helper macro for timing a function in c - beware of optimisation

c timeit

#define TIMEIT(F, N, REPS) \
{\
    clock_t start = clock();\
    for (int i = 0; i < N; ++i)\
    {\
    }\
    clock_t diff_loop = clock() - start;\
    double results[REPS];\
    for (int i = 0; i < REPS; ++i)\
    {\
        clock_t start = clock();\
        for (int j = 0; j < N; ++j)\
        {\
            F;\
        }\
        clock_t diff = clock() - start - diff_loop;\
        results[i] = ((double)diff) / CLOCKS_PER_SEC;\
    }\
    res = meana(results, REPS);\
    if (maxa(results, REPS) - mina(results, REPS) > 0.1 * res)\
    {\
        fprintf(stderr, "spread large\n");\
    }\
}\