为什么if语句和变量声明比循环中的加法更快？

Question

if we have if statements with declarations of variables like this:

#include <iostream>
#include <ctime>

using namespace std;

int main() {

    int res = 0;

    clock_t begin = clock();
    for(int i=0; i<500500000; i++) {
        if(i%2 == 0) {int fooa; fooa = i;}
        if(i%2 == 0) {int foob; foob = i;}
        if(i%2 == 0) {int fooc; fooc = i;}
    }
    clock_t end = clock();
    double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;

    cout << elapsed_secs << endl;

    return 0;
}

the result is:

1.44

Process returned 0 (0x0)   execution time : 1.463 s
Press any key to continue.

but, if it is:

#include <iostream>
#include <ctime>

using namespace std;

int main() {

    int res = 0;

    clock_t begin = clock();
    for(int i=0; i<500500000; i++) {
        res++;
        res--;
        res++;
    }
    clock_t end = clock();
    double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;

    cout << elapsed_secs << endl;

    return 0;
}

the result is:

3.098

Process returned 0 (0x0)   execution time : 3.115 s
Press any key to continue.

why does adding or subtracting takes more time to run than a if statement with a variable declaration?

Answer 1

The difference is almost certainly due to compiler optimization. You'll have to look at the assembly to make sure, but here is my take on what happens:

In the first example it's trivial for the optimizer to realize that the bodies of the if s have no effect. In each a variable local to the if is declared, assigned to and immediately destroyed. So the if s get optimized away, leaving an empty for loop that gets optimized away as well.

The sitiuation in the second example is not that trivial on the whole. What is trivial is that the body of the loop boils down to a single res++ , which most likely will be further optimized to ++res . But because res is not local to the loop the optimizer has to consider the whole main() function to realize that the loop has no effect. Most likely it fails to do so.

Conclusion: In its current form the measurements are meaningless. Disabling optimization also won't help because you'll never do that for a production build. If you really want to dive into this I suggest watching CppCon 2015: Chandler Carruth "Tuning C++: Benchmarks, and CPUs, and Compilers! Oh My!" for great advice on how to handle the optimizer in these kinds of siutuations.