In the test case below, the subtraction can safely be folded to zero because a is a restricted pointer (as is effectively also b), *a is read, and if (a == b) were true, the call g(b) couldn't also modify *a either via *(int*)b or by any other means; as a result, a == b must either be false or g(b) cannot modify *a.  Clang folds the subtraction but GCC does not.

$ cat c.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout c.c
void g (void *);

int f (int * restrict a, void * /* restrict */ b)
{
  int t = *a;

  g (b);

  return *a - t;   // can be folded to zero
}

;; Function f (f, funcdef_no=0, decl_uid=1933, cgraph_uid=1, symbol_order=0)

f (int * restrict a, void * restrict b)
{
  int t;
  int _1;
  int _7;

  <bb 2> [local count: 1073741824]:
  t_4 = *a_3(D);
  g (b_5(D));
  _1 = *a_3(D);
  _7 = _1 - t_4;
  return _7;

}