test_pack_unpack_roundtrip_for_nans failing on RISC-V buildbot

[furkanonder]

Bug report

Bug description:

The test_capi module's test_float are failing on RISC-V build bot

======================================================================
FAIL: test_pack_unpack_roundtrip_for_nans (test.test_capi.test_float.CAPIFloatTest.test_pack_unpack_roundtrip_for_nans) (data=b'\x7f\x93\x16\x02', size=4, endian=0)
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Traceback (most recent call last):
  File "/home/ubuntu/buildarea/3.x.onder-riscv64.installed/build/target/lib/python3.14/test/test_capi/test_float.py", line 216, in test_pack_unpack_roundtrip_for_nans
    self.assertTrue(math.isnan(value))
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
AssertionError: False is not true
======================================================================
FAIL: test_pack_unpack_roundtrip_for_nans (test.test_capi.test_float.CAPIFloatTest.test_pack_unpack_roundtrip_for_nans) (data=b'\x7f\x93\x16\x02', size=4, endian=1)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/ubuntu/buildarea/3.x.onder-riscv64.installed/build/target/lib/python3.14/test/test_capi/test_float.py", line 216, in test_pack_unpack_roundtrip_for_nans
    self.assertTrue(math.isnan(value))
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
AssertionError: False is not true
======================================================================
FAIL: test_pack_unpack_roundtrip_for_nans (test.test_capi.test_float.CAPIFloatTest.test_pack_unpack_roundtrip_for_nans) (data=b'\xff\xd5n\x92', size=4, endian=0)
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Traceback (most recent call last):
  File "/home/ubuntu/buildarea/3.x.onder-riscv64.installed/build/target/lib/python3.14/test/test_capi/test_float.py", line 217, in test_pack_unpack_roundtrip_for_nans
    self.assertEqual(data1, data2)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
AssertionError: b'\xff\xd5n\x92' != b'\x7f\xc0\x00\x00'
======================================================================
FAIL: test_pack_unpack_roundtrip_for_nans (test.test_capi.test_float.CAPIFloatTest.test_pack_unpack_roundtrip_for_nans) (data=b'\xff\xd5n\x92', size=4, endian=1)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/ubuntu/buildarea/3.x.onder-riscv64.installed/build/target/lib/python3.14/test/test_capi/test_float.py", line 217, in test_pack_unpack_roundtrip_for_nans
    self.assertEqual(data1, data2)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
AssertionError: b'\x92n\xd5\xff' != b'\x00\x00\xc0\x7f'
======================================================================
FAIL: test_pack_unpack_roundtrip_for_nans (test.test_capi.test_float.CAPIFloatTest.test_pack_unpack_roundtrip_for_nans) (data=b'\xff\x94\x9e\x8f', size=4, endian=0)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/ubuntu/buildarea/3.x.onder-riscv64.installed/build/target/lib/python3.14/test/test_capi/test_float.py", line 216, in test_pack_unpack_roundtrip_for_nans
    self.assertTrue(math.isnan(value))
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
AssertionError: False is not true
======================================================================

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-133304: workaround for RISC-V in PyFloat_Pack4/Unpack4() #133328

[furkanonder]

bisect to 6157135

CC: @skirpichev @vstinner

[skirpichev]

An example:

FAIL: test_pack_unpack_roundtrip_for_nans (test.test_capi.test_float.CAPIFloatTest.test_pack_unpack_roundtrip_for_nans) (data=b'\xff\xd5n\x92', size=4, endian=0)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/ubuntu/buildarea/3.x.onder-riscv64.installed/build/target/lib/python3.14/test/test_capi/test_float.py", line 217, in test_pack_unpack_roundtrip_for_nans
    self.assertEqual(data1, data2)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
AssertionError: b'\xff\xd5n\x92' != b'\x7f\xc0\x00\x00'

Here lost all payload of quiet nan together with a sign bit. For signaling nan - lost payload means it's not a nan anymore, that explains math.isnan()'s failures.

Seems to be a platform bug, which reveal new patch. I suspect bad things happens in the PyFloat_Unpack4(), after we return float x.

[skirpichev]

Ok, indeed the culpit is "If a NaN value is converted to a different floating-point type, the result is the canonical NaN of the new type" from RISC-V spec. (also: "The canonical NaN has a positive sign and all significand bits clear except the MSB, a.k.a. the quiet bit.") N.B. we unset MSB bit (after L2188) for signaling nans - making a positive infinities for risc-v...

I see following options: 1) filter out this non-IEEE platform in this test (not sure how to do this properly), 2) copy sign bit & payload from x after L2188 in PyFloat_Pack4().

[skirpichev]

PR is ready: #133328 (implements option (2) workaround).

[vstinner]

Fixed by f16f06f.

[vstinner]

Thanks @furkanonder for the report and thanks @skirpichev for the fix!