print a newline using tail call optimization

Question

There's this unanswered question in the Igor Zhirkov's book Low-Level Programming :

"Try to rewrite print_newline without calling print_char or copying its code. Hint: read about tail call optimization.".

I've spent some times reading about tail call optimization, but I can't figure out what would be the proper way of doing it.

Original code (without tail-call): print_newline => print_char => print_string=>string_length=>print_string

string_length:
    xor rax, rax
.loop:
    cmp byte [rdi+rax], 0   ; if rdi =  0 (not an address), then seg fault
    je .end 
    inc rax
    jmp .loop 
.end:
    ret
print_string:
    push rdi
    call string_length
    pop rsi
    mov rdx, rax
    mov rax, 1
    mov rdi, 1
    syscall
    ret
print_char:
    push rdi
    mov rdi, rsp 
    call print_string 
    pop rdi
    ret
print_newline:
    mov rdi, 10
    jmp print_char

What could be a tail-call optimization of that ?

Answer 1

For the sake of reference I will provide the solution in this thread too.

1. Without optimizations

print_newline:
    mov rdi, 10
    call print_char
    ret
print_char:
    push rdi
    mov rdi, rsp 
    call print_string 
    pop rdi
    ret

2. A tail call optimization. A sequence of instructions call X and ret can always be substituted by jmp X.

print_newline:
    mov rdi, 10
    jmp print_char
print_char:
    push rdi
    mov rdi, rsp 
    call print_string 
    pop rdi
    ret

3. Just falling into print_char; basically making a subroutine with two entry points.

print_newline:
    mov rdi, 10
print_char:
    push rdi
    mov rdi, rsp 
    call print_string 
    pop rdi
    ret

Here is a more in-depth explanation in my blog