[PATCH] D138958: [clang] Better UX for Clang’s unwind-affecting attributes

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert added a comment.

In D138958#4045567 <https://reviews.llvm.org/D138958#4045567>, @efriedma wrote:

> From an IR semantics standpoint, I'm not sure the `memory(none)` marking is right if the function throws an exception.  LangRef doesn't explicitly exclude the possibility, but take the following:
>
>   void f() { throw 1; }
>
> It gets lowered to:
>
>   define dso_local void @_Z1fv() local_unnamed_addr #0 {
>   entry:
>     %exception = tail call ptr @__cxa_allocate_exception(i64 4) #1
>     store i32 1, ptr %exception, align 16, !tbaa !5
>     tail call void @__cxa_throw(ptr nonnull %exception, ptr nonnull @_ZTIi, ptr null) #2
>     unreachable
>   }

If you mark this one as `readonly/none` we might, at some point, see the mismatch in the body and declare it UB. 
>From the outside, it should almost be fine since it's not-`nounwind` and the memory that is accessed is freshly allocated.
However, it still would be a problem waiting to happen to have escaping memory being written in a `readonly/none` function.

That said, I think we anyway want a `memory` category to express all accesses are to freshly allocated memory that may escape the function. This is a common pattern.
So `memory(allocated, inaccessiblemem)` would express that the allocation does access some inaccesible memory and the function will also access the newly allocated memory.
Its arguably not as good as `readnone` but I'm not sure how to get there. 
The best idea I have off the top of my head is a dedicated `exception` category for `memory` such that it won't interfere with anything but other exceptions, which it already does due to `unwind`.
Thus, `pure/const` -> `memory(exception).


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