cleanup & comments

Author: skirpichev

Lib/test/test_capi/test_float.py

@@ -203,6 +203,9 @@ def test_pack_unpack_roundtrip_for_nans(self):
                         data1 = data if endian == BIG_ENDIAN else data[::-1]
                         value = unpack(data1, endian)
                         if signaling and sys.platform == 'win32':
+                            # On this platform sNaN becomes qNaN when returned
+                            # from function.  That's a known bug, e.g.
+                            # https://developercommunity.visualstudio.com/t/155064
                             value = _testcapi.float_set_snan(value)
                         data2 = pack(size, value, endian)
                         self.assertTrue(math.isnan(value))

Objects/floatobject.c

@@ -2192,15 +2192,15 @@ PyFloat_Pack4(double x, char *data, int le)
         if (isinf(y) && !isinf(x))
             goto Overflow;
 
-        /* correct y if x was a sNaN, transformed to qNaN by assignment */
+        /* correct y if x was a sNaN, transformed to qNaN by conversion */
         if (isnan(x)) {
             uint64_t v;
 
             memcpy(&v, &x, 8);
             if ((v & (1ULL << 51)) == 0) {
                 uint32_t *py = (uint32_t *)&y;
 
-                *py -= 1 << 22; /* make sNaN */
+                *py &= ~(1 << 22); /* make sNaN */
             }
         }
 
@@ -2491,11 +2491,11 @@ PyFloat_Unpack4(const char *data, int le)
             uint32_t v;
 
             memcpy(&v, &x, 4);
-            if ((v & (1<<22)) == 0) {
+            if ((v & (1 << 22)) == 0) {
                 double y = x; /* will make qNaN double */
                 uint64_t *py = (uint64_t *)&y;
 
-                *py -= (1ULL<<51); /* make sNaN */
+                *py &= ~(1ULL << 51); /* make sNaN */
                 return y;
             }
         }