int *x;
  int *y;
  // ...
  *x = 5;

Because of aliasing, x and y might point to the same memory location, so the compiler must assume that when you modified x to 5, y also potentially got modifed, so at next access it will read y again from memory and discard any cached value in a register it might have.

The compiler will try proving that no such aliasing is present, by tracking your pointer usage, but it's not always possible, in those cases it will assume the worse and re-read values from memory.