[PATCH] D63753: [Sema] Instead of rejecting C unions with non-trivial fields, detect attempts to destruct/initialize/copy them.

[Akira Hatanaka via Phabricator via cfe-commits]

ahatanak marked an inline comment as done.
ahatanak added inline comments.


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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.


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https://reviews.llvm.org/D63753