Why two identical pointers do not compare equal with -O1? [duplicate]

#include <stdio.h>

int main(void)
{
    int a, b;
    int *p = &a;

#ifdef __clang__
    int *q = &b + 1;
#elif __GNUC__
    int *q = &b - 1;
#endif

    printf("%p %p %d\n", (void *)p, (void *)q, p == q);
}

C11 § 6.5.9 \ 6 says that

Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.

I have tested it four different ways:

1. Clang 9.0.1 with -01 option;
2. Clang 9.0.1 without any options;
3. GCC 9.2.0 with -01 option;
4. GCC 9.2.9 without any options.

The results are the following:

$ ./prog_clang
0x7ffebf0a65d4 0x7ffebf0a65d4 1
$ ./prog_clang_01
0x7ffd9931b9bc 0x7ffd9931b9bc 1
$ ./prog_gcc
0x7ffea055a980 0x7ffea055a980 1
$ ./prog_gcc_01
0x7fffd5fa5490 0x7fffd5fa5490 0

What is the correct behavior in this case?