ahatanak marked an inline comment as done. ahatanak added inline comments.
================ Comment at: lib/Sema/SemaExpr.cpp:16218 + checkNonTrivialCUnion(E->getType(), E->getExprLoc(), + Sema::NTCUC_LValueToRValueVolatile); + ---------------- rjmccall wrote: > ahatanak wrote: > > rjmccall wrote: > > > It looks like you're generally warning about this based on the specific > > > context that forced an lvalue-to-rvalue conversion. I'm not sure > > > `volatile` is special except that we actually perform the load even in > > > unused-value contexts. Is the assumption that you've exhaustively > > > covered all the other contexts of lvalue-to-rvalue conversions whose > > > values will actually be used? That seems questionable to me. > > Yes, that was my assumption. All the other contexts where lvalue-to-rvalue > > conversion is performed and the result is used are already covered by other > > calls sites of `checkNonTrivialCUnion`, which informs the users that the > > struct/union is being used in an invalid context. > > > > Do you have a case in mind that I didn't think of where a lvalue-to-rvalue > > conversion requires a non-trivial initialization/destruction/copying of a > > union but clang fails to emit any diagnostics? > > > > Also I realized that lvalue-to-rvalue conversion of volatile types doesn't > > always require non-trivial destruction, so I think `CheckDestruct` > > shouldn't be set in this case. > > > > ``` > > void foo(U0 *a, volatile U0 *b) { > > // this doesn't require destruction. > > // this is perfectly valid if U0 is non-trivial to destruct but trivial > > to copy. > > *a = *b; > > } > > ``` > > > > For the same reason, I think `CheckDestruct` shouldn't be set for function > > returns (but it should be set for function parameters since they are > > destructed by the callee). > There are a *lot* of places that trigger lvalue-to-rvalue conversion. Many > of them aren't legal with structs (in C), but I'm worried about approving a > pattern with the potential to be wrong by default just because we didn't > think about some weird case. As an example, casts can trigger > lvalue-to-rvalue conversion; I think the only casts allowed with structs are > the identity cast and the cast to `void`, but those are indeed allowed. Now, > a cast to `void` means the value is ignored, so we can elide a non-volatile > load in the operand, and an identity cast isn't terminal; if the idea is that > we're checking all the *terminal* uses of a struct r-value, then we're in > much more restricted territory (and don't need to worry about things like > commas and conditional operators that can propagate values out). But this > still worries me. > > I'm not sure we need to be super-pedantic about destructibility vs. > copyability in some of this checking. It's certain possible in C++, but I > can't imagine what sort of *primitive* type would be trivially copyable but > not trivially destructible. (The reverse isn't true: something like a > relative pointer would be non-trivially copyable but still trivially > destructible.) > > Is there any way to make this check cheaper so that we can immediately avoid > any further work for types that are obviously copyable/destructible? All the > restricted types are (possibly arrays of) record types, right? I'm not sure I fully understand what you are saying, but by "cheaper", do you mean fewer and simpler rules for allowing or disallowing non-trivial unions even when that might result in rejecting unions used in contexts in which non-trivial initialization/destruction/copying is not required? If so, we can probably diagnose any lvalue-to-rvalue conversions regardless of whether the source is volatile if the type is either non-trivial to copy or destruct. Repository: rC Clang CHANGES SINCE LAST ACTION https://reviews.llvm.org/D63753/new/ https://reviews.llvm.org/D63753 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits