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

["leni536 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

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

Lénárd Szolnoki <leni536 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |leni536 at gmail dot com

--- Comment #33 from Lénárd Szolnoki <leni536 at gmail dot com> ---
(In reply to joseph@codesourcery.com from comment #32)
> On Fri, 28 Oct 2022, jakub at gcc dot gnu.org via Gcc-bugs wrote:
> 
> > > That said, if C allows us to limit to 128bits then let's do that for now.
> > > 32bit targets will still see all the complication when we give that a stab.
> > 
> > I'm afraid once we define BITINT_MAXWIDTH, it will become part of the ABI, so
> > we can't increase it afterwards.
> 
> I don't think it's part of the ABI; I think it's always OK to increase 
> BITINT_MAXWIDTH, as long as the wider types don't need more alignment than 
> the previous choice of max_align_t.

It's not part of the ABI until people put _BitInt(BITINT_MAXWIDTH) on ABI
boundaries of their libraries. If a ridiculously large BITINT_MAXWIDTH does
nothing more than discourages usages of _BitInt(BITINT_MAXWIDTH) in general,
than that's already great. We don't need an other intmax.

Also I don't want to think about the max N for _BitInt(N), similarly how I
don't want to think about the max N for int[N]. There might be implementation
limits, but it should be high enough so I don't have to think about those for
everyday coding.

> Thus, starting with a 128-bit limit (or indeed a 64-bit limit on 32-bit 
> platforms, so that all the types fix within existing modes supported for 
> arithmetic), and adding support for wider _BitInt later, would be a 
> reasonable thing to do.

I disagree.

> (You still have ABI considerations even with such a limit: apart from the 
> padding question, on x86_64 the ABI says _BitInt(128) is 64-bit aligned 
> but __int128 is 128-bit aligned.)
> 
> > Anyway, I'm afraid we probably don't have enough time to implement this
> > properly in stage1, so might need to target GCC 14 with it.  Unless somebody
> > spends on it
> > the remaining 2 weeks full time.
> 
> I think https://gcc.gnu.org/pipermail/gcc/2022-October/239704.html is 
> still current as a list of C2x language features likely not to make it 
> into GCC 13.  (I hope to get auto and constexpr done in the next two 
> weeks, and the other C2x language features not on that list are done.)