asm 中 ADD 的奇怪行为

Question

The output of this code:

int i=0;
while(i<5)
    {
    asm volatile
        (
        "addl $1,%0"
        :"=r"(i)
        :
        :"memory"
        );
    printf("%d\n",i);
    }

is like this:

2
3
3
3
.
.
.

But it should be like this:

1
2
3
4
5

Why is that so? I can't seem to understand where the problem is.

Answer 1

You are specifying i as an output-only operand. That is, the compiler uses the output of the register %0 for the variable, but the current value if i is not copied to the register at the beginning.

Specify i as input as well, for the same register:

asm volatile
    (
    "addl $1,%0"
    :"=r"(i)
    :"0"(i)
    :"memory"
    );