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FPU: Fix comparison of remainder in square root code

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The square root procedure needs to compare B - R^2 with 2R + 1 to
decide whether to increment the square root estimate R by 1.  It
currently does this by putting 2R + 1 in B and using the pcmpb_lt
and pcmpb_eq signals.  This is not correct because the comparisons
that generate those signals have a 2-bit shift embedded into them.
Instead, put 2R + 1 into C and use pcmpc_lt/eq, which don't have
the 2-bit shift.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
This commit is contained in:
Paul Mackerras
2025-12-10 08:37:02 +11:00
parent f3b9566ae2
commit 41988e3b5f
1 changed files with 11 additions and 5 deletions
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16
fpu.vhdl
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@@ -1303,6 +1303,9 @@ begin
end if; end if;
-- Compare P with zero and with B -- Compare P with zero and with B
-- This has a 2-bit shift in it (p(59..4) compared to b(57..2))
-- because it's used in the FP division code to determine whether
-- to increment the quotient at bit 2 (DP_RBIT).
px_nz := or (r.p(UNIT_BIT + 1 downto 4)); px_nz := or (r.p(UNIT_BIT + 1 downto 4));
pcmpb_eq := '0'; pcmpb_eq := '0';
if r.p(59 downto 4) = r.b.mantissa(UNIT_BIT + 1 downto DP_RBIT) then if r.p(59 downto 4) = r.b.mantissa(UNIT_BIT + 1 downto DP_RBIT) then
@@ -1314,6 +1317,9 @@ begin
elsif unsigned(r.p(59 downto 4)) < unsigned(r.b.mantissa(UNIT_BIT + 1 downto DP_RBIT)) then elsif unsigned(r.p(59 downto 4)) < unsigned(r.b.mantissa(UNIT_BIT + 1 downto DP_RBIT)) then
pcmpb_lt := '1'; pcmpb_lt := '1';
end if; end if;
-- Compare P with zero and with C
-- This is used in the square root and integer division code
-- to decide whether to increment the result by 1
pcmpc_eq := '0'; pcmpc_eq := '0';
if r.p = r.c.mantissa then if r.p = r.c.mantissa then
pcmpc_eq := '1'; pcmpc_eq := '1';
@@ -2271,29 +2277,29 @@ begin
when SQRT_11 => when SQRT_11 =>
-- compute P = A - R * R (remainder) -- compute P = A - R * R (remainder)
-- also put 2 * R + 1 into B for comparison with P -- also put 2 * R + 1 into C for comparison with P
msel_1 <= MUL1_R; msel_1 <= MUL1_R;
msel_2 <= MUL2_R; msel_2 <= MUL2_R;
msel_add <= MULADD_A; msel_add <= MULADD_A;
msel_inv <= '1'; msel_inv <= '1';
f_to_multiply.valid <= r.first; f_to_multiply.valid <= r.first;
shiftin := '1'; shiftin := '1';
set_b := r.first; set_c := r.first;
if multiply_to_f.valid = '1' then if multiply_to_f.valid = '1' then
v.state := SQRT_12; v.state := SQRT_12;
end if; end if;
when SQRT_12 => when SQRT_12 =>
-- test if remainder is 0 or >= B = 2*R + 1 -- test if remainder is 0 or >= C = 2*R + 1
set_r := '0'; set_r := '0';
opsel_c <= CIN_INC; opsel_c <= CIN_INC;
if pcmpb_lt = '1' then if pcmpc_lt = '1' then
-- square root is correct, set X if remainder non-zero -- square root is correct, set X if remainder non-zero
v.x := r.p(UNIT_BIT + 2) or px_nz; v.x := r.p(UNIT_BIT + 2) or px_nz;
else else
-- square root needs to be incremented by 1 -- square root needs to be incremented by 1
set_r := '1'; set_r := '1';
v.x := not pcmpb_eq; v.x := not pcmpc_eq;
end if; end if;
v.state := FINISH; v.state := FINISH;