target/mips: Fix minor bug in FPU

Wrong type of NaN was generated for IEEE 754-2008 by MADDF.<D|S> and
MSUBF.<D|S> instructions when the arguments were (Inf, Zero, NaN) or
(Zero, Inf, NaN).

The if-else statement establishes if the system conforms to IEEE
754-1985 or IEEE 754-2008, and defines different behaviors depending
on that. In case of IEEE 754-2008, in mentioned cases of inputs,
<MADDF|MSUBF>.<D|S> returns the input value 'c' [2] (page 53) and
raises floating point exception 'Invalid Operation' [1] (pages 349,
350).

These scenarios were tested and the results in QEMU emulation match
the results obtained on the machine that has a MIPS64R6 CPU.

[1] MIPS Architecture for Programmers Volume II-a: The MIPS64
    Instruction Set Reference Manual, Revision 6.06
[2] MIPS Architecture for Programmers Volume IV-j: The MIPS64
    SIMD Architecture Module, Revision 1.12

Signed-off-by: Mateja Marjanovic <mateja.marjanovic@rt-rk.com>
Message-Id: <1553008916-15274-2-git-send-email-mateja.marjanovic@rt-rk.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[AJB: fixed up commit message]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

1 files changed, 16 insertions, 8 deletions

diff --git a/fpu/softfloat-specialize.h b/fpu/softfloat-specialize.h
index 16c0bcb6fa..7b8895726c 100644
--- a/fpu/softfloat-specialize.h
+++ b/fpu/softfloat-specialize.h
@@ -495,15 +495,15 @@ static int pickNaNMulAdd(FloatClass a_cls, FloatClass b_cls, FloatClass c_cls,
         return 1;
     }
 #elif defined(TARGET_MIPS)
-    /* For MIPS, the (inf,zero,qnan) case sets InvalidOp and returns
-     * the default NaN
-     */
-    if (infzero) {
-        float_raise(float_flag_invalid, status);
-        return 3;
-    }
-
     if (snan_bit_is_one(status)) {
+        /*
+         * For MIPS systems that conform to IEEE754-1985, the (inf,zero,nan)
+         * case sets InvalidOp and returns the default NaN
+         */
+        if (infzero) {
+            float_raise(float_flag_invalid, status);
+            return 3;
+        }
         /* Prefer sNaN over qNaN, in the a, b, c order. */
         if (is_snan(a_cls)) {
             return 0;
@@ -519,6 +519,14 @@ static int pickNaNMulAdd(FloatClass a_cls, FloatClass b_cls, FloatClass c_cls,
             return 2;
         }
     } else {
+        /*
+         * For MIPS systems that conform to IEEE754-2008, the (inf,zero,nan)
+         * case sets InvalidOp and returns the input value 'c'
+         */
+        if (infzero) {
+            float_raise(float_flag_invalid, status);
+            return 2;
+        }
         /* Prefer sNaN over qNaN, in the c, a, b order. */
         if (is_snan(c_cls)) {
             return 2;