首先请注意，并非所有语言都支持它。

TCO适用于递归的特殊情况。它的要点是，如果你在函数中做的最后一件事是调用自己(例如，它从“尾部”位置调用自己)，这可以被编译器优化为像迭代一样而不是标准的递归。

You see, normally during recursion, the runtime needs to keep track of all the recursive calls, so that when one returns it can resume at the previous call and so on. (Try manually writing out the result of a recursive call to get a visual idea of how this works.) Keeping track of all the calls takes up space, which gets significant when the function calls itself a lot. But with TCO, it can just say "go back to the beginning, only this time change the parameter values to these new ones." It can do that because nothing after the recursive call refers to those values.

看这里:

http://tratt.net/laurie/tech_articles/articles/tail_call_optimization

你可能知道，递归函数调用会对堆栈造成严重破坏;堆栈空间很容易很快用完。尾部调用优化是一种方法，通过它你可以创建一个使用常量堆栈空间的递归式算法，因此它不会不断增长，你会得到堆栈错误。

尾部调用优化可以避免为函数分配新的堆栈框架，因为调用函数将简单地返回从被调用函数获得的值。最常见的用法是尾部递归，其中为利用尾部调用优化而编写的递归函数可以使用常量堆栈空间。

Scheme是少数几种在规范中保证任何实现都必须提供这种优化的编程语言之一，因此这里有Scheme中的阶乘函数的两个示例:

(define (fact x)
  (if (= x 0) 1
      (* x (fact (- x 1)))))

(define (fact x)
  (define (fact-tail x accum)
    (if (= x 0) accum
        (fact-tail (- x 1) (* x accum))))
  (fact-tail x 1))

第一个函数不是尾部递归的，因为当进行递归调用时，函数需要跟踪调用返回后需要与结果进行的乘法运算。因此，堆栈看起来如下所示:

(fact 3)
(* 3 (fact 2))
(* 3 (* 2 (fact 1)))
(* 3 (* 2 (* 1 (fact 0))))
(* 3 (* 2 (* 1 1)))
(* 3 (* 2 1))
(* 3 2)
6

相反，尾部递归阶乘的堆栈跟踪如下所示:

(fact 3)
(fact-tail 3 1)
(fact-tail 2 3)
(fact-tail 1 6)
(fact-tail 0 6)
6

正如您所看到的，对于每次调用fact-tail，我们只需要跟踪相同数量的数据，因为我们只是返回我们直接到达顶部的值。这意味着即使我要调用(事实1000000)，我只需要与(事实3)相同的空间。对于非尾部递归事实，情况并非如此，如此大的值可能会导致堆栈溢出。

首先请注意，并非所有语言都支持它。

TCO适用于递归的特殊情况。它的要点是，如果你在函数中做的最后一件事是调用自己(例如，它从“尾部”位置调用自己)，这可以被编译器优化为像迭代一样而不是标准的递归。

You see, normally during recursion, the runtime needs to keep track of all the recursive calls, so that when one returns it can resume at the previous call and so on. (Try manually writing out the result of a recursive call to get a visual idea of how this works.) Keeping track of all the calls takes up space, which gets significant when the function calls itself a lot. But with TCO, it can just say "go back to the beginning, only this time change the parameter values to these new ones." It can do that because nothing after the recursive call refers to those values.

GCC C最小可运行示例与x86拆装分析

让我们看看GCC如何通过查看生成的程序集自动执行尾部调用优化。

这将作为一个非常具体的例子，说明其他答案(如https://stackoverflow.com/a/9814654/895245)中提到的优化可以将递归函数调用转换为循环。

这反过来又节省了内存并提高了性能，因为内存访问通常是现在使程序变慢的主要原因。

作为输入，我们给GCC一个基于阶乘的非优化朴素堆栈:

tail_call.c

#include <stdio.h>
#include <stdlib.h>

unsigned factorial(unsigned n) {
    if (n == 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

int main(int argc, char **argv) {
    int input;
    if (argc > 1) {
        input = strtoul(argv[1], NULL, 0);
    } else {
        input = 5;
    }
    printf("%u\n", factorial(input));
    return EXIT_SUCCESS;
}

GitHub上游。

编译和分解:

gcc -O1 -foptimize-sibling-calls -ggdb3 -std=c99 -Wall -Wextra -Wpedantic \
  -o tail_call.out tail_call.c
objdump -d tail_call.out

其中-foptimize-sibling-calls是根据man gcc泛化尾部调用的名称:

   -foptimize-sibling-calls
       Optimize sibling and tail recursive calls.

       Enabled at levels -O2, -O3, -Os.

如上所述:我如何检查gcc是否执行尾递归优化?

我选择-O1是因为:

优化不是用-O0完成的。我怀疑这是因为缺少必要的中间转换。 -O3产生了非常高效的代码，虽然它也是尾部调用优化。

使用-fno-optimize-sibling-calls进行反汇编:

0000000000001145 <factorial>:
    1145:       89 f8                   mov    %edi,%eax
    1147:       83 ff 01                cmp    $0x1,%edi
    114a:       74 10                   je     115c <factorial+0x17>
    114c:       53                      push   %rbx
    114d:       89 fb                   mov    %edi,%ebx
    114f:       8d 7f ff                lea    -0x1(%rdi),%edi
    1152:       e8 ee ff ff ff          callq  1145 <factorial>
    1157:       0f af c3                imul   %ebx,%eax
    115a:       5b                      pop    %rbx
    115b:       c3                      retq
    115c:       c3                      retq

-foptimize-sibling-calls:

0000000000001145 <factorial>:
    1145:       b8 01 00 00 00          mov    $0x1,%eax
    114a:       83 ff 01                cmp    $0x1,%edi
    114d:       74 0e                   je     115d <factorial+0x18>
    114f:       8d 57 ff                lea    -0x1(%rdi),%edx
    1152:       0f af c7                imul   %edi,%eax
    1155:       89 d7                   mov    %edx,%edi
    1157:       83 fa 01                cmp    $0x1,%edx
    115a:       75 f3                   jne    114f <factorial+0xa>
    115c:       c3                      retq
    115d:       89 f8                   mov    %edi,%eax
    115f:       c3                      retq

两者之间的关键区别在于:

the -fno-optimize-sibling-calls uses callq, which is the typical non-optimized function call. This instruction pushes the return address to the stack, therefore increasing it. Furthermore, this version also does push %rbx, which pushes %rbx to the stack. GCC does this because it stores edi, which is the first function argument (n) into ebx, then calls factorial. GCC needs to do this because it is preparing for another call to factorial, which will use the new edi == n-1. It chooses ebx because this register is callee-saved: What registers are preserved through a linux x86-64 function call so the subcall to factorial won't change it and lose n. the -foptimize-sibling-calls does not use any instructions that push to the stack: it only does goto jumps within factorial with the instructions je and jne. Therefore, this version is equivalent to a while loop, without any function calls. Stack usage is constant.

在Ubuntu 18.10, GCC 8.2中测试。

TCO (Tail Call Optimization) is the process by which a smart compiler can make a call to a function and take no additional stack space. The only situation in which this happens is if the last instruction executed in a function f is a call to a function g (Note: g can be f). The key here is that f no longer needs stack space - it simply calls g and then returns whatever g would return. In this case the optimization can be made that g just runs and returns whatever value it would have to the thing that called f.

这种优化可以使递归调用占用恒定的堆栈空间，而不是爆炸。

示例:这个阶乘函数不是TCOptimizable:

from dis import dis

def fact(n):
    if n == 0:
        return 1
    return n * fact(n-1)


dis(fact)
  2           0 LOAD_FAST                0 (n)
              2 LOAD_CONST               1 (0)
              4 COMPARE_OP               2 (==)
              6 POP_JUMP_IF_FALSE       12

  3           8 LOAD_CONST               2 (1)
             10 RETURN_VALUE

  4     >>   12 LOAD_FAST                0 (n)
             14 LOAD_GLOBAL              0 (fact)
             16 LOAD_FAST                0 (n)
             18 LOAD_CONST               2 (1)
             20 BINARY_SUBTRACT
             22 CALL_FUNCTION            1
             24 BINARY_MULTIPLY
             26 RETURN_VALUE

这个函数除了在它的return语句中调用另一个函数之外还做其他事情。

下面这个函数是TCOptimizable:

def fact_h(n, acc):
    if n == 0:
        return acc
    return fact_h(n-1, acc*n)

def fact(n):
    return fact_h(n, 1)


dis(fact)
  2           0 LOAD_GLOBAL              0 (fact_h)
              2 LOAD_FAST                0 (n)
              4 LOAD_CONST               1 (1)
              6 CALL_FUNCTION            2
              8 RETURN_VALUE

这是因为在这些函数中最后发生的事情是调用另一个函数。