ahatanak 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;
> 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
> - 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
Comment at: lib/CodeGen/CGNonTrivialStruct.cpp:193
+ TrivialFieldIsVolatile |= FT.isVolatileQualified();
+ if (Start == End)
> 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?
cfe-commits mailing list