fpu/softfloat: check for Inf / x or 0 / x before /0

The re-factoring of div_floats changed the order of checking meaning
an operation like -inf/0 erroneously raises the divbyzero flag.
IEEE-754 (2008) specifies this should only occur for operations on
finite operands.

We fix this by moving the check on the dividend being Inf/0 to before
the divisor is zero check.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Message-id: 20180416135442.30606-1-alex.bennee@linaro.org
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Reviewed-by: NBastian Koppelmann <kbastian@mail.uni-paderborn.de>
Tested-by: NBastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

fpu/softfloat.c

@@ -1146,6 +1146,11 @@ static FloatParts div_floats(FloatParts a, FloatParts b, float_status *s)
         a.cls = float_class_dnan;
         return a;
     }
+    /* Inf / x or 0 / x */
+    if (a.cls == float_class_inf || a.cls == float_class_zero) {
+        a.sign = sign;
+        return a;
+    }
     /* Div 0 => Inf */
     if (b.cls == float_class_zero) {
         s->float_exception_flags |= float_flag_divbyzero;
@@ -1153,11 +1158,6 @@ static FloatParts div_floats(FloatParts a, FloatParts b, float_status *s)
         a.sign = sign;
         return a;
     }
-    /* Inf / x or 0 / x */
-    if (a.cls == float_class_inf || a.cls == float_class_zero) {
-        a.sign = sign;
-        return a;
-    }
     /* Div by Inf */
     if (b.cls == float_class_inf) {
         a.cls = float_class_zero;