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SF bug 434186: 0x80000000/2 != 0x80000000>>1

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i_divmod:  New and simpler algorithm.  Old one returned gibberish on most
boxes when the numerator was -sys.maxint-1.  Oddly enough, it worked in the
release (not debug) build on Windows, because the compiler optimized away
some tricky sign manipulations that were incorrect in this case.
Makes you wonder <wink> ...
Bugfix candidate.
This commit is contained in:
Tim Peters 2001-06-18 19:21:11 +00:00
parent 888aa26819
commit 1dad6a86de
2 changed files with 24 additions and 23 deletions
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40
Objects/intobject.c
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@ -434,38 +434,32 @@ int_mul(PyObject *v, PyObject *w)
}
static int
i_divmod(register long xi, register long yi,
i_divmod(register long x, register long y,
long *p_xdivy, long *p_xmody)
{
long xdivy, xmody;
if (yi == 0) {
if (y == 0) {
PyErr_SetString(PyExc_ZeroDivisionError,
"integer division or modulo by zero");
return -1;
}
if (yi < 0) {
if (xi < 0) {
if (yi == -1 && -xi < 0) {
/* most negative / -1 */
err_ovf("integer division");
return -1;
}
xdivy = -xi / -yi;
}
else
xdivy = - (xi / -yi);
/* (-sys.maxint-1)/-1 is the only overflow case. */
if (y == -1 && x < 0 && x == -x) {
err_ovf("integer division");
return -1;
}
else {
if (xi < 0)
xdivy = - (-xi / yi);
else
xdivy = xi / yi;
}
xmody = xi - xdivy*yi;
if ((xmody < 0 && yi > 0) || (xmody > 0 && yi < 0)) {
xmody += yi;
xdivy -= 1;
xdivy = x / y;
xmody = x - xdivy * y;
/* If the signs of x and y differ, and the remainder is non-0,
* C89 doesn't define whether xdivy is now the floor or the
* ceiling of the infinitely precise quotient. We want the floor,
* and we have it iff the remainder's sign matches y's.
*/
if (xmody && ((y ^ xmody) < 0) /* i.e. and signs differ */) {
xmody += y;
--xdivy;
assert(xmody && ((y ^ xmody) >= 0));
}
*p_xdivy = xdivy;
*p_xmody = xmody;