https://github.com/python/cpython/commit/23d85a2a3fb029172ea15c6e596f64f8c2868ed3
commit: 23d85a2a3fb029172ea15c6e596f64f8c2868ed3
branch: main
author: Sergey B Kirpichev <[email protected]>
committer: vstinner <[email protected]>
date: 2025-11-12T11:06:29+01:00
summary:
gh-141042: fix sNaN's packing for mixed floating-point formats (#141107)
files:
A Misc/NEWS.d/next/C_API/2025-11-06-06-28-14.gh-issue-141042.brOioJ.rst
M Lib/test/test_capi/test_float.py
M Objects/floatobject.c
diff --git a/Lib/test/test_capi/test_float.py b/Lib/test/test_capi/test_float.py
index 983b991b4f163d..df7017e6436a69 100644
--- a/Lib/test/test_capi/test_float.py
+++ b/Lib/test/test_capi/test_float.py
@@ -29,6 +29,23 @@
NAN = float("nan")
+def make_nan(size, sign, quiet, payload=None):
+ if size == 8:
+ payload_mask = 0x7ffffffffffff
+ i = (sign << 63) + (0x7ff << 52) + (quiet << 51)
+ elif size == 4:
+ payload_mask = 0x3fffff
+ i = (sign << 31) + (0xff << 23) + (quiet << 22)
+ elif size == 2:
+ payload_mask = 0x1ff
+ i = (sign << 15) + (0x1f << 10) + (quiet << 9)
+ else:
+ raise ValueError("size must be either 2, 4, or 8")
+ if payload is None:
+ payload = random.randint(not quiet, payload_mask)
+ return i + payload
+
+
class CAPIFloatTest(unittest.TestCase):
def test_check(self):
# Test PyFloat_Check()
@@ -202,16 +219,7 @@ def test_pack_unpack_roundtrip_for_nans(self):
# HP PA RISC uses 0 for quiet, see:
# https://en.wikipedia.org/wiki/NaN#Encoding
signaling = 1
- quiet = int(not signaling)
- if size == 8:
- payload = random.randint(signaling, 0x7ffffffffffff)
- i = (sign << 63) + (0x7ff << 52) + (quiet << 51) + payload
- elif size == 4:
- payload = random.randint(signaling, 0x3fffff)
- i = (sign << 31) + (0xff << 23) + (quiet << 22) + payload
- elif size == 2:
- payload = random.randint(signaling, 0x1ff)
- i = (sign << 15) + (0x1f << 10) + (quiet << 9) + payload
+ i = make_nan(size, sign, not signaling)
data = bytes.fromhex(f'{i:x}')
for endian in (BIG_ENDIAN, LITTLE_ENDIAN):
with self.subTest(data=data, size=size, endian=endian):
@@ -221,6 +229,32 @@ def test_pack_unpack_roundtrip_for_nans(self):
self.assertTrue(math.isnan(value))
self.assertEqual(data1, data2)
+ @unittest.skipUnless(HAVE_IEEE_754, "requires IEEE 754")
+ @unittest.skipUnless(sys.maxsize != 2147483647, "requires 64-bit mode")
+ def test_pack_unpack_nans_for_different_formats(self):
+ pack = _testcapi.float_pack
+ unpack = _testcapi.float_unpack
+
+ for endian in (BIG_ENDIAN, LITTLE_ENDIAN):
+ with self.subTest(endian=endian):
+ byteorder = "big" if endian == BIG_ENDIAN else "little"
+
+ # Convert sNaN to qNaN, if payload got truncated
+ data = make_nan(8, 0, False, 0x80001).to_bytes(8, byteorder)
+ snan_low = unpack(data, endian)
+ qnan4 = make_nan(4, 0, True, 0).to_bytes(4, byteorder)
+ qnan2 = make_nan(2, 0, True, 0).to_bytes(2, byteorder)
+ self.assertEqual(pack(4, snan_low, endian), qnan4)
+ self.assertEqual(pack(2, snan_low, endian), qnan2)
+
+ # Preserve NaN type, if payload not truncated
+ data = make_nan(8, 0, False, 0x80000000001).to_bytes(8,
byteorder)
+ snan_high = unpack(data, endian)
+ snan4 = make_nan(4, 0, False, 16384).to_bytes(4, byteorder)
+ snan2 = make_nan(2, 0, False, 2).to_bytes(2, byteorder)
+ self.assertEqual(pack(4, snan_high, endian), snan4)
+ self.assertEqual(pack(2, snan_high, endian), snan2)
+
if __name__ == "__main__":
unittest.main()
diff --git
a/Misc/NEWS.d/next/C_API/2025-11-06-06-28-14.gh-issue-141042.brOioJ.rst
b/Misc/NEWS.d/next/C_API/2025-11-06-06-28-14.gh-issue-141042.brOioJ.rst
new file mode 100644
index 00000000000000..22a1aa1f405318
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2025-11-06-06-28-14.gh-issue-141042.brOioJ.rst
@@ -0,0 +1,3 @@
+Make qNaN in :c:func:`PyFloat_Pack2` and :c:func:`PyFloat_Pack4`, if while
+conversion to a narrower precision floating-point format --- the remaining
+after truncation payload will be zero. Patch by Sergey B Kirpichev.
diff --git a/Objects/floatobject.c b/Objects/floatobject.c
index 1fefb12803ec19..ef613efe4e7f44 100644
--- a/Objects/floatobject.c
+++ b/Objects/floatobject.c
@@ -2030,6 +2030,10 @@ PyFloat_Pack2(double x, char *data, int le)
memcpy(&v, &x, sizeof(v));
v &= 0xffc0000000000ULL;
bits = (unsigned short)(v >> 42); /* NaN's type & payload */
+ /* set qNaN if no payload */
+ if (!bits) {
+ bits |= (1<<9);
+ }
}
else {
sign = (x < 0.0);
@@ -2202,16 +2206,16 @@ PyFloat_Pack4(double x, char *data, int le)
if ((v & (1ULL << 51)) == 0) {
uint32_t u32;
memcpy(&u32, &y, 4);
- u32 &= ~(1 << 22); /* make sNaN */
+ /* if have payload, make sNaN */
+ if (u32 & 0x3fffff) {
+ u32 &= ~(1 << 22);
+ }
memcpy(&y, &u32, 4);
}
#else
uint32_t u32;
memcpy(&u32, &y, 4);
- if ((v & (1ULL << 51)) == 0) {
- u32 &= ~(1 << 22);
- }
/* Workaround RISC-V: "If a NaN value is converted to a
* different floating-point type, the result is the
* canonical NaN of the new type". The canonical NaN here
@@ -2222,6 +2226,10 @@ PyFloat_Pack4(double x, char *data, int le)
/* add payload */
u32 -= (u32 & 0x3fffff);
u32 += (uint32_t)((v & 0x7ffffffffffffULL) >> 29);
+ /* if have payload, make sNaN */
+ if ((v & (1ULL << 51)) == 0 && (u32 & 0x3fffff)) {
+ u32 &= ~(1 << 22);
+ }
memcpy(&y, &u32, 4);
#endif
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