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[3.13] gh-113841: fix possible undefined division by 0 in _Py_c_pow() (GH-127211) (#127216)

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Note, that transformed expression is not an equivalent for original one (1/exp(-x) != exp(x) in general for floating-point numbers). Though, the difference seems to be ~1ULP for good libm implementations.

It's more interesting why division was used from beginning. Closest algorithm I've found (no error checks, of course;)) - it's Algorithm 190 from ACM: https://dl.acm.org/doi/10.1145/366663.366679. It uses subtraction in the exponent.

(cherry picked from commit f7bb658124)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
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Miss Islington (bot) 2024-12-02 15:42:02 +01:00 • committed by GitHub
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commit f41d8d89e7
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5
Lib/test/test_complex.py
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@ -301,6 +301,11 @@ class ComplexTest(ComplexesAreIdenticalMixin, unittest.TestCase):
except OverflowError:
pass
# gh-113841: possible undefined division by 0 in _Py_c_pow()
x, y = 9j, 33j**3
with self.assertRaises(OverflowError):
x**y
def test_pow_with_small_integer_exponents(self):
# Check that small integer exponents are handled identically
# regardless of their type.