Re: Executing code at compilation time

Dear Patryk and Diego,


        thank you very much for the kind replies. In particular, Patryk,
thank you very much for pointing me to Partial Evaluation.
        About the small program that I had posted:


int main(int argc, char** argv) {
    int i = 0;
    int sum = 0;
    for (; i < 32767; i++) {
        sum += i;
    }
    printf("The sum is %d\n", sum);
}


        following Diego's hint, "gcc -O1 sl1.c -fdump-tree-all" seems to
reveal that the loop is resolved after sparse conditional constant
propagation. One interesting fact is that the program below is not
optimized by either O1, O2 or O3:


int main(int argc, char** argv) {
    int i = 0;
    int sum = 0;
    for (; i < 32767; i++) {
        sum -= i;
    }
    printf("The sum is %d\n", sum);
}


And the only difference is the "sum -= i". If I decrease the loop
limit to, say, 10, then the loop is completely resolved, what leads me
to believe that the responsible opts are, in this case, a combination
of loop unrolling with constant propagation, as Patryk had pointed. If
you allow me a last question, why the loop:


    for (; i < 32767; i++) {
        sum += i;
    }


is fully resolved, whereas:


    for (; i < 32767; i++) {
        sum -= i; // see the minus
    }


is not?


All the best,


Fernando

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