xy.c:5:6: Expression has undefined behavior (value of left operand x is
             modified by right operand y ^= x ^= y): x ^= y ^= x ^= y
  Code has unspecified behavior. Order of evaluation of function parameters or
  subexpressions is not defined, so if a value is used and modified in
  different places not separated by a sequence point constraining evaluation
  order, then the result of the expression is unspecified. (Use -evalorder to
  inhibit warning)