Detailed explanation of C++ function optimization: optimization principles and common optimization techniques

Optimizing C functions follows the principles: optimize critical path code, focus on hot functions, and balance performance and readability. Common optimization techniques include: inlining functions to eliminate function call overhead; reducing indirect calls to improve direct access speed; optimizing loops to improve efficiency; virtual function rewriting to prevent indirect calls; using object pools to avoid frequent memory allocation.

Detailed explanation of C function optimization: optimization principles and common optimization techniques

Optimization principles

When optimizing C functions, follow the following principles:

• Optimize the critical path code in a timely manner: Focus on optimizing the code path with the longest execution time.
• Focus on hot functions: Optimize frequently called functions.
• Balance performance and code readability: When optimizing code, maintain its readability and avoid excessive complexity.
• Use performance analysis tools: Use performance analyzers (such as gprof, Valgrind) to identify performance bottlenecks.

Common optimization techniques

1. Inline functions

Insert the code of small functions directly into the call point , eliminating the overhead of function calls.

inline void Swap(int& a, int& b) {
  int temp = a;
  a = b;
  b = temp;
}

2. Reduce indirect calls

Access objects directly through pointers or references and avoid indirect calls through pointers.

struct Point {
  int x, y;
};

void MovePoint(const Point& point) {
  // 间接调用：
  point->x++;

  // 直接调用：
  // point.x++;  // 只在 C++11 以上的版本中可用
  (*point).x++;
}

3. Optimize loops

Use range for loops and manual loop unrolling to improve loop efficiency.

// 手动循环展开：
for (int i = 0; i < n; i++) {
  Array1[i] *= Factor;
  Array2[i] /= Factor;
}

// 范围 for 循环：
for (auto& e : Array1) {
  e *= Factor;
}
for (auto& e : Array2) {
  e /= Factor;
}

4. Virtual function overriding

If a derived class overrides a virtual function, the virtual function pointer of the base class will no longer point to the implementation of the derived class, thus Causes indirect calls. This indirection can be eliminated by using the final keyword to specify that a virtual function cannot be overridden.

class Base {
 public:
  virtual void Display() final;  // 不能被派生类重写
};

5. Object pool

For objects that are frequently created and destroyed, using an object pool can avoid frequent memory allocation and release operations.

class ObjectPool {
 public:
  std::vector<std::shared_ptr<Object>> objects;

  std::shared_ptr<Object> Acquire() {
    if (objects.empty()) {
      objects.push_back(std::make_shared<Object>());
    }
    auto object = objects.back();
    objects.pop_back();
    return object;
  }

  void Release(std::shared_ptr<Object>& object) {
    objects.push_back(object);
  }
};

Practical case

Consider the following example function:

int SumArray(const int* array, int size) {
  int sum = 0;
  for (int i = 0; i < size; i++) {
    sum += array[i];
  }
  return sum;
}

After optimization:

• Inline functions: Because the function body is smaller, it can be inlined to eliminate function call overhead.
• Range for loop: Using range for loop can improve loop efficiency.
• Virtual function elimination: If no derived class overrides the SumArray function, you can use the final keyword to eliminate virtual function indirect calls.

inline int SumArray(const int* array, int size) {
    int sum = 0;
    for (auto e : array) {
        sum += e;
    }
    return sum;
}

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