Rust assembly of Option<i32>

Question

This code compiled with -C opt-level=3 in https://gcc.godbolt.org/z/jqEjasPTW

#[no_mangle]
pub fn match1(num: i32) -> i32 {
   if num == 10 {99} else {11}
}

#[no_mangle]
pub fn match2(num: Option<i32>) -> i32 {
    match num {
        Some(a) => a,
        _ => 11
    }
}
 

produces this assembly code:

match1:
        cmp     edi, 10
        mov     ecx, 99
        mov     eax, 11
        cmove   eax, ecx
        ret

match2:
        cmp     edi, 1
        mov     eax, 11
        cmove   eax, esi
        ret

why in match2 compares the register edi with 1 ?

Answer 1

No assembly expert here but I guess this is what's happening there (in pseudo-code):

var eax = 11; // assume it's None

if (edi == 1) { // it's Some
  eax = esi; // esi contains the "content" of Some, that is `a`
}

return eax;
  

So the comparison of edi with 1 is basically done to check if it's Some. Of course edi and esi are the function parameters that contain, respectively, a value indicating if it's Some and the value of Some.