Miscompiling error resulting to go into a forloop when the condition is false

[alliaces]

Hi !
I'm encoutering a problem from clang19.1.0 (clang20.1.0 also has it) with this kind of code :
https://godbolt.org/z/WWdqcvj6h

double recursiveEval(int n, const double a) {
    return n <= 0 ? 0 : a + recursiveEval(n - 1, a);
}

double eval(const std::vector<double>& emptyVector) {
    std::vector<double> vec;
    for (const double& it : emptyVector) {
        std::cerr << "BIG ERROR !! I should not be here" << std::endl;
        vec.push_back(recursiveEval(it, it));
    }
    return recursiveEval(vec.size(), vec[0]);
}


int main(int argc, char **argv) {
    std::vector<double> coefs;

    eval(coefs);
    return 0;
}

It segfaults but that is not the problem. The problem is we go into the forloop whereas the vector emptyVector is empty.

The problem disappears when (any of these):

• vec or emptyVector are not empty
• vec[0] is replaced by a prvalue
• the line return recursiveEval(vec.size(), vec[0]); is deleted (replaced by any return).

Thanks for your help

[efriedma-quic]

It segfaults but that is not the problem

Yes, it is the problem: undefined behavior can do anything, including running your error printing code.

You might be interested in -fno-delete-null-pointer-checks.

[pinskia]

I can't remember if time traveling for undefined behavior is valid behavior in c++ or not.

In this case the undefined behavior happens after the loop, so entering the loop would be time traveling.