Is this use of unions strictly conforming?

Question

Given the code:

struct s1 {unsigned short x;};
struct s2 {unsigned short x;};
union s1s2 { struct s1 v1; struct s2 v2; };

static int read_s1x(struct s1 *p) { re         


        

Answer 1

It is not about conforming or not conforming - it one of the optimisation "traps". All of your data structures have been optimised out and you pass the same pointer to optimised out data so the the execution tree is reduced to simple printf of the value.

  sub rsp, 8
  mov esi, 4321
  mov edi, OFFSET FLAT:.LC0
  xor eax, eax
  call printf
  xor eax, eax
  add rsp, 8
  ret

to change it you need to make this "transfer" function to be side effect prone and force the real assignments. It will force optimizer to not reduce those nodes in the execution tree:

int test(union s1s2 *p1, union s1s2 *p2, volatile union s1s2 *p3)
/* ....*/

main:
  sub rsp, 8
  mov esi, 1234
  mov edi, OFFSET FLAT:.LC0
  xor eax, eax
  call printf
  xor eax, eax
  add rsp, 8
  ret

it is quite trivial test just artificially made a bit more complicated.