fpu: Check for default_nan_mode before calling pickNaNMulAdd

If the target sets default_nan_mode then we're always going to return
the default NaN, and pickNaNMulAdd() no longer has any side effects.
For consistency with pickNaN(), check for default_nan_mode before
calling pickNaNMulAdd().

When we convert pickNaNMulAdd() to allow runtime selection of the NaN
propagation rule, this means we won't have to make the targets which
use default_nan_mode also set a propagation rule.

Since RiscV always uses default_nan_mode, this allows us to remove
its ifdef case from pickNaNMulAdd().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241202131347.498124-3-peter.maydell@linaro.org

fpu/softfloat-parts.c.inc

@@ -77,9 +77,13 @@ static FloatPartsN *partsN(pick_nan_muladd)(FloatPartsN *a, FloatPartsN *b,
         float_raise(float_flag_invalid | float_flag_invalid_imz, s);
     }
 
+    if (s->default_nan_mode) {
+        which = 3;
+    } else {
         which = pickNaNMulAdd(a->cls, b->cls, c->cls, infzero, s);
+    }
 
-    if (s->default_nan_mode || which == 3) {
+    if (which == 3) {
         parts_default_nan(a, s);
         return a;
     }

fpu/softfloat-specialize.c.inc

@@ -475,6 +475,13 @@ static int pickNaN(FloatClass a_cls, FloatClass b_cls,
 static int pickNaNMulAdd(FloatClass a_cls, FloatClass b_cls, FloatClass c_cls,
                          bool infzero, float_status *status)
 {
+    /*
+     * We guarantee not to require the target to tell us how to
+     * pick a NaN if we're always returning the default NaN.
+     * But if we're not in default-NaN mode then the target must
+     * specify.
+     */
+    assert(!status->default_nan_mode);
 #if defined(TARGET_ARM)
     /* For ARM, the (inf,zero,qnan) case sets InvalidOp and returns
      * the default NaN
@@ -578,8 +585,6 @@ static int pickNaNMulAdd(FloatClass a_cls, FloatClass b_cls, FloatClass c_cls,
     } else {
         return 1;
     }
-#elif defined(TARGET_RISCV)
-    return 3; /* default NaN */
 #elif defined(TARGET_S390X)
     if (infzero) {
         return 3;