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

John McCall rjmccall at apple.com
Mon Apr 8 11:33:14 PDT 2013

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.


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