Issues when using extended asm in C on Mac M1

Question

I'm trying to use extended assembly on MacOS (Apple M1). I run into a very un-understandable issue, here is my code :

#include <stdio.h>

int main(void)
{
    int a = 1;
    int b = 0;

    printf("%d\n", a);
    printf("%d\n", b);

    asm (
        " mov x1, x1"
        : "=r" (b)
        : "r" (a));

    printf("%d\n", a);
    printf("%d\n", b);

    return 0;
}

Here is the result of it :

❯ make && ./hello_world_asm
gcc    -c -o hello_world_asm.o hello_world_asm.c
gcc -Wall hello_world_asm.c -o hello_world_asm
1
0
1
1

How is that possible? I really don't get it. I don't even use the registers bounded to a and b but still, b is taking a's value.

I tried to change the inner assembly, if I don't directly put any value in the %0 (b bounded register), it'll just take a's.