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

[Jakub Jelinek <jakub@redhat.com>]

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+.

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

	Jakub