[wahern]

I think the point is that if the `argc <= 2` path is unreachable, then that means argc is always greater than 2, permitting the compiler to optimize the entire block to just:

  return printf("%s: we see %s", argv[0], argv[1]);

IOW, the conditional has been elided. But you're right in that the wording of the complaint doesn't match the example. The author presumably had in mind some of the more infamous NULL pointer-related optimizations, without spending the time to put together a properly analogous example.

[dtolnay]

I interpreted the author's characterization to be about something like:

  1  if (argc <= 2)
  2    puts("A");
  3  puts("B");
  4  if (argc <= 2)
  5    unreachable();
  6  else
  7    return puts("C");
  8  return puts("D");

in which not just lines 4-6,8 go away (as you said) but also lines 1-2.

It makes sense to me but I can see why the author would characterize this situation as "license to use an unreachable annotation on one code path to justify removing an entirely different code path that is not marked unreachable". In a different world one might expect A to be printed "before the UB happens".

[masklinn]

On the other hand, that has been the behaviour of optimising compilers in the face of UBs for years at this point, decades maybe. The linux kernel was hit by a deref' constraint propagation back in 2009 or so.

This is a behaviour I would absolutely expect from the construct, I would even qualify it as "the point".

[rrobukef]

I find this especially surprising because line 2 may be exactly the reason why line 5 is unreachable. E.g. if puts("A") contractually throws an exception you cannot just remove it.

What am I missing in this example?

[zefix]

C does not have exceptions...

[rrobukef]

But it has long_jmp, right?