What are the conditions for tail recursion optimization of C++ functions?

The conditions for tail recursive optimization (TCO) in C are as follows: the tail recursive call must be the last action of the function. A function's parameters and local variables must remain unchanged across tail-recursive calls. The compiler must support TCO. In a practical case, TCO is used to convert the tail recursive call of the factorial calculation function into a while loop, which improves performance.

Conditions for tail recursive optimization of C functions

Tail recursive optimization (TCO) is a compiler optimization technology that will Tail-recursive function calls are converted into jump instructions, thus avoiding the additional overhead of the function call stack.

In order for the tail recursive call of a function to be optimized by the compiler, the following conditions need to be met:

• The tail recursive call must be the last action of the function. For example, the following function can be tail-recursive optimized:

int factorial(int n) {
  if (n <= 1) {
    return 1;
  } else {
    return n * factorial(n - 1);  // 尾递归调用
  }
}

• The parameters and local variables of the function must remain unchanged across tail-recursive calls. For example, the following function cannot be tail-recursive optimized:

int sum(int n) {
  int result = 0;
  if (n > 0) {
    result += n;  // 局部变量 result 在尾递归调用中发生变化
    return sum(n - 1);
  } else {
    return result;
  }
}

• The compiler must support TCO. Most modern C compilers, such as Clang and GCC, support TCO. However, please note that not all compilers support TCO.

Practical case

Consider the following function, which uses recursion to calculate the factorial:

int factorial(int n) {
  if (n == 0) {
    return 1;
  } else {
    return n * factorial(n - 1);
  }
}

This function satisfies all conditions for tail-recursive optimization. We can use TCO to optimize this function and improve its performance.

int factorial(int n) {
  while (n > 0) {
    n = n * factorial(n - 1);  // 转换为迭代
  }
  return 1;
}

After using TCO, the tail recursive call of the function is converted into a while loop. This eliminates the overhead of function calls and improves performance.

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