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FPU: Simplify sign calculation in FP multiply-add instructions

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By starting out with result_sign = +/- sign of B, we avoid the need to
flip the result sign in a few places.

This also simplifies DO_FMADD state a bit by having DO_ZERO_DEN go to
DO_FMUL state for floating multiply-add where B is zero.  (The
RENORM_A2 and RENORM_C2 states already do this.)

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
pull/442/head
Paul Mackerras 1 year ago
parent 707dd619a0
commit a3613d863b
1 changed files with 11 additions and 13 deletions
Show Stats Download Patch File Download Diff File

24
fpu.vhdl
Unescape Escape View File

@ -952,7 +952,7 @@ begin
when "11100" | "11101" | "11110" | "11111" => --fmadd family
v.is_multiply := '1';
v.is_addition := '1';
v.result_sign := e_in.fra(63) xor e_in.frc(63);
v.result_sign := e_in.frb(63) xnor e_in.insn(1);
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);
@ -1253,6 +1253,7 @@ begin
v.fpscr(FPSCR_VXIDI) := '1';
qnan_result := '1';
else
sign_inv := r.is_multiply and r.is_subtract;
v.result_class := INFINITY;
end if;
arith_done := '1';
@ -1265,6 +1266,7 @@ begin
v.fpscr(FPSCR_VXISI) := '1';
qnan_result := '1';
else
sign_inv := r.is_multiply and r.is_subtract;
v.result_class := INFINITY;
end if;
arith_done := '1';
@ -1281,7 +1283,6 @@ begin
v.result_class := ZERO;
arith_done := '1';
else
sign_inv := r.is_multiply and r.is_subtract;
v.result_class := INFINITY;
arith_done := '1';
end if;
@ -1308,9 +1309,6 @@ begin
elsif r.is_addition = '1' then
-- result is +/- B
v.opsel_a := AIN_B;
if r.is_multiply = '1' then
rsgn_op := RSGN_SUB;
end if;
v.state := EXC_RESULT;
else
v.result_class := ZERO;
@ -1318,7 +1316,6 @@ begin
end if;
elsif r.use_c = '1' and r.c.class = ZERO then
v.opsel_a := AIN_B;
rsgn_op := RSGN_SUB;
v.state := EXC_RESULT;
else
-- B is zero, other operands are finite
@ -1349,6 +1346,14 @@ begin
if r.use_a = '1' and (r.use_b = '0' or r.use_c = '0') then
v.opsel_a := AIN_A;
end if;
if r.use_b = '1' and r.b.class = ZERO and r.use_c = '1' then
-- turn fmadd/sub into fmul
v.opsel_a := AIN_A;
rsgn_op := RSGN_SUB;
v.state := DO_FMUL;
else
v.state := r.exec_state;
end if;
-- input selection for denorm cases
case r.insn(5 downto 1) is
when "10010" => -- fdiv
@ -1367,7 +1372,6 @@ begin
end if;
when others =>
end case;
v.state := r.exec_state;
end if;

when DO_ILLEGAL =>
@ -1792,14 +1796,9 @@ begin
v.state := RENORM_A;
elsif r.c.mantissa(UNIT_BIT) = '0' then
v.state := RENORM_C;
elsif r.b.class = ZERO then
-- no addend, degenerates to multiply
f_to_multiply.valid <= '1';
v.state := MULT_1;
elsif r.madd_cmp = '0' then
-- addend is bigger, do multiply first
-- if subtracting, sign is opposite to initial estimate
rsgn_op := RSGN_SUB;
f_to_multiply.valid <= '1';
v.first := '1';
v.state := FMADD_0;
@ -2006,7 +2005,6 @@ begin
-- product is bigger here
-- shift B right and use it as the addend to the multiplier
-- for subtract, multiplier does B - A * C
rsgn_op := RSGN_SUB;
re_sel2 <= REXP2_B;
re_set_result <= '1';
-- set shift to b.exp - result_exp + 64

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