reg(d_in.decode.output_reg_a, d_in.insn);
v.d.is_modulus := not d_in.insn(8);
v.d.is_32bit := not d_in.insn(2);
if d_in.insn(8) = '1' then
signed_division := d_in.insn(6);
else
signed_division := d_in.insn(10);
end if;
v.d.is_signed := signed_division;
if d_in.insn(2) = '0' then
-- 64-bit forms
v.d.is_32bit := '0';
if d_in.insn(8) = '1' and d_in.insn(7) = '0' then
v.d.is_extended := '1';
end if;
if signed_division = '1' and dividend(63) = '1' then
v.d.neg_result := '1';
v.d.dividend := std_ulogic_vector(- signed(dividend));
else
v.d.dividend := dividend;
end if;
if signed_division = '1' and divisor(63) = '1' then
if d_in.insn(8) = '1' then
v.d.neg_result := not v.d.neg_result;
end if;
v.d.divisor := std_ulogic_vector(- signed(divisor));
else
v.d.divisor := divisor;
end if;
v.d.dividend := decoded_reg_a.data;
v.d.divisor := decoded_reg_b.data;
else
-- 32-bit forms
v.d.is_32bit := '1';
if signed_division = '1' and dividend(31) = '1' then
v.d.neg_result := '1';
absdend := std_ulogic_vector(- signed(dividend(31 downto 0)));
else
absdend := dividend(31 downto 0);
end if;
if d_in.insn(8) = '1' and d_in.insn(7) = '0' then -- extended forms
v.d.dividend := absdend & x"00000000";
v.d.dividend := decoded_reg_a.data(31 downto 0) & x"00000000";
elsif signed_division = '1' and decoded_reg_a.data(31) = '1' then
-- sign extend to 64 bits
v.d.dividend := x"ffffffff" & decoded_reg_a.data(31 downto 0);
else
v.d.dividend := x"00000000" & absdend;
end if;
if signed_division = '1' and divisor(31) = '1' then
if d_in.insn(8) = '1' then
v.d.neg_result := not v.d.neg_result;
v.d.dividend := x"00000000" & decoded_reg_a.data(31 downto 0);
end if;
v.d.divisor := x"00000000" & std_ulogic_vector(- signed(divisor(31 downto 0)));
if signed_division = '1' and decoded_reg_b.data(31) = '1' then
v.d.divisor := x"ffffffff" & decoded_reg_b.data(31 downto 0);
else
v.d.divisor := x"00000000" & divisor(31 downto 0);
v.d.divisor := x"00000000" & decoded_reg_b.data(31 downto 0);
end if;
end if;
v.d.rc := decode_rc(d_in.decode.rc, d_in.insn);

21
divider.vhdl
Unescape Escape View File

@ -24,10 +24,12 @@ architecture behaviour of divider is
signal sresult : std_ulogic_vector(63 downto 0);
signal qbit : std_ulogic;
signal running : std_ulogic;
signal signcheck : std_ulogic;
signal count : unsigned(6 downto 0);
signal neg_result : std_ulogic;
signal is_modulus : std_ulogic;
signal is_32bit : std_ulogic;
signal extended : std_ulogic;
signal rc : std_ulogic;
signal write_reg : std_ulogic_vector(4 downto 0);

@ -64,7 +66,7 @@ begin
running <= '0';
count <= "0000000";
elsif d_in.valid = '1' then
if d_in.is_extended = '1' then
if d_in.is_extended = '1' and not (d_in.is_signed = '1' and d_in.dividend(63) = '1') then
dend <= d_in.dividend & x"0000000000000000";
else
dend <= x"0000000000000000" & d_in.dividend;
@ -72,12 +74,27 @@ begin
div <= unsigned(d_in.divisor);
quot <= (others => '0');
write_reg <= d_in.write_reg;
neg_result <= d_in.neg_result;
neg_result <= '0';
is_modulus <= d_in.is_modulus;
extended <= d_in.is_extended;
is_32bit <= d_in.is_32bit;
rc <= d_in.rc;
count <= "0000000";
running <= '1';
signcheck <= d_in.is_signed and (d_in.dividend(63) or d_in.divisor(63));
elsif signcheck = '1' then
signcheck <= '0';
neg_result <= dend(63) xor (div(63) and not is_modulus);
if dend(63) = '1' then
if extended = '1' then
dend <= std_ulogic_vector(- signed(dend(63 downto 0))) & x"0000000000000000";
else
dend <= x"0000000000000000" & std_ulogic_vector(- signed(dend(63 downto 0)));
end if;
end if;
if div(63) = '1' then
div <= unsigned(- signed(div));
end if;
elsif running = '1' then
if count = "0111111" then
running <= '0';

132
divider_tb.vhdl
Unescape Escape View File

@ -44,7 +44,7 @@ begin
d1.write_reg <= "10001";
d1.dividend <= x"0000000010001000";
d1.divisor <= x"0000000000001111";
d1.neg_result <= '0';
d1.is_signed <= '0';
d1.is_32bit <= '0';
d1.is_extended <= '0';
d1.is_modulus <= '0';
@ -55,7 +55,7 @@ begin

d1.valid <= '0';

for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -79,7 +79,7 @@ begin

d1.valid <= '0';

for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -105,27 +105,15 @@ begin
ra := std_ulogic_vector(resize(signed(pseudorand(dlength * 8)), 64));
rb := std_ulogic_vector(resize(signed(pseudorand(vlength * 8)), 64));

if ra(63) = '1' then
d1.dividend <= std_ulogic_vector(- signed(ra));
else
d1.dividend <= ra;
end if;
if rb(63) = '1' then
d1.divisor <= std_ulogic_vector(- signed(rb));
else
d1.divisor <= rb;
end if;
if ra(63) = rb(63) then
d1.neg_result <= '0';
else
d1.neg_result <= '1';
end if;
d1.is_signed <= '1';
d1.valid <= '1';

wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -154,13 +142,13 @@ begin

d1.dividend <= ra;
d1.divisor <= rb;
d1.neg_result <= '0';
d1.is_signed <= '0';
d1.valid <= '1';

wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -187,28 +175,16 @@ begin
ra := std_ulogic_vector(resize(signed(pseudorand(dlength * 8)), 64));
rb := std_ulogic_vector(resize(signed(pseudorand(vlength * 8)), 64));

if ra(63) = '1' then
d1.dividend <= std_ulogic_vector(- signed(ra));
else
d1.dividend <= ra;
end if;
if rb(63) = '1' then
d1.divisor <= std_ulogic_vector(- signed(rb));
else
d1.divisor <= rb;
end if;
if ra(63) = rb(63) then
d1.neg_result <= '0';
else
d1.neg_result <= '1';
end if;
d1.is_signed <= '1';
d1.is_extended <= '1';
d1.valid <= '1';

wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -216,15 +192,18 @@ begin
end loop;
assert d2.valid = '1';

if unsigned(d1.divisor) > unsigned(d1.dividend) then
if rb /= x"0000000000000000" then
d128 := ra & x"0000000000000000";
q128 := std_ulogic_vector(signed(d128) / signed(rb));
if q128(127 downto 63) = x"0000000000000000" & '0' or
q128(127 downto 63) = x"ffffffffffffffff" & '1' then
behave_rt := q128(63 downto 0);
assert to_hstring(behave_rt) = to_hstring(d2.write_reg_data)
report "bad divde expected " & to_hstring(behave_rt) & " got " & to_hstring(d2.write_reg_data) & " for ra = " & to_hstring(ra) & " rb = " & to_hstring(rb);
assert ppc_cmpi('1', behave_rt, x"0000") & x"0000000" = d2.write_cr_data
report "bad CR setting for divde";
end if;
end if;
end loop;
end loop;
end loop;
@ -239,14 +218,14 @@ begin

d1.dividend <= ra;
d1.divisor <= rb;
d1.neg_result <= '0';
d1.is_signed <= '0';
d1.is_extended <= '1';
d1.valid <= '1';

wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -254,7 +233,7 @@ begin
end loop;
assert d2.valid = '1';

if unsigned(d1.divisor) > unsigned(d1.dividend) then
if unsigned(rb) > unsigned(ra) then
d128 := ra & x"0000000000000000";
q128 := std_ulogic_vector(unsigned(d128) / unsigned(rb));
behave_rt := q128(63 downto 0);
@ -275,21 +254,9 @@ begin
ra := std_ulogic_vector(resize(signed(pseudorand(dlength * 8)), 64));
rb := std_ulogic_vector(resize(signed(pseudorand(vlength * 8)), 64));

if ra(63) = '1' then
d1.dividend <= std_ulogic_vector(- signed(ra));
else
d1.dividend <= ra;
end if;
if rb(63) = '1' then
d1.divisor <= std_ulogic_vector(- signed(rb));
else
d1.divisor <= rb;
end if;
if ra(63) = rb(63) then
d1.neg_result <= '0';
else
d1.neg_result <= '1';
end if;
d1.is_signed <= '1';
d1.is_extended <= '0';
d1.is_32bit <= '1';
d1.valid <= '1';
@ -297,7 +264,7 @@ begin
wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -326,7 +293,7 @@ begin

d1.dividend <= ra;
d1.divisor <= rb;
d1.neg_result <= '0';
d1.is_signed <= '0';
d1.is_extended <= '0';
d1.is_32bit <= '1';
d1.valid <= '1';
@ -334,7 +301,7 @@ begin
wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -361,21 +328,9 @@ begin
ra := std_ulogic_vector(resize(signed(pseudorand(dlength * 8)), 32)) & x"00000000";
rb := std_ulogic_vector(resize(signed(pseudorand(vlength * 8)), 64));

if ra(63) = '1' then
d1.dividend <= std_ulogic_vector(- signed(ra));
else
d1.dividend <= ra;
end if;
if rb(63) = '1' then
d1.divisor <= std_ulogic_vector(- signed(rb));
else
d1.divisor <= rb;
end if;
if ra(63) = rb(63) then
d1.neg_result <= '0';
else
d1.neg_result <= '1';
end if;
d1.is_signed <= '1';
d1.is_extended <= '0';
d1.is_32bit <= '1';
d1.valid <= '1';
@ -383,7 +338,7 @@ begin
wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -391,13 +346,16 @@ begin
end loop;
assert d2.valid = '1';

if unsigned(d1.divisor(31 downto 0)) > unsigned(d1.dividend(63 downto 32)) then
if rb /= x"0000000000000000" then
behave_rt := std_ulogic_vector(signed(ra) / signed(rb));
if behave_rt(63 downto 31) = x"00000000" & '0' or
behave_rt(63 downto 31) = x"ffffffff" & '1' then
assert behave_rt(31 downto 0) = d2.write_reg_data(31 downto 0)
report "bad divwe expected " & to_hstring(behave_rt) & " got " & to_hstring(d2.write_reg_data) & " for ra = " & to_hstring(ra) & " rb = " & to_hstring(rb);
assert ppc_cmpi('0', behave_rt, x"0000") & x"0000000" = d2.write_cr_data
report "bad CR setting for divwe";
end if;
end if;
end loop;
end loop;
end loop;
@ -412,7 +370,7 @@ begin

d1.dividend <= ra;
d1.divisor <= rb;
d1.neg_result <= '0';
d1.is_signed <= '0';
d1.is_extended <= '0';
d1.is_32bit <= '1';
d1.valid <= '1';
@ -420,7 +378,7 @@ begin
wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -428,7 +386,7 @@ begin
end loop;
assert d2.valid = '1';

if unsigned(d1.divisor(31 downto 0)) > unsigned(d1.dividend(63 downto 32)) then
if unsigned(rb(31 downto 0)) > unsigned(ra(63 downto 32)) then
behave_rt := std_ulogic_vector(unsigned(ra) / unsigned(rb));
assert behave_rt(31 downto 0) = d2.write_reg_data(31 downto 0)
report "bad divweu expected " & to_hstring(behave_rt) & " got " & to_hstring(d2.write_reg_data) & " for ra = " & to_hstring(ra) & " rb = " & to_hstring(rb);
@ -447,17 +405,9 @@ begin
ra := std_ulogic_vector(resize(signed(pseudorand(dlength * 8)), 64));
rb := std_ulogic_vector(resize(signed(pseudorand(vlength * 8)), 64));

if ra(63) = '1' then
d1.dividend <= std_ulogic_vector(- signed(ra));
else
d1.dividend <= ra;
end if;
if rb(63) = '1' then
d1.divisor <= std_ulogic_vector(- signed(rb));
else
d1.divisor <= rb;
end if;
d1.neg_result <= ra(63);
d1.is_signed <= '1';
d1.is_extended <= '0';
d1.is_32bit <= '0';
d1.is_modulus <= '1';
@ -466,7 +416,7 @@ begin
wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -495,7 +445,7 @@ begin

d1.dividend <= ra;
d1.divisor <= rb;
d1.neg_result <= '0';
d1.is_signed <= '0';
d1.is_extended <= '0';
d1.is_32bit <= '0';
d1.is_modulus <= '1';
@ -504,7 +454,7 @@ begin
wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -531,17 +481,9 @@ begin
ra := std_ulogic_vector(resize(signed(pseudorand(dlength * 8)), 64));
rb := std_ulogic_vector(resize(signed(pseudorand(vlength * 8)), 64));

if ra(63) = '1' then
d1.dividend <= std_ulogic_vector(- signed(ra));
else
d1.dividend <= ra;
end if;
if rb(63) = '1' then
d1.divisor <= std_ulogic_vector(- signed(rb));
else
d1.divisor <= rb;
end if;
d1.neg_result <= ra(63);
d1.is_signed <= '1';
d1.is_extended <= '0';
d1.is_32bit <= '1';
d1.is_modulus <= '1';
@ -550,7 +492,7 @@ begin
wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;
@ -579,7 +521,7 @@ begin

d1.dividend <= ra;
d1.divisor <= rb;
d1.neg_result <= '0';
d1.is_signed <= '0';
d1.is_extended <= '0';
d1.is_32bit <= '1';
d1.is_modulus <= '1';
@ -588,7 +530,7 @@ begin
wait for clk_period;

d1.valid <= '0';
for j in 0 to 64 loop
for j in 0 to 65 loop
wait for clk_period;
if d2.valid = '1' then
exit;

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