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bpo-46018: Ensure that math.expm1 does not raise on underflow (GH-29997)

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(cherry picked from commit 3363e1cb05)

Co-authored-by: Steve Dower <steve.dower@python.org>
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Miss Islington (bot) 2021-12-09 11:37:06 -08:00 • committed by GitHub
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2 changed files with 7 additions and 2 deletions
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1
Misc/NEWS.d/next/Library/2021-12-09-00-44-42.bpo-46018.hkTI7v.rst Normal file
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@ -0,0 +1 @@
Ensure that :func:`math.expm1` does not raise on underflow.

8
Modules/mathmodule.c
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@ -977,9 +977,13 @@ is_error(double x)
* On some platforms (Ubuntu/ia64) it seems that errno can be * On some platforms (Ubuntu/ia64) it seems that errno can be
* set to ERANGE for subnormal results that do *not* underflow * set to ERANGE for subnormal results that do *not* underflow
* to zero. So to be safe, we'll ignore ERANGE whenever the * to zero. So to be safe, we'll ignore ERANGE whenever the
* function result is less than one in absolute value. * function result is less than 1.5 in absolute value.
*
* bpo-46018: Changed to 1.5 to ensure underflows in expm1()
* are correctly detected, since the function may underflow
* toward -1.0 rather than 0.0.
*/ */
if (fabs(x) < 1.0) if (fabs(x) < 1.5)
result = 0; result = 0;
else else
PyErr_SetString(PyExc_OverflowError, PyErr_SetString(PyExc_OverflowError,