#endif
#ifdef builtin_ilog32_nz
-#define ilog32(_v) (builtin_ilog32_nz(_v)&-!!(_v))
+/* This used to be builtin_ilog32_nz(_v)&-!!(_v), which means it zeroes out
+ * the undefined builtin_ilog32_nz(0) return. But clang UndefinedBehaviorSantizer
+ * complains, so do the branch: */
+#define ilog32(_v) ((_v) ? builtin_ilog32_nz(_v) : 0)
#define ilog32_nz(_v) builtin_ilog32_nz(_v)
#else
#define ilog32_nz(_v) ilog32(_v)
#endif /* builtin_ilog32_nz */
#ifdef builtin_ilog64_nz
-#define ilog64(_v) (builtin_ilog64_nz(_v)&-!!(_v))
+#define ilog32(_v) ((_v) ? builtin_ilog32_nz(_v) : 0)
#define ilog64_nz(_v) builtin_ilog64_nz(_v)
#else
#define ilog64_nz(_v) ilog64(_v)