What does this assembly instruction do?

Question

push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x18(%rbp)
mov    %rsi,-0x20(%rbp)
mov    -0x18(%rbp),%rax
mov    (%rax),%eax
mov    %eax,-0x4(%rbp)
mov    -0x20(%rbp),%rax
mov    (%rax),%edx
mov    -0x18(%rbp),%rax
mov    %edx,(%rax)
mov    -0x20(%rbp),%rax
mov    -0x4(%rbp),%edx
mov    %edx,(%rax)
mov    -0x18(%rbp),%rax
mov    (%rax),%edx
mov    -0x20(%rbp),%rax
mov    (%rax),%eax
add    %edx,%eax
pop    %rbp
retq  

I am just looking for someone to confirm my thinking, what I am seeing happen is that parameter 1 is taken and put 18 below rbp and parameter two is taken and put 20 below rbp and then it seems to me that the parameters are set to each other as in x=y and y=x however at the very end the second parameter is set to rax and then added to edx which i believe is the first parameter and then returned. Is this correct or am i way off?

Answer 1

Yes, that seems correct. The equivalent C code would look something like:

int func(int *arg1, int *arg2)
{
    int temp = *arg1;
    *arg1 = *arg2;
    *arg2 = temp;
    return *arg1 + *arg2;
}

The use of stack-based storage indexed off of rbp is what we called local storage. We can give each use a name to make it easier to see what is going on. Let's call the value at rbp-0x18 local_arg1, rbp-0x20 local_arg2 and rbp-0x4 local_temp. By calling convention, rdi is the first argument to the function and rsi is the second.

Adding comments where pointer dereferencing is occurring, the disassembly is then

push   %rbp
mov    %rsp,%rbp         
mov    %rdi,local_arg1
mov    %rsi,local_arg2
mov    local_arg1,%rax
mov    (%rax),%eax        ; dereference the pointer i.e. eax = *arg1
mov    %eax,local_temp
mov    local_arg2,%rax
mov    (%rax),%edx        ; edx = *arg2
mov    local_arg1,%rax
mov    %edx,(%rax)        ; *arg1 = edx
mov    local_arg2,%rax
mov    local_temp,%edx
mov    %edx,(%rax)        ; *arg2 = edx
mov    local_arg1,%rax
mov    (%rax),%edx        ; edx = *arg1
mov    local_arg2,%rax
mov    (%rax),%eax        ; eax = *arg2
add    %edx,%eax
pop    %rbp
retq