stdatomic.h function on plain int - yields undefined or implementation defined behaviour?

With GCC something like this compiles and probably works like expected:

#include <stdatomic.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    (void)argc;

    int x   = 23;
    // XXX
    // instead of:
    // _Atomic int x = 23;

    int exp = atoi(argv[1]);
    int y   = atoi(argv[2]);

    printf("x = %d, exp = %d, y = %d\n", x, exp, y);

    bool b = atomic_compare_exchange_strong(&x, &exp, y);

    printf("x = %d, exp = %d, y = %d => %s\n", x, exp, y, b ? "true" : "false");
    return 0;
}

The C reference says about such atomic functions:

This is a generic function defined for all atomic object types A. The argument is pointer to a volatile atomic type to accept addresses of both non-volatile and volatile (e.g. memory-mapped I/O) atomic variables. C is the non-atomic type corresponding to A.

This implies that the atomic functions like atomic_compare_exchange_strong() are not defined for non-atomic types.

Since int is a non-atomic type does the above code yields undefined behavior?

Or does the C standard specifies this as implementation defined behavior?

(where an implementation is able to define int as atomic type, as well - thus basically making it equivalent to _Atomic int)