Recursive implementation of C++ functions: how to optimize in different compilers?

Optimization methods for recursion in C include: Tail call optimization (TCO): Replace recursive calls with loops to eliminate the risk of stack overflow, supported in GCC and Clang compilers. Tail Recursion Elimination (TRE): Completely eliminates all recursive calls and replaces them with loops, suitable for languages ​​or compilers that do not support TCO, such as in MSVC.

Recursive implementation of C functions: how to optimize in different compilers

Recursion is a method that allows a function to call itself method, which enables concise code and efficient algorithms. However, if used incorrectly, recursion can cause performance issues, particularly stack overflows and slow execution.

In order to optimize the performance of recursive functions, the following methods can be used:

• Tail call optimization (TCO): Tail call means that the function is outside of itself There are no calls to any other language names. TCO allows the compiler to replace recursive calls with loops, thereby eliminating the risk of stack overflows and improving performance.
• Tail Recursion Elimination (TRE): TRE is a more radical technique that completely eliminates all recursive calls and replaces them with loops. TRE is suitable for languages ​​or compilers that do not have tail call semantics.

Implementing TCO and TRE in C

In C, the implementation of TCO and TRE varies from compiler to compiler. Here are examples of implementing these optimizations in different compilers:

GCC and Clang

The GCC and Clang compilers support TCO. To enable TCO, optimization level -O2 or higher is required.

// GCC 和 Clang 中的尾调用递归

#include <iostream>

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

int main() {
  std::cout << factorial(5) << std::endl;
  return 0;
}

MSVC

The MSVC compiler does not support TCO. To optimize recursive functions, you can use TRE. To enable TRE, optimization level /O2 or higher is required.

// MSVC 中的尾递归消除

#include <iostream>

int factorial(int n) {
  int result = 1;
  while (n > 0) {
    result *= n;
    n--;
  }
  return result;
}

int main() {
  std::cout << factorial(5) << std::endl;
  return 0;
}

Practical case

Consider a function that needs to calculate the Fibonacci sequence. The Fibonacci Sequence is a recursively defined sequence in which each number is the sum of the previous two numbers.

The following is a C function optimized with TRE to calculate Fibonacci numbers:

// TRE 优化的斐波那契数计算

int fib(int n) {
  if (n == 0)
    return 0;
  if (n == 1)
    return 1;

  int a = 0, b = 1, c;
  while (n > 1) {
    c = a + b;
    a = b;
    b = c;
    n--;
  }
  return b;
}

By applying TRE, the performance of this function has been significantly improved, eliminating the risk of stack overflow and shortening the time the execution time.

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