GeezerVison

CC.version: gcc (Debian 12.2.0-14) 12.2.0

builtins.float.double_format: IEEE, little-endian
builtins.float.float_format: IEEE, little-endian

platform.architecture: 32bit ELF
platform.freedesktop_os_release[NAME]: Debian GNU/Linux
platform.freedesktop_os_release[VERSION]: 12 (bookworm)
platform.libc_ver: glibc 2.36
platform.platform: Linux-6.1.0-33-686-pae-i686-with-glibc2.36

sys.float_info: sys.float_info(max=1.7976931348623157e+308, max_exp=1024, max_10_exp=308, min=2.2250738585072014e-308, min_exp=-1021, min_10_exp=-307, dig=15, mant_dig=53, epsilon=2.220446049250313e-16, radix=2, rounds=1)
sys.float_repr_style: short

sys.maxsize: 2147483647

FAIL: test_pack_unpack_roundtrip_for_nans (test.test_capi.test_float.CAPIFloatTest.test_pack_unpack_roundtrip_for_nans)
(data=b'\x7f\x98Pr', size=4, endian=0)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/test_capi/test_float.py", line 213, in test_pack_unpack_roundtrip_for_nans
    self.assertEqual(data1, data2)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
AssertionError: b'\x7f\x98Pr' != b'\x7f\xd8Pr'

======================================================================
FAIL: test_pack_unpack_roundtrip_for_nans (test.test_capi.test_float.CAPIFloatTest.test_pack_unpack_roundtrip_for_nans) 
(data=b'|\xd6', size=2, endian=0)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.14/test/test_capi/test_float.py", line 213, in test_pack_unpack_roundtrip_for_nans
    self.assertEqual(data1, data2)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
AssertionError: b'|\xd6' != b'~\xd6'

======================================================================
FAIL: test_pack_unpack_roundtrip_for_nans (test.test_capi.test_float.CAPIFloatTest.test_pack_unpack_roundtrip_for_nans) 
(data=b'\x7f\xf2h\xf8\x1bU*\xc1', size=8, endian=0)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.14/test/test_capi/test_float.py", line 213, in test_pack_unpack_roundtrip_for_nans
    self.assertEqual(data1, data2)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
AssertionError: b'\x7f\xf2h\xf8\x1bU*\xc1' != b'\x7f\xfah\xf8\x1bU*\xc1'

./configure CFLAGS="-m32" LDFLAGS="-m32"
sed -i -e 's!#define PY_HAVE_PERF_TRAMPOLINE 1!#undef PY_HAVE_PERF_TRAMPOLINE!g' pyconfig.h
make
./python -m test -v test_capi.test_float

(gdb) n
33	    double d = ((PyFloatObject *)obj)->ob_fval;
(gdb) n
35	    switch (size)
(gdb) p ((PyFloatObject *)obj)->ob_fval
$3 = nan(0x5e5454683683)
(gdb) p d
$4 = nan(0x85e5454683683)

float_set_snan(): v=fffbce9e80000000 (before)
float_set_snan(): v=fff3ce9e80000000 (sNaN)
float_pack():     v=fff3ce9e80000000
PyFloat_Pack4():  v=fffbce9e80000000

diff --git a/Modules/_testcapi/float.c b/Modules/_testcapi/float.c
index 2feeb205d8..b382883fa8 100644
--- a/Modules/_testcapi/float.c
+++ b/Modules/_testcapi/float.c
@@ -181,9 +181,12 @@ _testcapi_float_set_snan(PyObject *module, PyObject *obj)
     }
     uint64_t v;
     memcpy(&v, &d, 8);
-    v &= ~(1ULL << 51); /* make sNaN */
-    memcpy(&d, &v, 8);
-    return PyFloat_FromDouble(d);
+    if (v & (1ULL << 51)) {
+        v -= (1ULL << 51); /* make sNaN */
+    }
+    PyObject *ret = PyFloat_FromDouble(0.0);
+    memcpy(&(((PyFloatObject *)ret)->ob_fval), &v, 8);
+    return ret;
 }
 
 static PyMethodDef test_methods[] = {