[cfe-dev] Adding support for multiple non-virtual inheritance for -cxx-abi microsoft

Richard Smith richard at metafoo.co.uk
Mon Apr 8 17:24:36 PDT 2013

On Mon, Apr 8, 2013 at 11:33 AM, John McCall <rjmccall at apple.com> wrote:

> On Apr 8, 2013, at 11:08 AM, Reid Kleckner <rnk at google.com> wrote:
> > On Mon, Apr 8, 2013 at 2:02 PM, Timur Iskhodzhanov <timurrrr at google.com>
> wrote:
> >> 2013/4/8 John McCall <rjmccall at apple.com>:
> >>> I also find it curious that MSVC uses a thunk for member pointers,
> since
> >>> the required this-adjustment is already plainly expressible in the
> member
> >>> pointer value.
> >> Me too actually.
> >> Reid, wdyt?
> >
> > I think it allows them to avoid the union between non-virtual methods
> > and virtual methods.  Seems a bit cleaner and more obvious to me, but
> > it has tradeoffs in terms of code size at the call site and the number
> > of conditional vs. indirect branches that you have to do:
> > indirect to thunk and indirect through vtable, vs conditional between
> > two indirect calls
> Oh, does MSVC not do the union thing?  They always make a thunk to do
> the virtual call?
> If so, this "thunk" is potentially quite a bit more than just a thunk — it
> may
> actually have ABI pointer-equality requirements on it for e.g. member
> pointer equality tests.

I don't believe there's any need for that -- comparisons on pointers to
virtual member functions produce unspecified results (see [expr.eq]p2,
second-last sentence).
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