Re: [musl] [C23 128 bit 4/4] C23: implement proper support for int128_t and uint128_t

[Rich Felker <dalias@libc.org>]

On Wed, May 31, 2023 at 05:07:00PM +0200, Jₑₙₛ Gustedt wrote:
> Rich,
> 
> on Wed, 31 May 2023 10:57:24 -0400 you (Rich Felker <dalias@libc.org>)
> wrote:
> 
> > On Wed, May 31, 2023 at 04:55:45PM +0200, Jₑₙₛ Gustedt wrote:
> > > Rich,
> > > 
> > > on Wed, 31 May 2023 10:41:29 -0400 you (Rich Felker
> > > <dalias@libc.org>) wrote:
> > >   
> > > > On Wed, May 31, 2023 at 04:36:43PM +0200, Jₑₙₛ Gustedt wrote:  
> >  [...]  
> >  [...]  
> >  [...]  
> > > > 
> > > > Can you cite that?  
> > > 
> > > sure, almost by heart, since I wrote that ;-)
> > > 
> > >   … with the possible exceptions of signed bit-precise integer types
> > >   and of signed extended integer types that are wider than `long
> > > long` and that are referred by the type definition for an exact
> > > width integer type
> > >   
> > > > Because I don't see it. I still see that intmax_t
> > > > has to be at least as wide as all the intN_t.  
> > > 
> > > I seems that you read that the wrong way around.  
> > 
> > OK, so AIUI based on this exception it's permitted but not required to
> > offer int128_t.
> 
> yes
> 
> But compilers can never offer it if there is not minimal C library
> support, which we are doing here. This is the only way we found in
> yearlong discussions in WG14 to get us out of this intmax_t ABI trap.
> 
> For the 128 bit types in particular this answers numerous requests by
> users who want to have these in different contexts and where quite
> frustrated that compilers have these since decades but where not able
> to announce them officially, not even as extended integer types.

OK, well this whole thread/topic then is a request/proposal for
extended functionality, not part of C23 support, and needs to be
considered as such.

I'm sorry there seems to have been a misunderstanding here. I'm trying
to understand where you're coming from, and I think now you were
probably looking at the intmax_t situation as if int128_t was
something we wanted to offer, but couldn't because of ABI
requirements. Whereas I was always looking at compounding library
support for larger and larger types as an odious requirement that
intmax_t helped us avoid.

I *do* think there is demand for being able to compute with
larger-than-int64 integer types, and this is a good thing, but it's a
problem _BitInt solves entirely without imposing any heavy library
requirements. I don't think "printing a 128 bit number in decimal" is
very useful functionality. Hex, maybe, but then 256+ is really what
you would want (for key material etc).

I'm sorry for sending you down a road of implementing this stuff in a
way that'd be plausible for inclusion in musl based on a
misunderstanding that it was a requirement for C23 support.

Rich