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FPU: Move most result_sign computation out of state machine

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This moves the computation of r.result_sign out of the various
states for most instructions.  Now the sign is mostly computed in the
first cycle (when e_in.valid is true).

The set of operations done on r.result_sign in the state machine are
now restricted to 5 (other than no change): invert, xor with
r.is_subtract, or set to the sign of A, B or C.

Similarly r.is_subtract and r.negate are computed in the first cycle
now.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This commit is contained in:
Paul Mackerras
2024-01-17 21:05:30 +11:00
parent c5abe3c0a9
commit 955fa561fb
1 changed files with 64 additions and 45 deletions
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109
fpu.vhdl
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@@ -811,7 +811,6 @@ begin
variable mshift : signed(EXP_BITS-1 downto 0);
variable need_check : std_ulogic;
variable msb : std_ulogic;
variable is_add : std_ulogic;
variable set_a : std_ulogic;
variable set_a_exp : std_ulogic;
variable set_a_mant : std_ulogic;
@@ -889,6 +888,7 @@ begin
v.divmod := '0';
v.is_sqrt := '0';
v.is_multiply := '0';
v.is_subtract := '0';
fpin_a := '0';
fpin_b := '0';
fpin_c := '0';
@@ -896,6 +896,8 @@ begin
v.use_b := e_in.valid_b;
v.use_c := e_in.valid_c;
v.round_mode := '0' & r.fpscr(FPSCR_RN+1 downto FPSCR_RN);
v.result_sign := '0';
v.negate := '0';
case e_in.op is
when OP_FP_ARITH =>
fpin_a := e_in.valid_a;
@@ -913,6 +915,25 @@ begin
if e_in.insn(5 downto 1) = "01111" then
v.round_mode := "001";
end if;
case e_in.insn(5 downto 1) is
when "10100" | "10101" => -- fadd and fsub
v.result_sign := e_in.fra(63);
if unsigned(e_in.fra(62 downto 52)) <= unsigned(e_in.frb(62 downto 52)) then
v.result_sign := e_in.frb(63) xnor e_in.insn(1);
end if;
v.is_subtract := not (e_in.fra(63) xor e_in.frb(63) xor e_in.insn(1));
when "11001" => -- fmul
v.result_sign := e_in.fra(63) xor e_in.frc(63);
when "11100" | "11101" | "11110" | "11111" => --fmadd family
v.result_sign := e_in.fra(63) xor e_in.frc(63);
v.is_subtract := not (e_in.fra(63) xor e_in.frb(63) xor
e_in.frc(63) xor e_in.insn(1));
v.negate := e_in.insn(2);
when "10010" => -- fdiv
v.result_sign := e_in.fra(63) xor e_in.frb(63);
when others =>
v.result_sign := e_in.frb(63);
end case;
when OP_FP_CMP =>
fpin_a := e_in.valid_a;
fpin_b := e_in.valid_b;
@@ -921,6 +942,12 @@ begin
v.fp_rc := e_in.rc;
opcbits := e_in.insn(10) & e_in.insn(8) & e_in.insn(4) & e_in.insn(2) & e_in.insn(1);
exec_state := misc_decode(to_integer(unsigned(opcbits)));
case opcbits is
when "10110" => -- fcfid
v.result_sign := e_in.frb(63);
when others =>
v.result_sign := '0';
end case;
when OP_FP_MOVE =>
v.fp_rc := e_in.rc;
fpin_a := e_in.valid_a;
@@ -928,22 +955,49 @@ begin
fpin_c := e_in.valid_c;
if e_in.insn(5) = '0' then
exec_state := DO_FMR;
if e_in.insn(9) = '1' then
v.result_sign := '0'; -- fabs
elsif e_in.insn(8) = '1' then
v.result_sign := '1'; -- fnabs
elsif e_in.insn(7) = '1' then
v.result_sign := e_in.frb(63); -- fmr
elsif e_in.insn(6) = '1' then
v.result_sign := not e_in.frb(63); -- fneg
else
v.result_sign := e_in.fra(63); -- fcpsgn
end if;
else
exec_state := DO_FSEL;
v.result_sign := e_in.frb(63);
end if;
when OP_DIV =>
v.integer_op := '1';
is_32bint := e_in.single;
if e_in.single = '0' then
v.result_sign := e_in.is_signed and (e_in.fra(63) xor e_in.frb(63));
else
v.result_sign := e_in.is_signed and (e_in.fra(31) xor e_in.frb(31));
end if;
exec_state := DO_IDIVMOD;
when OP_DIVE =>
v.integer_op := '1';
v.divext := '1';
is_32bint := e_in.single;
if e_in.single = '0' then
v.result_sign := e_in.is_signed and (e_in.fra(63) xor e_in.frb(63));
else
v.result_sign := e_in.is_signed and (e_in.fra(31) xor e_in.frb(31));
end if;
exec_state := DO_IDIVMOD;
when OP_MOD =>
v.integer_op := '1';
v.divmod := '1';
is_32bint := e_in.single;
if e_in.single = '0' then
v.result_sign := e_in.is_signed and e_in.fra(63);
else
v.result_sign := e_in.is_signed and e_in.fra(31);
end if;
exec_state := DO_IDIVMOD;
when others =>
exec_state := DO_ILLEGAL;
@@ -951,7 +1005,6 @@ begin
v.quieten_nan := '1';
v.tiny := '0';
v.denorm := '0';
v.is_subtract := '0';
v.add_bsmall := '0';
v.int_ovf := '0';
v.div_close := '0';
@@ -1096,7 +1149,6 @@ begin
case r.state is
when IDLE =>
v.invalid := '0';
v.negate := '0';
if e_in.valid = '1' then
v.opsel_a := AIN_B;
v.busy := '1';
@@ -1319,24 +1371,12 @@ begin
re_sel2 <= REXP2_B;
re_set_result <= '1';
v.quieten_nan := '0';
if r.insn(9) = '1' then
v.result_sign := '0'; -- fabs
elsif r.insn(8) = '1' then
v.result_sign := '1'; -- fnabs
elsif r.insn(7) = '1' then
v.result_sign := r.b.negative; -- fmr
elsif r.insn(6) = '1' then
v.result_sign := not r.b.negative; -- fneg
else
v.result_sign := r.a.negative; -- fcpsgn
end if;
v.writing_fpr := '1';
v.instr_done := '1';
when DO_FRI => -- fri[nzpm]
-- r.opsel_a = AIN_B
v.result_class := r.b.class;
v.result_sign := r.b.negative;
re_sel2 <= REXP2_B;
re_set_result <= '1';
-- set shift to exponent - 52
@@ -1365,7 +1405,6 @@ begin
when DO_FRSP =>
-- r.opsel_a = AIN_B, r.shift = 0
v.result_class := r.b.class;
v.result_sign := r.b.negative;
re_sel2 <= REXP2_B;
re_set_result <= '1';
-- set shift to exponent - -126
@@ -1398,7 +1437,6 @@ begin
-- instr bit 1: 1=round to zero 0=use fpscr[RN]
-- r.opsel_a = AIN_B
v.result_class := r.b.class;
v.result_sign := r.b.negative;
re_sel2 <= REXP2_B;
re_set_result <= '1';
rs_sel1 <= RSH1_B;
@@ -1441,12 +1479,10 @@ begin
when DO_FCFID =>
-- r.opsel_a = AIN_B
v.result_sign := '0';
if r.insn(8) = '0' and r.b.negative = '1' then
-- fcfid[s] with negative operand, set R = -B
opsel_ainv <= '1';
carry_in <= '1';
v.result_sign := '1';
end if;
v.result_class := r.b.class;
re_con2 <= RECON2_UNIT;
@@ -1462,7 +1498,6 @@ begin
when DO_FADD =>
-- fadd[s] and fsub[s]
-- r.opsel_a = AIN_A
v.result_sign := r.a.negative;
v.result_class := r.a.class;
re_sel1 <= REXP1_A;
re_set_result <= '1';
@@ -1472,13 +1507,10 @@ begin
rs_sel2 <= RSH2_A;
v.fpscr(FPSCR_FR) := '0';
v.fpscr(FPSCR_FI) := '0';
is_add := r.a.negative xor r.b.negative xor r.insn(1);
v.is_subtract := not is_add;
if r.a.class = FINITE and r.b.class = FINITE then
v.add_bsmall := r.exp_cmp;
v.opsel_a := AIN_B;
if r.exp_cmp = '0' then
v.result_sign := r.b.negative xnor r.insn(1);
if r.a.exponent = r.b.exponent then
v.state := ADD_2;
else
@@ -1491,7 +1523,7 @@ begin
else
if r.a.class = NAN or r.b.class = NAN then
v.state := NAN_RESULT;
elsif r.a.class = INFINITY and r.b.class = INFINITY and is_add = '0' then
elsif r.a.class = INFINITY and r.b.class = INFINITY and r.is_subtract = '1' then
-- invalid operation, construct QNaN
v.fpscr(FPSCR_VXISI) := '1';
qnan_result := '1';
@@ -1502,7 +1534,6 @@ begin
else
-- result is +/- B
v.opsel_a := AIN_B;
v.result_sign := r.b.negative xnor r.insn(1);
v.state := EXC_RESULT;
end if;
end if;
@@ -1510,7 +1541,6 @@ begin
when DO_FMUL =>
-- fmul[s]
-- r.opsel_a = AIN_A unless C is denorm and A isn't
v.result_sign := r.a.negative xor r.c.negative;
v.result_class := r.a.class;
v.fpscr(FPSCR_FR) := '0';
v.fpscr(FPSCR_FI) := '0';
@@ -1550,7 +1580,6 @@ begin
v.result_class := r.a.class;
v.fpscr(FPSCR_FR) := '0';
v.fpscr(FPSCR_FI) := '0';
v.result_sign := r.a.negative xor r.b.negative;
re_sel1 <= REXP1_A;
re_sel2 <= REXP2_B;
re_neg2 <= '1';
@@ -1599,7 +1628,6 @@ begin
v.result_sign := r.c.negative;
else
v.opsel_a := AIN_B;
v.result_sign := r.b.negative;
end if;
v.quieten_nan := '0';
v.state := EXC_RESULT;
@@ -1607,7 +1635,6 @@ begin
when DO_FSQRT =>
-- r.opsel_a = AIN_B
v.result_class := r.b.class;
v.result_sign := r.b.negative;
v.fpscr(FPSCR_FR) := '0';
v.fpscr(FPSCR_FI) := '0';
re_sel2 <= REXP2_B;
@@ -1643,7 +1670,6 @@ begin
when DO_FRE =>
-- r.opsel_a = AIN_B
v.result_class := r.b.class;
v.result_sign := r.b.negative;
v.fpscr(FPSCR_FR) := '0';
v.fpscr(FPSCR_FI) := '0';
re_sel2 <= REXP2_B;
@@ -1669,7 +1695,6 @@ begin
when DO_FRSQRTE =>
-- r.opsel_a = AIN_B
v.result_class := r.b.class;
v.result_sign := r.b.negative;
v.fpscr(FPSCR_FR) := '0';
v.fpscr(FPSCR_FI) := '0';
re_sel2 <= REXP2_B;
@@ -1708,7 +1733,6 @@ begin
-- fmadd, fmsub, fnmadd, fnmsub
-- r.opsel_a = AIN_A if A is denorm, else AIN_C if C is denorm,
-- else AIN_B
v.result_sign := r.a.negative;
v.result_class := r.a.class;
-- put a.exp + c.exp into result_exp
re_sel1 <= REXP1_A;
@@ -1718,9 +1742,6 @@ begin
rs_sel1 <= RSH1_B;
v.fpscr(FPSCR_FR) := '0';
v.fpscr(FPSCR_FI) := '0';
is_add := r.a.negative xor r.c.negative xor r.b.negative xor r.insn(1);
v.negate := r.insn(2);
v.is_subtract := not is_add;
if r.a.class = FINITE and r.c.class = FINITE and
(r.b.class = FINITE or r.b.class = ZERO) then
-- Make sure A and C are normalized
@@ -1730,13 +1751,13 @@ begin
v.state := RENORM_C;
elsif r.b.class = ZERO then
-- no addend, degenerates to multiply
v.result_sign := r.a.negative xor r.c.negative;
f_to_multiply.valid <= '1';
v.is_multiply := '1';
v.state := MULT_1;
elsif r.madd_cmp = '0' then
-- addend is bigger, do multiply first
v.result_sign := r.b.negative xnor r.insn(1);
-- if subtracting, sign is opposite to initial estimate
v.result_sign := r.result_sign xor r.is_subtract;
f_to_multiply.valid <= '1';
v.first := '1';
v.state := FMADD_0;
@@ -1753,21 +1774,20 @@ begin
v.fpscr(FPSCR_VXIMZ) := '1';
qnan_result := '1';
elsif r.a.class = INFINITY or r.c.class = INFINITY then
if r.b.class = INFINITY and is_add = '0' then
if r.b.class = INFINITY and r.is_subtract = '1' then
-- invalid operation, construct QNaN
v.fpscr(FPSCR_VXISI) := '1';
qnan_result := '1';
else
-- result is infinity
v.result_class := INFINITY;
v.result_sign := r.a.negative xor r.c.negative;
arith_done := '1';
end if;
else
-- Here A is zero, C is zero, or B is infinity
-- Result is +/-B in all of those cases
v.opsel_a := AIN_B;
v.result_sign := r.b.negative xnor r.insn(1);
v.result_sign := r.result_sign xor r.is_subtract;
v.state := EXC_RESULT;
end if;
end if;
@@ -1970,7 +1990,7 @@ begin
-- product is bigger here
-- shift B right and use it as the addend to the multiplier
-- for subtract, multiplier does B - A * C
v.result_sign := r.a.negative xor r.c.negative xor r.is_subtract;
v.result_sign := r.result_sign xor r.is_subtract;
re_sel2 <= REXP2_B;
re_set_result <= '1';
-- set shift to b.exp - result_exp + 64
@@ -2638,7 +2658,6 @@ begin
when DO_IDIVMOD =>
-- r.opsel_a = AIN_B
v.result_sign := r.is_signed and (r.a.negative xor (r.b.negative and not r.divmod));
if r.b.class = ZERO then
-- B is zero, signal overflow
v.int_ovf := '1';
@@ -3168,7 +3187,7 @@ begin
end case;
rsign := v.result_sign;
rsign := r.result_sign;
if zero_divide = '1' then
v.fpscr(FPSCR_ZX) := '1';
end if;
@@ -3191,10 +3210,10 @@ begin
v.writing_fpr := '1';
v.update_fprf := '1';
end if;
if v.is_subtract = '1' and v.result_class = ZERO then
if r.is_subtract = '1' and v.result_class = ZERO then
rsign := r.round_mode(0) and r.round_mode(1);
end if;
if v.negate = '1' and v.result_class /= NAN then
if r.negate = '1' and v.result_class /= NAN then
rsign := not rsign;
end if;
v.instr_done := '1';