Issue #12973: Fix int.__pow__ overflow checks that invoked undefined behaviour, thereby producing incorrect results on Clang.
diff --git a/Misc/NEWS b/Misc/NEWS
index 2b67883..72d7a09 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -9,6 +9,11 @@
Core and Builtins
-----------------
+- Issue #12973: Fix overflow checks that invoked undefined behaviour in
+ int.__pow__. These overflow checks were causing int.__pow__ to produce
+ incorrect results with recent versions of Clang, as a result of the
+ compiler optimizing the check away.
+
- Issue #12266: Fix str.capitalize() to correctly uppercase/lowercase
titlecased and cased non-letter characters.
diff --git a/Objects/intobject.c b/Objects/intobject.c
index 7d70bfb..e518e74 100644
--- a/Objects/intobject.c
+++ b/Objects/intobject.c
@@ -751,7 +751,13 @@
while (iw > 0) {
prev = ix; /* Save value for overflow check */
if (iw & 1) {
- ix = ix*temp;
+ /*
+ * The (unsigned long) cast below ensures that the multiplication
+ * is interpreted as an unsigned operation rather than a signed one
+ * (C99 6.3.1.8p1), thus avoiding the perils of undefined behaviour
+ * from signed arithmetic overflow (C99 6.5p5). See issue #12973.
+ */
+ ix = (unsigned long)ix * temp;
if (temp == 0)
break; /* Avoid ix / 0 */
if (ix / temp != prev) {
@@ -764,7 +770,7 @@
iw >>= 1; /* Shift exponent down by 1 bit */
if (iw==0) break;
prev = temp;
- temp *= temp; /* Square the value of temp */
+ temp = (unsigned long)temp * temp; /* Square the value of temp */
if (prev != 0 && temp / prev != prev) {
return PyLong_Type.tp_as_number->nb_power(
(PyObject *)v, (PyObject *)w, (PyObject *)z);