Re: [musl] patches for C23

["Jₑₙₛ Gustedt" <jens.gustedt@inria.fr>]

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Rich,

on Wed, 3 May 2023 15:33:26 -0400 you (Rich Felker <dalias@libc.org>)
wrote:

> On Wed, May 03, 2023 at 08:46:56PM +0200, Jₑₙₛ Gustedt wrote:
> > Rich,
> > 
> > on Wed, 3 May 2023 13:28:02 -0400 you (Rich Felker
> > <dalias@libc.org>) wrote:
> >   
> > > On Wed, May 03, 2023 at 05:11:11PM +0200, Jₑₙₛ Gustedt wrote:  
>  [...]  
>  [...]  
> > >  [...]  
> > >  [...]    
>  [...]  
>  [...]  
> > > 
> > > Again, there are not multiple versions of musl with different
> > > features depending on which compiler was used to compile them.
> > > There is one unified feature set. There are not configure-time or
> > > compile-time decisions about which features to support.  
> > 
> > This sounds a bit dogmatic  
> 
> Yes, it's one of the core principles of musl: that we don't have
> build-time-selectable feature-set like uclibc did.
> 
> > and also unrealistic. As said the dependency
> > on compiler builtins undermines that approach. Future versions of
> > gcc and clang will soon support `va_start` with only one parameter
> > for example. Musl will just be dependent on that compiler feature.  
> 
> No it won't. None of the code in musl calls or needs to call va_start
> with one parameter. You're confusing

??

> header-level stuff that a c23
> application might depend on, with build dependencies of libc.
> 
> > How will you do with optional features, then? For example decimal
> > floating point? This will never be added to musl? (Nobody will
> > probably backport support for them to very old gcc versions, for
> > example, or even to more recent versions of clang)  
> 
> Decimal float math library will likely be left to a third-party
> library implementation.
> 
> Decimal float in printf, if that becomes a thing, will be done the
> same way as int128: stub to pop the arguments, and 100% integer code
> to actually work with the data.

>  [...]  
> > > 
> > > The compiler used to compile musl and the compiler used to
> > > compile the application using musl have nothing to do with each
> > > other except sharing a baseline ABI target.  
> > 
> > Yes, exactly. And one supporting `__int128` and the other that
> > doesn't basically wouldn't interfere.  
> 
> The premise here is that applications and libc are being built by
> possibly different people with different tools. If I have a system
> built with gcc 5.3, I can't build C23 applications, but I might get a
> dynamically-linked C23 binary from someone who can. That binary needs
> to run with my musl-1.2.7 (made-up number) libc.so because the C
> language version the binary was generated from (or whether it was even
> C at all) is irrelevant. The interface surface is just the musl ABI
> surface.
> 
> > For the support of `__int128`: gcc has this since ages on 64 bit
> > archs, is there any such arch out there where this support is
> > changing according to versions of gcc that are still in use? So if
> > we make the  
> 
> We also support pcc, cparser+libfirm, etc. on archs they support. Not
> just gcc. And gcc back to 3.x.
> 
> > availability of `__int128` dependent on `UINTPTR_WIDTH` being 64,
> > would that be acceptable for you? Or an even more dependent approach
> > with special casing architectures where this is available since
> > always?  
> 
> It's not really "special casing archs where this is available since
> always". It's more like the other way around, "not special casing
> archs where __int128 is a guaranteed part of the baseline psABI". For
> those we can just let the default C implementation be used. For the
> rest we need a (completely trivial) asm stub that pops the arg
> according to the variadic argument ABI for the arch. This really isn't
> that big a deal. It's a few instructions at most.

I would still prefer that on those archs where there is `__int128` or
`_BitInt(128)` (for the latter basically all C23 compilers, I think)
that the default is done with that compiler support. We should leave
to the compiler people what they do best ;-)

This leaves us with fallback code to write that will probably rarely
be used. Also, I have difficulties to asses the effort that is
needed. There are the `printf`, `scanf` and the new bit-fiddeling
interfaces. For the latter the current proposal is to have them
implemented as shallow static inline functions. That would a bit
complicated without compiler support.

In all to me this sounds like a substantial effort in implementation
and coordination. What is the way forward, here?

Thanks
Jₑₙₛ

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