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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 dereferen
by thayne 2y 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.
- krackers 2y 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 https://research.swtch.com/ub#null
- thayne 2y agoSure. That's why I said "the behavior you would probably expect", but that isn't necessarily what happens.