[PATCH] D67122: [UBSan][clang][compiler-rt] Applying non-zero offset to nullptr is undefined behaviour

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D67122#1656205 <https://reviews.llvm.org/D67122#1656205>, @lebedev.ri wrote:

> In D67122#1656189 <https://reviews.llvm.org/D67122#1656189>, @aaron.ballman wrote:
>
> > One fear I have with this is in expansions of the  `offsetof` macro, where it is a common implementation strategy to cast a null pointer to be of the correct type when calculating member offsets. Do you think you will be able to distinguish between null pointer additions that the user wrote directly (which is UB) as opposed to null pointer additions that come from the implementation (which is not UB)?
>
>
> Can you show a snippet on godbolt?


https://godbolt.org/z/5DHL2E

This will show that Clang has a `__builtin_offsetof()` that gets used. I'm worried about situations where there is no `__builtin_offsetof()` but the canonical reference implementation is used instead (which looks identical to what initializes `bad` in my link).


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