使用尾调用优化打印换行符 [英] print a newline using tail call optimization

问题描述

Igor Zhirkov 的著作 Low-Level Programming 中有这个未解决的问题:

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

"尝试在不调用print_char或复制其代码的情况下重写print_newline.提示:阅读关于尾调用优化.".

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

我花了一些时间阅读有关尾调用优化的文章，但我不知道正确的做法是什么.

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.

原始代码(无尾调用):print_newline =>print_char =>print_string=>string_length=>print_string

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

什么是尾调用优化?

推荐答案

为了参考，我也会在这个线程中提供解决方案.

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

1. 没有优化

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

1. 尾调用优化.call X 和 ret 指令序列总是可以被 jmp X 替换.

1. 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

1. 刚刚陷入print_char；基本上是创建一个带有两个入口点的子程序.

1. 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

这里有更深入的解释在我的博客

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