How Do I Use Labels In GCC Inline Assembly?

Question

I\'m trying to learn x86-64 inline assembly and decided to implement this very simple swap method that simply orders a and b in ascending order:

Answer 1

You cannot just put a bunch of asm statements inline like that. The optimizer is free to re-order, duplicate, and drop them based on what constraints it knows. (In your case, it knows none.)

So firstly, you should consolidate the asm together, with proper read/write/clobber constraints. Secondly, there is a special asm goto form that gives assembly to C-level labels.

void swap(int *a, int *b) {
    int tmp1, tmp2;
    asm(
        "mov (%2), %0\n"
        "mov (%3), %1\n"
        : "=r" (tmp1), "=r" (tmp2)
        : "r" (a), "r" (b)
        : "memory"   // pointer in register doesn't imply that the pointed-to memory has to be "in sync"
        // or use "m" memory source operands to let the compiler pick the addressing mode
    );
    asm goto(
        "cmp %1, %0\n"
        "jle %l4\n"
        "mov %1, (%2)\n"
        "mov %0, (%3)\n"
        :
        : "r" (tmp1), "r" (tmp2), "r" (a), "r" (b)
        : "cc", "memory"
        : L1
    );
L1:
    return;
}