On: The difference between undefined behavior and ill-...

[nlewycky]

The compiler may remove the nullptr check in:

  ptr->foo = 1;
  if (ptr == nullptr)
     return;

but it may not remove the nullptr check in:

  if (ptr == nullptr)
     return;
  ptr->foo = 1;

[thayne → nlewycky]

To explain why it can be removed in the former:

Since it is UB to dereference a null pointer, the compiler can assume that ptr isn't null after it is dereferenced[1]. Therefore, the if condition will always be false.

In fact if ptr is null, unless the foo field has a very large offset, the behavior you would probably expect would be for the dereference to segfault before reaching the if, so it doesn't really matter if it is optimized away.

[krackers → thayne]

>so it doesn't really matter if it is optimized away.

There are cases in which the optimization can result in behavior other than segfaulting, see https://research.swtch.com/ub#null

[thayne → krackers]

Sure. That's why I said "the behavior you would probably expect", but that isn't necessarily what happens.

[Bjartr → nlewycky]

I guess I've misunderstood that other story then, thanks.