fpu: Fix NaN encoding for E4M3 in parts64_uncanon

There is only one NaN fractional encoding for E4M3.  Retain the
incoming sign, but force the outgoing fraction to the unique value.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

diff --git a/fpu/softfloat-parts.c.inc b/fpu/softfloat-parts.c.inc
index 559e40d1969f58445594f0f5bf3d9c601f3b0b9a..d0064f5d222ea69167e040e7fc1947fbe3020510 100644 (file)
--- a/fpu/softfloat-parts.c.inc
+++ b/fpu/softfloat-parts.c.inc
@@ -278,11 +278,16 @@ static void partsN(uncanon_e4m3_overflow)(FloatPartsN *p, float_status *s,
                                           const FloatFmt *fmt, bool saturate)
 {
     assert(N == 64);
+    p->exp = fmt->exp_max;
     if (saturate) {
-        p->exp = fmt->exp_max;
         p->frac_hi = E4M3_NORMAL_FRAC_MAX;
     } else {
-        *p = partsN(default_nan)(s);
+        /*
+         * The class isn't actually used after this point in uncanon,
+         * but for clarity while debugging, don't leave it set to normal.
+         */
+        p->cls = float_class_qnan;
+        p->frac_hi = E4M3_NAN_FRAC;
     }
 }
 
@@ -507,9 +512,24 @@ static void partsN(uncanon)(FloatPartsN *p, float_status *s,
             return;
         case float_class_qnan:
         case float_class_snan:
-            assert(fmt->exp_max_kind != float_expmax_normal);
             p->exp = fmt->exp_max;
-            fracN(shr)(p, fmt->frac_shift);
+            switch (fmt->exp_max_kind) {
+            case float_expmax_e4m3:
+                /*
+                 * There is only one NaN encoding for E4M3, and with a
+                 * conversion from another format, the input NaN fraction
+                 * may not apply.
+                 */
+                assert(N == 64);
+                p->frac_hi = E4M3_NAN_FRAC;
+                /* fall through */
+            case float_expmax_ieee:
+                fracN(shr)(p, fmt->frac_shift);
+                break;
+            case float_expmax_normal:
+            default:
+                g_assert_not_reached();
+            }
             return;
         default:
             break;

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index be6b02d866cf610e55f5e98c2b462fbde6f211a9..2c3bf012132c943ad58cfa91542d3f10ab2301e7 100644 (file)
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -499,6 +499,8 @@ const FloatFmt float8_e4m3_params = {
 
 /* 110 << frac_shift, with the implicit bit set */
 #define E4M3_NORMAL_FRAC_MAX  0xe000000000000000ull
+/* 111 << frac_shift, no implicit bit */
+#define E4M3_NAN_FRAC         0x7000000000000000ull
 
 const FloatFmt float8_e5m2_params = {
     FLOAT_PARAMS(5, 2)