GCC 4.8.2 auto-vectorization fail due to cout

Question

My code is as follows and I use GCC 4.8.2:

#include <iostream>
#include <stdint.h>
#include <sys/time.h>
#include <ctime>

using namespace std;

int main(int argc, char *argv[]) {
    struct timespec time_start={0, 0},time_end={0, 0};
    uint8_t bitmap[20240];
    int cost;
    clock_gettime(CLOCK_REALTIME, &time_start);
    for (int i = 0; i < 20240; ++i) {
        bitmap[i >> 3] |= 1 << (i&7);
    }
    clock_gettime(CLOCK_REALTIME, &time_end);
    cost = time_end.tv_nsec - time_start.tv_nsec;
    cout << "case COST: " << cost << endl;
    clock_gettime(CLOCK_REALTIME, &time_start);
    for (int i = 0; i < 20240; ++i) {
        bitmap[i >> 3] &= 1 << (i&7);
    }
    clock_gettime(CLOCK_REALTIME, &time_end);
    cost = time_end.tv_nsec - time_start.tv_nsec;
    cout << "case COST: " << cost << endl;
    int a = bitmap[1];
    std::cout << "TEST: " << a << endl;

}

I compile it with

gcc -lstdc++ -std=c++11 -O2 -ftree-vectorize -ftree-vectorizer-verbose=7 -fopt-info test.cpp

and I get test.cpp:14: note: not vectorized: not enough data-refs in basic block..

Then I run the binary a.out and get COST more than 20000.

If I delete std::cout << "TEST: " << a << endl;, this code is vectorized and COST is less than 100.

Anyone can help me.

Answer 1

In the statement

std::cout << "TEST: " << a << endl

You are initializing an ostream object which involves storage. You also use std::endl which is not the same as using \n. When you delete that statement, all this cost is not involved.

The statement before the last cout is also optimized away (removed) by the compiler because the value of a is not being used anywhere.

int a = bitmap[1];

Moreover, both the for loops are optimized away by the compiler because the bitmap values calculated by both the for loops will NOT be used when you remove the last cout statement. And there is no need of the bitmap array as well.

You can see the assembly that is generated for your code with the compiler version and options you have given here. And you can see clearly what happens when you comment out and uncomment the cout statement.