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Issue #25402: in int-to-decimal-string conversion, reduce intermediate storage requirements and relax restriction on converting large integers. Patch by Serhiy Storchaka.

This commit is contained in:
Mark Dickinson 2016-08-29 17:26:43 +01:00
parent 583c6e860c
commit 4e1de16f88
2 changed files with 15 additions and 8 deletions
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4
Misc/NEWS
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@ -10,6 +10,10 @@ What's New in Python 3.6.0 beta 1
Core and Builtins
-----------------
- Issue #25402: In int-to-decimal-string conversion, improve the estimate
of the intermediate memory required, and remove an unnecessarily strict
overflow check. Patch by Serhiy Storchaka.
- Issue #27214: In long_invert, be more careful about modifying object
returned by long_add, and remove an unnecessary check for small longs.
Thanks Oren Milman for analysis and patch.

19
Objects/longobject.c
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@ -1591,6 +1591,7 @@ long_to_decimal_string_internal(PyObject *aa,
Py_ssize_t size, strlen, size_a, i, j;
digit *pout, *pin, rem, tenpow;
int negative;
int d;
enum PyUnicode_Kind kind;
a = (PyLongObject *)aa;
@ -1608,15 +1609,17 @@ long_to_decimal_string_internal(PyObject *aa,
But log2(a) < size_a * PyLong_SHIFT, and
log2(_PyLong_DECIMAL_BASE) = log2(10) * _PyLong_DECIMAL_SHIFT
> 3 * _PyLong_DECIMAL_SHIFT
> 3.3 * _PyLong_DECIMAL_SHIFT
size_a * PyLong_SHIFT / (3.3 * _PyLong_DECIMAL_SHIFT) =
size_a + size_a / d < size_a + size_a / floor(d),
where d = (3.3 * _PyLong_DECIMAL_SHIFT) /
(PyLong_SHIFT - 3.3 * _PyLong_DECIMAL_SHIFT)
*/
if (size_a > PY_SSIZE_T_MAX / PyLong_SHIFT) {
PyErr_SetString(PyExc_OverflowError,
"int too large to format");
return -1;
}
/* the expression size_a * PyLong_SHIFT is now safe from overflow */
size = 1 + size_a * PyLong_SHIFT / (3 * _PyLong_DECIMAL_SHIFT);
d = (33 * _PyLong_DECIMAL_SHIFT) /
(10 * PyLong_SHIFT - 33 * _PyLong_DECIMAL_SHIFT);
assert(size_a < PY_SSIZE_T_MAX/2);
size = 1 + size_a + size_a / d;
scratch = _PyLong_New(size);
if (scratch == NULL)
return -1;