libffi GCC vs. end user development

[David Ayers <ayers at fsfe dot org>]

Hello folks,

Anthony Green mentioned that libffi as shipped with GCC is currently for
use in gcj.  The GNU ObjC runtime which also comes with GCC could also
use libffi to implement it's forwarding which currently based on
builtin_apply.  Currently this is hardly used as far as I'm aware.
GNUstep for example builds on top of libffi (or ffcall for older
platforms and architectures that libffi doesn't support).

I've been on and off on looking at how ObjC forwarding could be
implemented with libffi by libobjc which would enable some long standing
PR's to be fixed.  The first candidate would be to use the version that
currently already resides in GCC to avoid an external dependency.

So my questions are:
1) What exactly is the relation between GCC's libffi and gcj?
2) Is GCC's libffi only needed for the compiler or also for libjava
runtime?
3) Has it been modified in a way that it can only be useful for gcj?
4) If it is used by the runtime, would you expect issues if it was also
used by the libobjc runtime and a third party ObjC Programm/Library also
used the stock libffi?

Thank you for your time.
Cheers,
David Ayers