FPU: Reorganize NaN and infinity handling and improve arch compliance

The architecture specifies that an invalid operation exception for
signalling NaN (VXSNAN) can occur in the same instructions as an
invalid operation exception for infinity times zero (VXIMZ) in the
case of a multiply-add instruction where B is a signalling NaN, and
one of A and C is infinity and the other is zero.  This moves the
invalid operation tests around so as to handle this case correctly.
It also restructures the infinity and NaN cases to simplify the logic
a little.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

fpu.vhdl

@@ -838,6 +838,7 @@ begin
         variable set_y       : std_ulogic;
         variable set_s       : std_ulogic;
         variable qnan_result : std_ulogic;
+        variable invalid_mul : std_ulogic;
         variable px_nz       : std_ulogic;
         variable pcmpb_eq    : std_ulogic;
         variable pcmpb_lt    : std_ulogic;
@@ -1217,6 +1218,7 @@ begin
                 -- At least one floating-point operand is infinity or NaN
                 v.fpscr(FPSCR_FR) := '0';
                 v.fpscr(FPSCR_FI) := '0';
+                invalid_mul := '0';
 
                 if (r.a.class = NAN and r.a.mantissa(QNAN_BIT) = '0') or
                     (r.b.class = NAN and r.b.mantissa(QNAN_BIT) = '0') or
@@ -1225,6 +1227,15 @@ begin
                     v.fpscr(FPSCR_VXSNAN) := '1';
                     invalid := '1';
                 end if;
+                -- Check for this case here since VXIMZ can be set along with VXSNAN
+                if r.is_multiply = '1' and
+                    ((r.a.class = INFINITY and r.c.class = ZERO) or
+                     (r.a.class = ZERO and r.c.class = INFINITY)) then
+                    v.fpscr(FPSCR_VXIMZ) := '1';
+                    qnan_result := '1';
+                    invalid_mul := '1';
+                end if;
+
                 if r.a.class = NAN or r.b.class = NAN or r.c.class = NAN then
                     if r.int_result = '1' then
                         v.state := INT_OFLOW;
@@ -1241,51 +1252,32 @@ begin
                     end if;
 
                 else
-                    if r.a.class = INFINITY then
+                    if (r.a.class = INFINITY or r.c.class = INFINITY) and invalid_mul = '0' then
-                        if r.is_multiply = '1' and r.c.class = ZERO then
+                        sign_inv := r.is_multiply and r.is_subtract;
-                            -- invalid operation, construct QNaN
+                        if r.is_subtract = '1' and r.b.class = INFINITY then
-                            v.fpscr(FPSCR_VXIMZ) := '1';
-                            qnan_result := '1';
-                        elsif r.is_subtract = '1' and r.b.class = INFINITY then
                             v.fpscr(FPSCR_VXISI) := '1';
                             qnan_result := '1';
-                        elsif r.is_inverse = '1' and r.b.class = INFINITY then
-                            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';
+                    end if;
-                    elsif r.c.class = INFINITY then
+                    if r.is_inverse = '1' and r.a.class = INFINITY and r.b.class = INFINITY then
-                        if r.is_multiply = '1' and r.a.class = ZERO then
+                        v.fpscr(FPSCR_VXIDI) := '1';
-                            -- invalid operation, construct QNaN
+                        qnan_result := '1';
-                            v.fpscr(FPSCR_VXIMZ) := '1';
+                    end if;
-                            qnan_result := '1';
+                    if r.b.class = INFINITY and r.is_sqrt = '1' and r.b.negative = '1' then
-                        elsif r.is_subtract = '1' and r.b.class = INFINITY then
+                        v.fpscr(FPSCR_VXSQRT) := '1';
-                            v.fpscr(FPSCR_VXISI) := '1';
+                        qnan_result := '1';
-                            qnan_result := '1';
+                    end if;
-                        else
+                    if r.b.class = INFINITY and r.is_inverse = '1' then
-                            sign_inv := r.is_multiply and r.is_subtract;
+                        -- fdiv, fre, frsqrte
-                            v.result_class := INFINITY;
+                        v.result_class := ZERO;
-                        end if;
-                        arith_done := '1';
                     else
-                        -- r.b.class = INFINITY
+                        v.result_class := INFINITY;
-                        if r.int_result = '1' then
+                    end if;
-                            -- fcti*
+                    if r.b.class = INFINITY and r.int_result = '1' then
-                            v.state := INT_OFLOW;
+                        -- fcti*
-                        elsif r.is_sqrt = '1' and r.b.negative = '1' then
+                        v.state := INT_OFLOW;
-                            v.fpscr(FPSCR_VXSQRT) := '1';
+                    else
-                            qnan_result := '1';
+                        arith_done := '1';
-                        elsif r.is_inverse = '1' then
-                            -- fdiv, fre, frsqrte
-                            v.result_class := ZERO;
-                            arith_done := '1';
-                        else
-                            v.result_class := INFINITY;
-                            arith_done := '1';
-                        end if;
                     end if;
                 end if;