long unknown(unsigned long x) { if (___________) return _________; unsigned long nx = _____________________; long rv = unknown(x); return ___________________; } 0000000000000000 : unknown: testq %rdi, %rdi jne .L8 movl$0, %eax ret .L8: pushq %rbx movq %rdi, %rax xorq $1, %rax andl $1, %eax movq %rax, %rbx shrq %rdi call unknown addq %rbx, %rax popq %rbx ret 

What value does unknown() store in the calleed-saved register %rbx? Fill in the missing expressions in the C code shown above. Hint: "unknown 31" returns 0 while "unknown -31" returns 4.