Re: wide function pointer type

[Florian Weimer <fweimer@redhat.com>]

* Martin Uecker:

> Am Montag, den 18.10.2021, 09:36 +0200 schrieb Florian Weimer:
>> * Martin Uecker via Libffi-discuss:
>> 
>> > In particular, I would be good to know whether
>> > implementing a perfectly forwarding stub for
>> > a variadic functions that loads the static
>> > is possible on all architectures. I assume so,
>> > but I am not entirely sure.
>> 
>> Are you asking if it is possible to consume an argument in a variadic
>> wrapper and forward the remaining arguments to a variadic function?  The
>> answer to that is no.
>
> Yes, I assumed this does not work. The question 
> is more whether architectures exist where the
> existing ABI states that the static chain has to be
> passed as a first argument (which would then need to be
> removed when calling a regular function).
>
> The implementation most architectures
> would use is that 1) the static chain is loaded from
> the wide pointer 2) stored in the designated register
> or stack slot and then 3) a call is performed
> using the same calling convention as used for
> regular functions.

Many ABIs do not actually specify a register for the chain pointer.  For
GCC's nested function extension, there are no ABI concerns because the
nested function definition and the thunk generation are always built
from the same translation unit.  So GCC can just pick a custom calling
convention, similar to what it does for other local, non-escaping
functions.

For obvious reasons, the proposal does not achieve interoperability with
std::function from C++, so there is no ABI concern, either.

Thanks,
Florian