How to optimize function performance in C++?

Optimizing function performance in C includes: 1. Inline functions: directly inserted into the calling position to eliminate function calling overhead. 2. Avoid dynamic memory allocation: allocate and reuse memory buffers in advance to reduce allocation and release operations. 3. Use constant references: Ensure that the object cannot be modified, allowing the compiler to perform more optimizations. 4. Unroll the loop: Copy the loop body statement to the outside to eliminate branch prediction overhead. In practice, by applying these techniques, the speed of the array sum function is significantly improved.

Optimized Function Performance in C

In C programming, function performance is critical to the overall efficiency of the application . By applying specific optimization techniques, we can significantly increase the execution speed of a function without affecting functionality.

Inline function

Inline function is a function that is inserted directly into the calling location by the compiler. This eliminates the overhead of function calls, including stack frame allocation and return address handling. Inlining is suitable for functions that have relatively low execution overhead, such as getter and setter methods.

inline int sum(int a, int b) {
  return a + b;
}

Avoid dynamic memory allocation

Dynamic memory allocation is a relatively expensive operation. By allocating and reusing memory buffers ahead of time, we can reduce unnecessary allocation and deallocation operations.

// 在栈上分配数组
int arr[100];

// 在堆上动态分配数组
int* arr = new int[100]; // 避免频繁分配和释放

Using constant references

Constant references provide references to constant objects, ensuring that the objects pointed to by the references will not be modified. This allows the compiler to perform more optimizations such as constant propagation and inlining.

void foo(const int& x) {
  // 由于 x 是常量引用，编译器可以内联函数
}

Unrolling a loop

Loop unrolling is a technique that copies some or all of the statements in the loop body to the outside of the loop. This eliminates branch prediction overhead, thereby improving loop performance.

// 未展开的循环
for (int i = 0; i < 100; i++) {
  a[i] = b[i] + c[i];
}

// 展开循环
for (int i = 0; i < 100; i += 4) {
  a[i] = b[i] + c[i];
  a[i+1] = b[i+1] + c[i+1];
  a[i+2] = b[i+2] + c[i+2];
  a[i+3] = b[i+3] + c[i+3];
}

Practical case

The following is an example function for summing arrays. By applying the above optimization techniques, we can significantly improve its performance:

// 未优化的函数
int sum(int* arr, int size) {
  int sum = 0;
  for (int i = 0; i < size; i++) {
    sum += arr[i];
  }
  return sum;
}

// 优化后的函数
inline int sum_optimized(const int* arr, int size) {
  int sum = 0;
  int block_size = 8; // 展开循环的块大小
  for (int i = 0; i < size - block_size + 1; i += block_size) {
    sum += arr[i] + arr[i+1] + arr[i+2] + arr[i+3];
    sum += arr[i+4] + arr[i+5] + arr[i+6] + arr[i+7];
  }
  for (int i = size - block_size + 1; i < size; i++) {
    sum += arr[i];
  }
  return sum;
}

By optimizing the function, we can significantly increase the speed of the sum operation. The performance of any function in C code can be improved by careful consideration and application of appropriate techniques.

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