[65/99] softfloat: Handle default NaN mode after pickNaNMulAdd, not before

Commit Message

Michael Roth July 23, 2018, 8:17 p.m. UTC

From: Peter Maydell <[e-mail]>

It is implementation defined whether a multiply-add of
(0,inf,qnan) or (inf,0,qnan) raises InvalidaOperation or
not, so we let the target-specific pickNaNMulAdd function
handle this. This means that we must do the "return the
default NaN in default NaN mode" check after the call,
not before. Correct the ordering, and restore the comment
from the old propagateFloat64MulAddNaN() that warned about
this corner case.

This fixes a regression from 2.11 for Arm guests where we would
incorrectly fail to set the Invalid flag for these cases.

Cc: [e-mail]
Signed-off-by: Peter Maydell <[e-mail]>
Reviewed-by: Richard Henderson <[e-mail]>
Reviewed-by: Alex Bennée <[e-mail]>
Tested-by: Alex Bennée <[e-mail]>
Message-id: [e-mail]
(cherry picked from commit 1839189bbf89889076aadf0c793c1b57977b28d7)
Signed-off-by: Michael Roth <[e-mail]>
---
 fpu/softfloat.c | 48 ++++++++++++++++++++++++++++--------------------
 1 file changed, 28 insertions(+), 20 deletions(-)

Patch

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index 70e0c40a1c..8401b37bd4 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -602,34 +602,42 @@  static FloatParts pick_nan(FloatParts a, FloatParts b, float_status *s)
 static FloatParts pick_nan_muladd(FloatParts a, FloatParts b, FloatParts c,
                                   bool inf_zero, float_status *s)
 {
+    int which;
+
     if (is_snan(a.cls) || is_snan(b.cls) || is_snan(c.cls)) {
         s->float_exception_flags |= float_flag_invalid;
     }
 
+    which = pickNaNMulAdd(is_qnan(a.cls), is_snan(a.cls),
+                          is_qnan(b.cls), is_snan(b.cls),
+                          is_qnan(c.cls), is_snan(c.cls),
+                          inf_zero, s);
+
     if (s->default_nan_mode) {
+        /* Note that this check is after pickNaNMulAdd so that function
+         * has an opportunity to set the Invalid flag.
+         */
         a.cls = float_class_dnan;
-    } else {
-        switch (pickNaNMulAdd(is_qnan(a.cls), is_snan(a.cls),
-                              is_qnan(b.cls), is_snan(b.cls),
-                              is_qnan(c.cls), is_snan(c.cls),
-                              inf_zero, s)) {
-        case 0:
-            break;
-        case 1:
-            a = b;
-            break;
-        case 2:
-            a = c;
-            break;
-        case 3:
-            a.cls = float_class_dnan;
-            return a;
-        default:
-            g_assert_not_reached();
-        }
+        return a;
+    }
 
-        a.cls = float_class_msnan;
+    switch (which) {
+    case 0:
+        break;
+    case 1:
+        a = b;
+        break;
+    case 2:
+        a = c;
+        break;
+    case 3:
+        a.cls = float_class_dnan;
+        return a;
+    default:
+        g_assert_not_reached();
     }
+    a.cls = float_class_msnan;
+
     return a;
 }