c++ - Undefined behavior causing time travel

Question

One example of this article from a msdn blog made me ticker:

It says that this function:

void unwitting(bool door_is_open)
{
    if (door_is_open) {
        walk_on_in();
    } else {
        ring_bell();

        // wait for the door to open using the fallback value
        fallback = value_or_fallback(nullptr);
        wait_for_door_to_open(fallback);
    }
}

Can be optimized into this one:

void unwitting(bool door_is_open)
{
    walk_on_in();
}

Because calling value_or_fallback(nullptr) is undefined behavior (this is proven earlier in the article).

Now what I don’t understand is this: the run time enters undefined behavior only when it reaches that line. Shouldn’t the happen-before / happen-after concept applies here, in the sense that all observable effects of the first paragraph have be resolved before the run time enters UB?

Answer 1

There is a flow in the reasoning.

When a compiler writer says: we use Undefined Behavior to optimize a program, there are two different interpretations:

• most people hear: we identify Undefined Behavior and decide we can do whatever we want (*)
• the compiler writer meant: we assume Undefined Behavior does not occur

Thus, in your case:

• dereferencing a nullptr is Undefined Behavior
• thus executing value_or_fallback(nullptr) is Undefined Behavior
• thus executing the else branch is Undefined Behavior
• thus door_is_open being false is Undefined Behavior

And since Undefined Behavior does not occur (the programmer swears she will follow the terms of use), door_is_open is necessarily true and the compiler can elide the else branch.

(*) I am slightly annoyed that Raymond Chen actually formulated it this way...