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gh-107868 Add an O(1) fastpath for sum(range(...)) #107870

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gh-107868 Add an O(1) fastpath for sum(range(...)) #107870

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10ffc47
add fastpath for sum(range(...))
mcognetta Aug 11, 2023
bd2c5f9
fix assert
mcognetta Aug 11, 2023
2a0b4a3
fix Py_DECREF
mcognetta Aug 11, 2023
86bdcbf
📜🤖 Added by blurb_it.
blurb-it[bot] Aug 11, 2023
1bf1688
fix news and acks
mcognetta Aug 11, 2023
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4 changes: 4 additions & 0 deletions Lib/test/test_builtin.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1626,8 +1626,12 @@ def test_sum(self):

self.assertEqual(sum(range(10), 1000), 1045)
self.assertEqual(sum(range(10), start=1000), 1045)
self.assertAlmostEqual(sum(range(10), 0.1), 45.1)
self.assertEqual(sum(range(10), 2**31-5), 2**31+40)
self.assertEqual(sum(range(10), 2**63-5), 2**63+40)
self.assertEqual(sum(range(-11, 13, 3)), -4)
self.assertEqual(sum(range(-11, -131, -2)), -4200)
self.assertEqual(sum(range(-100, -200, 13)), 0)

self.assertEqual(sum(i % 2 != 0 for i in range(10)), 5)
self.assertEqual(sum((i % 2 != 0 for i in range(10)), 2**31-3),
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1 change: 1 addition & 0 deletions Misc/ACKS
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Expand Up @@ -350,6 +350,7 @@ Hervé Coatanhay
Riccardo Coccioli
Nick Coghlan
Josh Cogliati
Marco Cognetta
Noam Cohen
Dave Cole
Terrence Cole
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2023-08-11-16-44-29.gh-issue-107868.BgT7zE.rst
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@@ -0,0 +1 @@
Reduce ``sum(range(...))`` from ``O(n)`` to ``O(1)`` time . Patch by Marco Cognetta.
59 changes: 59 additions & 0 deletions Python/bltinmodule.c
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Expand Up @@ -2489,13 +2489,72 @@ This function is intended specifically for use with numeric values and may
reject non-numeric types.
[clinic start generated code]*/

static PyObject* range_sum_fastpath(PyObject* module, PyObject *range)
{
PyObject* length = builtin_len(module, range);

if (PyObject_RichCompareBool(length, PyLong_FromLong(0), Py_EQ)) {
Py_DECREF(length);
return PyLong_FromLong(0);
}

// compute start * length + step * length * (length - 1) // 2
// [ a ]
// [ b ]
// [ c ]
// [ d ]
// [ e ]
// [ result ]

PyObject* start = PyObject_GetAttrString(range, "start");
PyObject* step = PyObject_GetAttrString(range, "step");

PyObject* one = PyLong_FromLong(1);
PyObject* a = PyNumber_Subtract(length, one);

PyObject* b = PyNumber_Multiply(a, length);

PyObject* two = PyLong_FromLong(2);
PyObject* c = PyNumber_FloorDivide(b, two);

PyObject* d = PyNumber_Multiply(step, c);

PyObject* e = PyNumber_Multiply(length, start);

PyObject* result = PyNumber_Add(d, e);
Comment on lines +2509 to +2524
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@Eclips4 Eclips4 Aug 11, 2023

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All of these calls can return NULL. You should prevent these situations.

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@mcognetta mcognetta Aug 11, 2023

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To be clear, do you mean adding something like:

if (one == NULL) {
    Py_DECREF(one);
    return NULL;
}

after every one of these? I would have to do it after

        PyObject* rangesum =  range_sum_fastpath(module, iterable);
        result = PyNumber_Add(result, rangesum);
        Py_DECREF(rangesum);
        return result;

as well then, I think.


Py_DECREF(length);
Py_DECREF(start);
Py_DECREF(step);

Py_DECREF(one);
Py_DECREF(a);
Py_DECREF(b);
Py_DECREF(two);
Py_DECREF(c);
Py_DECREF(d);
Py_DECREF(e);

return result;
}

static PyObject *
builtin_sum_impl(PyObject *module, PyObject *iterable, PyObject *start)
/*[clinic end generated code: output=df758cec7d1d302f input=162b50765250d222]*/
{
PyObject *result = start;
PyObject *temp, *item, *iter;

if (PyRange_Check(iterable)) {
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@Eclips4 Eclips4 Aug 11, 2023 •
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builtin_sum_impl written with the usage of Argument Clinic. So, you should run AC on this file to re-generate checksum's (you can see this at the beggining of this function).
More details about the Argument Clinic you can read here:
https://docs.python.org/3.13/howto/clinic.html

if (result == NULL) {
result = PyLong_FromLong(0);
}
PyObject* rangesum = range_sum_fastpath(module, iterable);
result = PyNumber_Add(result, rangesum);
Py_DECREF(rangesum);
return result;
}

iter = PyObject_GetIter(iterable);
if (iter == NULL)
return NULL;
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