Question

Suppose we have a loop that runs for 100 times. Does using unsigned char instead of int for its counter make a difference? And also using i += 1U instead of i++? Or do compilers take care of that?

Answer 1

In simple case int vs unsigned char gives the same code for me:

    for ( unsigned char i = 0; i < 100; ++i ) {
01081001  mov         esi,64h  
01081006  jmp         main+10h (1081010h)  
01081008  lea         esp,[esp]  
0108100F  nop  
        std::cout << 1 << std::endl;
01081010  mov         eax,dword ptr [__imp_std::endl (108204Ch)]  
01081015  mov         ecx,dword ptr [__imp_std::cout (1082050h)]  
0108101B  push        eax  
0108101C  push        1  
0108101E  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (1082044h)]  
01081024  mov         ecx,eax  
01081026  call        dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (1082048h)]  
0108102C  dec         esi  
0108102D  jne         main+10h (1081010h)  
    }
    return 0; 

You should profile your code and then optimize slow parts. Do not start with premature optimization.