What are the code refactoring techniques in C++ function performance optimization?

Code refactoring is a key step in improving the performance of C functions and involves changing the structure and organization of the code without changing functionality. Specific techniques include: 1. Inlining functions to eliminate function call overhead; 2. Eliminating virtual function calls to eliminate virtual table lookup overhead; 3. Loop unrolling to reduce branch prediction overhead; 4. Pointer elimination to reduce indirect memory access overhead. Practical examples show the process of applying these techniques to optimize function performance.

Code refactoring skills in C function performance optimization

Code refactoring is one of the key steps to improve function performance. It involves changing the structure and organization of your code without changing its functionality. Here are several code refactoring techniques for C function performance optimization:

1. Inline functions:

Inline functions expand the function body at the call point to eliminate the overhead of function calls. This is especially useful for small functions that are called frequently.

// 原始代码
void foo(int a, int b) {
  return a + b;
}

// 内联后
inline int foo(int a, int b) {
  return a + b;
}

2. Eliminate virtual function calls:

Virtual function calls will generate virtual table lookup overhead. This overhead can be eliminated by replacing virtual functions with static methods or inline functions.

// 原始代码
class Base {
 public:
  virtual int foo();
};

class Derived : public Base {
 public:
  int foo() {
    return 42;
  }
};

// 重构后
class Base {
 public:
  int foo() {
    return 42;
  }
};

class Derived : public Base {
  // 现在 foo() 是静态方法
  static int foo() {
    return 42;
  }
};

3. Loop unrolling:

Loop unrolling copies the loop code into itself to reduce branch prediction overhead. This is especially useful for compact loops with a fixed number of iterations.

// 原始代码
for (int i = 0; i < 100; i++) {
  a[i] += 1;
}

// 展开后
for (int i = 0; i < 100; i++) {
  a[i] += 1;
  a[i] += 1;
  a[i] += 1;
  a[i] += 1;
  /* ... */
}

4. Pointer elimination:

The use of pointers will generate indirect memory access overhead. This overhead can be reduced by eliminating pointers using references or smart pointers.

// 原始代码
int* foo() {
  return &a;
}

// 重构后
int& foo() {
  return a;
}

Practical case:

The following is a practical case of C function performance optimization code reconstruction:

Original function:

int factorial(int n) {
  if (n <= 0) {
    throw invalid_argument("n must be positive");
  }

  int result = 1;
  for (int i = 1; i <= n; i++) {
    result *= i;
  }

  return result;
}

After reconstruction:

inline int factorial(int n) {
  if (n <= 0) {
    return 1;  // 改为返回 1 以处理 n == 0 的情况
  }

  return n * factorial(n - 1);  // 使用尾递归优化
}

The reconstructed function eliminates the overhead of virtual function calls, uses tail recursion optimization, and eliminates repeated multiplication operations. This significantly improves function performance.

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