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bpo-46311: Clean up PyLong_FromLong and PyLong_FromLongLong #30496

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Mar 1, 2022
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bpo-46311: Clean up PyLong_FromLong and PyLong_FromLongLong #30496

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5cf8d94
Clean up PyLong_FromLong and PyLong_FromLongLong
mdickinson Jan 9, 2022
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2022-01-09-11-59-04.bpo-30496.KvuuGT.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fixed a minor portability issue in the implementation of
:c:func:`PyLong_FromLong`, and added a fast path for single-digit integers
to :c:func:`PyLong_FromLongLong`.
72 changes: 31 additions & 41 deletions Objects/longobject.c
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Expand Up @@ -274,44 +274,40 @@ _PyLong_Negate(PyLongObject **x_p)
}

/* Create a new int object from a C long int */

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This is a minor consistency fix, following the Boy Scout Rule: the prevailing style in this file is to leave a blank line between the comment-description and the definition. But the change is not strictly necessary for this PR, and I can revert if reviewers prefer.

PyObject *
PyLong_FromLong(long ival)
{
PyLongObject *v;
unsigned long abs_ival, t;
int ndigits;

/* Handle small and medium cases. */
if (IS_SMALL_INT(ival)) {
return get_small_int((sdigit)ival);
}
unsigned long abs_ival;
int sign;
if (ival < 0) {
/* negate: can't write this as abs_ival = -ival since that
invokes undefined behaviour when ival is LONG_MIN */
abs_ival = 0U-(twodigits)ival;
sign = -1;
}
else {
abs_ival = (unsigned long)ival;
sign = 1;
}
/* Fast path for single-digit ints */
if (!(abs_ival >> PyLong_SHIFT)) {
if (-(long)PyLong_MASK <= ival && ival <= (long)PyLong_MASK) {
return _PyLong_FromMedium((sdigit)ival);
}
/* Must be at least two digits.
* Do shift in two steps to avoid undefined behavior. */
unsigned long t = (abs_ival >> PyLong_SHIFT) >> PyLong_SHIFT;
Py_ssize_t ndigits = 2;

/* Count digits (at least two - smaller cases were handled above). */
abs_ival = ival < 0 ? 0U-(unsigned long)ival : (unsigned long)ival;
/* Do shift in two steps to avoid possible undefined behavior. */
t = abs_ival >> PyLong_SHIFT >> PyLong_SHIFT;
ndigits = 2;
while (t) {
++ndigits;
t >>= PyLong_SHIFT;
}
PyLongObject *v = _PyLong_New(ndigits);

/* Construct output value. */
v = _PyLong_New(ndigits);
if (v != NULL) {
digit *p = v->ob_digit;
Py_SET_SIZE(v, ndigits * sign);
Py_SET_SIZE(v, ival < 0 ? -ndigits : ndigits);
t = abs_ival;
while (t) {
*p++ = Py_SAFE_DOWNCAST(
t & PyLong_MASK, unsigned long, digit);
*p++ = (digit)(t & PyLong_MASK);
t >>= PyLong_SHIFT;
}
}
Expand Down Expand Up @@ -1107,38 +1103,32 @@ PyObject *
PyLong_FromLongLong(long long ival)
{
PyLongObject *v;
unsigned long long abs_ival;
unsigned long long t; /* unsigned so >> doesn't propagate sign bit */
int ndigits = 0;
int negative = 0;
unsigned long long abs_ival, t;
int ndigits;

/* Handle small and medium cases. */
if (IS_SMALL_INT(ival)) {
return get_small_int((sdigit)ival);
}

if (ival < 0) {
/* avoid signed overflow on negation; see comments
in PyLong_FromLong above. */
abs_ival = (unsigned long long)(-1-ival) + 1;
negative = 1;
}
else {
abs_ival = (unsigned long long)ival;
if (-(long long)PyLong_MASK <= ival && ival <= (long long)PyLong_MASK) {
return _PyLong_FromMedium((sdigit)ival);
}

/* Count the number of Python digits.
We used to pick 5 ("big enough for anything"), but that's a
waste of time and space given that 5*15 = 75 bits are rarely
needed. */
t = abs_ival;
/* Count digits (at least two - smaller cases were handled above). */
abs_ival = ival < 0 ? 0U-(unsigned long long)ival : (unsigned long long)ival;
/* Do shift in two steps to avoid possible undefined behavior. */
t = abs_ival >> PyLong_SHIFT >> PyLong_SHIFT;
ndigits = 2;
while (t) {
++ndigits;
t >>= PyLong_SHIFT;
}

/* Construct output value. */
v = _PyLong_New(ndigits);
if (v != NULL) {
digit *p = v->ob_digit;
Py_SET_SIZE(v, negative ? -ndigits : ndigits);
Py_SET_SIZE(v, ival < 0 ? -ndigits : ndigits);
t = abs_ival;
while (t) {
*p++ = (digit)(t & PyLong_MASK);
Expand Down