SF bug 434186: 0x80000000/2 != 0x80000000>>1
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.
diff --git a/Objects/intobject.c b/Objects/intobject.c
index 9a1f0e6..de28156 100644
--- a/Objects/intobject.c
+++ b/Objects/intobject.c
@@ -434,38 +434,32 @@
}
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;