[PATCH] D81865: [clang] Use string tables for static diagnostic descriptions

Mon Jul 27 11:29:08 PDT 2020

dblaikie added a comment.

In D81865#2176148 <https://reviews.llvm.org/D81865#2176148>, @froydnj wrote:

> In D81865#2176014 <https://reviews.llvm.org/D81865#2176014>, @bkramer wrote:
>
> > Nice, those relocations have annoyed me for years. I'm worried about whether the way you're accessing StaticDiagInfoDescriptionStringTable might be undefined behavior. I won't block this change on that though.
>
>
> Is there somebody who should review that particular bit?  My understanding is that it is OK, but my understanding of the C++ standard is not necessarily complete, and I'd like to get a second opinion.

I believe this falls under the (using cppreference ( https://en.cppreference.com/w/cpp/language/union ) , since it's a bit easier to read) UB clause: " It's undefined behavior to read from the member of the union that wasn't most recently written. Many compilers implement, as a non-standard language extension, the ability to read inactive members of a union."

Last member written to was the "StringTable" member, but then it's read from the "String" member, so that'd be UB. Commonly supported, but UB - not sure if we have a general statement that we depend on this behavior in LLVM, even though it's non-standard (but it's possible that we do make such an assumption about the compiler that's building LLVM). It'd be nice to avoid that, though - and not too difficult, I think - I /believe/ it's valid to take a pointer to an object, cast it to char*, compute a pointer to some specific member and then cast it back to the right type and access. But I could be wrong there. @rsmith would be the person to give an authoritative answer.

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