rjmccall added inline comments.
================
Comment at: lib/Sema/SemaExpr.cpp:16218
+ checkNonTrivialCUnion(E->getType(), E->getExprLoc(),
+ Sema::NTCUC_LValueToRValueVolatile);
+
----------------
ahatanak wrote:
> 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.
Sorry, that point was separate from the discussion of `volatile` and
lvalue-to-rvalue conversions. I mean that you're changing a lot of core paths
in Sema, and it would be nice if we could very quickly decide based on the type
that no restrictions apply instead of having to make a function call, a switch,
and a bunch of other calls in order to realize that e.g. `void*` never needs
additional checking. Currently you have a `!CPlusPlus` check in front of all
the `checkNonTrivialCUnion` calls; I would like something that reliably avoids
doing this work for the vast majority of types that are not restricted, even in
C.
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D63753/new/
https://reviews.llvm.org/D63753
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