A test about atomic_int in g++ 4.8.1 compiler

Question

I have the following test in MinGW in my winx , compiled by :

g++ -std=c++11 -DTEST2 testatomic1.cpp -lpthread -o testatomic1.exe

g++ -std=c++11 -DTEST4 testatomic1.cpp -lpthread -o testatomic1.exe

atomic_int  totalx(0);

void *test_func0(void *arg)
{
    #ifdef TEST2
    for(i=0;i<100000000;++i){
        totalx += 1 ;  
    }//for  
    #endif
    #ifdef TEST4
    for(i=0;i<100000000;++i){
        atomic_fetch_add(&totalx, 1); 
    }//for  
    #endif 
}

int main(int argc, const char *argv[])
{
    pthread_t id[3];
    int iCPU =1  ;
    pthread_create(&id[0],NULL,test_func0,(void *)(long)iCPU );
    pthread_create(&id[1],NULL,test_func0,(void *)(long)iCPU );
    pthread_create(&id[2],NULL,test_func0,(void *)(long)iCPU );

    int i ;
    for(i=0;i<3;++i){
        pthread_join(id[i],NULL);
    }
    #ifdef TEST2
    cout << "TEST2 totalx=" << totalx << endl  ;
    #endif
    #ifdef TEST4 
    cout << "TEST4 totalx=" << totalx << endl  ;
    #endif
}//main 

This test run many times in TEST2 and TEST4 defined , and answer are 300000000 all , in TEST4 , it works as I expect , TEST2 is strange to me , totalx += 1 ; without any memory model , can produce the right answer ...then why bother has load , store function ? just define an atomic_int var will do the job right , I wonder it is too easy ... Am I miss something ?

Answer 1

The C++11 standard defines atomic_XXX as a typedef of std::atomic<XXX> ([atomics.types.generic] p7). std::atomic<int> has an operator+=(int) that is defined to have the same effects as atomic_fetch_add ([atomics.types.operations.pointer] p27). So your two test sequences are defined to have identical effects.

The atomic_XXX types are for C11 compatibility - you may have an easier time simply using std::atomic<XXX>.