[SOLVED] GCC optimalizations ignore for loop condition

Issue

so I wrote this code:

#include <iostream>

constexpr
int fibonacci (int n) {
    int a = 0;
    int b = 1;

    for(auto i = 0; i < n; i++) {
        b += a;
        a = b - a;
    }
    return b;
}

template<int N, int (T)(int)>
struct array {
    using type = decltype(T(0));

    constexpr array() : arr() {
        for (auto i = 0; i < N; ++i) {
            arr[i] = T(i);
        }
    }
    const type &operator[](int i) const { return arr[i]; }

private:
    type arr[N];
};

int main() {
    constexpr auto x = array<10, fibonacci>();

    for (int i = 0; i < 11; i++) {
        std::cout << i << " " << x[i] << std::endl;
    }
}

And without optimizations it works as expected, prints 11 values with last one being random value. But as soon I move to -O2 I get randomly long list of numbers finished with crash and segmentation fault. I checked this result on godbolt.org (https://godbolt.org/z/4MqjbPbxE) and it seems that it is not a problem in, for example, clang.

My question is, is this a bug in gcc? Why would optimizations remove/not check condition in for loop?

Solution

You are invoking undefined behavior by using out-of-range x[i].

Allocate one more to avoid out-of-range access.

In other words,

    constexpr auto x = array<10, fibonacci>();

should be

    constexpr auto x = array<11, fibonacci>();

Defining constant to avoid typo is better:

int main() {
    constexpr int num = 11;
    constexpr auto x = array<num, fibonacci>();

    for (int i = 0; i < num; i++) {
        std::cout << i << " " << x[i] << std::endl;
    }
}

Answered By - MikeCAT