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:
> > > 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.
> > Instead of checking `!CPlusPlus`, we can call `isNonTrivialPrimitiveCType`
> > (which is deleted in this patch) to do a cheaper check of whether the type
> > requires any non-trivial default-initialize/destruct/copy functions. The
> > function still uses the copy visitor, so if we want to make the check even
> > cheaper, we can add a flag to `RecordDecl` that indicates whether it
> > contains a non-trivial union.
> I think it would be sufficient for the fast path to just check for a C record
> type (i.e. `!isa<CXXRecordDecl>(RT->getDecl())`). But it doesn't seem
> unreasonable to record non-copyable/destructible/defaultable types on the
> `RecordDecl` if we have the bits.
I don't think we can call `!isa<CXXRecordDecl>` if the type is an array? I
created a new function `hasNonTrivialPrimitiveCStruct` instead that checks
whether the type is a non-trivial C struct or union.
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D63753/new/
https://reviews.llvm.org/D63753
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