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GH-100425: Improve accuracy of builtin sum() for float inputs (GH-100426)

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Raymond Hettinger 2022-12-23 14:35:58 -08:00 • committed by GitHub
parent 1ecfd1ebf1
commit 5d84966cce
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6 changed files with 45 additions and 8 deletions
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21
Python/bltinmodule.c
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@ -2532,6 +2532,7 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
if (PyFloat_CheckExact(result)) {
double f_result = PyFloat_AS_DOUBLE(result);
double c = 0.0;
Py_SETREF(result, NULL);
while(result == NULL) {
item = PyIter_Next(iter);
@ -2539,10 +2540,25 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
Py_DECREF(iter);
if (PyErr_Occurred())
return NULL;
/* Avoid losing the sign on a negative result,
and don't let adding the compensation convert
an infinite or overflowed sum to a NaN. */
if (c && Py_IS_FINITE(c)) {
f_result += c;
}
return PyFloat_FromDouble(f_result);
}
if (PyFloat_CheckExact(item)) {
f_result += PyFloat_AS_DOUBLE(item);
// Improved Kahan–Babuška algorithm by Arnold Neumaier
// https://www.mat.univie.ac.at/~neum/scan/01.pdf
double x = PyFloat_AS_DOUBLE(item);
double t = f_result + x;
if (fabs(f_result) >= fabs(x)) {
c += (f_result - t) + x;
} else {
c += (x - t) + f_result;
}
f_result = t;
_Py_DECREF_SPECIALIZED(item, _PyFloat_ExactDealloc);
continue;
}
@ -2556,6 +2572,9 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
continue;
}
}
if (c && Py_IS_FINITE(c)) {
f_result += c;
}
result = PyFloat_FromDouble(f_result);
if (result == NULL) {
Py_DECREF(item);