Assembly language to C code?

Question

I'm trying to figure out what this block of assembly code is doing:

mystery:
.LFB0
    testl   %edi, %edi
    je      .L3
    subq    $8, %rsp
    shrl    $2, %edi
    call mystery
    addq    $1, %rax
    jmp     .L2
.L3:
    movl    $l, %eax
    ret
.L2:
    addq    $8, %rsp
    ret

So far I have this as the C code:

long mystery(unsigned n){
    if(n==0)
       return 1;

And I get that there is a recursive function going on here while n!=0 but I don't understand what the %rsp register is doing

Answer 1

Here's my guess:

#include <inttypes.h>

int64_t mystery(int32_t n) {
    if (n == 0) {
        return 1;
    }

    return 1 + mystery(n / 4);
}

I don't know what the $l is in the line:

movl    $l, %eax

so my above solution assumes it's a typo for $1 . Enlighten me.