Specifying -xalias without a list assumes that your program does not violate the Fortran aliasing rules. It is equivalent to asserting no% for all the aliasing keywords.
The compiler default, when compiling without specifying -xalias, is:
-xalias=no%dummy,craypointer,no%actual,no%overindex,no%ftnpointer
If your program uses Cray pointers but conforms to the Fortran aliasing rules whereby the pointer references cannot result in aliasing, even in ambiguous situations, compiling with -xalias may result in generating better optimized code.