[3.13] gh-113841: fix possible undefined division by 0 in _Py_c_pow() (GH-127211) #127216
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…ythonGH-127211) `x**y == 1/x**-y ` thus changing `/=` to `*=` by negating the exponent. (cherry picked from commit f7bb658) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
This was referenced Nov 24, 2024
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@gpshead do you still want to change the commit message in this backport (as mentioned in the original PR after merge?) |
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(Description was updated with a proposal.) |
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Thanks @miss-islington for the PR, and @Yhg1s for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12. |
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…_pow() (pythonGH-127211) (pythonGH-127216) 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 f7bb658) (cherry picked from commit f41d8d8) Co-authored-by: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
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GH-127530 is a backport of this pull request to the 3.12 branch. |
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…GH-127211) (GH-127216) (GH-127530) [3.13] gh-113841: fix possible undefined division by 0 in _Py_c_pow() (GH-127211) (GH-127216) 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 f7bb658) (cherry picked from commit f41d8d8) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
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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 f7bb658)
Co-authored-by: Sergey B Kirpichev skirpichev@gmail.com
_Py_c_pow()#113841