Why is this C++ program slower than Node.js equivalent? - c++

Question

I'm learning C++ and decided to remake an old Node.js program to see how much faster it would be, as C++ should to my knowledge be a lot faster due to being compiled.
This program is very simple, it is just to find prime numbers. It uses the exact same logic as my Node.js program, but it takes 8 to 9 seconds whereas the Node.js took only 4 to 5 seconds.
#include <iostream>
#include <string>
#include <ctime>
using namespace std;
// Declare functions
int main();
bool isPrime(int num);
bool is6n_1(int num);
// Define variables
int currentNum = 5; // Start at 5 so we iterate over odd numbers, we add 2 and 3 manually
int primesToFind = 1000000;
int primesFound = 2;
int* primes = NULL;
// Main
int main() {
// Create dynamic memory primes array
primes = new int[1000000];
primes[0] = 2;
primes[1] = 3;
cout << "Finding primes..." << endl;
time_t start_time = time(NULL);
// Main execution loop
for (; primesFound < primesToFind; currentNum += 2) {
if (isPrime(currentNum)) {
primes[primesFound] = currentNum;
primesFound++;
}
}
time_t end_time = time(NULL);
cout << "Finished" << endl;
cout << end_time - start_time << endl;
return 0;
}
// Check whether a number is prime
// Dependant on primes[]
bool isPrime(int num) {
// We divide it by every previous prime number smaller than the sqrt
// and check the remainder
for (int i = 1; i <= sqrt(num) && i < primesFound; i++) { // Start i at 1 to skip the first unnecessary modulo with 2
if (num % primes[i] == 0) { // because we increment by 2
return false;
}
}
return true;
}
Because I'm so new at C++ I don't know if this is due to inefficient code (probably) or because of some settings in the compiler or in Visual Studio IDE.
I'm using Visual Studio 2019 community, release version and x64 architecture with O2 optimization.
How can I make this program faster?

Answer 1

Regarding compiler settings, I can only say:
Use x64, not x86 (32-bit) as a target
Use Release as configuration, not Debug
Enable aggressive optimizations
(Edit) It seems you already have these settings in the compiler, so there should be nothing obvious left to do regarding the compiler.
Also, there is probably a lot of optimization possible, because it does not seem that you are using the Sieve of Eratosthenes. You can then further skip all multiples of two, and increase the step size to tree.
And you would of course have to provide the node.js code. I am almost certain that it does not use the exact same logic.