--disable-new-gxx-abi

[Joe Buck]

I wrote:

>     Joe> Hmm.  But for the most common case -- single inheritance with
>     Joe> no virtual base clases -- isn't the code sequence to do a
>     Joe> virtual function call exactly the same with old and new ABIs?

Mark writes:
> Yes.

But there is a major caveat here:

>   - The functions are numbered differently, so you have to compute the
>     place to look into the vtable a little differently.

This may mean that even in the case of single inheritance, the wrong
function gets called.