expensive_branching.cpp

/* Naive (conditional) and optimized (branchless) absolute value */

#include <ctime>
#include <iostream>
#include <functional>

using namespace std;

int abs_naive(const int i) {
	return i < 0 ? -i : i;
}

int abs_opt(const int i) {
	int value = i;
	int temp = i >> 31;
	value ^= temp;
	value += temp & 1;
	return value;
}

double time_it(const long num_iter, const std::function<int(int)> f, const int num_to_eval = -101) {
	
    volatile int result = 0;
    clock_t begin1 = clock();
    
    for (long i = 0; i < num_iter; i++) {
    	result = f(num_to_eval);
    }

    clock_t end1 = clock();
    double elapsed_secs1 = double(end1 - begin1) / CLOCKS_PER_SEC;
    return elapsed_secs1;
}

int main() {

	const long numiter = 100000000;

	cout << "\nTiming in seconds for " << numiter << " iterations" << endl << endl;

	std::cout << "Naive: " << time_it(numiter, abs_naive) << "\n";

	std::cout << "Optimized: " << time_it(numiter, abs_opt) << "\n\n";

	std::cout << "abs_naive(-101) = " << abs_naive(-101) << "\n";
	std::cout << "abs_naive(101) = " << abs_naive(101) << "\n";
	std::cout << "abs_opt(-101) = " << abs_opt(-101) << "\n";
	std::cout << "abs_opt(101) = " << abs_opt(101) << "\n\n";

}

Compile with

g++ abs.cpp -O3 --std=c++11 -o abs && ./abs

Output:

Timing in seconds for 100000000 iterations

Naive: 0.283806
Optimized: 0.23181

abs_naive(-101) = 101
abs_naive(101) = 101
abs_opt(-101) = 101
abs_opt(101) = 101