[tekne]

No, that's actually UB. The important bit here is "compiler defined" -- UB means the compiler is allowed to assume it never happens while compiling.

Consider, for example, an implementation defined function f() -- which can also diverge/crash horribly, etc.

If I write

    if p {
      print("p is true")
    } else {
      g()
    }

    if p {
      f()
    }

Then either we: - print p is true and execute f - do nothing

This is true regardless of if f immediately crashes the computer, nasal demons, whatever -- that's implementation defined.

UB means f may never happen.

And that means the compiler may optimize this to just:

    g()

Notice the difference here -- the print never happens!, and g always happens.

You can see why this is concerning when you write code like

    if dry_run {
      print("would run rm -rf /")
    } else {
      run("rm -rf /")
    }

    if dry_run {
      // oops: some_debug_string is NULL and will segfault!
      print(some_debug_string);
    }

[tyg13]

I see what you're going for, but I don't see how your example is UB. If `p` is a pointer, and, after your `if (p)` check, `p` is dereferenced unconditionally, then yes, your check for `p == NULL` could be removed, and the code under the `if` would be removed as well. But the example you've constructed is not UB.

[Quekid5]

You misunderstood their example, I think.

If doesn't matter what 'p' is in their example. The point is: if 'f' is undefined behavior (rather than just impl-defined), then the optimizer concludes that the "if p { f() }" can never happen... which means that we're allowed to assume that 'if p { ... } else { ... }' (in the first part of the example) will always take the else branch. The compiler will optimize accordingly and just always call g() unconditionally.