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gh-82088: Improve performance of PyLong_As*() for multi-digit ints #135585

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gh-82088: Improve performance of PyLong_As*() for multi-digit ints #135585

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eendebakpt Jun 16, 2025
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Merge branch 'pylong_aslong' of github.com:eendebakpt/cpython into py…
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Improve performance of pylong_as_xxx conversions
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83 changes: 58 additions & 25 deletions Objects/longobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -511,9 +511,9 @@ PyLong_FromDouble(double dval)
long
PyLong_AsLongAndOverflow(PyObject *vv, int *overflow)
{
/* This version by Tim Peters */
/* This version originally by Tim Peters */
PyLongObject *v;
unsigned long x, prev;
unsigned long x;
long res;
Py_ssize_t i;
int sign;
Expand Down Expand Up @@ -556,14 +556,28 @@ PyLong_AsLongAndOverflow(PyObject *vv, int *overflow)
res = -1;
i = _PyLong_DigitCount(v);
sign = _PyLong_NonCompactSign(v);
x = 0;

digit *digits = v->long_value.ob_digit;
assert(i >= 2);
/* unroll 1 digit */
--i;
assert(ULONG_MAX >= ((1UL << PyLong_SHIFT) - 1));
x = digits[i];

#if ((ULONG_MAX >> PyLong_SHIFT)) >= ((1UL << PyLong_SHIFT) - 1)
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We can also unroll the second digit for the other methods. It gains a bit of performance at the cost of a bit of extra code. The unrolling can be factored out to a helper function (the loop itself is not so easy because there are small differences in how to handle the overflow)

/* unroll another digit */
x <<= PyLong_SHIFT;
--i;
x |= digits[i];
#endif


while (--i >= 0) {
prev = x;
x = (x << PyLong_SHIFT) | v->long_value.ob_digit[i];
if ((x >> PyLong_SHIFT) != prev) {
if (x > SIZE_MAX >> PyLong_SHIFT) {
*overflow = sign;
goto exit;
}
x = (x << PyLong_SHIFT) | v->long_value.ob_digit[i];
}
/* Haven't lost any bits, but casting to long requires extra
* care (see comment above).
Expand Down Expand Up @@ -627,7 +641,7 @@ PyLong_AsInt(PyObject *obj)
Py_ssize_t
PyLong_AsSsize_t(PyObject *vv) {
PyLongObject *v;
size_t x, prev;
size_t x;
Py_ssize_t i;
int sign;

Expand All @@ -646,12 +660,19 @@ PyLong_AsSsize_t(PyObject *vv) {
}
i = _PyLong_DigitCount(v);
sign = _PyLong_NonCompactSign(v);
x = 0;

digit *digits = v->long_value.ob_digit;
assert(i >= 2);
/* unroll 1 digit */
--i;
assert(ULONG_MAX >= ((1UL << PyLong_SHIFT) - 1));
x = digits[i];

while (--i >= 0) {
prev = x;
x = (x << PyLong_SHIFT) | v->long_value.ob_digit[i];
if ((x >> PyLong_SHIFT) != prev)
if (x > SIZE_MAX >> PyLong_SHIFT) {
goto overflow;
}
x = (x << PyLong_SHIFT) | digits[i];
}
/* Haven't lost any bits, but casting to a signed type requires
* extra care (see comment above).
Expand All @@ -677,7 +698,7 @@ unsigned long
PyLong_AsUnsignedLong(PyObject *vv)
{
PyLongObject *v;
unsigned long x, prev;
unsigned long x;
Py_ssize_t i;

if (vv == NULL) {
Expand Down Expand Up @@ -708,13 +729,19 @@ PyLong_AsUnsignedLong(PyObject *vv)
return (unsigned long) -1;
}
i = _PyLong_DigitCount(v);
x = 0;

digit *digits = v->long_value.ob_digit;
assert(i >= 2);
/* unroll 1 digit */
--i;
assert(ULONG_MAX >= ((1UL << PyLong_SHIFT) - 1));
x = digits[i];

while (--i >= 0) {
prev = x;
x = (x << PyLong_SHIFT) | v->long_value.ob_digit[i];
if ((x >> PyLong_SHIFT) != prev) {
if (x > SIZE_MAX >> PyLong_SHIFT) {
goto overflow;
}
x = (x << PyLong_SHIFT) | digits[i];
}
return x;
overflow:
Expand All @@ -731,7 +758,7 @@ size_t
PyLong_AsSize_t(PyObject *vv)
{
PyLongObject *v;
size_t x, prev;
size_t x;
Py_ssize_t i;

if (vv == NULL) {
Expand All @@ -753,16 +780,22 @@ PyLong_AsSize_t(PyObject *vv)
return (size_t) -1;
}
i = _PyLong_DigitCount(v);
x = 0;

digit *digits = v->long_value.ob_digit;
assert(i >= 2);
/* unroll 1 digit */
--i;
assert(ULONG_MAX >= ((1UL << PyLong_SHIFT) - 1));
x = digits[i];

while (--i >= 0) {
prev = x;
x = (x << PyLong_SHIFT) | v->long_value.ob_digit[i];
if ((x >> PyLong_SHIFT) != prev) {
PyErr_SetString(PyExc_OverflowError,
"Python int too large to convert to C size_t");
return (size_t) -1;
if (x > SIZE_MAX >> PyLong_SHIFT) {
PyErr_SetString(PyExc_OverflowError,
"Python int too large to convert to C size_t");
return (size_t) -1;
}
x = (x << PyLong_SHIFT) | digits[i];
}
}
return x;
}

Expand Down
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