Comment by crote

The problem is that the function call as a whole is UB. Having the original example compile to the equivalent of

  volatile int x;
  int a = x;
  int b = x;
  printf("%x %d\n", a, b);

is equally valid as

  volatile int x;
  int a = x;
  int b = x;
  printf("%x %d\n", b, a);

, and neither needs to have the same output as your proposed fix.

C could've specified something like "arguments are evaluated left-to-right" or "if two arguments have the same expression, the expression is [only evaluated once]/[always evaluated twice]". But it didn't, so the developer is left gingerly navigating a minefield every time they use volatile.