[Bug c/102989] Implement C2x's n2763 (_BitInt)

["segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102989

--- Comment #21 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #19)
> (In reply to Segher Boessenkool from comment #16)
> > (In reply to Jakub Jelinek from comment #15)
> > > PowerPC I think does, not sure about s390.
> > 
> > Does what?
> 
> Published psABI which ought to specify how to pass/return _BitInt(N) and
> unsigned _BitInt(N).

psABI is an x86 thing?  But there are various ABIs for PowerPC that have
public documentation, six or so, and GCC has support for most of those.

None of them are "processor specific" (most are OS specific, instead), and
they differ in very fundamental things, in places.  They are much related
as well of course, either because there is an obvious choice, or history.

Many of those ABIs have not seen updates for decades, and are unlikely to
anymore.  OTOH the GCC support for them has been updated over time, there
often is only one sane choice anyway.

We'll make decisions on what ELFv2 will do for _Bitint when it is closer
in time than it is now.  The only interesting choice is whether values in
memory have undefined bits -- and they likely should, simply because all
other padding bits are undefined as well.