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GH-100425: Timing experiment: For builtin_sum, try replacing Fast2Sum with 2Sum #100860

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GH-100425: Timing experiment: For builtin_sum, try replacing Fast2Sum with 2Sum #100860

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24 changes: 17 additions & 7 deletions Python/bltinmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2427,6 +2427,19 @@ builtin_vars_impl(PyObject *module, PyObject *object)
return d;
}

typedef struct{ double hi; double lo; } DoubleLength;

static inline DoubleLength
twosum(double a, double b)
{
double s = a + b;
double ap = s - b;
double bp = s - a;
double da = a - ap;
double db = b - bp;
double t = da + db;
return (DoubleLength) {s, t};
}

/*[clinic input]
sum as builtin_sum
Expand Down Expand Up @@ -2561,14 +2574,11 @@ builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
if (PyFloat_CheckExact(item)) {
// Improved Kahan–Babuška algorithm by Arnold Neumaier
// https://www.mat.univie.ac.at/~neum/scan/01.pdf
// but with Fast2Sum replaced with 2Sum.
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;
DoubleLength t = twosum(f_result, x);
f_result = t.hi;
c += t.lo;
_Py_DECREF_SPECIALIZED(item, _PyFloat_ExactDealloc);
continue;
}
Expand Down