/* Approximate version when we don't have long arithmetic (on 64-bit archs) */
-#define HALF_POS_INT (((I_)1) << (BITS_IN (I_) / 2))
+/* If we have n-bit words then we have n-1 bits after accounting for the
+ * sign bit, so we can fit the result of multiplying 2 (n-1)/2-bit numbers */
+#define HALF_POS_INT (((I_)1) << ((BITS_IN (I_) - 1) / 2))
#define HALF_NEG_INT (-HALF_POS_INT)
#define mulIntMayOflo(a,b) \