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bpo-37907: Slightly improve performance of PyLong_AsSsize_t() with large longs #15363

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bpo-37907: Slightly improve performance of PyLong_AsSsize_t() with large longs #15363

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bpo-37907: Slightly improve performance of PyLong_AsSsize_t() with la…
sir-sigurd Aug 20, 2019
99ab95d
use limits consts
sir-sigurd Aug 27, 2022
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2019-08-21-21-19-38.bpo-37907.0SNcSw.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Slightly improve performance of :c:func:`PyLong_AsSsize_t`,
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This is a completely trivial internal change. I don't think it needs a NEWS entry.

:c:func:`PyLong_AsSize_t`, :c:func:`PyLong_AsLong` and
:c:func:`PyLong_AsUnsignedLong` with large longs. Patch by Sergey Fedoseev.
29 changes: 13 additions & 16 deletions Objects/longobject.c
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Expand Up @@ -484,7 +484,7 @@ PyLong_AsLongAndOverflow(PyObject *vv, int *overflow)
{
/* This version by Tim Peters */
PyLongObject *v;
unsigned long x, prev;
unsigned long x;
long res;
Py_ssize_t i;
int sign;
Expand Down Expand Up @@ -527,12 +527,11 @@ PyLong_AsLongAndOverflow(PyObject *vv, int *overflow)
i = -(i);
}
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Would it be possible to start at "x = v->ob_digit[0];" and "i=1;" to avoid starting the loop with x=0 which means one useless if at the first iteration.

With 64-bit unsigned long and PyLong_SHIFT, we can even combine two digits without having to check for overflow, no?

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we can even combine two digits without having to check for overflow, no?

Something like:

/* in the default: case, i >= 2 */
assert(i >= 2);
#if ((ULONG_MAX >> PyLong_SHIFT)) >= ((1UL << PyLong_SHIFT) - 1)
  /* use 2 digits */
  --i;
  x= digit[i];
  x <<=PyLong_SHIFT;
  --i;
  x |= digit[i];
#else
  /* use 1 digit */
  --i
  assert(ULONG_MAX >= ((1UL << PyLong_SHIFT) - 1);
  x= digit[i];
#endif
while (--i >= 0) { ... }

while (--i >= 0) {
prev = x;
x = (x << PyLong_SHIFT) | v->ob_digit[i];
if ((x >> PyLong_SHIFT) != prev) {
if (x > ULONG_MAX >> PyLong_SHIFT) {
*overflow = sign;
goto exit;
}
x = (x << PyLong_SHIFT) | v->ob_digit[i];
}
/* Haven't lost any bits, but casting to long requires extra
* care (see comment above).
Expand Down Expand Up @@ -596,7 +595,7 @@ _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 @@ -623,10 +622,10 @@ PyLong_AsSsize_t(PyObject *vv) {
i = -(i);
}
while (--i >= 0) {
prev = x;
x = (x << PyLong_SHIFT) | v->ob_digit[i];
if ((x >> PyLong_SHIFT) != prev)
if (x > SIZE_MAX >> PyLong_SHIFT) {
goto overflow;
}
x = (x << PyLong_SHIFT) | v->ob_digit[i];
}
/* Haven't lost any bits, but casting to a signed type requires
* extra care (see comment above).
Expand All @@ -652,7 +651,7 @@ unsigned long
PyLong_AsUnsignedLong(PyObject *vv)
{
PyLongObject *v;
unsigned long x, prev;
unsigned long x;
Py_ssize_t i;

if (vv == NULL) {
Expand All @@ -677,14 +676,13 @@ PyLong_AsUnsignedLong(PyObject *vv)
case 1: return v->ob_digit[0];
}
while (--i >= 0) {
prev = x;
x = (x << PyLong_SHIFT) | v->ob_digit[i];
if ((x >> PyLong_SHIFT) != prev) {
if (x > ULONG_MAX >> PyLong_SHIFT) {
PyErr_SetString(PyExc_OverflowError,
"Python int too large to convert "
"to C unsigned long");
return (unsigned long) -1;
}
x = (x << PyLong_SHIFT) | v->ob_digit[i];
}
return x;
}
Expand All @@ -696,7 +694,7 @@ 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 @@ -721,13 +719,12 @@ PyLong_AsSize_t(PyObject *vv)
case 1: return v->ob_digit[0];
}
while (--i >= 0) {
prev = x;
x = (x << PyLong_SHIFT) | v->ob_digit[i];
if ((x >> PyLong_SHIFT) != prev) {
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) | v->ob_digit[i];
}
return x;
}
Expand Down