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by nlewycky 688 days ago
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;
2 comments
thayne 688 days ago
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.

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krackers 688 days ago
>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

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thayne 686 days ago
Sure. That's why I said "the behavior you would probably expect", but that isn't necessarily what happens.
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Bjartr 688 days ago
I guess I've misunderstood that other story then, thanks.
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