MMX SSE extensions for for loop

I have GCC 9.2 compiler. If i use MMX or SSE/AVX extension you will have you code run in parallel, so it will be faster. How to tell the compiler to use this instructions I have a code snippet i want to parallel:

char max(char * a, int n){
    char max = (*a);
    for (int i = 0 ; i< n ; ++i){
            if (max < a[i]){
                max = a[i];
            }
    }
    return max;
}

it generates code using SSE extension but don't use pmaxub why