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k_mul(): White-box testing turned up that (ah+al)*(bh+bl) can, in rare

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cases, overflow the allocated result object by 1 bit.  In such cases,
it would have been brought back into range if we subtracted al*bl and
ah*bh from it first, but I don't want to do that because it hurts cache
behavior.  Instead we just ignore the excess bit when it appears -- in
effect, this is forcing unsigned mod BASE**(asize + bsize) arithmetic
in a case where that doesn't happen all by itself.
This commit is contained in:
Tim Peters 2002-08-12 19:30:26 +00:00
parent 3747a0f04c
commit d8b2173ef9
1 changed files with 11 additions and 3 deletions
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14
Objects/longobject.c
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@ -1663,7 +1663,7 @@ k_mul(PyLongObject *a, PyLongObject *b)
*/
/* 1. Allocate result space. */
ret = _PyLong_New(asize + bsize);
ret = _PyLong_New(asize + bsize + 1);
if (ret == NULL) goto fail;
#ifdef Py_DEBUG
/* Fill with trash, to catch reference to uninitialized digits. */
@ -1727,7 +1727,15 @@ k_mul(PyLongObject *a, PyLongObject *b)
Py_DECREF(t2);
if (t3 == NULL) goto fail;
/* Add t3. */
/* Add t3. Caution: t3 can spill one bit beyond the allocated
* result space; it's t3-al*bl-ah*bh that always fits. We have
* to arrange to ignore the hight bit.
*/
if (t3->ob_size > i) {
assert(t3->ob_size == i+1); /* just one digit over */
assert(t3->ob_digit[t3->ob_size - 1] == 1); /* & just one bit */
--t3->ob_size; /* ignore the overflow bit */
}
(void)v_iadd(ret->ob_digit + shift, i, t3->ob_digit, t3->ob_size);
Py_DECREF(t3);
@ -1761,7 +1769,7 @@ long_mul(PyLongObject *v, PyLongObject *w)
return Py_NotImplemented;
}
#if 0
#if 1
if (Py_GETENV("KARAT") != NULL)
z = k_mul(a, b);
else