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FPU: Move condition register calculations to an explicit data path

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Instead of calculating v.cr_result in the state machine, we now have
the state machine set a 'cr_op' variable which then controls what
computation the CR data path does to set v.cr_result.  The CR data
path also handles updating the XERC result bits for integer operations
(division and modulus).

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This commit is contained in:
Paul Mackerras
2024-03-13 09:45:46 +11:00
parent bbc485f336
commit fcfdbc449c
1 changed files with 154 additions and 117 deletions
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271
fpu.vhdl
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@@ -304,6 +304,13 @@ architecture behaviour of fpu is
constant RCLS_TZERO : std_ulogic_vector(1 downto 0) := "10";
constant RCLS_TINF : std_ulogic_vector(1 downto 0) := "11";
constant CROP_NONE : std_ulogic_vector(2 downto 0) := "000";
constant CROP_FCMP : std_ulogic_vector(2 downto 0) := "001";
constant CROP_MCRFS : std_ulogic_vector(2 downto 0) := "010";
constant CROP_FTDIV : std_ulogic_vector(2 downto 0) := "100";
constant CROP_FTSQRT : std_ulogic_vector(2 downto 0) := "101";
constant CROP_INTRES : std_ulogic_vector(2 downto 0) := "110";
constant arith_decode : decode32 := (
-- indexed by bits 5..1 of opcode
2#01000# => DO_FRI,
@@ -875,6 +882,10 @@ begin
variable is_nan_inf : std_ulogic;
variable is_zero_den : std_ulogic;
variable set_reg_ind : std_ulogic;
variable cr_op : std_ulogic_vector(2 downto 0);
variable cr_result : std_ulogic_vector(3 downto 0);
variable set_cr : std_ulogic;
variable set_fpcc : std_ulogic;
begin
v := r;
v.complete := '0';
@@ -1144,6 +1155,7 @@ begin
carry_in <= '0';
misc_sel <= "000";
fpscr_mask := (others => '1');
cr_op := CROP_NONE;
update_fx := '0';
arith_done := '0';
invalid := '0';
@@ -1160,6 +1172,8 @@ begin
set_c := '0';
set_r := '1';
set_s := '0';
set_cr := '0';
set_fpcc := '0';
f_to_multiply.is_signed <= '0';
f_to_multiply.valid <= '0';
msel_1 <= MUL1_A;
@@ -1361,6 +1375,8 @@ begin
v.instr_done := '1';
when DO_MCRFS =>
cr_op := CROP_MCRFS;
set_cr := '1';
j := to_integer(unsigned(insn_bfa(r.insn)));
for i in 0 to 7 loop
if i = j then
@@ -1373,94 +1389,56 @@ begin
v.instr_done := '1';
when DO_FTDIV =>
v.cr_result := "0000";
-- set result_exp to the exponent of B
re_sel2 <= REXP2_B;
re_set_result <= '1';
if r.a.class = INFINITY or r.b.class = ZERO or r.b.class = INFINITY or
(r.b.class = FINITE and r.b.denorm = '1') then
v.cr_result(2) := '1';
end if;
if r.a.class = NAN or r.a.class = INFINITY or
r.b.class = NAN or r.b.class = ZERO or r.b.class = INFINITY or
(r.a.class = FINITE and r.a.exponent <= to_signed(-970, EXP_BITS)) then
v.cr_result(1) := '1';
v.instr_done := '1';
else
cr_op := CROP_FTDIV;
if (r.a.class = ZERO or r.a.class = FINITE) and r.b.class = FINITE then
v.doing_ftdiv := "11";
v.first := '1';
v.state := FTDIV_1;
v.instr_done := '0';
else
set_cr := '1';
v.instr_done := '1';
end if;
when DO_FTSQRT =>
cr_op := CROP_FTSQRT;
set_cr := '1';
v.instr_done := '1';
v.cr_result := "0000";
if r.b.class = ZERO or r.b.class = INFINITY or
(r.b.class = FINITE and r.b.denorm = '1') then
v.cr_result(2) := '1';
end if;
if r.b.class = NAN or r.b.class = INFINITY or r.b.class = ZERO
or r.b.negative = '1' or r.b.exponent <= to_signed(-970, EXP_BITS) then
v.cr_result(1) := '1';
end if;
when DO_FCMP =>
-- fcmp[uo]
-- Prepare to subtract mantissas, put B in R
opsel_a <= AIN_B;
opsel_b <= BIN_ZERO;
set_r := '1';
v.instr_done := '1';
update_fx := '1';
re_sel2 <= REXP2_B;
re_set_result <= '1';
if (r.a.class = NAN and r.a.mantissa(QNAN_BIT) = '0') or
(r.b.class = NAN and r.b.mantissa(QNAN_BIT) = '0') then
-- Signalling NAN
v.fpscr(FPSCR_VXSNAN) := '1';
if r.insn(6) = '1' and r.fpscr(FPSCR_VE) = '0' then
v.fpscr(FPSCR_VXVC) := '1';
end if;
invalid := '1';
v.cr_result := "0001"; -- unordered
elsif r.a.class = NAN or r.b.class = NAN then
if r.insn(6) = '1' then
cr_op := CROP_FCMP;
if r.a.class = NAN or r.b.class = NAN then
if (r.a.class = NAN and r.a.mantissa(QNAN_BIT) = '0') or
(r.b.class = NAN and r.b.mantissa(QNAN_BIT) = '0') then
-- Signalling NAN
v.fpscr(FPSCR_VXSNAN) := '1';
if r.insn(6) = '1' and r.fpscr(FPSCR_VE) = '0' then
v.fpscr(FPSCR_VXVC) := '1';
end if;
invalid := '1';
elsif r.insn(6) = '1' then
-- fcmpo
v.fpscr(FPSCR_VXVC) := '1';
invalid := '1';
end if;
v.cr_result := "0001"; -- unordered
elsif r.a.class = ZERO and r.b.class = ZERO then
v.cr_result := "0010"; -- equal
elsif r.a.negative /= r.b.negative then
v.cr_result := r.a.negative & r.b.negative & "00";
elsif r.a.class = ZERO then
-- A and B are the same sign from here down
v.cr_result := not r.b.negative & r.b.negative & "00";
elsif r.a.class = INFINITY then
if r.b.class = INFINITY then
v.cr_result := "0010";
else
v.cr_result := r.a.negative & not r.a.negative & "00";
end if;
elsif r.b.class = ZERO then
-- A is finite from here down
v.cr_result := r.a.negative & not r.a.negative & "00";
elsif r.b.class = INFINITY then
v.cr_result := not r.b.negative & r.b.negative & "00";
elsif r.a.exponent > r.b.exponent then
-- A and B are both finite from here down
v.cr_result := r.a.negative & not r.a.negative & "00";
elsif r.a.exponent /= r.b.exponent then
-- A exponent is smaller than B
v.cr_result := not r.a.negative & r.a.negative & "00";
else
-- Prepare to subtract mantissas, put B in R
v.cr_result := "0000";
v.instr_done := '0';
v.state := CMP_1;
end if;
v.fpscr(FPSCR_FL downto FPSCR_FU) := v.cr_result;
if r.a.class = FINITE and r.b.class = FINITE and
r.a.negative = r.b.negative and
r.a.exponent = r.b.exponent then
v.state := CMP_1;
else
set_cr := '1';
set_fpcc := '1';
v.instr_done := '1';
end if;
when DO_MTFSB =>
-- mtfsb{0,1}
@@ -1878,15 +1856,9 @@ begin
v.state := CMP_2;
when CMP_2 =>
if r.r(63) = '1' then
-- A is smaller in magnitude
v.cr_result := not r.a.negative & r.a.negative & "00";
elsif (r_hi_nz or r_lo_nz) = '0' then
v.cr_result := "0010";
else
v.cr_result := r.a.negative & not r.a.negative & "00";
end if;
v.fpscr(FPSCR_FL downto FPSCR_FU) := v.cr_result;
cr_op := CROP_FCMP;
set_cr := '1';
set_fpcc := '1';
v.instr_done := '1';
when MULT_1 =>
@@ -2086,10 +2058,10 @@ begin
-- We go through this state up to two times; the first sees if
-- B.exponent is in the range [-1021,1020], and the second tests
-- whether B.exp - A.exp is in the range [-1022,1020].
v.cr_result(1) := exp_tiny or exp_huge;
-- set shift to a.exp
rs_sel2 <= RSH2_A;
rs_sel2 <= RSH2_A; -- set shift to a.exp
cr_op := CROP_FTDIV;
if exp_tiny = '1' or exp_huge = '1' or r.a.class = ZERO or r.first = '0' then
set_cr := '1';
v.instr_done := '1';
else
v.doing_ftdiv := "10";
@@ -3057,58 +3029,26 @@ begin
v.state := IDIV_OVFCHK;
end if;
when IDIV_OVFCHK =>
opsel_r <= RES_MISC;
misc_sel <= "000";
if r.single_prec = '0' then
sign_bit := r.r(63);
else
sign_bit := r.r(31);
end if;
v.int_ovf := sign_bit xor r.result_sign;
if v.int_ovf = '1' then
v.state := IDIV_ZERO;
else
v.state := IDIV_DONE;
end if;
set_r := sign_bit xor r.result_sign;
v.state := IDIV_DONE;
when IDIV_DONE =>
v.xerc_result := v.xerc;
if r.oe = '1' then
v.xerc_result.ov := '0';
v.xerc_result.ov32 := '0';
v.writing_xer := '1';
end if;
if r.m32b = '0' then
v.cr_result(3) := r.r(63);
v.cr_result(2 downto 1) := "00";
if r.r = 64x"0" then
v.cr_result(1) := '1';
else
v.cr_result(2) := not r.r(63);
end if;
else
v.cr_result(3) := r.r(31);
v.cr_result(2 downto 1) := "00";
if r.r(31 downto 0) = 32x"0" then
v.cr_result(1) := '1';
else
v.cr_result(2) := not r.r(31);
end if;
end if;
v.cr_result(0) := v.xerc.so;
cr_op := CROP_INTRES;
set_cr := '1';
v.writing_fpr := '1';
v.instr_done := '1';
when IDIV_ZERO =>
opsel_r <= RES_MISC;
misc_sel <= "000";
set_r := '1';
v.xerc_result := v.xerc;
if r.oe = '1' then
v.xerc_result.ov := r.int_ovf;
v.xerc_result.ov32 := r.int_ovf;
v.xerc_result.so := r.xerc.so or r.int_ovf;
v.writing_xer := '1';
end if;
v.cr_result := "001" & v.xerc_result.so;
v.writing_fpr := '1';
v.instr_done := '1';
v.state := IDIV_DONE;
end case;
@@ -3525,6 +3465,103 @@ begin
v.shift := rsh_in1 + rsh_in2 + (rs_neg1 or rs_neg2);
end if;
-- Condition register data path
cr_result := "0000";
case cr_op is
when CROP_FCMP =>
if r.a.class = NAN or r.b.class = NAN then
cr_result := "0001"; -- unordered
elsif r.a.class = ZERO and r.b.class = ZERO then
cr_result := "0010"; -- equal
elsif r.a.negative /= r.b.negative then
cr_result := r.a.negative & r.b.negative & "00";
elsif r.a.class = INFINITY and r.b.class = INFINITY then
-- A and B are the same sign from here down
cr_result := "0010";
elsif r.a.class = ZERO then
cr_result := not r.b.negative & r.b.negative & "00";
elsif r.a.class = INFINITY then
cr_result := r.a.negative & not r.a.negative & "00";
elsif r.b.class = ZERO then
-- A is finite from here down
cr_result := r.a.negative & not r.a.negative & "00";
elsif r.b.class = INFINITY then
cr_result := not r.b.negative & r.b.negative & "00";
elsif r.a.exponent > r.b.exponent then
-- A and B are both finite from here down
cr_result := r.a.negative & not r.a.negative & "00";
elsif r.a.exponent /= r.b.exponent then
-- A exponent is smaller than B
cr_result := not r.a.negative & r.a.negative & "00";
elsif r.r(63) = '1' then
-- A is smaller in magnitude
cr_result := not r.a.negative & r.a.negative & "00";
elsif (r_hi_nz or r_lo_nz) = '0' then
cr_result := "0010";
else
cr_result := r.a.negative & not r.a.negative & "00";
end if;
when CROP_MCRFS =>
j := to_integer(unsigned(insn_bfa(r.insn)));
for i in 0 to 7 loop
if i = j then
k := (7 - i) * 4;
cr_result := r.fpscr(k + 3 downto k);
end if;
end loop;
when CROP_FTDIV =>
if r.a.class = INFINITY or r.b.class = ZERO or r.b.class = INFINITY or
(r.b.class = FINITE and r.b.denorm = '1') then
cr_result(2) := '1';
end if;
if r.a.class = NAN or r.a.class = INFINITY or
r.b.class = NAN or r.b.class = ZERO or r.b.class = INFINITY or
(r.a.class = FINITE and r.a.exponent <= to_signed(-970, EXP_BITS)) or
(r.doing_ftdiv(1) = '1' and (exp_tiny or exp_huge) = '1') then
cr_result(1) := '1';
end if;
when CROP_FTSQRT =>
if r.b.class = ZERO or r.b.class = INFINITY or
(r.b.class = FINITE and r.b.denorm = '1') then
cr_result(2) := '1';
end if;
if r.b.class = NAN or r.b.class = INFINITY or r.b.class = ZERO
or r.b.negative = '1' or r.b.exponent <= to_signed(-970, EXP_BITS) then
cr_result(1) := '1';
end if;
when CROP_INTRES =>
v.xerc_result := v.xerc;
if r.oe = '1' then
v.xerc_result.ov := r.int_ovf;
v.xerc_result.ov32 := r.int_ovf;
v.xerc_result.so := r.xerc.so or r.int_ovf;
v.writing_xer := '1';
end if;
if r.m32b = '0' then
cr_result(3) := r.r(63);
if r.r = 64x"0" then
cr_result(1) := '1';
else
cr_result(2) := not r.r(63);
end if;
else
cr_result(3) := r.r(31);
if r.r(31 downto 0) = 32x"0" then
cr_result(1) := '1';
else
cr_result(2) := not r.r(31);
end if;
end if;
cr_result(0) := v.xerc_result.so;
when others =>
end case;
if set_cr = '1' then
v.cr_result := cr_result;
end if;
if set_fpcc = '1' then
v.fpscr(FPSCR_FL downto FPSCR_FU) := cr_result;
end if;
if r.update_fprf = '1' then
v.fpscr(FPSCR_C downto FPSCR_FU) := result_flags(r.res_sign, r.result_class,
r.r(UNIT_BIT) and not r.denorm);