Do we need to reopen B1?

[Christophe de Dinechin]

Jason Merrill wrote:
>  >      , as long as they are emitted with the corresponding
>  >      virtual table.
> 
> No; this would disallow emitting them with the function, if the function
> and vtable are defined in different places.  I don't think we need to
> specify this.

Ooops, you are correct. The sentence should indeed be "as long as they are
emitted whenever the main entry point is emitted."

> 
>  >      An existing compiler which uses thunks could be
>  >      converted to use this ABI by only adding support for the vcall
>  >      offsets (in the case of virtual inheritance), and by creating
>  >      a named entry point and a thunk for each possible adjustment
>  >      from a secondary base to the derived class (in the case of
>  >      multiple, non-virtual inheritance).
> 
> I don't understand "a named entry point and a thunk".  In this case, the
> thunk is the entry point.

The 'entry point' denotes a label with linkage. The 'thunk' denotes the code. I
agree the wording is not that good.


> 
>  >      A more efficient implementation would be to emit all of the
>  >      adjusting entry points immediately before the non-adjusting
>  >      entry point to the function. Such thunks can also be combined
>  >      in the form of multiple 'Add' instructions that fall through
>  >      to the main entry point.
> 
> You used 'thunk'.  :)

Ooops.

> 
>  >      Another implementation yet is to use
>  >      predication rather than branches to reach the main function
>  >      [Note: I'm not sure this actually works without a calling
>  >      convention on predicates...].
> 
> I had been thinking about something like
> 
> foo:
>    add *p
>    set predicate
> bar:
>    [if not set] add *q
>    ...
> 
> but I'd be perfectly happy to lose the predication language.
> 
> Jason

The problem is that you can't go to 'bar' unless you know that the predicate is
not set. That's what I called a "calling convention on predicates". Something
like "p777 is always reset at the time a virtual call branch is made", where
p777 would be part of the ABI.



Regards
Christophe