2023-10-11  Jakub Jelinek  <jakub@redhat.com>

	* wide-int.h (widest_int_storage <N>::write_val): If l is small
	and there is space in u.val array, store a canary value at the
	end when checking.
	(widest_int_storage <N>::set_len): Check the canary hasn't been
	overwritten.

--- gcc/wide-int.h.jj	2023-10-11 12:05:47.718059477 +0200
+++ gcc/wide-int.h	2023-10-11 13:51:56.081552500 +0200
@@ -1635,6 +1635,8 @@ widest_int_storage <N>::write_val (unsig
       u.valp = XNEWVEC (HOST_WIDE_INT, l);
       return u.valp;
     }
+  else if (CHECKING_P && l < WIDE_INT_MAX_INL_ELTS)
+    u.val[l] = HOST_WIDE_INT_UC (0xbaaaaaaddeadbeef);
   return u.val;
 }
 
@@ -1650,6 +1652,9 @@ widest_int_storage <N>::set_len (unsigne
       memcpy (u.val, valp, l * sizeof (u.val[0]));
       XDELETEVEC (valp);
     }
+  else if (len && len < WIDE_INT_MAX_INL_ELTS)
+    gcc_checking_assert ((unsigned HOST_WIDE_INT) u.val[len]
+			 == HOST_WIDE_INT_UC (0xbaaaaaaddeadbeef));
   len = l;
   /* There are no excess bits in val[len - 1].  */
   STATIC_ASSERT (N % HOST_BITS_PER_WIDE_INT == 0);