rsmith added inline comments.
================
Comment at: lib/AST/ASTContext.cpp:2200
+ Layout.getBaseClassOffset(Base->getAsCXXRecordDecl());
+ CharUnits BaseSize = Context.getTypeSizeInChars(Base);
+ if (BaseOffset != CurOffset)
----------------
erichkeane wrote:
> rsmith wrote:
> > On reflection, I don't think this is right, and likewise I don't think the
> > check for unique object representations of the base class above is quite
> > right.
> >
> > A class can be standard-layout but not POD for the purposes of layout (eg,
> > by declaring a special member function). If so, the derived class can reuse
> > the base class's tail padding, and if it does, the derived class can have
> > unique object representations even when the base class does not. Example:
> >
> > ```
> > struct A { ~A(); int n; char a; }; struct B : A { char b, c, d; };
> > ```
> >
> > Here, under the Itanium C++ ABI, `sizeof(B) == 8`, and `B` has unique
> > object representations even though its base class `A` does not.
> >
> >
> > This should be fairly easy to fix. One approach would be to change the
> > recursive call to `hasUniqueObjectRepresentations` above to return the
> > actual size occupied by the struct and its fields (rather than checking for
> > tail padding), and add that size here instead of using the base size. Or
> > you could query the DataSize in the record layout rather than getting the
> > (complete object) type size (but you'd then still need to check that
> > there's no bits of tail padding before the end of the dsize, since we still
> > pad out trailing bit-fields in the dsize computation).
> According to the standard, the above case isn't, because it is non-trivial,
> right? 9.1 requires that "T" (B in your case) be trivially copyable, which
> it isn't, right?
>
> The predicate condition for a template specialization
> has_unique_object_representations<T> shall be
> satisfied if and only if:
> (9.1) - T is trivially copyable, and
> (9.2) - any two objects of type T with the same value have the same
> object representation
Fixed example:
```
struct A { int &r; char a; }; struct B : A { char b[7]; };
```
https://reviews.llvm.org/D39347
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
