From 150125de3fb079b33b3ee2e2c216c032656bbaf2 Mon Sep 17 00:00:00 2001
From: Peter Maydell <peter.maydell@linaro.org>
Date: Mon, 14 May 2018 07:24:25 -0400
Subject: [PATCH] softfloat: Handle default NaN mode after pickNaNMulAdd, not
 before

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.

Backports commit 1839189bbf89889076aadf0c793c1b57977b28d7 from qemu
---
 qemu/fpu/softfloat.c | 52 +++++++++++++++++++++++++-------------------
 1 file changed, 30 insertions(+), 22 deletions(-)

diff --git a/qemu/fpu/softfloat.c b/qemu/fpu/softfloat.c
index 23d93c74..0da2a954 100644
--- a/qemu/fpu/softfloat.c
+++ b/qemu/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;
     }
 
-    if (s->default_nan_mode) {
-        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();
-        }
+    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);
 
-        a.cls = float_class_msnan;
+    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;
+        return a;
     }
+
+    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;
 }