When developing software, it’s essential to optimize code for performance. Benchmarking is a valuable technique for evaluating the execution time of different code snippets. In this blog post, we’ll explore a technique called recursive templates that can be used for benchmarking template code in C++.

The Recursive Template Approach

The recursive template approach involves using templates and compile-time recursion to generate code variations with different parameter values. This technique allows us to test different scenarios without duplicating code manually. The idea is to have a base template that defines the logic and recursively generate template instantiations for different parameter values.

Let’s consider a simple example to better understand the recursive template approach. Suppose we have a template function called fibonacci that calculates the nth Fibonacci number. We want to benchmark this function for different input values.

template<int N>
struct Fibonacci {
    static constexpr int value = Fibonacci<N - 1>::value + Fibonacci<N - 2>::value;
};

template<>
struct Fibonacci<0> {
    static constexpr int value = 0;
};

template<>
struct Fibonacci<1> {
    static constexpr int value = 1;
};

In the above code snippet, we define a template class Fibonacci that calculates the nth Fibonacci number using compile-time recursion. We have specializations for the base cases where N is 0 or 1, and the generic case which recursively calculates the Fibonacci value.

To benchmark the fibonacci function for different values, we can define a new template class Benchmark that recursively generates instantiations of the Fibonacci class with different parameter values.

template<int N>
struct Benchmark {
    static void run() {
        constexpr int result = Fibonacci<N>::value;
        // Perform any necessary operations on `result`
        // e.g., print or store the result
        run<N - 1>();
    }
};

template<>
struct Benchmark<0> {
    static void run() {}
};

In the Benchmark class, we define a run() function that calculates the Fibonacci number for the current value of N using Fibonacci<N>::value. We can perform any necessary operations on the result inside this function. After that, we recursively invoke the run() function for the next value N - 1. We also provide a specialization for the base case where N is 0 to terminate the recursion.

Using Recursive Templates for Benchmarking

To use recursive templates for benchmarking, we can create an instantiation of the Benchmark class with the desired initial parameter value.

int main() {
    Benchmark<10>::run();
    
    return 0;
}

In the above code snippet, we create an instance of Benchmark<10> to benchmark the fibonacci function for N = 10. The recursive template approach will generate 10 instances of the Fibonacci class with N ranging from 0 to 10.

Conclusion

The recursive template approach provides a powerful technique for benchmarking template code in C++. By using compile-time recursion and template specialization, we can generate code variations for different parameter values without duplicating code manually. This approach allows us to easily benchmark and optimize template code. Consider using recursive templates in your projects when you need to perform benchmarking of template code for different scenarios.

#C++ #RecursiveTemplates #CodeBenchmarking