8000 bpo-29882: Add an efficient popcount method for integers by niklasf · Pull Request #771 · python/cpython · GitHub
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Merged
merged 13 commits into from
May 29, 2020
+135 −2
Merged

bpo-29882: Add an efficient popcount method for integers #771

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74e3ea6
bpo-29882: Add an efficient popcount method for integers
niklasf Mar 10, 2017
16cc8e5
Merge remote-tracking branch 'upstream/master' into bpo-29882-popcount
mdickinson May 26, 2020
19da190
Update 'sign bit' and versionadded in docs
mdickinson May 26, 2020
070ac45
Add entry to whatsnew document
mdickinson May 26, 2020
b87f162
Doc: use positive example, mention population count
mdickinson May 27, 2020
48084e7
Minor cleanups of the core code
mdickinson May 27, 2020
4c6819a
Fix conflicts with master
mdickinson May 27, 2020
729737c
Move popcount_digit closer to where it's used
mdickinson May 27, 2020
79f4d16
Use z instead of self after conversion
mdickinson May 27, 2020
e3228cf
Add 'absolute value' and 'population count' to docstring
mdickinson May 27, 2020
a0fed82
Fix clinic error about missing summary line
mdickinson May 27, 2020
46d8bc9
Merge remote-tracking branch 'upstream/master' into bpo-29882-popcount
mdickinson May 29, 2020
3e8422b
Ensure popcount_digit is portable with 64-bit ints
mdickinson May 29, 2020
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Minor cleanups of the core code
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@mdickinson
mdickinson committed May 27, 2020
commit 48084e75790a44f36bc61ab3488e0d118e52c476
25 changes: 14 additions & 11 deletions Objects/longobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5447,30 +5447,33 @@ static PyObject *
int_bit_count_impl(PyObject *self)
/*[clinic end generated code: output=2e571970daf1e5c3 input=a428900d3e39a606]*/
{
Py_ssize_t ndigits, i, bit_count = 0;
PyLongObject *result, *x, *y;

assert(self != NULL);
assert(PyLong_Check(self));

ndigits = Py_ABS(Py_SIZE(self));
PyLongObject *z = (PyLongObject *)self;
Py_ssize_t ndigits = Py_ABS(Py_SIZE(self));
Py_ssize_t bit_count = 0;

for (i = 0; i < ndigits && i < PY_SSIZE_T_MAX/PyLong_SHIFT; i++) {
bit_count += popcount_digit(((PyLongObject *)self)->ob_digit[i]);
/* Each digit has up to PyLong_SHIFT ones, so the accumulated bit count
from the first PY_SSIZE_T_MAX/PyLong_SHIFT digits can't overflow a
Py_ssize_t. */
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Nice comment, thanks! Before the comment, I was surprised by the two loops.

Py_ssize_t ndigits_fast = Py_MIN(ndigits, PY_SSIZE_T_MAX/PyLong_SHIFT);
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I proposed to work use size_t for bit_count, but then I realzied that i is a Py_ssize_t, so it's better to leave the code as it is: work on Py_ssize_t for i and bit_count.

for (Py_ssize_t i = 0; i < ndigits_fast; i++) {
bit_count += popcount_digit(z->ob_digit[i]);
}

result = (PyLongObject *)PyLong_FromSsize_t(bit_count);
PyObject *result = PyLong_FromSsize_t(bit_count);
if (result == NULL) {
return NULL;
}

/* Use Python integers if bit_count would overflow. */
for (; i < ndigits; i++) {
x = (PyLongObject *)PyLong_FromLong(popcount_digit(((PyLongObject *)self)->ob_digit[i]));
for (Py_ssize_t i = ndigits_fast; i < ndigits; i++) {
PyObject *x = PyLong_FromLong(popcount_digit(z->ob_digit[i]));
if (x == NULL) {
goto error;
}
y = (PyLongObject *)long_add(result, x);
PyObject *y = long_add((PyLongObject *)result, (PyLongObject *)x);
Py_DECREF(x);
if (y == NULL) {
goto error;
Expand All @@ -5479,7 +5482,7 @@ int_bit_count_impl(PyObject *self)
result = y;
}

return (PyObject *)result;
return result;

error:
Py_DECREF(result);
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