Re: [committed] libstdc++: Fix detection of std::format support for __float128 [PR107693]

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 15 Nov 2022 at 14:42, Jakub Jelinek wrote:
>
> On Tue, Nov 15, 2022 at 02:31:19PM +0000, Jonathan Wakely via Gcc-patches wrote:
> > Tested x86_64-linux and x86_64-w64-mingw32. Pushed to trunk.
> >
> > -- >8 --
> >
> > std::format gives linker errors on targets that define __float128 but
> > do not support using it with std::to_chars. This improves the handling
> > of 128-bit flaoting-point types so they are disabled if unsupportable.
> >
> > libstdc++-v3/ChangeLog:
> >
> >       PR libstdc++/107693
> >       * include/std/format (_GLIBCXX_FORMAT_F128): Define to 2 when
> >       basic_format_arg needs to use its _M_f128 member.
> >       (__extended_floating_point, __floating_point): Replace with ...
> >       (__formattable_floating_point): New concept.
> >       * testsuite/std/format/functions/format.cc: Check whether
> >       __float128 is supported. Also test _Float128.
>
> > --- a/libstdc++-v3/include/std/format
> > +++ b/libstdc++-v3/include/std/format
>
> > +#elif __FLT128_DIG__ && defined(__GLIBC_PREREQ) // see floating_to_chars.cc
>
> I'd just use here
> #elif __FLT128_DIG__ && defined(_GLIBCXX_HAVE_FLOAT128_MATH)
> instead.
>
> The reason for defined(__GLIBC_PREREQ) in floating_{to,from}_chars.cc
> is that I didn't want to make the ABI of linux libstdc++.so.6 dependent
> on whether gcc was built against glibc 2.26+ or older glibc.
> So, the symbols exist in libstdc++.so.6 even for older glibcs, but it will
> actually only work properly (without losing precision; otherwise it will
> just go through long double) if at runtime one uses glibc 2.26+.

Yes, and my intention was that std::format would also support
__float128, with the same imprecise behaviour. But could just limit
std::format support to when std::to_chars works properly.

> But in the headers, defined(_GLIBCXX_HAVE_FLOAT128_MATH) is used everywhere
> else (which is true only when compiling against glibc 2.26+).