FPU: Do result processing on denorm short-circuit results when FPSCR[UE] is set

Results that are tiny (i.e., in the denorm range) need special
processing when underflow exceptions are enabled, including in the
cases where the result is just one of the input operands, such as for
a fmadd with A or C equal to zero.  To make sure this gets done, go to
FINISH state rather than returning the relevant input operand as the
result.  The same logic is now used when the result needs to be rounded
to single precision.

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

fpu.vhdl

@@ -98,6 +98,7 @@ architecture behaviour of fpu is
         zero_divide   : std_ulogic;
         new_fpscr     : std_ulogic_vector(31 downto 0);
         immed_result  : std_ulogic;      -- result is an input, zero, infinity or NaN
+        need_finish   : std_ulogic;      -- result needs further processing
         qnan_result   : std_ulogic;
         result_sel    : std_ulogic_vector(2 downto 0);
         result_class  : fp_number_class;
@@ -833,6 +834,7 @@ begin
         e.zero_divide := '0';
         e.new_fpscr := (others => '0');
         e.immed_result := '0';
+        e.need_finish := '0';
         e.qnan_result := '0';
         e.result_sel := AIN_ZERO;
         e.result_class := FINITE;
@@ -912,6 +914,10 @@ begin
                 e.result_sel := AIN_B;
                 e.result_class := r.b.class;
                 -- r.result_sign is already correct
+                if r.b.class = FINITE and r.int_result = '0' and
+                    (r.single_prec = '1' or (r.fpscr(FPSCR_UE) = '1' and r.b.denorm = '1')) then
+                    e.need_finish := '1';
+                end if;
             else
                 e.result_class := ZERO;
             end if;
@@ -926,6 +932,10 @@ begin
             e.immed_result := '1';
             e.result_sel := AIN_B;
             e.result_class := r.b.class;
+            if r.b.class = FINITE and r.int_result = '0' and
+                (r.single_prec = '1' or (r.fpscr(FPSCR_UE) = '1' and r.b.denorm = '1')) then
+                e.need_finish := '1';
+            end if;
 
         elsif r.use_b = '1' and r.b.class = ZERO and r.is_multiply = '0' then
             -- B is zero, other operands are finite
@@ -939,6 +949,10 @@ begin
                 -- fadd, result is A
                 e.result_sel := AIN_A;
                 e.rsgn_op := RSGN_SEL;
+                if r.a.class = FINITE and r.int_result = '0' and
+                    (r.single_prec = '1' or (r.fpscr(FPSCR_UE) = '1' and r.a.denorm = '1')) then
+                    e.need_finish := '1';
+                end if;
             else
                 -- other things, result is zero
                 e.result_class := ZERO;
@@ -3108,9 +3122,9 @@ begin
                     arith_done := '1';
                 else
                     misc_sel <= "111";
-                    if r.single_prec = '1' and scinfo.result_class = FINITE and r.int_result = '0' then
-                        -- we have to do the equivalent of frsp on the result
-                        v.state := DO_FRSP_2;
+                    if scinfo.need_finish = '1' then
+                        -- we have to do rounding or underflow exception processing on the result
+                        v.state := FINISH;
                     else
                         arith_done := '1';
                     end if;