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:
> > 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]; };
> > ```
> AH! I got caught up by the destructor as the reason it wasn't unique, but
> the actual thing you meant is that "A" has tail padding, so it is NOT unique.
> However, on Itanium, the tail padding gets used when inheriting from it.
>
> Do I have that correct? I just have to fix the behavior of
> inheriting-removing-tail-padding?
You have fallen into a trap :) There can be (up to 7 bits of) padding between
the end of the members and the end of the data size, specifically if the struct
ends in a bit-field. You could check for that before you return
`CurOffsetInBits` at the end of this function, but I think it'd be better to
store the size in `Bases` and just use that down here rather than grabbing and
using the data size.
https://reviews.llvm.org/D39347
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