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. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D81865/new/ https://reviews.llvm.org/D81865 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
