rjmccall added inline comments.
Comment at: include/clang/AST/Type.h:1121
+ /// after it is moved, as opposed to a truely destructive move in which the
+ /// source object is placed in an uninitialized state.
+ PrimitiveCopyKind isNonTrivialToPrimitiveDestructiveMove() const;
> rjmccall wrote:
> > "truly"
> > Hmm. Now that I'm thinking more about it, I'm not sure there's any point
> > in tracking non-triviality of a C++-style destructive move separately from
> > the non-triviality of a copy. It's hard to imagine that there would ever
> > be a non-C++ type that primitively has non-trivial copies but trivial
> > C++-style moves or vice-versa. Type-based destructors imply that the type
> > represents some kind of resource, and a C++-style move will always be
> > non-trivial for resource types because ownership of the resource needs to
> > be given up by the old location. Otherwise, a type might be non-trivial to
> > copy but not destroy because there's something special about how it's
> > stored (like volatility), but it's hard to imagine what could possibly
> > cause it to be non-trivial to destroy but not copy.
> > If we were tracking non-triviality of an *unsafe* destructive move, one
> > that leaves the source in an uninitialized state, that's quite different.
> > I think there are three reasonable options here:
> > - Ignore the argument I just made about the types that we're *likely* to
> > care about modeling and generalize your tracking to also distinguish
> > construction from assignment. In such an environment, I think you can
> > absolutely make an argument that it's still interesting to track C++-style
> > moves separately from copies.
> > - Drop the tracking of destructive moves completely. If you want to keep
> > the method around, find, but it can just call
> > `isNonTrivialToPrimitiveCopy()`.
> > - Change the tracking of *destructive* moves to instead track
> > *deinitializing* moves. The implementation would stop considering
> > `__strong` types to be non-trivial to move.
> > But as things stand today, I do not see any point in separately tracking
> > triviality of C++-style destructive moves.
> The second option seems most reasonable to me. We can always make changes if
> someone comes up with a type that requires tracking destructive moves
Well, we already have a type that would have a trivial deinitializing move: ARC
`__strong` pointers. What we don't have is anything in IRGen that currently
would take advantage of that fact. Anyway, I agree that we can wait to start
tracking this until we add such code to the compiler.
Comment at: lib/CodeGen/CGNonTrivialStruct.cpp:193
+ TrivialFieldIsVolatile |= FT.isVolatileQualified();
+ if (Start == End)
> rjmccall wrote:
> > ahatanak wrote:
> > > rjmccall wrote:
> > > > I feel like maybe volatile fields should be individually copied instead
> > > > of being aggregated into a multi-field memcpy. This is a more natural
> > > > interpretation of the C volatile rules than we currently do. In fact,
> > > > arguably we should really add a PrimitiveCopyKind enumerator for
> > > > volatile fields (that are otherwise trivially-copyable) and force all
> > > > copies of structs with volatile fields into this path precisely so that
> > > > we can make a point of copying the volatile fields this way.
> > > > (Obviously that part is not something that's your responsibility to do.)
> > > >
> > > > To get that right with bit-fields, you'll need to propagate the actual
> > > > FieldDecl down. On the plus side, that should let you use
> > > > EmitLValueForField to do the field projection in the common case.
> > > I added method visitVolatileTrivial that copies volatile fields
> > > individually. Please see test case test_copy_constructor_Bitfield1 in
> > > test/CodeGenObjC/strong-in-c-struct.m.
> > Okay, great! I like the name.
> > Does this mean we're now copying all structs that contain volatile fields
> > with one of these helper functions? If so, please add a C test case
> > testing just that. Also, you should retitle this review and stress that
> > we're changing how *all* non-trivial types are copied, and that that
> > includes both volatile and ARC-qualified fields.
> No, the current patch doesn't copy volatile fields of a struct individually
> unless the struct is a non-trivial type (which means its primitive copy kind
> is PCK_Struct). I'll look into today how I can force structs with volatile
> fields that are not non-trivial to be copied using the helper functions.
> It seems like we would need a boolean flag in RecordDecl that tracks the
> presence of volatile fields in the struct or one of its subobjects. I assume
> we want to copy volatile fields individually in C++ too, in which case the
> flag needs to be set in both C and C++ mode. Is that right?
Oh, I see, there's specific logic to not flag the struct trivial if a field is
merely VolatileTrivial. Okay, that works for me; we can think about the right
thing to do in a follow-up commit. It probably needs to be raised as an RFC on
The C++ standard has explicit language about triviality that says that volatile
fields don't make defaulted copy-constructors non-trivial. Whatever we do here
can't break that. But that doesn't necessarily imply that we should continue
to use a simple memcpy to copy the struct; we could still use the C logic to
copy "trivial" C++ structs with volatile members.
If we do try to apply this rule to C++, you will need to propagate these bits
appropriately for C++ types, including from bases.
Comment at: lib/Sema/SemaDecl.cpp:15424
+ if (FT.isNonTrivialToPrimitiveCopy() != QualType::PCK_Trivial &&
+ FT.isNonTrivialToPrimitiveCopy() != QualType::PCK_VolatileTrivial)
Please compute isNonTrivialToPrimitiveCopy once here, it's not trivial.
...arguably we should figure out a way to do all these type checks once and
then set the bits from that. We can do that as a separate patch, though.
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