gh-130317: Fix test_pack_unpack_roundtrip_for_nans() on x86

* Only test 64-bit double on x86.
* Fix _testcapi.float_pack(): use memcpy() to preserve the sNaN bit.

Author: vstinner

Lib/test/test_capi/test_float.py

@@ -183,8 +183,14 @@ def test_pack_unpack_roundtrip(self):
     def test_pack_unpack_roundtrip_for_nans(self):
         pack = _testcapi.float_pack
         unpack = _testcapi.float_unpack
+        arch_32bit = (sys.maxsize == 2147483647)
+        if (not arch_32bit) or (sys.platform == 'win32'):
+            sizes = (2, 4, 8)
+        else:
+            sizes = (8,)
+
         for _ in range(1000):
-            for size in (2, 4, 8):
+            for size in sizes:
                 sign = random.randint(0, 1)
                 signaling = random.randint(0, 1)
                 quiet = int(not signaling)
@@ -202,7 +208,7 @@ def test_pack_unpack_roundtrip_for_nans(self):
                     with self.subTest(data=data, size=size, endian=endian):
                         data1 = data if endian == BIG_ENDIAN else data[::-1]
                         value = unpack(data1, endian)
-                        if signaling and sys.platform == 'win32':
+                        if signaling and arch_32bit:
                             # On this platform sNaN becomes qNaN when returned
                             # from function.  That's a known bug, e.g.
                             # https://developercommunity.visualstudio.com/t/155064

Modules/_testcapi/clinic/float.c.h

@@ -15,17 +15,25 @@ module _testcapi
 _testcapi.float_pack
 
     size: int
-    d: double
+    obj: object
     le: int
     /
 
 Test PyFloat_Pack2(), PyFloat_Pack4() and PyFloat_Pack8()
 [clinic start generated code]*/
 
 static PyObject *
-_testcapi_float_pack_impl(PyObject *module, int size, double d, int le)
-/*[clinic end generated code: output=7899bd98f8b6cb04 input=52c9115121999c98]*/
+_testcapi_float_pack_impl(PyObject *module, int size, PyObject *obj, int le)
+/*[clinic end generated code: output=7eb936d25b14e105 input=78f017f3ca316740]*/
 {
+    if (!PyFloat_Check(obj)) {
+        PyErr_SetString(PyExc_ValueError, "float-point number expected");
+        return NULL;
+    }
+    double d;
+    // gh-130317: memcpy() is needed to preserve the sNaN flag on x86 (32-bit).
+    memcpy(&d, &((PyFloatObject *)obj)->ob_fval, 8);
+
     switch (size)
     {
     case 2: