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Special case parser produces incorrect test case for signbit expected behavior for NaN #250

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@oleksandr-pavlyk

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@oleksandr-pavlyk

test_special_cases.py::test_unary derives tests from docstring. For signbit it says:

  • If x_i is NaN and the sign bit of x_i is 0, the result is False.
  • If x_i is NaN and the sign bit of x_i is 1, the result is True.

The parser of these strings is not able to handle the distinction:

(dev_dpctl) opavlyk@opavlyk-mobl:~/repos/array-api-tests$ SYCL_CACHE_PERSISTENT=1 ONEAPI_DEVICE_SELECTOR=*:cpu ARRAY_API_TESTS_MODULE=dpctl.tensor ipython
Python 3.9.12 (main, Jun  1 2022, 11:38:51)
Type 'copyright', 'credits' or 'license' for more information
IPython 8.18.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import warnings

In [2]: warnings.filterwarnings("ignore")

In [3]: import array_api_tests.test_special_cases as tsc

In [4]: signbit_params = [f for f in tsc.unary_params if f.values[0] == "signbit"]

In [5]: sb_f, sb_t = signbit_params[-1].values[-1], signbit_params[-2].values[-1]

In [6]: (sb_f, sb_t)
Out[6]: (UnaryCase(<x_i is NaN -> True>), UnaryCase(<x_i is NaN -> False>))

In [7]: p_nan = float('nan')

In [8]: n_nan = -p_nan

In [9]: {repr(case):(case.check_result(p_nan, 0.0), case.check_result(p_nan, 1.0)) for case in [sb_f, sb_t]}
Out[9]:
{'UnaryCase(<x_i is NaN -> True>)': (False, True),
 'UnaryCase(<x_i is NaN -> False>)': (True, False)}

In [10]: {repr(case):(case.check_result(n_nan, 0.0), case.check_result(n_nan, 1.0)) for case in [sb_f, sb_t]}
Out[10]:
{'UnaryCase(<x_i is NaN -> True>)': (False, True),
 'UnaryCase(<x_i is NaN -> False>)': (True, False)}

We can use NumPy to verify that the sign bit of p_nan is not set, while the sign bit of n_nan is:

In [11]: import numpy as np

In [12]: hex(np.asarray(p_nan, dtype="f4").view(np.uint32))
Out[12]: '0x7fc00000'

In [13]: hex(np.asarray(n_nan, dtype="f4").view(np.uint32))
Out[13]: '0xffc00000'

In [14]: np.signbit(p_nan), np.signbit(n_nan)
Out[14]: (False, True)

This is causing unexpected test failure in array-api-test suite for dpctl when its __array_api_version__ is set to "2023.12".

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